Tree-ring δ15N as an indicator of nitrogen dynamics in stands with N2-fixing Alnus rubra

Tree-ring δ15N may depict site-specific, long-term patterns in nitrogen (N) dynamics under N2-fixing species, but field trials with N2-fixing tree species are lacking and the relationship of temporal patterns in tree-ring δ15N to soil N dynamics is controversial. We examined whether the tree-ring δ15N of N2-fixing red alder (Alnus rubra Bong.) would mirror N accretion rates and δ15N of soils and whether the influence of alder-fixed N could be observed in the wood of a neighboring conifer. We sampled a 27-year-old replacement series trial on south-eastern Vancouver Island, with red alder and coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) planted in five proportions (0/100, 11/89, 25/75, 50/50 and 100/0) at a uniform stem density. An escalation in forest floor N content was evident with an increasing proportion of red alder, equivalent to a difference of ~750 kg N ha-1 between 100% Douglas-fir versus 100% alder. The forest floor horizon also had high δ15N values in treatments with more red alder. Red alder had a consistent quadratic fit in tree-ring δ15N over time, with a net increase of $\sim$1.5‰, on average, from initial values, followed by a plateau or slight decline. Douglas-fir tree-ring δ15N, in contrast, was largely unchanged over time (in three of four plots) but was significantly higher in the 50/50 mix. The minor differences in current leaf litter N content and δ15N between alder and Douglas-fir, coupled with declining growth in red alder, suggests the plateau or declining trend in alder tree-ring δ15N could coincide with lower N2-fixation rates, potentially by loss in alder vigor at canopy closure, or down-regulation via nitrate availability.

Introduction of non-native Douglas fir reduces leaf damage on beech saplings and mature trees in European beech forests

Recent ecological research suggests that, in general, mixtures are more resistant to insect herbivores and pathogens than monocultures. However, we know little about mixtures with non-native trees, where enemy release could lead to patterns that differ from commonly observed relationships among native species. This becomes particularly relevant when considering that adaptation strategies to climate change increasingly promote a larger share of non-native tree species, such as North American Douglas fir in Central Europe. We studied leaf damage on European beech (Fagus sylvatica) saplings and mature trees across a wide range of site conditions in monocultures and mixtures with phylogenetically distant conifers native Norway spruce (Picea abies) and non-native Douglas fir (Pseudotsuga menziesii). We analyzed leaf herbivory and pathogen damage in relation to tree diversity and composition effects, as well as effects of environmental factors and plant characteristics. We observed lower sapling herbivory and tree sucking damage on beech in non-native Douglas fir mixtures than in beech monocultures, probably due to a lower herbivore diversity on Douglas fir trees, and higher pathogen damage on beech saplings in Norway spruce than Douglas fir mixtures, possibly because of higher canopy openness. Our findings suggest that for low diversity gradients, tree diversity effects on leaf damage can strongly depend on tree species composition, in addition to modifications caused by feeding guild and tree ontogeny. Moreover, we found that nutrient capacity modulated the effects of tree diversity, composition, and environmental factors, with different responses in sites with low or high nutrient capacity. The existence of contrasting diversity effects based on tree species composition provides important information on our understanding of the relationships between tree diversity and plant-herbivore interactions in light of non-native tree species introductions. Especially with recent Norway spruce die-off, the planting of Douglas fir as replacement is likely to strongly increase in Central Europe. Our findings suggest that mixtures with Douglas fir could benefit the survival or growth rates of beech saplings and mature trees due to lower leaf damage, emphasizing the need to clearly identify and compare the potential benefits and ecological trade-offs of non-native tree species in forest management under ongoing environmental change.

Predictive accuracy of post-fire conifer death declines over time in models based on crown and bole injury

A key uncertainty of empirical models of post-fire tree mortality is understanding the drivers of elevated post-fire mortality several years following fire, known as delayed mortality. Delayed mortality can represent a substantial fraction of mortality, particularly for large trees that are a conservation focus in western US coniferous forests. Current post-fire tree mortality models have undergone limited evaluation of how injury level and time since fire interact to influence model accuracy and predictor variable importance. Less severe injuries potentially serve as an indicator for vulnerability to additional stressors such as bark beetle attack or moisture stress. We used a collection of 164,293 individual tree records to examine post-fire tree mortality in eight western USA conifers: Abies concolor, Abies grandis, Calocedrus decurrens, Larix occidentalis, Pinus contorta, Pinus lambertiana, Pinus ponderosa, and Pseudotsuga menziesii. We evaluated the importance of fire injury predictors on discriminating between surviving trees versus immediate and delayed post-fire mortality. We fit balanced random forest models for each species using cumulative tree mortality from 1 to 5-years post-fire. We compared these results to multi-class random forest models using first-year mortality, 2-5-year mortality, and survival 5-years post-fire as a response variable. Crown volume scorched, diameter at breast height, and relative bark char height, were used as predictor variables. The cumulative mortality models all predicted trees that died within 1-year of fire with high accuracy but failed to predict 2-5-year mortality. The multi-class models were an improvement but had lower accuracy for predicting 2-5-year mortality. Multi-class model accuracies ranged from 85% to 95% across all species for predicting 1-year post-fire mortality, 42%-71% for predicting 2-5-year mortality, and 64%-85% for predicting trees that lived past 5-years. Our study highlights the differences in tree species tolerance to fire injury and suggests that including second-order predictors such as beetle attack or climatic water stress before and after fire will be critical to improve accuracy and better understand the mechanisms and patterns of fire-caused tree death. Random forest models have potential for management applications such as post-fire harvesting and simulating future stand dynamics.

Differences in branch hydraulic architecture related to the aridity of growing sites and seed sources of coastal Douglas-fir saplings

To better understand hydraulic adaptations of coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) to local climate, we examined genetic (G) and environmental (E) responses of branch hydraulic architecture of 7-year-old saplings from dry and wet climates of origin grown at a relatively dry and a relatively wet common garden site in western Oregon. We sampled 2 years of branch growth from three dry-source and three wet-source families grown at both sites (72 branches, total). Overall, only 4 of the 11 traits had significant genetic (G) effects, whereas 9 traits had significant environmental (E) effects (P < 0.05). Both dry and wet sources had higher leaf-specific conductance (kl) at the dry than the wet site, but the values were achieved by different mechanisms and driven by G × E effects for leaf area/sapwood area (Al/As), shoot length (L), specific conductivity (Ks) and leaf-specific conductivity (Kl). Dry sources achieved higher kl in the dry site through higher Kl (via a lower Al/As and no change in Ks) with no difference in L. Wet sources achieved higher kl at the dry site through no difference in Kl (via no effect on Al/As, despite decreases in Al and As, and lower Ks) with lower L. Vulnerability to embolism (measured as percentage loss of conductivity at 4 MPa) had no G effect but an E effect, with slightly lower values at the dry site. Specific leaf area had G and E effects, with lower values for the dry sources and site. There were no G or E effects on wood density. The different responses of dry and wet sources to site aridity suggest that populations are differentially adapted to the aridity of growing sites. Population variation in response to aridity should be considered when selecting seed sources for establishing forests for future climates.

Physiological responses of Douglas-fir to climate and forest disturbances as detected by cellulosic carbon and oxygen isotope ratios

Swiss needle cast (SNC), caused by a fungal pathogen, Nothophaeocryptopus gaeumannii, is a major forest disease of Douglas-fir (Pseudotsuga menziesii) stands of the Pacific Northwest (PNW). There is mounting concern that the current SNC epidemic occurring in Oregon and Washington will continue to increase in severity, frequency and spatial extent with future warming. Nothophaeocryptopus gaeumannii occurs wherever its host is found, but very little is known about the history and spatial distribution of SNC and its effects on growth and physiological processes of mature and old-growth forests within the Douglas-fir region of the PNW. Our findings show that stem growth and physiological responses of infected Douglas-fir to climate and SNC were different between sites, growth periods and disease severity based on cellulosic stable carbon and oxygen isotope ratios and ring width data in tree rings. At a coastal Oregon site within the SNC impact zone, variations in stem growth and Δ13C were primarily influenced by disproportional reductions in stomatal conductance (gs) and assimilation (A) caused by a loss of functioning stomates through early needle abscission and stomatal occlusion by pseudothecia of N. gaeumannii. At the less severely infected inland sites on the west slopes of Oregon's Cascade Range, stem growth correlated negatively with δ18O and positively with Δ13C, indicating that gs decreased in response to high evaporative demand with a concomitant reduction in A. Current- and previous-years summer vapor pressure deficit was the principal seasonal climatic variable affecting radial stem growth and the dual stable isotope ratios at all sites. Our results indicate that rising temperatures since the mid-1970s has strongly affected Douglas-fir growth in the PNW directly by a physiological response to higher evaporative demand during the annual summer drought and indirectly by a major SNC epidemic that is expanding regionally to higher latitudes and higher elevations.

