Delta(13)C and tree-ring width reflect different drought responses in Quercus ilex and Pinus halepensis

Differential growth rate, water-use efficiency and climate sensitivity between males and females of Ilex aquifolium in north-western Spain

BACKGROUND AND AIMS

Dioecious plant species, i.e. those in which male and female functions are housed in different individuals, are particularly vulnerable to global environmental changes. For long-lived plant species, such as trees, long-term studies are imperative to understand how growth patterns and their sensitivity to climate variability affect the sexes differentially.

METHODS

Here, we explore long-term intersexual differences in wood traits, namely radial growth rates and water-use efficiency quantified as stable carbon isotope abundance of wood cellulose, and their climate sensitivity in Ilex aquifolium trees growing in a natural population in north-western Spain.

KEY RESULTS

We found that sex differences in secondary growth rates were variable over time, with males outperforming females in both radial growth rates and water-use efficiency in recent decades. Summer water stress significantly reduced the growth of female trees in the following growing season, whereas the growth of male trees was favoured primarily by cloudy and rainy conditions in the previous autumn and winter combined with low cloud cover and warm conditions in summer. Sex-dependent lagged correlations between radial growth and water availability were found, with a strong association between tree growth and cumulative water availability in females at 30 months and in males at 10 months.

CONCLUSIONS

Overall, our results point to greater vulnerability of female trees to increasing drought, which could lead to sex-ratio biases threatening population viability in the future.

Isotope and morphometrical evidence reveals the technological package associated with agriculture adoption in western Europe

This study aimed to reconstruct the environmental conditions and the crop management practices and plant characteristics when agriculture appeared in western Europe. We analyzed oak charcoal and a large number of cereal caryopsides recovered from La Draga (Girona, Spain), an early (5300 to 4800 cal. BC) agricultural site from the Iberian Peninsula. The carbon isotope discrimination (Δ13C) values of oak, the dominant forest species in the region, indicates prevalence of a wet climate at the site. Further, we reconstructed crop management conditions, achievable yield, and crop characteristics through the analysis of Δ13C, nitrogen isotope composition (δ15N), nitrogen content, and the reconstructed weight of wheat and barley caryopsides, following protocols developed by our team [Araus et al., Nat. Commun. 5, 3953 (2014)] and comparison of these parameters with present-day organic agriculture in the region. In parallel, a regional perspective was achieved through the study of wheat and barley grains of seventeen Neolithic sites from the western Mediterranean. The results suggest that rather than small-garden cultivation, a more extensive agriculture was practiced under good water availability and moderate manuring. Moreover, results from La Draga evidence that grain weight and spike morphology were comparable to contemporary cereals. Growing conditions and the prevalence of improved crop traits indicate that agriculture was fairly consolidated at the time it reached the western edge of Europe.

Contrasting water-use strategies to climate warming in white birch and larch in a boreal permafrost region

The effects of rising atmospheric CO2 concentrations (Ca) with climate warming on intrinsic water-use efficiency and radial growth in boreal forests are still poorly understood. We measured tree-ring cellulose δ13C, δ18O, and tree-ring width in Larix dahurica (larch) and Betula platyphylla (white birch), and analyzed their relationships with climate variables in a boreal permafrost region of northeast China over past 68 years covering a pre-warming period (1951-1984; base period) and a warm period (1985-2018; warm period). We found that white birch but not larch significantly increased their radial growth over the warm period. The increased intrinsic water-use efficiency in both species was mainly driven by elevated Ca but not climate warming. White birch but not larch showed significantly positive correlations between tree-ring δ13C, δ18O and summer maximum temperature as well as vapor pressure deficit in the warm period, suggesting a strong stomatal response in the broad-leaved birch to temperature changes. The climate warming-induced radial growth enhancement in white birch is primarily associated with a conservative water-use strategy. In contrast, larch exhibits a profligate water-use strategy. It implies an advantage for white birch over larch in the warming permafrost regions.

