Changes in drought response strategies with ontogeny in Quercus rubra: implications for scaling from seedlings to mature trees

Context-dependent benefits of forest soil addition on Aleppo pine seedling performance under drought and grass competition

Seedling establishment under natural conditions is limited by numerous interacting factors. Here, we tested the combined effects of drought, herbaceous competition, and ectomycorrhizal inoculation on the performance of Aleppo pine seedlings grown in a net-house. The roots of all pine seedlings were strongly dominated by Geopora, a fungal genus known to colonize seedlings in dry habitats. Ectomycorrhizal fungi (EMF) inoculum significantly increased seedling height, biomass, and the number of side branches. However, under either competition or drought, the positive effect of EMF on seedling biomass and height was greatly reduced, while the effect on shoot branching was maintained. Further, under a combination of drought and competition, EMF had no influence on either plant growth or shape. The discrepancy in pine performance across treatments highlights the complexity of benefits provided to seedlings by EMF under ecologically relevant settings.

High risk, high gain? Trade-offs between growth and resistance to extreme events differ in northern red oak (Quercus rubra L.)

Information about the resistance and adaptive potential of tree species and provenances is needed to select suitable planting material in times of rapidly changing climate conditions. In this study, we evaluate growth responses to climatic fluctuations and extreme events for 12 provenances of northern red oak (Quercus rubra L.) that were tested across three trial sites with distinct environmental conditions in Germany. Six provenances each were sourced from the natural distribution in North America and from introduced stands in Germany. We collected increment cores of 16 trees per provenance and site. Dendroecological methods were used to compare provenance performance and establish climate-growth relationships to identify the main growth limiting factors. To evaluate the provenance response to extreme drought and frost events, three site-specific drought years were selected according to the Standardized Precipitation Evapotranspiration Index (SPEI) and 2010 as a year with an extreme late frost event. Resistance indices for these years were calculated and assessed in relation to overall growth performance. We observed a high variation in growth and in the climate sensitivity between sites depending on the prevailing climatic conditions, as well as a high intra-specific variation. Overall, summer drought and low temperatures in the early growing season appear to constrain the growth of red oak. The resistance of provenances within sites and extreme years showed considerable rank changes and interaction effects. We did not find a trade-off between growth and resistance to late frost, namely, fast growing provenances had a high frost hardiness. Further, there was no evidence for a trade-off between growth and drought hardiness. Still, responses to drought or late frost differ between provenances, pointing to dissimilar adaptive strategies. Provenances from introduced (i.e. German) stands represent suitable seed sources, as they combine a higher growth and frost hardiness compared to their North American counterparts. Drought hardiness was slightly higher in the slow-growing provenances. The results provide a better understanding of the variable adaptive strategies between provenances and help to select suitable planting material for adaptive forest management.

Disentangling the effect of growth from development in size-related trait scaling relationships

In plant ecology, the terms growth and development are often used interchangeably. Yet these constitute two distinct processes. Plant architectural traits (e.g. number of successive forks) can estimate development stages. Here, we show the importance of including the effect of development stages to better understand size-related trait scaling relationships (i.e. between height and stem diameter). We focused on one common savanna woody species (Senegalia nigrescens) from the Greater Kruger Area, South Africa. We sampled 406 individuals that experience different exposure to herbivory, from which we collected four traits: plant height, basal stem diameter, number of successive forks (proxy for development stage), and resprouting. We analysed trait relationships (using standardized major axis regression) between height and stem diameter, accounting for the effect of ontogeny, exposure to herbivory, and resprouting. The number of successive forks affects the scaling relationship between height and stem diameter, with the slope and strength of the relationship declining in more developed individuals. Herbivory exposure and resprouting do not affect the overall height-diameter relationship. However, when height and stem diameter were regressed separately against number of successive forks, herbivory exposure and resprouting had an effect. For example, resprouting individuals allocate more biomass to both primary and secondary growth than non-resprouting plants in more disturbed conditions. We stress the need to include traits related to ontogeny so as to disentangle the effect of biomass allocation to primary and secondary growth from that of development in plant functional relationships.

Populus euphratica has stronger regrowth ability than Populus pruinosa under salinity stress

Pest infestation and soil salinization levels are increasing due to climate change. Comprehending plant regrowth after insect damage and salinity stress is crucial to understanding climate change's multifactorial impacts on forest ecosystems. This study examined Populus euphratica and P. pruinosa regrowth after different defoliation levels combined with salinity stress. Specifically, the biomass and regrowth ability, non-structural carbohydrate (NSC) and nitrogen (N) pools in different organs and the whole plant, and the leaf Cl- concentration of both poplars were analyzed. Our results showed that after 50% defoliation and no salt addition, the regrowth of both species recovered similarly to the control level, while their regrowth was about 70% after 90% defoliation. However, under salinity stress, the regrowth (% leaf biomass) of P. euphratica was significantly higher than P. pruinose at either the 50% or 90% defoliation levels. Additionally, P. euphratica had more soluble sugar, starch, NSC and N pools in leaf, stem, root and whole plant than P. pruinose under salinity stress. The regrowth based on leaf biomass increased linearly with soluble sugar, starch, NSC and N pools, and decreased linearly with leaf Cl- concentration across different salinity and defoliation levels. These results indicated that defoliation significantly decreased NSC and N pools, limiting the growth of both poplars, and salinity stress exacerbated the negative effect. Furthermore, when suffering from salinity stress, P. euphratica with higher NSC and N pools exhibited stronger regrowth ability than P. pruinose.

Mechanistic links between physiology and spectral reflectance enable previsual detection of oak wilt and drought stress

Tree mortality due to global change-including range expansion of invasive pests and pathogens-is a paramount threat to forest ecosystems. Oak forests are among the most prevalent and valuable ecosystems both ecologically and economically in the United States. There is increasing interest in monitoring oak decline and death due to both drought and the oak wilt pathogen (Bretziella fagacearum). We combined anatomical and ecophysiological measurements with spectroscopy at leaf, canopy, and airborne levels to enable differentiation of oak wilt and drought, and detection prior to visible symptom appearance. We performed an outdoor potted experiment with Quercus rubra saplings subjected to drought stress and/or artificially inoculated with the pathogen. Models developed from spectral reflectance accurately predicted ecophysiological indicators of oak wilt and drought decline in both potted and field experiments with naturally grown saplings. Both oak wilt and drought resulted in blocked water transport through xylem conduits. However, oak wilt impaired conduits in localized regions of the xylem due to formation of tyloses instead of emboli. The localized tylose formation resulted in more variable canopy photosynthesis and water content in diseased trees than drought-stressed ones. Reflectance signatures of plant photosynthesis, water content, and cellular damage detected oak wilt and drought 12 d before visual symptoms appeared. Our results show that leaf spectral reflectance models predict ecophysiological processes relevant to detection and differentiation of disease and drought. Coupling spectral models that detect physiological change with spatial information enhances capacity to differentiate plant stress types such as oak wilt and drought.