Friendly neighbours: Hydraulic redistribution accounts for one quarter of water used by neighbouring drought stressed tree saplings

Hydraulic redistribution (HR) can buffer drought events of tree individuals, however, its relevance for neighbouring trees remains unclear. Here, we quantified HR to neighbouring trees in single- and mixed-species combinations. We hypothesized that uptake of HR water positively correlates with root length, number of root tips and root xylem hydraulic conductivity and that neighbours in single-species combinations receive more HR water than in phylogenetic distant mixed-species combinations. In a split-root experiment, a sapling with its roots split between two pots redistributed deuterium labelled water from a moist to a dry pot with an additional tree each. We quantified HR water received by the sapling in the dry pot for six temperate tree species. After 7 days, one quarter of the water in roots (2.1 ± 0.4 ml), stems (0.8 ± 0.2 ml) and transpiration (1.0 ± 0.3 ml) of the drought stressed sapling originated from HR. The amount of HR water transpired by the receiving plant stayed constant throughout the experiment. While the uptake of HR water increased with root length, species identity did not affect HR as saplings of Picea abies ((L.) Karst) and Fagus sylvatica (L.) in single- and mixed-species combinations received the same amount of HR water.

Projections of water, carbon, and nitrogen dynamics under future climate change in an old-growth Douglas-fir forest in the western Cascade Range using a biogeochemical model

Statistically downscaled climate change scenarios from four General Circulation Models for two Representative Concentration Pathways (RCP) were applied as inputs to a biogeochemical model, PnET-BGC, to examine potential future dynamics of water, carbon, and nitrogen in an old-growth Douglas-fir forest in the western Cascade Range. Projections show 56% to 77% increases in stomatal conductance throughout the year from 1986-2010 to 2076-2100, and 65% to 104% increases in leaf carbon assimilation between October and June over the same period. However, future dynamics of water and carbon under the RCP scenarios are affected by a 49% to 86% reduction in foliar biomass resulting from severe air temperature and humidity stress to the forest in summer. Important implications of future decreases in foliar biomass include 1) 20% to 71% decreases in annual transpiration which increase soil moisture by 7% to 15% in summer and fall; 2) decreases in photosynthesis by 77% and soil organic matter by 62% under the high radiative forcing scenario; and 3) altered foliar and soil carbon to nitrogen stoichiometry. Potential carbon dioxide fertilization effects on vegetation are projected to 1) amplify decreases in transpiration by 4% to 9% and increases in soil moisture in summer and fall by 1% to 2%; and 2) alleviate decreases in photosynthesis by 4%; while 3) having negligible effects on the dynamics of nitrogen. Our projections suggest that future decrease in transpiration and moderate water holding capacity may mitigate soil moisture stress to the old-growth Douglas-fir forest. Future increases in nitrogen concentration in soil organic matter are projected to alleviate the decrease in net nitrogen mineralization despite a reduction in decomposition of soil organic matter by the end of the century.

Drought effects on root and needle terpenoid content of a coastal and an interior Douglas fir provenance

Douglas fir (Pseudotsuga menziesii) is a conifer species that stores large amounts of terpenoids, mainly monoterpenoids in resin ducts of various tissues. The effects of drought on stored leaf terpenoid concentrations in trees are scarcely studied and published data are partially controversial, since reduced, unaffected or elevated terpenoid contents due to drought have been reported. Even less is known on the effect of drought on root terpenoids. In the present work, we investigated the effect of reduced water availability on the terpenoid content in roots and needles of Douglas fir seedlings. Two contrasting Douglas fir provenances were studied: an interior provenance (var. glauca) with assumed higher drought resistance, and a coastal provenance (var. menziesii) with assumed lower drought resistance. We tested the hypothesis that both provenances show specific patterns of stored terpenoids and that the patterns will change in response to drought in both, needles and roots. We further expected stronger changes in the less drought tolerant coastal provenance. For this purpose, we performed an experiment under controlled conditions, in which the trees were exposed to moderate and severe drought stress. According to our expectations, the study revealed clear provenance-specific terpenoid patterns in needles. However, such patterns were not detected in the roots. Drought slightly increased the needle terpenoid contents of the coastal but not of the interior provenance. We also observed increased terpenoid abundance mainly in roots of the moderately stressed coastal provenance. Overall, from the observed provenance-specific reactions with increased terpenoid levels in trees of the coastal origin in response to drought, we conclude on functions of terpenoids for abiotic stress tolerance that might be fulfilled by other, constitutively expressed mechanisms in drought-adapted interior provenances.

Silver fir and Douglas fir are more tolerant to extreme droughts than Norway spruce in south-western Germany

Improving our understanding of the potential of forest adaptation is an urgent task in the light of predicted climate change. Long-term alternatives for susceptible yet economically important tree species such as Norway spruce (Picea abies) are required, if the frequency and intensity of summer droughts will continue to increase. Although Silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) have both been described as drought-tolerant species, our understanding of their growth responses to drought extremes is still limited. Here, we use a dendroecological approach to assess the resistance, resilience, and recovery of these important central Europe to conifer species the exceptional droughts in 1976 and 2003. A total of 270 trees per species were sampled in 18 managed mixed-species stands along an altitudinal gradient (400-1200 m a.s.l.) at the western slopes of the southern and central Black Forest in southwest Germany. While radial growth in all species responded similarly to the 1976 drought, Norway spruce was least resistant and resilient to the 2003 summer drought. Silver fir showed the overall highest resistance to drought, similarly to Douglas fir, which exhibited the widest growth rings. Silver fir trees from lower elevations were more drought prone than trees at higher elevations. Douglas fir and Norway spruce, however, revealed lower drought resilience at higher altitudes. Although the 1976 and 2003 drought extremes were quite different, Douglas fir maintained consistently the highest radial growth. Although our study did not examine population-level responses, it clearly indicates that Silver fir and Douglas fir are generally more resistant and resilient to previous drought extremes and are therefore suitable alternatives to Norway spruce; Silver fir more so at higher altitudes. Cultivating these species instead of Norway spruce will contribute to maintaining a high level of productivity across many Central European mountain forests under future climate change.

Photosynthetic phenological variation may promote coexistence among co-dominant tree species in a Madrean sky island mixed conifer forest

Across much of western North America, forests are predicted to experience a transition from snow- to rain-dominated precipitation regimes due to anthropogenic climate warming. Madrean sky island mixed conifer forests receive a large portion of their precipitation from summertime convective storms and may serve as a lens into the future for snow-dominated forests after prolonged warming. To better understand the linkage between physiological traits, climate variation, and the structure and function of mixed conifer forests, we measured leaf photosynthetic (A) responses to controlled variation in internal CO2 concentration (Ci) to quantify interspecific phenological variation in A/Ci-derived ecophysiological traits among ponderosa pine (Pinus ponderosa Lawson and C. Lawson), southwestern white pine (Pinus strobiformis Engelm.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Species had similar, positive responses in net photosynthesis under ambient conditions (Anet) to the onset of summertime monsoonal precipitation, but during the cooler portions of the year P. ponderosa was able to maintain greater Anet than P. menziesii and P. strobiformis. Moreover, P. ponderosa had greater Anet in response to ephemerally favorable springtime conditions than either P. menziesii or P. strobiformis. Monsoonal precipitation was associated with a sharp rise in the maximum rates of electron transport (Jmax) and carboxylation (VCmax) in P. menziesii in comparison with P. ponderosa and P. strobiformis. In contrast, species shared similar low values of Jmax and VCmax in response to cool winter temperatures. Patterns of relative stomatal limitation followed predictions based on species' elevational distributions, reinforcing the role of stomatal behavior in maintaining hydraulic conductivity and shaping bioclimatic limits. Phenological variation in ecophysiologial traits among co-occurring tree species in a Madrean mixed conifer forest may promote temporal resource partitioning and thereby contribute to species' coexistence. Moreover, these results provide a physiological basis for predicting the ecological implications of North American mixed conifer forests currently transitioning from snow- to rain-dominated precipitation regimes.

Leaf water 18 O and 2 H enrichment along vertical canopy profiles in a broadleaved and a conifer forest tree

Distinguishing meteorological and plant-mediated drivers of leaf water isotopic enrichment is prerequisite for ecological interpretations of stable hydrogen and oxygen isotopes in plant tissue. We measured input and leaf water δ² H and δ¹⁸ O as well as micrometeorological and leaf morpho-physiological variables along a vertical gradient in a mature angiosperm (European beech) and gymnosperm (Douglas fir) tree. We used these variables and different enrichment models to quantify the influence of Péclet and non-steady state effects and of the biophysical drivers on leaf water enrichment. The two-pool model accurately described the diurnal variation of leaf water enrichment. The estimated unenriched water fraction was linked to leaf dry matter content across the canopy heights. Non-steady state effects and reduced stomatal conductance caused a higher enrichment of Douglas fir compared to beech leaf water. A dynamic effect analyses revealed that the light-induced vertical gradients of stomatal conductance and leaf temperature outbalanced each other in their effects on evaporative enrichment. We conclude that neither vertical canopy gradients nor the Péclet effect is important for estimates and interpretation of isotopic leaf water enrichment in hypostomatous trees. Contrarily, species-specific non-steady state effects and leaf temperatures as well as the water vapour isotope composition need careful consideration.