Quercus ilex L. dieback is genetically determined: Evidence provided by dendrochronology, δ13C and SSR genotyping

Quercus ilex L. dieback has been reported in several Mediterranean forests, revealing different degree of crown damages even in close sites, as observed in two Q. ilex forest stands in southern Tuscany (IT). In this work, we applied a novel approach combining dendrochronological, tree-ring δ13C and genetic analysis to test the hypothesis that different damage levels observed in a declining (D) and non-declining (ND) Q. ilex stands are connected to population features linked to distinct response to drought. Furthermore, we investigated the impact of two major drought events (2012 and 2017), that occurred in the last fifteen years in central Italy, on Q. ilex growth and intrinsic water use efficiency (WUEi). Overall, Q. ilex showed slightly different ring-width patterns between the two stands, suggesting a lower responsiveness to seasonal climatic variations for trees at D stand, while Q. ilex at ND stand showed changes in the relationship between climatic parameters and growth across time. The strong divergence in δ13C signals between the two stands suggested a more conservative use of water for Q. ilex at ND compared to D stand that may be genetically driven. Q. ilex at ND resulted more resilient to drought compared to trees at D, probably thanks to its safer water strategy. Genotyping analysis based on simple-sequence repeat (SSR) markers revealed the presence of different Q. ilex populations at D and ND stands. Our study shows intraspecific variations in drought response among trees grown in close. In addition, it highlights the potential of combining tree-ring δ13C data with SSR genotyping for the selection of seed-bearing genotypes aimed to preserve Mediterranean holm oak ecosystem and improve its forest management.

The high climate vulnerability of western Mediterranean forests

Understanding the effects of climate change is one of the most challenging goals for biodiversity conservation. The forests of Andalusia, in Southern Spain, are part of an important Mediterranean Basin biodiversity hotspot. However, great changes in climate are expected to occur in this region, and there is an increasing need to assess the vulnerability of its vegetation. We assess the vulnerability of twelve forest types in the region that are included in the European Directive 92/43/EEC as Habitats of Community Interest (HCI). HCI are natural habitat types which are in danger, have a small natural range, or present an outstanding example of a biogeographical regions in the European Union. We assessed vulnerability by analyzing the climate exposure level of each forest type under two global climate models (MRI-CGCM3, which predicts warmer and wetter conditions, and MIROC-ESM which predicts hotter and drier conditions), two emission scenarios (RCP4.5, a representative concentration pathway that predicts stable emissions of CO2, and RCP8.5, that predicts the highest CO2 emissions) by the mid- and end-century time periods. The vulnerability analysis also includes the sensitivity and adaptive capacity of the dominant tree species which compose each forest type. An overall vulnerability score was calculated for each forest type, model, scenario and time period. High-elevation forest types and those with high moisture requirements were more vulnerable to climate change, while forest types dominated by more thermophilic species were less vulnerable and more resilient. The worst climate impacts were predicted in the MIROC-ESM model and RCP8.5 scenario by the end of the century (2070-2100), while the least climatic stress was obtained in the MRI-CGCM3 model and RCP4.5 scenario by the mid-century (2040-2070), which still shows high potential stress for most forest types. By the end of the century, the climate exposure of the entire forest domain will range between 32 % in the least stressful situation (MRI-CGCM3 and RCP4.5), and 98 % in the most climatically stressful situation (MIROC-ESM and RCP8.5). However, the effects of climate change will be perceptible by the mid-century, with most of the HCI forest types suffering climate stress. The "Andalusian oak forest" and the "Corylus wet forest" types were the most vulnerable to climate change, while the "Mediterranean pine forest", the "Olea and Ceratonia forests" and the "oak forests" were the least vulnerable. This assessment identifies the vulnerable forest types to climate change in the south of the Iberian Peninsula, and provides context for natural resource managers in making decisions about how to adapt forests to the impacts of climate change.