Upscaling the effect of traits in response to drought: The relative importance of safety-efficiency and acquisitive-conservation functional axes

We tested the idea that functional trade-offs that underlie species tolerance to drought-driven shifts in community composition via their effects on demographic processes and subsequently on shifts in species' abundance. Using data from 298 tree species from tropical dry forests during the extreme ENSO-2015, we scaled-up the effects of trait trade-offs from individuals to communities. Conservative wood and leaf traits favoured slow tree growth, increased tree survival and positively impacted species abundance and dominance at the community-level. Safe hydraulic traits, on the other hand, were related to demography but did not affect species abundance and communities. The persistent effects of the conservative-acquisitive trade-off across organizational levels is promising for generalization and predictability of tree communities. However, the safety-efficient trade-off showed more intricate effects on performance. Our results demonstrated the complex pathways in which traits scale up to communities, highlighting the importance of considering a wide range of traits and performance processes.

Seasonal Differences in Leaf Photoprotective Potential between Adults and Juveniles of Two Mediterranean Perennials with Distinct Growth Forms: A Comparative Field Study

The photosynthetic differences between adult and juvenile Mediterranean plants were previously studied under field conditions, yet the corresponding differentiation of their photoprotective efficiency has not been sufficiently investigated. The present study aims to examine possible differences in the photoprotective potential between adults and juveniles of two native Mediterranean plants with distinct growth forms. Thus, the seasonal variations in individual carotenoids, electron transport rate (ETR), and non-photochemical quenching (NPQ) were monitored in fully exposed mature leaves from adults and juveniles of the winter deciduous tree Cercis siliquastrum L. and the evergreen sclerophyllous shrub Nerium oleander L. All plants were grown under apparently similar field conditions. In both species, juveniles displayed substantially lower ETR and increased NPQ values than adults in spring, with the differences intensifying during summer drought and diminishing in autumn. Concomitantly, juveniles showed significantly higher chlorophyll-based total carotenoids in spring and summer mainly due to the higher investment in xanthophyll cycle components (VAZ), in combination with an increased mid-day de-epoxidation state (DEPS) and partial retention of zeaxanthin in the dark. In N. oleander, although ETR was lower in juveniles during winter, NPQ was extremely low in both ages. In conclusion, juveniles exhibit enhanced photoprotection potential, especially in the summer, due to their reduced photochemical capacity. The photosynthetic superiority of adults during the favorable spring period may be attributed to the needs of the co-existing reproductive effort.

Age Determination and Growth Characteristics of the Potentilla griffithii: A Comparison of Two Different Habitats in Western Sichuan Plateau, China

This study proposes a rapid and non-destructive technique for determining the age of Potentilla griffithii individuals in the field by observing the sequence of leaf scars. Based on two- to three-year-old P. griffithii seedlings, planted in a common garden in the western Sichuan Plateau, China, the study found that the rates of basal leaf production were consistent, with leaves growing from March to April and falling off from October to December, leaving behind basal leaf scars. Thus, the age of individuals in situ could be determined by counting the leaf scars. Through this method, we determined the age structure and growth strategy of P. griffithii populations in two typical habitats in the western Sichuan Plateau. In open land habitats, the age structure of P. griffithii populations was relatively younger compared to understory habitats. In open land, P. griffithii tends to allocate more photosynthate terminal organs (leaves and fine roots) to absorbing more resources, as well as to its reproductive organs (flower stems and aggregate fruits), to expand the population. The P. griffithii population in the understory habitat is in its middle-age stage and concentrates more photosynthate in the coarse root part (e.g., the high coarse root mass fraction (FRMF)) to support the plant. Additionally, we found a significant correlation between P. griffithii plant age and various traits in open land habitats. Therefore, we conclude that plant age can be used as a good predictor of plant growth condition in open land. These results allow for predicting ecological processes, based on the ages and traits of P. griffithii plants, providing a theoretical basis to support the large-scale breeding of P. griffithii.

How woody plants adjust above- and below-ground traits in response to sustained drought

Future increases in drought severity and frequency are predicted to have substantial impacts on plant function and survival. However, there is considerable uncertainty concerning what drought adjustment is and whether plants can adjust to sustained drought. This review focuses on woody plants and synthesises the evidence for drought adjustment in a selection of key above-ground and below-ground plant traits. We assess whether evaluating the drought adjustment of single traits, or selections of traits that operate on the same plant functional axis (e.g. photosynthetic traits) is sufficient, or whether a multi-trait approach, integrating across multiple axes, is required. We conclude that studies on drought adjustments in woody plants might overestimate the capacity for adjustment to drier environments if spatial studies along gradients are used, without complementary experimental approaches. We provide evidence that drought adjustment is common in above-ground and below-ground traits; however, whether this is adaptive and sufficient to respond to future droughts remains uncertain for most species. To address this uncertainty, we must move towards studying trait integration within and across multiple axes of plant function (e.g. above-ground and below-ground) to gain a holistic view of drought adjustments at the whole-plant scale and how these influence plant survival.

Field development of Posidonia oceanica seedlings changes under predicted acidification conditions

Ocean acidification has been consistently evidenced to have profound and lasting impacts on marine species. Observations have shown seagrasses to be highly susceptible to future increased pCO₂ conditions, but the responses of early life stages as seedlings are poorly understood. This study aimed at evaluating how projected Mediterranean Sea acidification affects the survival, morphological and biochemical development of Posidonia oceanica seedlings through a long-term field experiment along a natural low pH gradient. Future ocean conditions seem to constrain the morphological development of seedlings. However, high pCO₂ exposures caused an initial increase in the degree of saturation of fatty acids in leaves and then improved the fatty acid adjustment increasing unsaturation levels in leaves (but not in seeds), suggesting a nutritional compound translocation. Results also suggested a P. oceanica structural components remodelling which may counteract the effects of ocean acidification but would not enhance seagrass seedling productivity.

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (T_leaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy T_leaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging T_leaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high T_leaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback.

Strategies to mitigate shifts in red oak (Quercus sect. Lobatae) distribution under a changing climate

Red oaks (Quercus sect. Lobatae) are a taxonomic group of hardwood trees, which occur in swamp forests, subtropical chaparral and savannahs from Columbia to Canada. They cover a wide range of ecological niches, and many species are thought to be able to cope with current trends in climate change. Genus Quercus encompasses ca. 500 species, of which ca. 80 make up sect. Lobatae. Species diversity is greatest within the southeastern USA and within the northern and eastern regions of Mexico. This review discusses the weak reproductive barriers between species of red oaks and the effects this has on speciation and niche range. Distribution and diversity have been shaped by drought adaptations common to the species of sect. Lobatae, which enable them to fill various xeric niches across the continent. Drought adaptive traits of this taxonomic group include deciduousness, deep tap roots, ring-porous xylem, regenerative stump sprouting, greater leaf thickness and smaller stomata. The complex interplay between these anatomical and morphological traits has given red oaks features of drought tolerance and avoidance. Here, we discuss physiological and genetic components of these adaptations to address how many species of sect. Lobatae reside within xeric sites and/or sustain normal metabolic function during drought. Although extensive drought adaptation appears to give sect. Lobatae a resilience to climate change, aging tree stands, oak life history traits and the current genetic structures place many red oak species at risk. Furthermore, oak decline, a complex interaction between abiotic and biotic agents, has severe effects on red oaks and is likely to accelerate species decline and fragmentation. We suggest that assisted migration can be used to avoid species fragmentation and increase climate change resilience of sect. Lobatae.