Species distribution models may misdirect assisted migration: insights from the introduction of Douglas-fir to Europe

Species distribution models (SDMs), which statistically relate species occurrence to climatic variables, are widely used to identify areas suitable for species growth under future climates and to plan for assisted migration. When SDMs are projected across times or spaces, it is assumed that species climatic requirements remain constant. However, empirical evidence supporting this assumption is rare, and SDM predictions could be biased. Historical human-aided movements of tree species can shed light on the reliability of SDM predictions in planning for assisted migration. We used Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), a North American conifer introduced into Europe during the mid-19th century, as a case-study to test niche conservatism. We combined transcontinental data sets of Douglas-fir occurrence and climatic predictors to compare the realized niches between native and introduced ranges. We calibrated a SDM in the native range and compared areas predicted to be climatically suitable with observed presences. The realized niches in the native and introduced ranges showed very limited overlap. The SDM calibrated in North America had very high predictive power in the native range, but failed to predict climatic suitability in Europe where Douglas-fir grows in climates that have no analogue in the native range. We review the ecological mechanisms and silvicultural practices that can trigger such shifts in realized niches. Retrospective analysis of tree species introduction revealed that the assumption of niche conservatism is erroneous. As a result, distributions predicted by SDM are importantly biased. There is a high risk that assisted migration programs may be misdirected and target inadequate species or introduction zones.

Thermotolerance and heat stress responses of Douglas-fir and ponderosa pine seedling populations from contrasting climates

Temperature and the frequency and intensity of heat waves are predicted to increase throughout the 21st century. Germinant seedlings are expected to be particularly vulnerable to heat stress because they are in the boundary layer close to the soil surface where intense heating occurs in open habitats. We quantified leaf thermotolerance and whole-plant physiological responses to heat stress in first-year germinant seedlings in two populations each of Pinus ponderosa P. and C. Lawson (PIPO) and Pseudotsuga menziesii (Mirb.) Franco (PSME) from climates with contrasting precipitation and temperature regimes. Thermotolerance of detached needles was evaluated using chlorophyll fluorescence (FV/FM, FO) and electrolyte leakage. PSME was more heat tolerant than PIPO according to both independent assessments of thermotolerance. Following exposure of whole seedlings to a simulated heat wave at 45 °C for 1 h in a growth chamber, we monitored FV/FM, photosynthesis, stomatal conductance, non-structural carbohydrates (NSCs) and carbon isotope ratios (δ13C) for 14 days. Heat treatment induced significant reductions in FV/FM in both species and a transient reduction in photosynthetic gas exchange only in PIPO 1 day after treatment. Heat treatment induced an increase in glucose + fructose concurrent with a decrease in starch in both species, whereas total NSC and sucrose were not affected by heat treatment. The negative relationship between glucose + fructose and starch observed in treated plants may be due to the conversion of starch to glucose + fructose to aid recovery from heat-induced damage. Populations from drier sites displayed greater δ13C values than those from wetter sites, consistent with higher intrinsic water-use efficiency and drought resistance of populations from drier climates. Thermotolerance and heat stress responses appeared to be phenotypically plastic and representative of the environment in which plants were grown, whereas intrinsic water-use efficiency appeared to reflect ecotypic differentiation and the climate of origin.

Variation in short-term and long-term responses of photosynthesis and isoprenoid-mediated photoprotection to soil water availability in four Douglas-fir provenances

For long-lived forest tree species, the understanding of intraspecific variation among populations and their response to water availability can reveal their ability to cope with and adapt to climate change. Dissipation of excess excitation energy, mediated by photoprotective isoprenoids, is an important defense mechanism against drought and high light when photosynthesis is hampered. We used 50-year-old Douglas-fir trees of four provenances at two common garden experiments to characterize provenance-specific variation in photosynthesis and photoprotective mechanisms mediated by essential and non-essential isoprenoids in response to soil water availability and solar radiation. All provenances revealed uniform photoprotective responses to high solar radiation, including increased de-epoxidation of photoprotective xanthophyll cycle pigments and enhanced emission of volatile monoterpenes. In contrast, we observed differences between provenances in response to drought, where provenances sustaining higher CO2 assimilation rates also revealed increased water-use efficiency, carotenoid-chlorophyll ratios, pools of xanthophyll cycle pigments, β-carotene and stored monoterpenes. Our results demonstrate that local adaptation to contrasting habitats affected chlorophyll-carotenoid ratios, pool sizes of photoprotective xanthophylls, β-carotene, and stored volatile isoprenoids. We conclude that intraspecific variation in isoprenoid-mediated photoprotective mechanisms contributes to the adaptive potential of Douglas-fir provenances to climate change.

Immediate and potential long-term effects of consecutive heat waves on the photosynthetic performance and water balance in Douglas-fir

The frequency and intensity of climatic extremes, such as heat waves, are predicted to increase globally, with severe implications for terrestrial carbon and water cycling. Temperatures may rise above critical thresholds that allow trees to function optimally, with unknown long-term consequences for forest ecosystems. In this context, we investigated how photosynthetic traits and the water balance in Douglas-fir are affected by exposure to three heat waves with temperatures about 12°C above ambient. Photosynthetic carboxylation efficiency (V_cmax) was mostly unaffected, but electron transport (J_max) and photosynthetic rates under saturating light (A_sat) were strongly influenced by the heat waves, with lagging limitations on photosynthesis still being observed six weeks after the last heat wave. We also observed lingering heat-induced inhibitions on transpiration, minimum stomatal conductance, and night-time stomatal conductance (g_s-night). Results from the stomatal models used to calculate minimum stomatal conductance were similar to g_s-night and indicated changes in leaf morphology, e.g. stomatal occlusions and alterations in epicuticular wax. Our results show Douglas-fir's ability to restrict water loss following heat stress, but at the price of reduced photosynthetic performance. Such limitations indicate potential long-term restrictions that heat waves can impose on tree development and functioning under extreme climatic conditions.

Volatile and Within-Needle Terpene Changes to Douglas-fir Trees Associated With Douglas-fir Beetle (Coleoptera: Curculionidae) Attack

Mass attack by tree-killing bark beetles (Curculionidae: Scolytinae) brings about large chemical changes in host trees that can have important ecological consequences. For example, mountain pine beetle (Dendroctonus ponderosae Hopkins) attack increases emission of terpenes by lodgepole pine (Pinus contorta Dougl. ex Loud.), affecting foliage flammability with consequences for wildfires. In this study, we measured chemical changes to Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) foliage in response to attack by Douglas-fir beetles (Dendroctonus pseudotsugae Hopkins) as trees die and crowns transitioned from green/healthy, to green-infested (year of attack), to yellow (year after attack), and red (2 yr after attack). We found large differences in volatile and within-needle terpene concentrations among crown classes and variation across a growing season. In general, emissions and concentrations of total and individual terpenes were greater for yellow and red needles than green needles. Douglas-fir beetle attack increased emissions and concentrations of terpene compounds linked to increased tree flammability in other conifer species and compounds known to attract beetles (e.g., [Formula: see text]-pinene, camphene, and D-limonene). There was little relationship between air temperature or within-needle concentrations of terpenes and emission of terpenes, suggesting that passive emission of terpenes (e.g., from dead foliage) does not fully explain changes in volatile emissions. The potential physiological causes and ecological consequences of these bark beetle-associated chemical changes are discussed.

Water availability as dominant control of heat stress responses in two contrasting tree species

Heat waves that trigger severe droughts are predicted to increase globally; however, we lack an understanding of how trees respond to the combined change of extreme temperatures and water availability. Here, we studied the impacts of two consecutive heat waves as well as post-stress recovery in young Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) and Robinia pseudoacacia L. (black locust) growing under controlled conditions. Responses were compared under water supply close to the long-term average and under reduced irrigation to represent drought. Exposure to high temperatures (+10 °C above ambient) and vapour pressure deficit strongly affected the trees in terms of water relations, photosynthesis and growth. Douglas-fir used water resources conservatively, and transpiration decreased in response to mild soil water limitation. In black locust, heat stress led to pronounced tree water deficits (stem diameter shrinkage), accompanied by leaf shedding to alleviate stress on the hydraulic system. The importance of water availability during the heat waves became further apparent by a concurrent decline in photosynthesis and stomatal conductance with increasing leaf temperatures in both species, reaching the lowest rates in the heat-drought treatments. Stress severity determined both the speed and the amount of recovery. Upon release of stress, photosynthesis recovered rapidly in drought-treated black locust, while it remained below control rates in heat (t = -2.4, P < 0.05) and heat-drought stressed trees (t = 2.96, P < 0.05). In Douglas-fir, photosynthesis recovered quickly, while water-use efficiency increased in heat-drought trees because stomatal conductance remained reduced (t = -2.92, P < 0.05). Moreover, Douglas-fir was able to compensate for stem-growth reductions following heat (-40%) and heat-drought stress (-68%), but most likely at the expense of storage and other growth processes. Our results highlight the importance of studying heat waves alongside changes in water availability. They further suggest that we should look beyond the actual stress event to identify lagged effects and acclimation processes that may determine tree resilience in the long term.