Beyond width and density: stable carbon and oxygen isotopes in cork-rings provide insights of physiological responses to water stress in Quercus suber L

As climate change increasingly affects forest ecosystems, detailed understanding of major effects is important to anticipate their consequences under future climate scenarios. The Mediterranean region is a prominent climate change hotspot, and evergreen cork oak (Quercus suber L.) woodlands are particularly climatically sensitive due to cork (bark) harvesting. Cork oak's drought avoidance strategy is well-known and includes structural and physiological adaptations that maximise soil water uptake and transport and limit water use, potentially leading to reduced stem and cork growth. Trees' responses to cope with water-limited conditions have been extensively described based on cork-rings width and, more recently, on cork-rings density, in dendroecological studies. However, so far, tree functional attributes and physiological strategies, namely photosynthetic metabolism adjustments affecting cork formation, have never been addressed and/or integrated on these previous cork-rings-based studies. In this study, we address the relation between carbon and oxygen stable isotopes of cork rings and precipitation and temperature, in two distinct locations of southwestern Portugal-the (wetter) Tagus basin peneplain and the (drier) Grândola mountains. We aimed at assessing whether the two climatic factors affect cork-ring isotopic composition under contrasting conditions of water availability, and, therefore, if carbon and oxygen signatures in cork can reflect tree functional (physiological and structural) responses to stressful conditions, which might be aggravated by climate change. Our results indicate differences between the study areas. At the drier site, the stronger statistically significant negative cork δ ¹³C correlations were found with mean temperature, whereas strong positive cork δ ¹⁸O correlations were fewer and found only with precipitation. Moreover, at the wetter site, cork rings are enriched in ¹⁸O and depleted in ¹³C, indicating, respectively, shallow groundwater as the water source for physiological processes related with biosynthesis of non-photosynthetic secondary tissues, such as suberin, and a weak stomatal regulation under high water availability, consistent with non-existent water availability constrains. In contrast, at the drier site, trees use water from deeper ground layers, depleted in ¹⁸O, and strongly regulate stomatal conductance under water stress, thus reducing photosynthetic carbon uptake and probably relying on stored carbon reserves for cork ring formation. These results suggest that although stable isotopes signatures in cork rings are not proxies for net growth, they may be (fairly) robust indicators of trees' physiological and structural adjustments to climate and environmental changes in Mediterranean environments.

Bark Transpiration Rates Can Reach Needle Transpiration Rates Under Dry Conditions in a Semi-arid Forest

Drought can cause tree mortality through hydraulic failure and carbon starvation. To prevent excess water loss, plants typically close their stomata before massive embolism formation occurs. However, unregulated water loss through leaf cuticles and bark continues after stomatal closure. Here, we studied the diurnal and seasonal dynamics of bark transpiration and how it is affected by tree water availability. We measured continuously for six months water loss and CO₂ efflux from branch segments and needle-bearing shoots in Pinus halepensis growing in a control and an irrigation plot in a semi-arid forest in Israel. Our aim was to find out how much passive bark transpiration is affected by tree water status in comparison with shoot transpiration and bark CO₂ emission that involve active plant processes, and what is the role of bark transpiration in total tree water use during dry summer conditions. Maximum daily water loss rate per bark area was 0.03-0.14 mmol m^-2 s^-1, which was typically ~76% of the shoot transpiration rate (on leaf area basis) but could even surpass the shoot transpiration rate during the highest evaporative demand in the control plot. Irrigation did not affect bark transpiration rate. Bark transpiration was estimated to account for 64-78% of total water loss in drought-stressed trees, but only for 6-11% of the irrigated trees, due to differences in stomatal control between the treatments. Water uptake through bark was observed during most nights, but it was not high enough to replenish the lost water during the day. Unlike bark transpiration, branch CO₂ efflux decreased during drought due to decreased metabolic activity. Our results demonstrate that although bark transpiration represents a small fraction of the total water loss through transpiration from foliage in non-stressed trees, it may have a large impact during drought.