Combined effects of warming and drought on plant biomass depend on plant woodiness and community type: a meta-analysis

Global warming and precipitation extremes (drought or increased precipitation) strongly affect plant primary production and thereby terrestrial ecosystem functioning. Recent syntheses show that combined effects of warming and precipitation extremes on plant biomass are generally additive, while individual experiments often show interactive effects, indicating that combined effects are more negative or positive than expected based on the effects of single factors. Here, we examined whether variation in biomass responses to single and combined effects of warming and precipitation extremes can be explained by plant growth form and community type. We performed a meta-analysis of 37 studies, which experimentally crossed warming and precipitation treatments, to test whether biomass responses to combined effects of warming and precipitation extremes depended on plant woodiness and community type (monocultures versus mixtures). Our results confirmed that the effects of warming and precipitation extremes were overall additive. However, combined effects of warming and drought on above- and belowground biomass were less negative in woody- than in herbaceous plant systems and more negative in plant mixtures than in monocultures. We further show that drought effects on plant biomass were more negative in greenhouse- than in field studies, suggesting that greenhouse experiments may overstate drought effects in the field. Our results highlight the importance of plant system characteristics to better understand plant responses to climate change.

Amplifying effects of recurrent drought on the dynamics of tree growth and water use in a subalpine forest

Despite recent advances in our understanding of drought impacts on tree functioning, we lack knowledge about the dynamic responses of mature trees to recurrent drought stress. At a subalpine forest site, we assessed the effects of three years of recurrent experimental summer drought on tree growth and water relations of Larix decidua Mill. and Picea abies (L. Karst.), two common European conifers representative for contrasting water-use strategies. We combined dendrometer and xylem sap flow measurements with analyses of xylem anatomy and non-structural carbohydrates and their carbon-isotope composition. Recurrent drought increased the effects of soil moisture limitation on growth and xylogenesis, and to a lesser extent on xylem sap flow. P. abies showed stronger growth responses to recurrent drought, reduced starch concentrations in branches and increased water-use efficiency when compared to L. decidua. Despite comparatively larger maximum tree water deficits than in P. abies, xylem formation of L. decidua was less affected by drought, suggesting a stronger capacity of rehydration or lower cambial turgor thresholds for growth. Our study shows that recurrent drought progressively increases impacts on mature trees of both species, which suggests that in a future climate increasing drought frequency could impose strong legacies on carbon and water dynamics of treeline species.

Trait coordination at leaf level explains the resistance to excess light stress in shade-tolerant tropical tree species

Plant functioning in response to the environment is an important issue for vegetation reassembly of degraded lands because of both low and excessive sunlight influence the performance of young plants. However, how shade-tolerant tree species deal with excess of light energy remains unclear due to the contrasting results among studies and the subjective classifications of species shade tolerance. From a quantitative classification of shade tolerance of 12 tropical tree species planted in the field under contrasting light conditions, we hypothesized that shade-tolerant species are capable of effective long-term acclimation to high-light conditions. Plant size and survival of species were measured at the beginning and 38 months after planting. We also measured functional traits associated with carbon economy and non-photochemical energy dissipation. Under high-light condition, more light-demanding species showed higher growth rates associated with higher values of functional traits that improve the CO2 assimilation capacity. By contrast, more shade-tolerant species showed higher survival that was associated with higher values of qN, leaf K contents and leaf thickness. The concomitant increase in these traits, as well as their greater plasticity to light in WUEi, Fv/Fm and qN, confers on these species strong photoprotection to avoid high-light stress and to persist under open field conditions. The results challenge the common assumption that only fast-growing and light-demanding tree species are suitable for vegetation reassembly in full sunlight conditions and to improve the environmental conditions for other species.

Effects of Forest Gap and Seed Size on Germination and Early Seedling Growth in Quercus acutissima Plantation in Mount Tai, China

Elucidating the influence mechanisms of seed germination and seedling growth is important for revealing the natural regeneration of forest plantations. We collected the seeds from 58-year-old Quercus acutissima Carruth. forest, and the seeds were further divided into three classes: large, medium, and small, and sown under the forest gaps (I, 197.82 m2; II, 91.85 m2, III, understory) to observe seed germination and early seedling growth. Precipitation in the study area and soil moisture content in the forest gaps were also observed during the trial period. The results showed that the precipitation in 2019 was similar to that in 2020; both were significantly lower than the precipitation in 2021. The difference in soil water content between gaps I and II was not significant, and both were significantly lower than III. The order of seedling emergence rate in gaps was II > III > I, but the minimum was almost close to zero in I. Large and medium seeds showed significantly greater emergence rate than small seeds. The seedlings of II had higher seedling height, ground diameter, ground diameter relative growth rate, seedling biomass, root surface area, and root volume than those of III. Large seeds had the highest ground diameter, ground diameter relative growth rate, biomass, root mass ratio, root shoot ratio, and root surface area. Correlation analysis showed that seedling biomass was significantly and positively correlated with root surface area and root volume, and significantly and negatively correlated with specific root length and specific root surface area. The regulation of soil moisture in the gap and the adaptability related to seed size were two key factors influencing the seed germination and early seedling growth of Q. acutissima.

Satellite data reveal differential responses of Swiss forests to unprecedented 2018 drought

Extreme events such as the summer drought of 2018 in Central Europe are projected to occur more frequently in the future and may cause major damages including increased tree mortality and negative impacts on forest ecosystem services. Here, we quantify the response of >1 million forest pixels of 10 × 10 m across Switzerland to the 2018 drought in terms of resistance, recovery, and resilience. We used the Normalized Difference Water Index (NDWI) derived from Sentinel-2 satellite data as a proxy for canopy water content and analyzed its relative change. We calculated NDWI change between the 2017 pre-drought and 2018 drought years (indicating resistance), 2018 and the 2019 post-drought (indicating recovery), and between 2017-2019 (indicating resilience). Analyzing the data from this large natural experiment, we found that for 4.3% of the Swiss forest the NDWI declined between 2017 and 2018, indicating areas with low resistance of the forest canopy to drought effects. While roughly 50% of this area recovered, in 2.7% of the forested area NDWI continued to decline from 2018 to 2019, suggesting prolonged negative effects or delayed damage. We found differential forest responses to drought associated with site topographic characteristics and forest stand characteristics, and to a lesser extent with climatic conditions and interactions between these drivers. Low drought resistance and high recovery were most prominent at forest edges, but also on south-facing slopes and lower elevations. Tree functional type was the most important driver of drought resilience, with most of the damage in stands with high conifer abundance. Our results demonstrate the suitability of satellite-based quantification of drought-induced forest damage at high spatial resolution across large areas. Such information is important to predict how local site characteristics may impact forest vulnerability to future extreme events and help in the search for appropriate adaptation strategies.

The xylem of anisohydric Quercus alba L. is more vulnerable to embolism than isohydric codominants

The coordination of plant leaf water potential (Ψ_L ) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem more conservatively regulate Ψ_L than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh. and Liriodendron tulipifera L., by synthesizing 1600 Ψ_L observations, 122 xylem embolism curves and xylem anatomical measurements across 10 forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated Ψ_L less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose Ψ_L regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought-prone future.