Host and habitat filtering in seedling root-associated fungal communities: taxonomic and functional diversity are altered in 'novel' soils

Climatic and land use changes have significant consequences for the distribution of tree species, both through natural dispersal processes and following management prescriptions. Responses to these changes will be expressed most strongly in seedlings near current species range boundaries. In northern temperate forest ecosystems, where changes are already being observed, ectomycorrhizal fungi contribute significantly to successful tree establishment. We hypothesised that communities of fungal symbionts might therefore play a role in facilitating, or limiting, host seedling range expansion. To test this hypothesis, ectomycorrhizal communities of interior Douglas-fir and interior lodgepole pine seedlings were analysed in a common greenhouse environment following growth in five soils collected along an ecosystem gradient. Currently, Douglas-fir's natural distribution encompasses three of the five soils, whereas lodgepole pine's extends much further north. Host filtering was evident amongst the 29 fungal species encountered: 7 were shared, 9 exclusive to Douglas-fir and 13 exclusive to lodgepole pine. Seedlings of both host species formed symbioses with each soil fungal community, thus Douglas-fir did so even where those soils came from outside its current distribution. However, these latter communities displayed significant taxonomic and functional differences to those found within the host distribution, indicative of habitat filtering. In contrast, lodgepole pine fungal communities displayed high functional similarity across the soil gradient. Taxonomic and/or functional shifts in Douglas-fir fungal communities may prove ecologically significant during the predicted northward migration of this species; especially in combination with changes in climate and management operations, such as seed transfer across geographical regions for forestry purposes.

Impact of climate change on cold hardiness of Douglas-fir (Pseudotsuga menziesii): environmental and genetic considerations

The success of conifers over much of the world's terrestrial surface is largely attributable to their tolerance to cold stress (i.e., cold hardiness). Due to an increase in climate variability, climate change may reduce conifer cold hardiness, which in turn could impact ecosystem functioning and productivity in conifer-dominated forests. The expression of cold hardiness is a product of environmental cues (E), genetic differentiation (G), and their interaction (G × E), although few studies have considered all components together. To better understand and manage for the impacts of climate change on conifer cold hardiness, we conducted a common garden experiment replicated in three test environments (cool, moderate, and warm) using 35 populations of coast Douglas-fir (Pseudotsuga menziesii var. menziesii) to test the hypotheses: (i) cool-temperature cues in fall are necessary to trigger cold hardening, (ii) there is large genetic variation among populations in cold hardiness that can be predicted from seed-source climate variables, (iii) observed differences among populations in cold hardiness in situ are dependent on effective environmental cues, and (iv) movement of seed sources from warmer to cooler climates will increase risk to cold injury. During fall 2012, we visually assessed cold damage of bud, needle, and stem tissues following artificial freeze tests. Cool-temperature cues (e.g., degree hours below 2 °C) at the test sites were associated with cold hardening, which were minimal at the moderate test site owing to mild fall temperatures. Populations differed 3-fold in cold hardiness, with winter minimum temperatures and fall frost dates as strong seed-source climate predictors of cold hardiness, and with summer temperatures and aridity as secondary predictors. Seed-source movement resulted in only modest increases in cold damage. Our findings indicate that increased fall temperatures delay cold hardening, warmer/drier summers confer a degree of cold hardiness, and seed-source movement from warmer to cooler climates may be a viable option for adapting coniferous forest to future climate.

Selecting Populations for Non-Analogous Climate Conditions Using Universal Response Functions: The Case of Douglas-Fir in Central Europe

Identifying populations within tree species potentially adapted to future climatic conditions is an important requirement for reforestation and assisted migration programmes. Such populations can be identified either by empirical response functions based on correlations of quantitative traits with climate variables or by climate envelope models that compare the climate of seed sources and potential growing areas. In the present study, we analyzed the intraspecific variation in climate growth response of Douglas-fir planted within the non-analogous climate conditions of Central and continental Europe. With data from 50 common garden trials, we developed Universal Response Functions (URF) for tree height and mean basal area and compared the growth performance of the selected best performing populations with that of populations identified through a climate envelope approach. Climate variables of the trial location were found to be stronger predictors of growth performance than climate variables of the population origin. Although the precipitation regime of the population sources varied strongly none of the precipitation related climate variables of population origin was found to be significant within the models. Overall, the URFs explained more than 88% of variation in growth performance. Populations identified by the URF models originate from western Cascades and coastal areas of Washington and Oregon and show significantly higher growth performance than populations identified by the climate envelope approach under both current and climate change scenarios. The URFs predict decreasing growth performance at low and middle elevations of the case study area, but increasing growth performance on high elevation sites. Our analysis suggests that population recommendations based on empirical approaches should be preferred and population selections by climate envelope models without considering climatic constrains of growth performance should be carefully appraised before transferring populations to planting locations with novel or dissimilar climate.

Defoliation of interior Douglas-fir elicits carbon transfer and stress signalling to ponderosa pine neighbors through ectomycorrhizal networks

Extensive regions of interior Douglas-fir (Pseudotsuga menziesii var. glauca, IDF) forests in North America are being damaged by drought and western spruce budworm (Choristoneura occidentalis). This damage is resulting from warmer and drier summers associated with climate change. To test whether defoliated IDF can directly transfer resources to ponderosa pine (Pinus ponderosae) regenerating nearby, thus aiding in forest recovery, we examined photosynthetic carbon transfer and defense enzyme response. We grew pairs of ectomycorrhizal IDF 'donor' and ponderosa pine 'receiver' seedlings in pots and isolated transfer pathways by comparing 35 μm, 0.5 μm and no mesh treatments; we then stressed IDF donors either through manual defoliation or infestation by the budworm. We found that manual defoliation of IDF donors led to transfer of photosynthetic carbon to neighboring receivers through mycorrhizal networks, but not through soil or root pathways. Both manual and insect defoliation of donors led to increased activity of peroxidase, polyphenol oxidase and superoxide dismutase in the ponderosa pine receivers, via a mechanism primarily dependent on the mycorrhizal network. These findings indicate that IDF can transfer resources and stress signals to interspecific neighbors, suggesting ectomycorrhizal networks can serve as agents of interspecific communication facilitating recovery and succession of forests after disturbance.

Climate-related genetic variation in drought-resistance of Douglas-fir (Pseudotsuga menziesii)

There is a general assumption that intraspecific populations originating from relatively arid climates will be better adapted to cope with the expected increase in drought from climate change. For ecologically and economically important species, more comprehensive, genecological studies that utilize large distributions of populations and direct measures of traits associated with drought-resistance are needed to empirically support this assumption because of the implications for the natural or assisted regeneration of species. We conducted a space-for-time substitution, common garden experiment with 35 populations of coast Douglas-fir (Pseudotsuga menziesii var. menziesii) growing at three test sites with distinct summer temperature and precipitation (referred to as 'cool/moist', 'moderate', or 'warm/dry') to test the hypotheses that (i) there is large genetic variation among populations and regions in traits associated with drought-resistance, (ii) the patterns of genetic variation are related to the native source-climate of each population, in particular with summer temperature and precipitation, (iii) the differences among populations and relationships with climate are stronger at the warm/dry test site owing to greater expression of drought-resistance traits (i.e., a genotype × environment interaction). During midsummer 2012, we measured the rate of water loss after stomatal closure (transpiration(min)), water deficit (% below turgid saturation), and specific leaf area (SLA, cm(2) g(-1)) on new growth of sapling branches. There was significant genetic variation in all plant traits, with populations originating from warmer and drier climates having greater drought-resistance (i.e., lower transpiration(min), water deficit and SLA), but these trends were most clearly expressed only at the warm/dry test site. Contrary to expectations, populations from cooler climates also had greater drought-resistance across all test sites. Multiple regression analysis indicated that Douglas-fir populations from regions with relatively cool winters and arid summers may be most adapted to cope with drought conditions that are expected in the future.

Douglas-fir seedlings exhibit metabolic responses to increased temperature and atmospheric drought

In the future, periods of strongly increased temperature in concert with drought (heat waves) will have potentially detrimental effects on trees and forests in Central Europe. Norway spruce might be at risk in the future climate of Central Europe. However, Douglas-fir is often discussed as an alternative for the drought and heat sensitive Norway spruce, because some provenances are considered to be well adapted to drier and warmer conditions. In this study, we identified the physiological and growth responses of seedlings from two different Douglas-fir provenances to increased temperature and atmospheric drought during a period of 92 days. We analysed (i) plant biomass, (ii) carbon stable isotope composition as an indicator for time integrated intrinsic water use efficiency, (iii) apparent respiratory carbon isotope fractionation as well as (iv) the profile of polar low molecular metabolites. Plant biomass was only slightly affected by increased temperatures and atmospheric drought but the more negative apparent respiratory fractionation indicated a temperature-dependent decrease in the commitment of substrate to the tricarboxylic acid cycle. The metabolite profile revealed that the simulated heat wave induced a switch in stress protecting compounds from proline to polyols. We conclude that metabolic acclimation successfully contributes to maintain functioning and physiological activity in seedlings of both Douglas-fir provenances under conditions that are expected during heat waves (i.e. elevated temperatures and atmospheric drought). Douglas-fir might be a potentially important tree species for forestry in Central Europe under changing climatic conditions.