Oak Competition Dominates Interspecific Interactions in Growth and Water-Use Efficiency in a Mixed Pine–Oak Mediterranean Forest

In the Mediterranean, mixed forests of Aleppo pine and holm oak are widespread. Generally considered a transition stage in the succession towards climax oak communities, niche segregation may also contribute to the prevalence of these communities. So far, there is increasing evidence of hydrological niche segregation, with the two species showing complementary water use and seasonal growth patterns. However, it remains unknown whether interspecific interactions affect the response to climate and the mid-term (decadal) growth and water-use efficiency of pines and oaks in mixed stands. Here, we combined tree-ring chronologies, built on different competition classes within a mixed stand, with a spatially explicit assessment of individual growth and wood carbon isotope discrimination (∆13C), as a proxy of intrinsic water-use efficiency, and compared these results with previously reported water uptake patterns. We found that competition with pines modulated the climate response of oaks, whereas pine climate response was insensitive to competition. On the other hand, pine density affected only pine growth, whereas oak competition affected both species. We conclude that the presence of pines had negligible or even positive effects on the oaks, but competition with neighbor oaks limited their ability to recover after drought. Conversely, pines experienced greater drought stress under competition, with both oaks and pines.

Short- and long-term responses of riparian cottonwoods (Populus spp.) to flow diversion: Analysis of tree-ring radial growth and stable carbon isotopes

Long duration tree-ring records with annual precision allow for the reconstruction of past growing conditions. Investigations limited to the most common tree-ring proxy of ring width can be difficult to interpret, however, because radial growth is affected by multiple environmental processes. Furthermore, studies of living trees may miss important effects of drought on tree survival and forest changes. Stable carbon isotopes can help distinguish drought from other environmental factors that influence tree-ring width and forest stand condition. We quantified tree-ring radial expansion and stable carbon isotope ratios (δ¹³C) in riparian cottonwoods (Populus angustifolia and P. angustifolia x P.trichocarpa) along Snake Creek in Nevada, USA. We investigated how hydrological drought affected tree growth and death at annual to half-century scales in a partially dewatered reach (DW) compared to reference reaches immediately upstream and downstream. A gradual decline in tree-ring basal area increment (BAI) began at DW concurrent to streamflow diversion in 1961. BAI at DW diverged from one reference reach immediately but not from the other until nearly 50 years later. In contrast, tree-ring δ¹³C had a rapid and sustained increase following diversion at DW only, providing the stronger and clearer drought signal. BAI and δ¹³C were not significantly correlated prior to diversion; after diversion they both reflected drought and were correlated for DW trees only. Cluster analyses distinguished all trees in DW from those in reference reaches based on δ¹³C, but BAI patterns left trees intermixed across reaches. Branch and tree mortality were also highest and canopy vigor was lowest in DW. Results indicate that water scarcity strongly limited cottonwood photosynthesis following flow diversion, thus reducing carbon assimilation, basal growth and survival. The dieback was not sudden, but occurred over decades as carbon deficits mounted and depleted streamflow left trees increasingly vulnerable to local meteorological drought.

Neighbor identity affects growth and survival of Mediterranean plants under recurrent drought

The increasing intensity and frequency of droughts predicted for the Mediterranean basin with ongoing climate change will impact plant communities and ecosystem functioning. This study investigated the effect of severe recurrent droughts and the role of the neighbor plant identity on the growth and survival of three abundant and co-existing species of a typical Mediterranean shrubland. Two juvenile plants, either of the same species or in all possible combinations of the two woody species Quercus coccifera and Cistus albidus and the perennial grass species Brachypodium retusum were grown together in rhizotrons under controlled watering regimes for two years. Compared to a treatment with only one drought cycle, three successive droughts reduced the relative growth rates (RGR) of shoots and roots in B. retusum, but not in woody species, and increased the mortality of the woody species, but not that of the grass. The survival of C. albidus and of B. retusum, but not of Q. coccifera, increased when the neighbor individual was a different species than when it was the same species. Our data suggest that both species composition and frequency of drought events will impact the dynamics of plant communities in Mediterranean shrublands under ongoing climate change. The abundance of dehydration sensitive woody species will likely decrease under more frequent drought events at the expense of dehydration-tolerant grass species, resulting in potentially strong changes in the functioning of these ecosystems.