Detecting forest response to droughts with global observations of vegetation water content

Droughts in a warming climate have become more common and more extreme, making understanding forest responses to water stress increasingly pressing. Analysis of water stress in trees has long focused on water potential in xylem and leaves, which influences stomatal closure and water flow through the soil-plant-atmosphere continuum. At the same time, changes of vegetation water content (VWC) are linked to a range of tree responses, including fluxes of water and carbon, mortality, flammability, and more. Unlike water potential, which requires demanding in situ measurements, VWC can be retrieved from remote sensing measurements, particularly at microwave frequencies using radar and radiometry. Here, we highlight key frontiers through which VWC has the potential to significantly increase our understanding of forest responses to water stress. To validate remote sensing observations of VWC at landscape scale and to better relate them to data assimilation model parameters, we introduce an ecosystem-scale analog of the pressure-volume curve, the non-linear relationship between average leaf or branch water potential and water content commonly used in plant hydraulics. The sources of variability in these ecosystem-scale pressure-volume curves and their relationship to forest response to water stress are discussed. We further show to what extent diel, seasonal, and decadal dynamics of VWC reflect variations in different processes relating the tree response to water stress. VWC can also be used for inferring belowground conditions-which are difficult to impossible to observe directly. Lastly, we discuss how a dedicated geostationary spaceborne observational system for VWC, when combined with existing datasets, can capture diel and seasonal water dynamics to advance the science and applications of global forest vulnerability to future droughts.

Sensitivity and threshold dynamics of Pinus strobus and Quercus spp. in response to experimental and naturally occurring severe droughts

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (Js), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016-17). Results showed that Js in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15 m3 m-3, whereas oak's (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m3 m-3-a finding consistent with pine's more isohydric strategy. Nevertheless, once oaks' moisture threshold was surpassed, Js declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks' growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species' contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

Higher biomass partitioning to absorptive roots improves needle nutrition but does not alleviate stomatal limitation of northern Scots pine

Harsh environmental conditions affect both leaf structure and root traits. However, shoot growth in high-latitude systems is predominately under photoperiod control while root growth may occur for as long as thermal conditions are favorable. The different sensitivities of these organs may alter functional relationships above- and belowground along environmental gradients. We examined the relationship between absorptive root and foliar traits of Scots pine trees growing in situ along a temperate-boreal transect and in trees grown in a long-term common garden at a temperate latitude. We related changes in foliar nitrogen, phosphorus, specific leaf area, needle mass and ¹³ C signatures to geographic trends in absorptive root biomass to better understand patterns of altered tree nutrition and water balance. Increased allocation to absorptive fine roots was associated with greater uptake of soil nutrients and subsequently higher needle nutrient contents in the northern provenances compared with more southern provenances when grown together in a common garden setting. In contrast, the leaf δ¹³ C in northern and southern provenances were similar within the common garden suggesting that higher absorptive root biomass fractions could not adequately increase water supply in warmer climates. These results highlight the importance of allocation within the fine-root system and its impacts on needle nutrition while also suggesting increasing stomatal limitation of photosynthesis in the context of anticipated climatic changes.

MicroRNA156-mediated changes in leaf composition lead to altered photosynthetic traits during vegetative phase change

Plant morphology and physiology change with growth and development. Some of these changes are due to change in plant size and some are the result of genetically programmed developmental transitions. In this study we investigate the role of the developmental transition, vegetative phase change (VPC), on morphological and photosynthetic changes. We used overexpression of microRNA156, the master regulator of VPC, to modulate the timing of VPC in Populus tremula × alba, Zea mays, and Arabidopsis thaliana to determine its role in trait variation independent of changes in size and overall age. Here, we find that juvenile and adult leaves in all three species photosynthesize at different rates and that these differences are due to phase-dependent changes in specific leaf area (SLA) and leaf N but not photosynthetic biochemistry. Further, we found juvenile leaves with high SLA were associated with better photosynthetic performance at low light levels. This study establishes a role for VPC in leaf composition and photosynthetic performance across diverse species and environments. Variation in leaf traits due to VPC are likely to provide distinct benefits under specific environments; as a result, selection on the timing of this transition could be a mechanism for environmental adaptation.

Space-for-time inferences about range-edge dynamics of tree species can be influenced by sampling biases

Differences between the distributions of tree saplings and adults in geographic or niche space have been used to infer climate change effects on tree range dynamics. Previous studies have reported narrower latitudinal or climatic niche ranges of juvenile trees compared to adults, concluding that tree ranges are contracting, contradicting climate-based predictions. However, more comprehensive sampling of adult trees than juvenile trees in most regional forest inventories could potentially bias ontogenetic comparisons. Here we first report spatial simulations showing that reduced sampling intensity can result in underestimates of range and niche limits, but that resampling the same number of individuals of different life stages can eliminate this bias. We then reanalyzed the U.S. Forest Inventory and Analysis data, comparing the range and niche limits between adult trees and saplings of 92 tree species, both using the original data and two resampling procedures. Resampling aimed to reduce sampling biases by controlling for either sampling area or the number of individuals sampled. Overall, these resampling procedures had a major influence on the estimation of range limits, most often by reducing, eliminating, or even reversing the tendency in the original analyses for saplings to have broader distributions than adult trees. These results indicate that previous conclusions that the distributions of juvenile trees were contracting in response to climate change were potentially artifacts of sampling in the underlying data. More generally, sampling effects involved in the estimation of geographic ranges and environmental niche widths need to be taken into account in studies comparing different life stages, and also likely in other types of distribution comparisons.

Leaf traits and performance vary with plant age and water availability in Artemisia californica

BACKGROUND AND AIMS

Leaf functional traits are strongly tied to growth strategies and ecological processes across species, but few efforts have linked intraspecific trait variation to performance across ontogenetic and environmental gradients. Plants are believed to shift towards more resource-conservative traits in stressful environments and as they age. However, uncertainty as to how intraspecific trait variation aligns with plant age and performance in the context of environmental variation may limit our ability to use traits to infer ecological processes at larger scales.

METHODS

We measured leaf physiological and morphological traits, canopy volume and flowering effort for Artemisia californica (California sagebrush), a dominant shrub species in the coastal sage scrub community, under conditions of 50, 100 and 150 % ambient precipitation for 3 years.

KEY RESULTS

Plant age was a stronger driver of variation in traits and performance than water availability. Older plants demonstrated trait values consistent with a more conservative resource-use strategy, and trait values were less sensitive to drought. Several trait correlations were consistent across years and treatments; for example, plants with high photosynthetic rates tended to have high stomatal conductance, leaf nitrogen concentration and light-use efficiency. However, the trade-off between leaf construction and leaf nitrogen evident in older plants was absent for first-year plants. While few traits correlated with plant growth and flowering effort, we observed a positive correlation between leaf mass per area and performance in some groups of older plants.

CONCLUSIONS

Overall, our results suggest that trait sensitivity to the environment is most visible during earlier stages of development, after which intraspecific trait variation and relationships may stabilize. While plant age plays a major role in intraspecific trait variation and sensitivity (and thus trait-based inferences), the direct influence of environment on growth and fecundity is just as critical to predicting plant performance in a changing environment.