Spatially nonrandom tree mortality and ingrowth maintain equilibrium pattern in an old-growth Pseudotsuga-Tsuga forest

Mortality processes in old-growth forests are generally assumed to be driven by gap-scale disturbance, with only a limited role ascribed to density-dependent mortality, but these assumptions are rarely tested with data sets incorporating repeated measurements. Using a 12-ha spatially explicit plot censused 13 years apart in an approximately 500-year-old Pseudotsuga-Tsuga forest, we demonstrate significant density-dependent mortality and spatially aggregated tree recruitment. However, the combined effect of these strongly nonrandom demographic processes was to maintain tree patterns in a state of dynamic equilibrium. Density-dependent mortality was most pronounced for the dominant late-successional species, Tsuga heterophylla. The long-lived, early-seral Pseudotsuga menziesii experienced an annual stem mortality rate of 0.84% and no new recruitment. Late-seral species Tsuga and Abies amabilis had nearly balanced demographic rates of ingrowth and mortality. The 2.34% mortality rate for Taxus brevifolia was higher than expected, notably less than ingrowth, and strongly affected by proximity to Tsuga. Large-diameter Tsuga structured both the regenerating conspecific and heterospecific cohorts with recruitment of Tsuga and Abies unlikely in neighborhoods crowded with large-diameter competitors (P < 0.001). Density-dependent competitive interactions strongly shape forest communities even five centuries after stand initiation, underscoring the dynamic nature of even equilibrial old-growth forests.

Resin duct size and density as ecophysiological traits in fire scars of Pseudotsuga menziesii and Larix occidentalis

BACKGROUND AND AIMS

Resin ducts (RDs) are features present in most conifer species as defence structures against pests and pathogens; however, little is known about RD expression in trees following fire injury. This study investigates changes in RD size and density in fire scars of Douglas fir (Pseudotsuga menziesii) and western larch (Larix occidentalis) as a means to evaluate the ecophysiological significance of traumatic resinosis for tree defence and survival.

METHODS

Transverse and tangential microsections were prepared for light microscopy and image analysis in order to analyse axial and radial RDs, respectively. Epithelial cells of RDs and fusiform rays associated with radial RDs were also examined. RDs were compared between normal xylem and wound xylem at four different section heights along the fire-injured stem.

KEY RESULTS

Following fire injury, P. menziesii axial RDs narrowed by 38-43 % in the first year after injury, and the magnitude of this change increased with stem height. Larix occidentalis axial RDs widened by 46-50 % in the second year after injury. Radial RDs were of equivalent size in P. menziesii, but widened by 162-214 % in L. occidentalis. Fusiform rays were larger following fire injury, by 4-14 % in P. menziesii and by 23-38 % in L. occidentalis. Furthermore, axial RD density increased in both species due to the formation of tangential rows of traumatic RDs, especially in the first and second years after injury. However, radial RD density did not change significantly.

CONCLUSIONS

These results highlight traumatic resinosis as a species-specific response. Pseudotsuga menziesii produce RDs of equivalent or reduced size, whereas L. occidentalis produce wider RDs in both the axial and radial duct system, thereby increasing resin biosynthesis and accumulation within the whole tree. Larix occidentalis thus appears to allocate more energy to defence than P. menziesii.

Climate refugia: joint inference from fossil records, species distribution models and phylogeography

Climate refugia, locations where taxa survive periods of regionally adverse climate, are thought to be critical for maintaining biodiversity through the glacial-interglacial climate changes of the Quaternary. A critical research need is to better integrate and reconcile the three major lines of evidence used to infer the existence of past refugia - fossil records, species distribution models and phylogeographic surveys - in order to characterize the complex spatiotemporal trajectories of species and populations in and out of refugia. Here we review the complementary strengths, limitations and new advances for these three approaches. We provide case studies to illustrate their combined application, and point the way towards new opportunities for synthesizing these disparate lines of evidence. Case studies with European beech, Qinghai spruce and Douglas-fir illustrate how the combination of these three approaches successfully resolves complex species histories not attainable from any one approach. Promising new statistical techniques can capitalize on the strengths of each method and provide a robust quantitative reconstruction of species history. Studying past refugia can help identify contemporary refugia and clarify their conservation significance, in particular by elucidating the fine-scale processes and the particular geographic locations that buffer species against rapidly changing climate.

Testing the link between community structure and function for ectomycorrhizal fungi involved in a global tripartite symbiosis

Alnus trees associate with ectomycorrhizal (ECM) fungi and nitrogen-fixing Frankia bacteria and, although their ECM fungal communities are uncommonly host specific and species poor, it is unclear whether the functioning of Alnus ECM fungal symbionts differs from that of other ECM hosts. We used exoenzyme root tip assays and molecular identification to test whether ECM fungi on Alnus rubra differed in their ability to access organic phosphorus (P) and nitrogen (N) when compared with ECM fungi on the non-Frankia host Pseudotsuga menziesii. At the community level, potential acid phosphatase (AP) activity of ECM fungal root tips from A. rubra was significantly higher than that from P. menziesii, whereas potential leucine aminopeptidase (LA) activity was significantly lower for A. rubra root tips at one of the two sites. At the individual species level, there was no clear relationship between ECM fungal relative root tip abundance and relative AP or LA enzyme activities on either host. Our results are consistent with the hypothesis that ECM fungal communities associated with Alnus trees have enhanced organic P acquisition abilities relative to non-Frankia ECM hosts. This shift, in combination with the chemical conditions present in Alnus forest soils, may drive the atypical structure of Alnus ECM fungal communities.

The impacts of water stress on phloem transport in Douglas-fir trees

Despite the critical role that phloem plays in a number of plant functional processes and the potential impact of water stress on phloem structural and phloem sap compositional characteristics, little research has been done to examine how water stress influences phloem transport. The objectives of this study were to develop a more accurate understanding of how water stress affects phloem transport in trees, both in terms of the short-term impacts of water stress on phloem sap composition and the longer-term impacts on sieve cell anatomical characteristics. Phloem sieve cell conductivity (kp) was evaluated along a gradient of tree height and xylem water potential in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees in order to evaluate the influence of water stress on phloem transport capacity. The Hagen-Poiseuille equation was used with measurements of sieve cell anatomical characteristics, water content of phloem sap, non-structural carbohydrate content of phloem sap and shoot water potential (Ψl) to evaluate impacts of water stress on kp. Based on regression analysis, for each 1 MPa decrease in mean midday Ψl, sieve cell lumen radius decreased by 2.63 µm MPa(-1). Although there was no significant trend in sucrose content with decreasing Ψl, glucose and fructose content increased significantly with water stress and sieve cell relative water content decreased by 13.5% MPa(-1), leading to a significant increase in sugar molar concentration of 0.46 mol l(-1) MPa(-1) and a significant increase in viscosity of 0.27 mPa s MPa(-1). Modeled kp was significantly influenced both by trends in viscosity as well as by water stress-related trends in sieve cell anatomy.

Forest floor leachate fluxes under six different tree species on a metal contaminated site

Trees play an important role in the biogeochemical cycling of metals, although the influence of different tree species on the mobilization of metals is not yet clear. This study examined effects of six tree species on fluxes of Cd, Zn, DOC, H(+) and base cations in forest floor leachates on a metal polluted site in Belgium. Forest floor leachates were sampled with zero-tension lysimeters in a 12-year-old post-agricultural forest on a sandy soil. The tree species included were silver birch (Betula pendula), oak (Quercus robur and Q. petraea), black locust (Robinia pseudoacacia), aspen (Populus tremula), Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii). We show that total Cd fluxes in forest floor leachate under aspen were slightly higher than those in the other species' leachates, yet the relative differences between the species were considerably smaller when looking at dissolved Cd fluxes. The latter was probably caused by extremely low H(+) amounts leaching from aspen's forest floor. No tree species effect was found for Zn leachate fluxes. We expected higher metal leachate fluxes under aspen as its leaf litter was significantly contaminated with Cd and Zn. We propose that the low amounts of Cd and Zn leaching under aspen's forest floor were possibly caused by high activity of soil biota, for example burrowing earthworms. Furthermore, our results reveal that Scots pine and oak were characterized by high H(+) and DOC fluxes as well as low base cation fluxes in their forest floor leachates, implying that those species might enhance metal mobilization in the soil profile and thus bear a potential risk for belowground metal dispersion.

Do ray cells provide a pathway for radial water movement in the stems of conifer trees?