Both Mature Patches and Expanding Areas of Juniperus thurifera Forests Are Vulnerable to Climate Change But for Different Reasons

Research Highlights: Water use efficiency (WUE) varied along a gradient of Juniperus thurifera (L.) forest expansion, being higher in recently colonised areas. Background and Objectives: WUE is a classic physiological process of plants that reflects the compromise between carbon assimilation and water loss and has a profound influence on their performance in water-limited environments. Forest expansion in Mediterranean regions associated with land abandonment can influence the WUE of plants due to the existence of two opposing gradients: one of favourable–unfavourable environmental conditions and another one of increased–decreased intraspecific competition, the former increasing and the latter decreasing towards the expanding front. The main objective of this study was to elucidate how the WUE of Juniperus thurifera varied along the stages of forest expansion and to provide insight on how this variation is influenced by intraspecific competition and abiotic factors. Materials and Methods: Seventeen plots at different distances from the mature forest core were selected at three sites located in the centre of the Iberian Peninsula. For 30 individuals within each plot, we measured biometric characteristics, age, tree vigour, and C/N ratio in leaves, and the leaf carbon isotope signature (δ13C (‰)) as a proxy for WUE. Around each individual, we scored the percentage cover of bare soil, stoniness, conspecifics, and other woody species. Results: WUE of J. thurifera individuals varied along the forest expansion gradient, being greater for the individuals at the expanding front than for those at the mature forest. WUE was influenced by the cover of conspecifics, tree age, and C/N ratio in leaves. This pattern reveals that less favourable environmental conditions (i.e., rocky soils and higher radiation due to lower vegetation cover) and younger trees at the expanding front are associated with increased WUE. The increased cover of conspecifics decreases irradiance at the mature forest, involving milder stress conditions than at the expanding front. Conclusions: Lower WUE in mature forests due to more favourable conditions and higher WUE due to abiotic stress at expanding fronts revealed high constraints on water economy of this tree species in these two contrasting situations. Climate change scenarios bringing increased aridity are a serious threat to Juniperus thurifera forests, affecting both mature and juvenile populations although in different ways, which deserve further research to fully unveil.

Biogeographic, Atmospheric, and Climatic Factors Influencing Tree Growth in Mediterranean Aleppo Pine Forests

There is a lack of knowledge on how tree species respond to climatic constraints like water shortages and related atmospheric patterns across broad spatial and temporal scales. These assessments are needed to project which populations will better tolerate or respond to global warming across the tree species distribution range. Warmer and drier conditions have been forecasted for the Mediterranean Basin, where Aleppo pine (Pinus halepensis Mill.) is the most widely distributed conifer in dry sites. This species shows plastic growth responses to climate, being particularly sensitive to drought. We evaluated how 32 Aleppo pine forests responded to climate during the second half of the 20th century by using dendrochronology. Climatic constraints of radial growth were inferred by fitting the Vaganov–Shashkin (VS-Lite) growth model to ring-width data from our Aleppo pine forest network. Our findings reported that Aleppo pine growth decreased and showed the highest common coherence among trees in dry, continental sites located in southeastern and eastern inland Spain and Algeria. In contrast, growth increased in wetter sites located in northeastern Spain. Overall, across the Aleppo pine network tree growth was enhanced by prior wet winters and cool and wet springs, whilst warm summers were associated with less growth. The relationships between site ring-width chronologies were higher in nearby forests. This explains why Aleppo pine growth was distinctly linked to indices of atmospheric circulation patterns depending on the geographical location of the forests. The western forests were more influenced by moisture and temperature conditions driven by the Western Mediterranean Oscillation (WeMO) and the Northern Atlantic Oscillation (NAO), the southern forests by the East Atlantic (EA) and the august NAO, while the Balearic, Tunisian and northeastern sites by the Arctic Oscillation (AO) and the Scandinavian pattern (SCA). The climatic constraints for Aleppo pine tree growth and its biogeographical variability were well captured by the VS-Lite model. The model performed better in dry and continental sites, showing strong growth coherence between trees and climatic limitations of growth. Further research using similar broad-scale approaches to climate–growth relationships in drought-prone regions deserves more attention.