Selection patterns on early-life phenotypic traits in Pinus sylvestris are associated with precipitation and temperature along a climatic gradient in Europe

Understanding the dynamics of selection is key to predicting the response of tree species to new environmental conditions in the current context of climate change. However, selection patterns acting on early recruitment stages and their climatic drivers remain largely unknown in most tree species, despite being a critical period of their life cycle. We measured phenotypic selection on Pinus sylvestris seed mass, emergence time and early growth rate over 2 yr in four common garden experiments established along the latitudinal gradient of the species in Europe. Significant phenotypic plasticity and among-population genetic variation were found for all measured phenotypic traits. Heat and drought negatively affected fitness in the southern sites, but heavy rainfalls also decreased early survival in middle latitudes. Climate-driven directional selection was found for higher seed mass and earlier emergence time, while the form of selection on seedling growth rates differed among sites and populations. Evidence of adaptive and maladaptive phenotypic plasticity was found for emergence time and early growth rate, respectively. Seed mass, emergence time and early growth rate have an adaptive role in the early stages of P. sylvestris and climate strongly influences the patterns of selection on these fitness-related traits.

Direct and correlated responses to artificial selection for growth and water-use efficiency in a Mediterranean pine

PREMISE

Persistence of tree populations in the face of global change relies on their capacity to respond to biotic and abiotic stressors through plastic or adaptive changes. Genetic adaptation will depend on the additive genetic variation within populations and the heritability of traits related to stress tolerance. Because traits can be genetically linked, selective pressure acting on one trait may lead to correlated responses in other traits.

METHODS

To test direct and correlated responses to selection for growth and drought tolerance in Pinus halepensis, we selected trees in a parental population for higher growth and greater water-use efficiency (WUE) and compared their offspring with the offspring of random trees from the parental population in two contrasting common gardens. We estimated direct responses to selection for growth and WUE and correlated responses for growth and tolerance to abiotic and biotic stressors.

RESULTS

We found a strong response to selection and high realized heritability for WUE, but no response to selection for growth. Correlated responses to selection in other life-history traits were not significant, except for concentration of some chemical defenses, which was greater in the offspring of mother trees selected for growth than in the offspring of unselected control trees.

CONCLUSIONS

The empirical evidence of direct responses to selection for high WUE suggests that P. halepensis has the potential to evolve in response to increasing drought stress. Contrary to expectations, the results are not conclusive of a potential negative impact of WUE and growth selection on other key life-history traits.

Phenotypic Plasticity of Drought Tolerance Traits in a Widespread Eucalypt (Eucalyptus obliqua)

Long-term studies of tree responses to drought stress help us to understand the capacity of species to adapt to their environment. In this study, we investigated how Eucalyptus obliqua adjusts physiological and morphological traits in response to seasonal and multi-year droughts. We monitored physiological and morphological traits over multiple years in undisturbed control and throughfall reduction plots in a eucalypt forest in south-eastern Australia. The throughfall reduction treatment did not induce significantly lower soil moisture in the throughfall reduction plots compared with the control plots. However, natural variability in precipitation and evaporative demand induced drought stress of varying intensity each summer in all plots. We observed a significant relationship between seasonal precipitation and leaf pre-dawn water potential (ΨPD), with less precipitation over summer, resulting in a decline in ΨPD and drought stress when ΨPD fell below −0.75 MPa. Eucalyptus obliqua responded to short-term summer drought through rapid leaf osmotic adjustment which lowered the leaf water potential at the turgor loss point beyond the minimum leaf water potential. Morphological adjustments, such as the reduction of leaf area to sapwood area (higher Huber Value) were moderate during the measurement period and only occurred under severe drought stress (pre-dawn water potential < −1.2 MPa). Overall, E. obliqua responded to short-term mild drought stress through physiological trait plasticity, while morphological adjustment only occurred under a more severe water deficit.

Adaptation to drought is coupled with slow growth, but independent from phenology in marginal silver fir (Abies alba Mill.) populations

Drought is one of the most important selection pressures for forest trees in the context of climate change. Yet, the different evolutionary mechanisms, and their environmental drivers, by which certain populations become more drought tolerant than others is still little understood. We studied adaptation to drought in 16 silver fir (Abies alba Mill.) populations from the French Mediterranean Alps by combining observations on seedlings from a greenhouse experiment (N = 8,199) and on adult tress in situ (N = 315). In the greenhouse, we followed half-sib families for four growing seasons for growth and phenology traits, and tested their water stress response in a "drought until death" experiment. Adult trees in the field were assessed for δ 13C, a proxy for water use efficiency, and genotyped at 357 SNP loci. SNP data was used to generate a null expectation for seedling trait divergence between populations in order to detect the signature of selection, and 31 environmental variables were used to identify the selective environment. We found that seedlings originating from populations with low soil water capacity grew more slowly, attained a smaller stature, and resisted water stress for a longer period of time in the greenhouse. Additionally, adult trees of these populations exhibited a higher water use efficiency as evidenced by their δ 13C. These results suggest a correlated evolution of the growth-drought tolerance trait complex. Population divergence in bud break phenology was adaptive only in the second growing season, and evolved independently from the growth-drought tolerance trait complex. Adaptive divergence in bud break phenology was principally driven by the inter- and intra-annual variation in temperature at the geographic origin of the population. Our results illustrate the different evolutionary strategies used by populations to cope with drought stress at the range limits across a highly heterogeneous landscape, and can be used to inform assisted migration programs.

Consequences of drought severity for tropical live oak (Quercus oleoides) in Mesoamerica

In two Costa Rican and three Honduran sites that vary in rainfall and soil properties, we used natural isotopes, a soil water balance model, and broad-scale climate-based drought indices to study shifts in water use with ontogeny from seedlings to mature tropical live oak (Quercus oleoides) trees. Water use patterns help to explain persistence of this broadly distributed species in Mesoamerica and to evaluate likely threats of ongoing climate changes. At the end of the dry season, soil δ¹⁸ O profiles can be described by one-phase exponential decay curves. Minimum values reflect geographic origins of the last significant rain event, and curvature is inversely related to canopy closure, demonstrating its role in controlling topsoil evaporation. Partitioning of soil water sources for transpiration was analyzed with a mixing model. In the Costa Rican sites, in a relatively dry year, saplings and mature trees took up water from the upper soil. In a relatively wet year in the Honduran sites, we observed deeper water extraction. In all sites, soil storage dampens extreme variation in water availability. The size dependence of water uptake with larger stems exploiting deeper layers is translated into variation in bulk leaf δ¹³ C-based water use efficiency (WUE) with the exception of mature trees. From 1932 to 2015, drought severity was evaluated with the Standardized Precipitation Evapotranspiration Index (SPEI) concurrently with simulations of the soil water balance model. Drought occurrence increased, regardless of the time period, averaged across 6, 12, or 24 months. All ontogenetic stages in all populations experienced frequent water limitation. We found evidence for linear trends toward aridification with increases of return periods of drought for October SPEI-24 declining from 42 to 6 yr in Costa Rica and from 21 to 7 yr in Honduras and recent occurrence of multiyear droughts from 2013 to 2016. October SPEI-12 and SPEI-24 were significantly related to the Oceanic Niño Indices demonstrating that local inter-annual variations in drought severity in Mesoamerica are modulated by large-scale climate forces. Drought severity in the near-term future depends on the extent to which the Pacific will adopt a more La Niña-like vs. a more El Niño-like state under ongoing climatic changes.