PREMISE OF THE STUDY

The pathway of radial water movement in tree stems presents an unknown with respect to whole-tree hydraulics. Radial profiles have shown substantial axial sap flow in deeper layers of sapwood (that may lack direct connection to transpiring leaves), which suggests the existence of a radial pathway for water movement. Rays in tree stems include ray tracheids and/or ray parenchyma cells and may offer such a pathway for radial water transport. This study investigated relationships between radial hydraulic conductivity (k(s-rad)) and ray anatomical and stem morphological characteristics in the stems of three conifer species whose distributions span a natural aridity gradient across the Cascade Mountain range in Oregon, United States.

METHODS

The k(s-rad) was measured with a high-pressure flow meter. Ray tracheid and ray parenchyma characteristics and water transport properties were visualized using autofluorescence or confocal microscopy.

KEY RESULTS

The k(s-rad) did not vary predictably with sapwood depth among species and populations. Dye tracer did not infiltrate ray tracheids, and infiltration into ray parenchyma was limited. Regression analyses revealed inconsistent relationships between k(s-rad) and selected anatomical or growth characteristics when ecotypes were analyzed individually and weak relationships between k(s-rad) and these characteristics when data were pooled by tree species.

CONCLUSIONS

The lack of significant relationships between k(s-rad) and the ray and stem morphologies we studied, combined with the absence of dye tracer in ray tracheid and limited movement of dye into ray parenchyma suggests that rays may not facilitate radial water transport in the three conifer species studied.

Applying the dual-isotope conceptual model to interpret physiological trends under uncontrolled conditions

The inter-relationships among δ(13)C and δ(18)O in tree ring cellulose and ring width have the potential to illuminate long-term physiological and environmental information in forest stands that have not been monitored. We examine how within-stand competition and environmental gradients affect ring widths and the stable isotopes of cellulose. We utilize a natural climate gradient across a catchment dominated by Douglas-fir and temporal changes in climate over an 8-year period. We apply a dual-isotope approach to infer physiological response of trees in differing crown dominance classes to temporal and spatial changes in environmental conditions using a qualitative conceptual model of the (13)C-(18)O relationship and by normalizing the data to minimize other variance. The δ(13)C and δ(18)O of cellulose were correlated with year-to-year variation in relative humidity and consistent with current isotope theory. Using a qualitative conceptual model of the (13)C-(18)O relationship and physiological knowledge about the species, we interpreted these changes as stomatal conductance responses to evaporative demand. Spatial variance between plots was not strong and seemed related to leaf nitrogen rather than any other environmental variable. Dominant trees responded to environmental gradients more consistently with current isotope theory as compared with other classes within the same stand. We found a correlation of stable isotopes with environmental variables is useful for assessing the impacts of environmental change over short time series and where growth varies only minimally with climate.

Comparison of leaf gas exchange and stable isotope signature of water-soluble compounds along canopy gradients of co-occurring Douglas-fir and European beech

Combined δ(13) C and δ(18) O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (A(n) ) between the investigated crown positions. Estimates of iWUE from δ(13) C of leaf water-soluble organic matter (WSOM) outperformed the estimates from phloem compounds. In the beech crown, δ(13) C of leaf WSOM coincided clearly with gas exchange measurements. The relationship was not as reliable in the Douglas-fir. The differences in δ(18) O between leaf and phloem material were found to correlate with stomatal conductance. The semi-quantitative model approach was applicable for comparisons of daily average A(n) between different crown positions and trees. Intracanopy gradients were more pronounced in the beech than in the Douglas-fir, which reached higher values of iWUE at the respective positions, particularly under dry air conditions.

Variation in water potential, hydraulic characteristics and water source use in montane Douglas-fir and lodgepole pine trees in southwestern Alberta and consequences for seasonal changes in photosynthetic capacity

Tree species response to climate change-induced shifts in the hydrological cycle depends on many physiological traits, particularly variation in water relations characteristics. We evaluated differences in shoot water potential, vulnerability of branches to reductions in hydraulic conductivity, and water source use between Pinus contorta Dougl. ex Loud. var. latifolia Engelm. (lodgepole pine) and Pseudotsuga menziesii (Mirb.) Franco (interior Douglas-fir), and determined the consequences for seasonal changes in photosynthetic capacity. The Douglas-fir site had soil with greater depth, finer texture and higher organic matter content than soil at the lodgepole pine site, all factors that increased the storage of soil moisture. While the measured xylem vulnerability curves were quite similar for the two species, Douglas-fir had lower average midday shoot water potentials than did lodgepole pine. This implied that lodgepole pine exhibited stronger stomatal control of transpiration than Douglas-fir, which helped to reduce the magnitude of the water potential gradient required to access water from drying soil. Stable hydrogen isotope measurements indicated that Douglas-fir increased the use of groundwater during mid-summer when precipitation inputs were low, while lodgepole pine did not. There was a greater reduction of photosynthetic carbon gain in lodgepole pine compared with Douglas-fir when the two tree species were exposed to seasonal declines in soil water content. The contrasting patterns of seasonal variation in photosynthetic capacity observed for the two species were a combined result of differences in soil characteristics at the separate sites and the inherent physiological differences between the species.

Tree-growth analyses to estimate tree species' drought tolerance

Climate change is challenging forestry management and practices. Among other things, tree species with the ability to cope with more extreme climate conditions have to be identified. However, while environmental factors may severely limit tree growth or even cause tree death, assessing a tree species' potential for surviving future aggravated environmental conditions is rather demanding. The aim of this study was to find a tree-ring-based method suitable for identifying very drought-tolerant species, particularly potential substitute species for Scots pine (Pinus sylvestris L.) in Valais. In this inner-Alpine valley, Scots pine used to be the dominating species for dry forests, but today it suffers from high drought-induced mortality. We investigate the growth response of two native tree species, Scots pine and European larch (Larix decidua Mill.), and two non-native species, black pine (Pinus nigra Arnold) and Douglas fir (Pseudotsuga menziesii Mirb. var. menziesii), to drought. This involved analysing how the radial increment of these species responded to increasing water shortage (abandonment of irrigation) and to increasingly frequent drought years. Black pine and Douglas fir are able to cope with drought better than Scots pine and larch, as they show relatively high radial growth even after irrigation has been stopped and a plastic growth response to drought years. European larch does not seem to be able to cope with these dry conditions as it lacks the ability to recover from drought years. The analysis of trees' short-term response to extreme climate events seems to be the most promising and suitable method for detecting how tolerant a tree species is towards drought. However, combining all the methods used in this study provides a complete picture of how water shortage could limit species.

Metal stress and decreased tree growth in response to biosolids application in greenhouse seedlings and in situ Douglas-fir stands

To assess physiological impacts of biosolids on trees, metal contaminants and phytochelatins were measured in Douglas-fir stands amended with biosolids in 1982. A subsequent greenhouse study compared these same soils to soils amended with fresh wastewater treatment plant biosolids. Biosolids-amended field soils had significantly higher organic matter, lower pH, and elevated metals even after 25 years. In the field study, no beneficial growth effects were detected in biosolids-amended stands and in the greenhouse study both fresh and historic biosolids amendments resulted in lower seedling growth rates. Phytochelatins - bioindicators of intracellular metal stress - were elevated in foliage of biosolids-amended stands, and significantly higher in roots of seedlings grown with fresh biosolids. These results demonstrate that biosolids amendments have short- and long-term negative effects that may counteract the expected tree growth benefits.

Assessment of uncertainty in functional-structural plant models

BACKGROUND AND AIMS

Constructing functional-structural plant models (FSPMs) is a valuable method for examining how physiology and morphology interact in determining plant processes. However, such models always have uncertainty concerned with whether model components have been selected and represented effectively, with the number of model outputs simulated and with the quality of data used in assessment. We provide a procedure for defining uncertainty of an FSPM and how this uncertainty can be reduced.

METHODS

An important characteristic of FSPMs is that typically they calculate many variables. These can be variables that the model is designed to predict and also variables that give indications of how the model functions. Together these variables are used as criteria in a method of multi-criteria assessment. Expected ranges are defined and an evolutionary computation algorithm searches for model parameters that achieve criteria within these ranges. Typically, different combinations of model parameter values provide solutions achieving different combinations of variables within their specified ranges. We show how these solutions define a Pareto Frontier that can inform about the functioning of the model.

KEY RESULTS

The method of multi-criteria assessment is applied to development of BRANCHPRO, an FSPM for foliage reiteration on old-growth branches of Pseudotsuga menziesii. A geometric model utilizing probabilities for bud growth is developed into a causal explanation for the pattern of reiteration found on these branches and how this pattern may contribute to the longevity of this species.

CONCLUSIONS

FSPMs should be assessed by their ability to simulate multiple criteria simultaneously. When different combinations of parameter values achieve different groups of assessment criteria effectively a Pareto Frontier can be calculated and used to define the sources of model uncertainty.

Assessment of uncertainty in functional-structural plant models

BACKGROUND AND AIMS

Constructing functional-structural plant models (FSPMs) is a valuable method for examining how physiology and morphology interact in determining plant processes. However, such models always have uncertainty concerned with whether model components have been selected and represented effectively, with the number of model outputs simulated and with the quality of data used in assessment. We provide a procedure for defining uncertainty of an FSPM and how this uncertainty can be reduced.