The Role of Population and Half-Sib Family on Driving Suitable Functional Traits for Quercus suber L. Forest Restoration

Research Highlights: Seedlings of different Quercus suber L. populations and half-sib families differ in their response to multiple stressors, which may have consequences on the future distribution of this Mediterranean species. Background and Objectives: Global change will likely increase the frequency and severity of drought in drylands. Plant species’ distributions will largely depend on their ability to respond to the combined effect of drought and other environmental stressors. Genetic diversity in morpho-functional traits are key components of this response. Yet, information on the response to multiple stresses is scarce for many iconic species. The present study assessed the variability in the response of populations and half-sib families of a Mediterranean acidophilous tree, cork oak, to drought and changes in soil conditions. Materials and Methods: We sampled acorns of half-sib families from two cork oak populations genetically introgressed with the alkaline-tolerant species Quercus ilex L., and from a non-introgressed cork oak population located in its core habitat. We germinated the acorns and subjected seedlings to contrasted levels of water availability and additions of calcium and magnesium carbonate, and assessed their morpho-physiological response. Results: Response to drought and soil chemistry composition differed between populations and families. For some traits, introgressed populations responded similarly to drought than the non-introgressed population. Conversely, the response to soil chemistry was not clearly related to introgression. When considering half-sib families within populations, the population effect diminished, which revealed the importance of intra-population variation. However, relevant traits for water scarcity adaptations, such as specific leaf area and root:shoot ratio, remained significantly different at the population level, which highlights the relevance of these traits for management. Conclusions: Our study shows that the adaptive management and restoration of cork oak forests should consider not only geographic provenances, but also half-sib lines within populations.

Morphological characteristics and variation of wood, cone and seed productions in the reforestation of Aleppo pine in Northeastern Tunisia using terrestrial and spatial index approaches

For several decades, forest management policies have encouraged land cover changes, with the establishment of tree cover such as Aleppo pine in natural or degraded ecosystems for soil protection and the firewood production. In order to investigate the importance of Aleppo pine trees in the ecosystem, this study was conducted in the reforestation of the Aleppo pine of Northeastern Tunisia. The production of wood, cones and seeds of Aleppo pine in the 6 date reforestation in Northeastern Tunisia were surveyed using 6 plots (20 tree/plot) spread over 6 different location. Our hypothesis was to determine the characteristics that can be decisive in estimating the production of Aleppo pine in reforestation in Tunisia using terrestrial and spatial measurements including (i) age, (ii) dendrometric characteristics, (iii) silvicultural treatments, (iv) stationary data and/ or (v) remote sensing parameter (NDVI: Normalized Difference Vegetation Index). We found that the Aleppo pine in Northeastern Tunisia and at young age trees (14–35 years) were more productive than the regeneration or reforestation stand, either young or old, in Northwest of Tunisia. Wood, cone and seed productions were significantly different amongst the plots (p <0.05). The NDVI was positively correlated with the production of wood, cones and seeds. Stand age, exposure and longitude were also positively correlated with the production. However, longitude and altitude were negatively correlated with the production, showing a negative effect on the morphological traits of trees and, subsequently, their growth. The regression analysis indicates that NDVI and age were the most determinant factors of seed production. This research suggests that reforestation planning, particularly the choice of altitude and latitude, may result in improved tree morphology that may increase Aleppo pine wood seed and cone crops.