Mechanisms of severe dieback and mortality in a classically drought-tolerant shrubland species (Arctostaphylos glauca)

PREMISE

Mortality events involving drought and pathogens in natural plant systems are on the rise due to global climate change. In Santa Barbara, California, United States, big berry manzanita (Arctostaphylos glauca) has experienced canopy dieback related to a multi-year drought and infection from fungal pathogens in the Botryosphaeriaceae family. A greenhouse experiment was conducted using Neofusicoccum australe to test the specific influences of drought and fungal infection on A. glauca.

METHODS

A full factorial design was used to compare four treatment groups (drought + inoculation; drought - inoculation; watering + inoculation; and control: watering - inoculation). Data were collected for 10 weeks on stress symptoms, changes in leaf fluorescence and photosynthesis, and mortality.

RESULTS

Results indicated significant effects of watering and inoculation treatments on net photosynthesis, dark-adapted fluorescence, and disease symptom severity (P < 0.05), and a strong correlation was found between physiological decline and visible stress (P < 0.0001). Mortality differed between treatments, with all groups except for the control experiencing mortality (43% mortality in drought - inoculation, 83% in watering - inoculation, and 100% in drought + inoculation). A Kaplan-Meier survival analysis showed drought + inoculation to have the least estimated survivorship compared to all other treatment groups.

CONCLUSIONS

In addition to a possible synergistic interaction between drought and fungal infection in disease onset and mortality rates in A. glauca, these results indicate that young, non-drought-stressed plants are susceptible to mortality from N. australe infection, with important implications for the future of wildland shrub communities.

Which oak provenances for the 22nd century in Western Europe? Dendroclimatology in common gardens

The current distribution area of the two sympatric oaks Quercus petraea and Q. robur covers most of temperate Western Europe. Depending on their geographic location, populations of these trees are exposed to different climate constraints, to which they are adapted. Comparing the performances of trees from contrasting populations provides the insight into their expected resilience to future climate change required for forest management. In this study, the descendants of 24 Q. petraea and two Q. robur provenances selected from sites throughout Europe were grown for 20 years in three common gardens with contrasting climates. The 2420 sampled trees allowed the assessments of the relationship between radial growth and climate. An analysis of 15-year chronologies of ring widths, with different combinations of climate variables, revealed different response patterns between provenances and between common gardens. As expected, provenances originating from sites with wet summers displayed the strongest responses to summer drought, particularly in the driest common garden. All provenances displayed positive significant relationships between the temperature of the previous winter and radial growth when grown in the common garden experiencing the mildest winter temperatures. Only eastern provenances from continental cold climates also clearly expressed this limitation of growth by cold winter temperatures in the other two common gardens. However, ecological distance, calculated on the basis of differences in climate between the site of origin and the common garden, was not clearly related to the radial growth responses of the provenances. This suggests that the gradient of genetic variability among the selected provenances was not strictly structured according to climate gradients. Based on these results, we provide guidelines for forest managers for the assisted migration of Quercus petraea and Q. robur provenances.

Allometric scaling laws linking biomass and rooting depth vary across ontogeny and functional groups in tropical dry forest lianas and trees

There are two theories about how allocation of metabolic products occurs. The allometric biomass partitioning theory (APT) suggests that all plants follow common allometric scaling rules. The optimal partitioning theory (OPT) predicts that plants allocate more biomass to the organ capturing the most limiting resource. Whole-plant harvests of mature and juvenile tropical deciduous trees, evergreen trees, and lianas and model simulations were used to address the following knowledge gaps: (1) Do mature lianas comply with the APT scaling laws or do they invest less biomass in stems compared to trees? (2) Do juveniles follow the same allocation patterns as mature individuals? (3) Is either leaf phenology or life form a predictor of rooting depth? It was found that: (1) mature lianas followed the same allometric scaling laws as trees; (2) juveniles and mature individuals do not follow the same allocation patterns; and (3) mature lianas had shallowest coarse roots and evergreen trees had the deepest. It was demonstrated that: (1) mature lianas invested proportionally similar biomass to stems as trees and not less, as expected; (2) lianas were not deeper-rooted than trees as had been previously proposed; and (3) evergreen trees had the deepest roots, which is necessary to maintain canopy during simulated dry seasons.

Correcting tree-ring δ13C time series for tree-size effects in eight temperate tree species

Stable carbon isotope ratios (δ13C) in tree rings have been widely used to study changes in intrinsic water-use efficiency (iWUE), sometimes with limited consideration of how C-isotope discrimination is affected by tree height and canopy position. Our goals were to quantify the relationships between tree size or tree microenvironment and wood δ13C for eight functionally diverse temperate tree species in northern New England and to better understand the physical and physiological mechanisms underlying these differences. We collected short increment cores in closed-canopy stands and analyzed δ13C in the most recent 5 years of growth. We also sampled saplings in both shaded and sun-exposed environments. In closed-canopy stands, we found strong tree-size effects on δ13C, with 3.7-7.2‰ of difference explained by linear regression vs height (0.11-0.28‰ m-1), which in some cases is substantially stronger than the effect reported in previous studies. However, open-grown saplings were often isotopically more similar to large codominant trees than to shade-grown saplings, indicating that light exposure contributes more to the physiological and isotopic differences between small and large trees than does height. We found that in closed-canopy forests, δ13C correlations with diameter at breast height were nonlinear but also strong, allowing a straightforward procedure to correct tree- or stand-scale δ13C-based iWUE chronologies for changing tree size. We demonstrate how to use such data to correct and interpret multi-decadal composite isotope chronologies in both shade-regenerated and open-grown tree cohorts, and we highlight the importance of understanding site history when interpreting δ13C time series.

Over-accumulation of abscisic acid in transgenic tomato plants increases the risk of hydraulic failure

Climate change threatens food security, and plant science researchers have investigated methods of sustaining crop yield under drought. One approach has been to overproduce abscisic acid (ABA) to enhance water use efficiency. However, the concomitant effects of ABA overproduction on plant vascular system functioning are critical as it influences vulnerability to xylem hydraulic failure. We investigated these effects by comparing physiological and hydraulic responses to water deficit between a tomato (Solanum lycopersicum) wild type control (WT) and a transgenic line overproducing ABA (sp12). Under well-watered conditions, the sp12 line displayed similar growth rate and greater water use efficiency by operating at lower maximum stomatal conductance. X-ray microtomography revealed that sp12 was significantly more vulnerable to xylem embolism, resulting in a reduced hydraulic safety margin. We also observed a significant ontogenic effect on vulnerability to xylem embolism for both WT and sp12. This study demonstrates that the greater water use efficiency in the tomato ABA overproducing line is associated with higher vulnerability of the vascular system to embolism and a higher risk of hydraulic failure. Integrating hydraulic traits into breeding programmes represents a critical step for effectively managing a crop's ability to maintain hydraulic conductivity and productivity under water deficit.