METHODS

An important characteristic of FSPMs is that typically they calculate many variables. These can be variables that the model is designed to predict and also variables that give indications of how the model functions. Together these variables are used as criteria in a method of multi-criteria assessment. Expected ranges are defined and an evolutionary computation algorithm searches for model parameters that achieve criteria within these ranges. Typically, different combinations of model parameter values provide solutions achieving different combinations of variables within their specified ranges. We show how these solutions define a Pareto Frontier that can inform about the functioning of the model.

KEY RESULTS

The method of multi-criteria assessment is applied to development of BRANCHPRO, an FSPM for foliage reiteration on old-growth branches of Pseudotsuga menziesii. A geometric model utilizing probabilities for bud growth is developed into a causal explanation for the pattern of reiteration found on these branches and how this pattern may contribute to the longevity of this species.

CONCLUSIONS

FSPMs should be assessed by their ability to simulate multiple criteria simultaneously. When different combinations of parameter values achieve different groups of assessment criteria effectively a Pareto Frontier can be calculated and used to define the sources of model uncertainty.

Estimation of canopy average mesophyll conductance using δ(13) C of phloem contents

Conductance to CO(2) inside leaves, known as mesophyll conductance (g(m)), imposes large limitations on photosynthesis. Because g(m) is difficult to quantify, it is often neglected in calculations of (13)C photosynthetic discrimination. The 'soluble sugar method' estimates g(m) via differences between observed photosynthetic discrimination, calculated from the δ(13)C of soluble sugars, and discrimination when g(m) is infinite. We expand upon this approach and calculate a photosynthesis-weighted average for canopy mesophyll conductance ((c) g(m)) using δ(13)C of stem phloem contents. We measured gas exchange at three canopy positions and collected stem phloem contents in mature trees of three conifer species (Pseudotsuga menziesii, Thuja plicata and Larix occidentalis). We generated species-specific and seasonally variable estimates of (c)g(m) . We found that (c)g(m) was significantly different among species (0.41, 0.22 and 0.09 mol m(-2) s(-1) for Larix, Pseudotsuga and Thuja, respectively), but was similar throughout the season. Ignoring respiratory and photorespiratory fractionations ((c)Δ(ef)) resulted in ≈30% underestimation of (c)g(m) in Larix and Pseudotsuga, but was innocuous in Thuja. Substantial errors (~1-4‰) in photosynthetic discrimination calculations were introduced by neglecting (c)g(m) and (c)Δ(ef) . Our method is easy to apply and cost-effective, captures species variation and would have captured seasonal variation had it existed. The method provides an average canopy value, which makes it suitable for parameterization of canopy-scale models of photosynthesis, even in tall trees.

Climate-related trends in sapwood biophysical properties in two conifers: avoidance of hydraulic dysfunction through coordinated adjustments in xylem efficiency, safety and capacitance

In the Pacific north-west, the Cascade Mountain Range blocks much of the precipitation and maritime influence of the Pacific Ocean, resulting in distinct climates east and west of the mountains. The current study aimed to investigate relationships between water storage and transport properties in populations of Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) adapted to both climates. Sapwood thickness, capacitance, vulnerability to embolism, and axial and radial conductivity were measured on samples collected from trunks of mature trees. The sapwood of ponderosa pine was three to four times thicker than Douglas-fir. Radial conductivity was higher in west-side populations of both species, but axial conductivity was higher in the east-side populations and in Douglas-fir. Eastern populations of both species had sapwood that was more vulnerable to embolism than west-side populations. Sapwood capacitance was similar between species, but was about twice as great in east-side populations (580 kg m⁻³ MPa⁻¹) as in west-side populations (274 kg m⁻³ MPa⁻¹). Capacitance was positively correlated with both mean embolism pressure and axial conductivity across species and populations, suggesting that coordinated adjustments in xylem efficiency, safety and water storage capacity may serve to avoid embolism along a gradient of increasing aridity.

Southward Pleistocene migration of Douglas-fir into Mexico: phylogeography, ecological niche modeling, and conservation of 'rear edge' populations

• Poleward Pleistocene plant migration has been an important process structuring modern temperate and boreal plant communities, but the contribution of equatorward migration remains poorly understood. Paleobotanical evidence suggests Miocene or Pleistocene origin for temperate 'sky island' plant taxa in Mexico. These 'rear edge' populations situated in a biodiversity hotspot may be an important reserve of genetic diversity in changing climates. • We used mtDNA sequences, cpDNA sequences and chloroplast microsatellites to test hypotheses of Miocene vs Pleistocene colonization of temperate Douglas-fir in Mexico, explore geographic patterns of molecular variation in relation to Pleistocene climate history using ecological niche models, and assess the taxonomic and conservation implications. • We found strong evidence for Pleistocene divergence of Douglas-fir in Mexico (958 thousand yr before present (ka) with the 90% highest posterior density interval ranging from 1.6 million yr before present (Ma) to 491 ka), consistent with the southward Pleistocene migration hypothesis. Genetic diversity was high and strongly partitioned among populations. Spatial patterns of molecular variation and ecological niche models suggest a complex late Pleistocene history involving periods of isolation and expansion along mountain corridors. • These results highlight the importance of southward Pleistocene migration in establishing modern high-diversity plant communities and provide critical insights into proposals to conserve the unique biodiversity of Mexican Douglas-fir and associated taxa.

Historical demography and phylogeography of a specialist bark beetle, Dendroctonus pseudotsugae Hopkins (Curculionidae: Scolytinae)

Contemporary distribution of North American species has been shaped by past glaciation events during the Quaternary period. However, their effects were not as severe in the southern Rocky Mountains and Northern Mexico as elsewhere in North America. In this context, we test hypotheses about the historical demography of Dendroctonus pseudotsugae, based on 136 haplotypes of mitochondrial cytochrome oxidase I. The phylogenetic analysis yielded four haplogroups corresponding to northwestern United States and southwestern Canada (NUS), southwestern United States (Arizona, SUS), northwestern Mexico (Sierra Madre Occidental, SMOC), and northeastern Mexico (Sierra Madre Oriental, SMOR). Predictions of demographic expansion were examined through neutrality tests against population growth and mismatch distribution. Results showed that the NUS and SMOC haplogroups have experienced demographic expansion events, whereas the SUS and SMOR haplogroups have not. Divergence times between pairs of haplogroups were estimated from early to middle Pleistocene. The longer divergence time between NUS and all other haplogroups could be the result of refugia within the Pacific Northwest and northern Rocky Mountains and long-term isolation from southernmost populations in Mexico. The results obtained in this study are in agreement with the evolutionary history of the host Douglas-fir, as the warmer climates of interglacial periods pushed conifers northward of Colorado, New Mexico, and Arizona, whereas environmental changes reduced the population size of Douglas-fir and forced fragmentation of distribution range southward into northern Mexico.

Xylem anisotropy and water transport--a model for the double sawcut experiment

Early experiments with overlapping cuts to the stems of trees demonstrated that lateral flow within the stem must be possible to allow such trees to maintain water flow to their leaves. We present a mathematical approach to considering lateral flow in stems by treating the xylem as an anisotropic medium for flow and develop an expression of its conductivity in the form of a tensor. In both 3D models of tracheid-bearing stems with cuts (incorporating this tensor analysis) and experimental stems with steadily deepening cuts, it is shown that flow can continue despite the presence of even strongly overlapping cuts through 90% of the stem. Such remaining conducting ability was, however, strongly dependent on values for radial and tangential conductivity (conductivity to lateral flow across the stem either radially with respect to the central axis or tangentially to the stem surface). Furthermore, the lateral flow around obstructing cuts was more dependent on tangential flow around the stem upstream and downstream of the cuts than on radial flow across the stem. The relative importance of tangential flow could be accounted for by a greater tangential conductivity, perhaps related to the predominance of pits on radial walls of tracheids, and the presence of non-conducting pith and early growth rings in the stems. These results demonstrate that a consideration of anisotropy in transport properties of the xylem will be important for future studies of flow in stems around naturally occurring geometric features such as branching points.

Defining how aging Pseudotsuga and Abies compensate for multiple stresses through multi-criteria assessment of a functional-structural model

Many hypotheses have been advanced about factors that control tree longevity. We use a simulation model with multi-criteria optimization and Pareto optimality to determine branch morphologies in the Pinaceae that minimize the effect of growth limitations due to water stress while simultaneously maximizing carbohydrate gain. Two distinct branch morphologies in the Pareto optimal space resemble Pseudotsuga menziesii (Mirb.) Franco and Abies grandis (Dougl. ex D. Don) Lindl., respectively. These morphologies are distinguished by their performance with respect to two pathways of compensation for hydraulic limitation: minimizing the mean path length to terminal foliage (Pseudotsuga) and minimizing the mean number of junction constrictions to terminal foliage (Abies). Within these two groups, we find trade-offs between the criteria for foliage display and the criteria for hydraulic functioning, which shows that an appropriate framework for considering tree longevity is how trees compensate, simultaneously, for multiple stresses. The diverse morphologies that are found in a typical old-growth conifer forest may achieve compensation in different ways. The method of Pareto optimization that we employ preserves all solutions that are successful in achieving different combinations of criteria. The model for branch development that we use simulates the process of delayed adaptive reiteration (DAR), whereby new foliage grows from suppressed buds within the established branch structure. We propose a theoretical synthesis for the role of morphology in the persistence of old Pseudotsuga based on the characteristics of branch morphogenesis found in branches simulated from the optimal set. (i) The primary constraint on branch growth for Pseudotsuga is the mean path length; (ii) as has been previously noted, DAR is an opportunistic architecture; and (iii) DAR is limited by the number of successive reiterations that can form. We show that Pseudotsuga morphology is not the only solution to old-growth constraints, and we suggest how the model results should be used to guide future empirical investigation based on the two contrasting morphologies and how the morphological contrast may relate to physiological processes. Our results show that multi-criteria optimization with Pareto optimality has promise to advance the use of models in theory development and in exploration of functional-structural trade-offs, particularly in complex biological systems with multiple limiting factors.

Height-related trends in stomatal sensitivity to leaf-to-air vapour pressure deficit in a tall conifer

Stomatal responses to leaf-to-air vapour pressure deficit (LVPD), leaf water potential components, and cuticular properties were characterized for Douglas-fir (Pseudotsuga menziesii) foliage collected from tree tops along a height gradient from 5 m to 58 m in order to explore height-related trends in stomatal sensitivity to LVPD and to investigate the role of bulk leaf turgor and leaf cuticle thickness in determining stomatal behaviour. There were three distinct phases in the response of stomatal conductance (g(s)) to changes in LVPD. At low LVPD, g(s) increased with increasing LVPD (phase one). During the second phase, g(s) was maximal at low to intermediate LVPD and during the third phase g(s) declined steadily as LVPD increased. The responsiveness of g(s) to LVPD exhibited a height-related pattern such that maximum g(s) (g(s-max)) occurred at progressively greater LVPD with increasing height (r2=0.55, P=0.006). Bulk leaf osmotic potential at full turgor decreased with height (r2=0.77, P=0.00016), and LVPD at g(s-max) and at maximum crown conductance (g(c-max)) in the field were significantly correlated with leaf turgor (r2=0.92, P=0.0093). Cuticle thickness increased by 0.044 microm for every metre increase in height (r2=0.78, P=0.00015). The observed trends in the response of g(s) to LVPD along a height gradient, and their consistency with height-related trends in foliar osmotic potential suggest that osmotic adjustment at the tops of tall trees influences the relationship between g(s) and LVPD.

Stem-mediated hydraulic redistribution in large roots on opposing sides of a Douglas-fir tree following localized irrigation

*Increasing evidence about hydraulic redistribution and its ecological consequences is emerging. Hydraulic redistribution results from an interplay between competing plant and soil water potential gradients. In this work, stem-mediated hydraulic redistribution was studied in a 53-year-old Douglas-fir tree during a period of drought. *Sap flux density measurements using the heat field deformation method were performed at four locations: in two large opposing roots and on two sides of the tree stem. Hydraulic redistribution was induced by localized irrigation on one of the measured roots, creating heterogeneous soil water conditions. *Stem-mediated hydraulic redistribution was detected during night-time conditions when water was redistributed from the wet side of the tree to the nonirrigated dry side. In addition to stem-mediated hydraulic redistribution, bidirectional flow in the dry root was observed, indicating radial sectoring in the xylem. *It was observed that, through stem-mediated hydraulic redistribution, Douglas-fir was unable to increase its transpiration despite the fact that sufficient water was available to one part of the root system. This resulted from the strong water potential gradient created by the dry soil in contact with the nonirrigated part of the root system. A mechanism of stem-mediated hydraulic redistribution is proposed and its possible implications are discussed.

Effects of growth medium, nutrients, water, and aeration on mycorrhization and biomass allocation of greenhouse-grown interior Douglas-fir seedlings

Commercial nursery practices usually fail to promote mycorrhization of interior Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco] seedlings in British Columbia, which may account for their poor performance following planting in the field. We tested the effects of four nursery cultivation factors (nitrogen fertilization, phosphorus fertilization, watering, and soil aeration) and field soil addition on mycorrhization, survival, growth, and biomass allocation of interior Douglas-fir seedlings in a series of greenhouse experiments. Where field soil was added to the growing medium, mycorrhization and root/shoot ratios were maximized at lower levels of mineral nutrient application and aeration. Where field soil was not added, mycorrhization was negligible across all fertilization and aeration treatments, but root/shoot ratio was maximized at lower levels of mineral nutrients and the highest level of aeration. Regardless of whether field soil was added, intermediate levels of soil water resulted in the best mycorrhizal colonization and root/shoot ratios. However, field soil addition reduced seedling mortality at the two lowest water levels. A cluster analysis placed ectomycorrhizal morphotypes into three groups (Mycelium radicis-atrovirens Melin, Wilcoxina, and mixed) based on their treatment response, with all but two morphotypes in the mixed group whose abundance was maximized under conditions common to advanced seedling establishment. For maximal mycorrhization and root development of interior Douglas-fir seedlings, nurseries should minimize addition of nitrogen and phosphorus nutrients, maximize aeration, provide water at moderate rates, and, where possible, add small amounts of field soil to the growing medium.

Stand-level gas-exchange responses to seasonal drought in very young versus old Douglas-fir forests of the Pacific Northwest, USA

This study examines how stand age affects ecosystem mass and energy exchange response to seasonal drought in three adjacent Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests. The sites include two early seral (ES) stands (0-15 years old) and an old-growth (OG) (approximately 450-500 years old) forest in the Wind River Experimental Forest, Washington, USA. We use eddy covariance flux measurements of carbon dioxide (F(NEE)), latent energy (lambdaE) and sensible heat (H) to derive evapotranspiration rate (E(T)), Bowen ratio (beta), water use efficiency (WUE), canopy conductance (G(c)), the Priestley-Taylor coefficient (alpha) and a canopy decoupling factor (Omega). The canopy and bulk parameters are examined to find out how ecophysiological responses to water stress, including changes in relative soil water content ((r)) and vapour pressure deficit (deltae), differ among the two forest successional stages. Despite different rainfall patterns in 2006 and 2007, we observed site-specific diurnal patterns of E(T), alpha, G(c), deltae and (r) during both years. The largest stand differences were (1) at the OG forest high morning G(c) (> 10 mm s(-1)) coincided with high net CO(2) uptake (F(NEE) = -9 to -6 micromol m(-2) s(-1)), but a strong negative response in OG G(c) to moderate deltae was observed later in the afternoons and subsequently reduced daily E(T) and (2) at the ES stands total E(T) was higher (+72 mm) because midday G(c) did not decrease until very low water availability levels ((r) < 30%) were reached at the end of the summer. Our results suggest that ES stands are more likely than mature forests to experience constraints on gas exchange if the dry season becomes longer or intensifies because water conserving ecophysiological responses were observed in the youngest stands only at the very end of the seasonal drought.

Physiological responses to fertilization recorded in tree rings: isotopic lessons from a long-term fertilization trial

Nitrogen fertilizer applications are common land use management tools, but details on physiological responses to these applications are often lacking, particularly for long-term responses over decades of forest management. We used tree ring growth patterns and stable isotopes to understand long-term physiological responses to fertilization using a controlled fertilization experiment begun in 1964 in Washington State (USA), in which three levels of nitrogen fertilizer were applied: 157, 314; and 471 kg/ha. Basal area increment (BAI) increased more than fourfold in the highest treatment to twofold in the lowest, and a significant increase in BAI was observed for 20 years. Latewood delta 13C sharply decreased by 1.4 per thousand after fertilization and was significantly lower than controls for four years, but no differences existed between fertilization levels, and the effect disappeared after four years, indicating that intrinsic water use efficiency (A/gs) increased in response to fertilization. Earlywood delta 13C showed similar trends but was more variable. Latewood delta 18O increased significantly above controls by approximately 2 per thousand in all treatments, but the duration differed with treatment level, with the effect being longer for higher levels of fertilization and lasting as long as nine years after fertilization. Because source water and relative humidity were the same between experimental plots, we interpreted the delta 18O increase with treatment as a decrease in leaf-level transpiration. Earlywood delta 18O did not show any treatment effects. Because the Pacific Northwest has a mediterranean climate with dry summers, we speculated that fertilization caused a substantial increase in leaf area, causing the trees to transpire themselves into drought stress during the late summer. We estimate from the delta 18O data that stomatal conductance (gs) was reduced by approximately 30%. Using the delta 13C data to estimate assimilation rates (A), A during the late season was also reduced by 20-30%. If leaf-level A decreased, but BAI increased, we estimated that leaf area on those trees must have increased by fourfold with the highest level of treatment within this stand. This increase in leaf area resulting from fertilization caused a hydraulic imbalance within the trees that lasted as long as nine years after treatment at the highest levels of fertilization.