Effects of soil rewatering on mesophyll and stomatal conductance and the associated mechanisms involving leaf anatomy and some physiological activities in Manchurian ash and Mongolian oak in the Changbai Mountains

The recoveries of mesophyll (g_m) and stomatal conductance to CO₂ (g_sc) after soil rewatering have received considerable attention in recent years, but the recovery mechanisms involving leaf anatomy and physiological activities are poorly understood. Moreover, it is also unclear whether leaf gas-phase conductance (g_ias) or liquid-phase conductance (g_liq) is the main factor promoting g_m recovery. By simultaneously using gas exchange and chlorophyll fluorescence, we measured the recoveries of g_m and g_sc in saplings of Manchurian ash (Fraxinus mandshurica Rupr.) and Mongolian oak (Quercus mongolica Fish. ex Ledeb) exposed to two initial water stress (medium water stress, MW, and severe water stress, SW) and following rewatering. Furthermore, leaf anatomical characteristics and the activities of aquaporin (AQP) and carbonic anhydrase (CA) were measured to explain the mechanisms of g_m and g_sc recoveries. The results showed that (i) both g_m and g_sc were partly recovered after rewatering, and the recoveries decreased with initial water stress in both species. (ii) The g_m recovery was much greater in Mongolian oak than in Manchurian ash, while the g_sc recovery was much greater in Manchurian ash. Consequently, the photosynthesis recovery in Manchurian ash was mostly affected by g_sc recovery, while that in Mongolian oak was mostly affected by g_m recovery. (iii) The g_m recovery mainly resulted from the great increase in leaf g_liq after rewatering rather than that in g_ias, as g_ias had a negative effect on g_m recovery. The stomatal opening status improved after rewatering, as the stomatal pore size (SS) increased, greatly promoting g_sc recovery. In addition, the activities of both AQP and CA increased after rewatering, which improved CO₂ transmembrane transports and greatly promoted g_m and g_sc recoveries.

Seed local adaptation and seedling plasticity account for Gleditsia triacanthos tree invasion across biomes

BACKGROUND AND AIMS

Phenotypic plasticity and local adaption can contribute to the success of invasive species. While the former is an environmentally induced trait, the latter involves a selection process to filter the best genotype for a location. We examined the evidence for phenotypic plasticity and local adaptation for seed and seedling traits of the invasive tree Gleditsia triacanthos, with three origins distributed along an approx. 10° latitude gradient across three biomes.

METHODS

In sub-tropical forests, dry woodlands and secondary temperate grasslands in Argentina, we harvested seeds from clusters of neighbouring trees (i.e. families) distributed within 15-20 km in each origin (biome). We manipulated the environmental conditions relevant to each biome, assuming that propagule availability did not represent an ecological barrier. In growth chambers, we evaluated seed imbibition and seed germination under different light, temperature and water potential. In a 2 year common garden, we evaluated the impact of resident vegetation removal on seedling survival and growth.

KEY RESULTS

Mean time to complete seed imbibition differed among origins; seeds from temperate grasslands reached full imbibition before seeds from dry woodlands and sub-tropical forests. Germination was always >70 %, but was differentially affected by water potential, and light quantity (dark-light) and quality (red-far red) among origins, suggesting local adaptation. In the common garden, vegetation removal rather than origin negatively affected seedling survival and enhanced seedling growth. Vegetation removal increased basal diameter, leaves per plant and spine number, and reduced the height:basal diameter ratio.

CONCLUSIONS

We conclude that local adaptation in seed germination traits and plastic changes in seedling allometry (e.g. height:diameter) may allow this tree to respond over the short and long term to changes in environmental conditions, and to contribute to shape G. triacanthos as a successful woody invader. Overall, our study revealed how local adaptation and plasticity can explain different aspects of tree invasion capacity across biomes.

Deepening Rooting Depths Improve Plant Water and Carbon Status of a Xeric Tree during Summer Drought

Exploring the effects of drought on trees of different sizes is an important research topic because the size-dependent mortality pattern of the major dominant species significantly affects the structure and function of plant communities. Here we studied the physiological performance and non-structural carbohydrates (NSCs) dynamics of a small xeric tree species, Haloxylon ammodendron (C.A.Mey.) of different tree size with varying rooting depth, during summer drought. We measured predawn (Ψpd) and midday (Ψm) leaf water potential, osmotic potential at saturated turgor (π100), and turgor lost point (Ψtlp), stomatal conductance (gs) at noon, maximum photochemical efficiency of photosystem II (Fv/Fm) in the morning, and NSCs concentration, from June–September. Our results demonstrated that the summer drought reduces the overall performance of physiological traits of the small young trees more than the larger adult trees. Ψpd, gs and Fv/Fm dropped larger in the small-diameter groups than the larger diameter groups. Substantial osmotic adjustments were observed in small size individuals (with lower π100 and Ψtlp) to cope with summer drought. Furthermore, mean concentration of NSCs for the leaf and shoot were higher in September than in July in every basal stem diameter classes suggested the leaf and shoot acted as reserve for NSC. However the root NSCs concentrations within each basal stem diameter class exhibited less increase in September than in the July. At the same time, the small young tress had lower root NSCs concentrations than the larger adult tree in both July and September. The contrasting root NSC concentrations across the basal stem diameter classes indicated that the roots of smaller trees may be more vulnerable to carbon starvation under non-lethal summer drought. The significant positive relationship between rooting depth and physiological traits & root NSCs concentration emphasize the importance of rooting depth in determining the seasonal variation of water status, gas exchange and NSCs.

Effect of Drought and Topographic Position on Depth of Soil Water Extraction of Pinus sylvestris L. var. mongolica Litv. Trees in a Semiarid Sandy Region, Northeast China

Drought and topographic position are the most important factors influencing tree growth and survival in semiarid sandy regions of Northeast China. However, little is known about how trees respond to drought in combination with topographic position by modifying the depth of soil water extraction. Therefore, we identified water sources for 33-year-old Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) trees growing at the top and bottom of sand dunes by comparing stable isotopes δ2H and δ18O in twig xylem water, soil water at various depths and groundwater during dry and wet periods. Needle carbon isotope composition (δ13C) was simultaneously measured to assess water use efficiency. Results showed that when soil moisture was low during the dry period, trees at the top used 40–300 cm soil water while trees at the bottom utilized both 40–300 cm soil water and possibly groundwater. Nevertheless, when soil moisture at 0–100 cm depth was higher during the wet period, it was the dominant water sources for trees at both the top and bottom. Moreover, needle δ13C in the dry period were significantly higher than those in the wet period. These findings suggested that trees at both the top and bottom adjust water uptake towards deeper water sources and improve their water use efficiency under drought condition. Additionally, during the dry period, trees at the top used shallower water sources compared with trees at the bottom, in combination with significantly higher needle δ13C, indicating that trees at the bottom applied a relatively more prodigal use of water by taking up deeper water (possibly groundwater) during drought conditions. Therefore, Mongolian pine trees at the top were more susceptible to suffer dieback under extreme dry years because of shallower soil water uptake and increased water restrictions. Nevertheless, a sharp decline in the groundwater level under extreme dry years had a strong negative impact on the growth and survival of Mongolian pine trees at the bottom due to their utilization of deeper water sources (possibly groundwater).

Increased root investment can explain the higher survival of seedlings of 'mesic' Quercus suber than 'xeric' Quercus ilex in sandy soils during a summer drought

In Mediterranean-type ecosystems, drought is considered the main ecological filter for seedling establishment. The evergreen oaks Quercus ilex L. and Quercus suber L. are two of the most abundant tree species in the Mediterranean Basin. Despite their shared evergreen leaf habit and ability to resist low soil water potentials, traditionally it has been suggested that Q. ilex is better suited to resist dry conditions than Q. suber. In this study, we examined how seedlings of Q. ilex and Q. suber grown in sandy soils responded to different levels of water availability using natural dry conditions and supplemental watering. Specifically, we estimated survival and water status of seedlings and explored the role of acorn mass and belowground biomass in seedling performance. To our surprise, Q. suber was better able to survive the summer drought in our experiment than Q. ilex. Nearly 55% of the Q. suber seedlings remained alive after a 2-month period without rain or supplemental water, which represents almost 20% higher survival than Q. ilex over the same period. At the end of the dry period, the surviving seedlings of Q. suber had strikingly higher water potential, potential maximum quantum yield of photosystem II (Fv/Fm) and stomatal conductance (gs) than those of Q. ilex. Acorn mass was associated with the probability of survival under dry conditions; however, it did not explain the differences in survival or water status between the species. In contrast, Q. suber had a higher root ratio and root:shoot ratio than Q. ilex and these traits were positively associated with predawn leaf water potential, Fv/Fm, gs and survival. Taken together, our results suggest that the higher relative investment in roots by Q. suber when growing in a sandy acidic substrate allowed this species to maintain better physiological status and overall condition than Q. ilex, increasing its probability of survival in dry conditions.

Diversification, adaptation, and community assembly of the American oaks (Quercus), a model clade for integrating ecology and evolution

Contents Summary 669 I. Model clades for the study and integration of ecology and evolution 670 II. Oaks: an important model clade 671 III. Insights from the history of the American oaks for understanding community assembly and ecosystem dominance 673 IV. Bridging the gap between micro- and macroevolutionary processes relevant to ecology 679 V. How do we reconcile evidence for adaptive evolution with niche conservatism and long-term stasis? 682 VI. High plasticity and within-population genetic variation contribute to population persistence 683 VII. Emerging technologies for tracking functional change 685 VIII. Conclusions 685 Acknowledgements 686 References 686 SUMMARY: Ecologists and evolutionary biologists are concerned with explaining the diversity and composition of the natural world and are aware of the inextricable linkages between ecological and evolutionary processes that maintain the Earth's life support systems. Yet examination of these linkages remains challenging due to the contrasting nature of focal systems and research approaches. Model clades provide a critical means to integrate ecology and evolution, as illustrated by the oaks (genus Quercus), an important model clade, given their ecological dominance, remarkable diversity, and growing phylogenetic, genomic, and ecological data resources. Studies of the clade reveal that their history of sympatric parallel adaptive radiation continues to influence community assembly today, highlighting questions on the nature and extent of coexistence mechanisms. Flexible phenology and hydraulic traits, despite evolutionary stasis, may have enabled adaptation to a wide range of environments within and across species, contributing to their high abundance and diversity. The oaks offer fundamental insights at the intersection of ecology and evolution on the role of diversification in community assembly processes, on the importance of flexibility in key functional traits in adapting to new environments, on factors contributing to persistence of long-lived organisms, and on evolutionary legacies that influence ecosystem function.

Species richness and traits predict overyielding in stem growth in an early-successional tree diversity experiment

Over the last two decades, empirical work has established that higher biodiversity can lead to greater primary productivity; however, the importance of different aspects of biodiversity in contributing to such relationships is rarely elucidated. We assessed the relative importance of species richness, phylogenetic diversity, functional diversity, and identity of neighbors for stem growth 3 yr after seedling establishment in a tree diversity experiment in eastern Minnesota. Generally, we found that community-weighted means of key functional traits (including mycorrhizal association, leaf nitrogen and calcium, and waterlogging tolerance) as well as species richness were strong, independent predictors of stem biomass growth. More phylogenetically diverse communities did not consistently produce more biomass than expected, and the trait values or diversity of individual functional traits better predicted biomass production than did a multidimensional functional diversity metric. Furthermore, functional traits and species richness best predicted growth at the whole-plot level (12 m² ), whereas neighborhood composition best predicted growth at the focal tree level (0.25 m² ). The observed effects of biodiversity on growth appear strongly driven by positive complementary effects rather than by species-specific selection effects, suggesting that synergistic species' interactions rather than the influence of a few important species may drive overyielding.

The Concentration of Non-structural Carbohydrates, N, and P in Quercus variabilis Does Not Decline Toward Its Northernmost Distribution Range Along a 1500 km Transect in China

Understanding the mechanisms that determine plant distribution range is crucial for predicting climate-driven range shifts. Compared to altitudinal gradients, less attention has been paid to the mechanisms that determine latitudinal range limit. To test whether intrinsic resource limitation contributes to latitudinal range limits of woody species, we investigated the latitudinal variation in non-structural carbohydrates (NSC; i.e., total soluble sugar plus starch) and nutrients (nitrogen and phosphorus) in mature and juvenile Chinese cork oak (Quercus variabilis Blume) along a 1500 km north-south transect in China. During the growing season and dormant season, leaves, branches, and fine roots were collected from both mature and juvenile oaks in seven sites along the transect. Tissue concentration of NSCs, N, and P did not decrease with increasing latitude irrespective of sampling season and ontogenetic stage. Furthermore, higher levels of NSCs and N in tissues of juveniles relative to mature trees were found during the dormant season. Partial correlation analysis also revealed that during the dormant season, soluble sugar, NSC, the ratio of soluble sugar to starch, and tissue nitrogen concentration were correlated positively with latitude but negatively with precipitation and mean temperature of dormant season. Our results suggest that carbon or nutrient availability may not be the driving factors of the latitudinal range limit of the studied species. Further studies should be carried out at the community or ecosystem level with multiple species to additionally test the roles of factors such as regeneration, competition, and disturbance in determining a species' northern distribution limit.

Assessment of shared alleles in drought-associated candidate genes among southern California white oak species (Quercus sect. Quercus)

BACKGROUND

Hybridization and introgression are common phenomena among oak species. These processes can be beneficial by introducing favorable genetic variants across species (adaptive introgression). Given that drought is an important stress, impacting physiological and morphological variation and limiting distributions, our goal was to identify drought-related genes that might exhibit patterns of introgression influenced by natural selection. Using RNAseq, we sequenced whole transcriptomes of 24 individuals from three oaks in southern California: (Quercus engelmannii, Quercus berberidifolia, Quercus cornelius-mulleri) and identified genetic variants to estimate admixture rates of all variants and those in drought genes.

RESULTS

We found 398,042 variants across all loci and 4352 variants in 139 drought candidate genes. STRUCTURE analysis of all variants revealed the majority of our samples were assignable to a single species, but with several highly admixed individuals. When using drought-associated variants, the same individuals exhibited less admixture and their allele frequencies were more polarized between Engelmann and scrub oaks than when using the total gene set. These findings are consistent with the hypothesis that selection may act differently on functional genes, such as drought-associated genes, and point to candidate genes that are suggestive of divergent selection among species maintaining adaptive differences. For example, the drought genes that showed the strongest bias against engelmannii-fixed oak variants in scrub oaks were related to sugar transporter, coumarate-coA ligases, glutathione S-conjugation, and stress response.

CONCLUSION

This pilot study illustrates that whole transcriptomes of individuals will provide useful data for identifying functional genes that contribute to adaptive divergence among hybridizing species.