Regime shifts of Mediterranean forest carbon uptake and reduced resilience driven by multidecadal ocean surface temperatures

The mechanisms translating global circulation changes into rapid abrupt shifts in forest carbon capture in semi-arid biomes remain poorly understood. Here, we report unprecedented multidecadal shifts in forest carbon uptake in semi-arid Mediterranean pine forests in Spain over 1950-2012. The averaged carbon sink reduction varies between 31% and 37%, and reaches values in the range of 50% in the most affected forest stands. Regime shifts in forest carbon uptake are associated with climatic early warning signals, decreased forest regional synchrony and reduced long-term carbon sink resilience. We identify the mechanisms linked to ocean multidecadal variability that shape regime shifts in carbon capture. First, we show that low-frequency variations of the surface temperature of the Atlantic Ocean induce shifts in the non-stationary effects of El Niño Southern Oscillation (ENSO) on regional forest carbon capture. Modelling evidence supports that the non-stationary effects of ENSO can be propagated from tropical areas to semi-arid Mediterranean biomes through atmospheric wave trains. Second, decadal changes in the Atlantic Multidecadal Oscillation (AMO) significantly alter sea-air heat exchanges, modifying in turn ocean vapour transport over land and land surface temperatures, and promoting sustained drought conditions in spring and summer that reduce forest carbon uptake. Third, we show that lagged effects of AMO on the winter North Atlantic Oscillation also contribute to the maintenance of long-term droughts. Finally, we show that the reported strong, negative effects of ocean surface temperature (AMO) on forest carbon uptake in the last decades are unprecedented over the last 150 years. Our results provide new, unreported explanations for carbon uptake shifts in these drought-prone forests and review the expected impacts of global warming on the profiled mechanisms.

How Does Water Availability Affect the Allocation to Bark in a Mediterranean Conifer?

Bark thickness is a key structural feature in woody plants in the protection against fire. We used 19 provenances of Pinus halepensis, an obligate-seeder species, in a replicated common garden at two environments contrasting in water availability to assess the interacting effects of site environment and population in the relative allocation to bark, expecting lower allocation at the drier site. Secondly, given the average fire frequency, we analyzed whether trees reached the critical absolute thickness soon enough for population persistence via aerial seed bank. Our analyses indicated that trees at the moister site allocated a rather fixed quantity of resources independent of tree size, and almost all populations reached critical absolute bark thickness to eventually survive fire. In contrast, at the drier site allocation to bark reduced with tree size, and most populations did not reach the critical bark thickness. Populations from areas with higher fire frequency had thicker basal bark, while those from areas with severe droughts and short vegetative periods, had thinner bark. In conclusion, drought-stressed trees have a higher risk to die from fires before achieving reproduction and building a sufficient aerial seed bank.

Wood Growth in Pure and Mixed Quercus ilex L. Forests: Drought Influence Depends on Site Conditions

Climate response of tree-species growth may be influenced by intra- and inter-specific interactions. The different physiological strategies of stress response and resource use among species may lead to different levels of competition and/or complementarity, likely changing in space and time according to climatic conditions. Investigating the drivers of inter- and intra-specific interactions under a changing climate is important when managing mixed and pure stands, especially in a climate change hot spot such as the Mediterranean basin. Mediterranean tree rings show intra-annual density fluctuations (IADFs): the links among their occurrence, anatomical traits, wood growth and stable isotope ratios can help understanding tree physiological responses to drought. In this study, we compared wood production and tree-ring traits in Quercus ilex L. dominant trees growing in two pure and two mixed stands with Pinus pinea at two sites in Southern Italy, on the basis of the temporal variation of cumulative basal area, intrinsic water use efficiency (WUEi), δ18O and IADF frequency in long tree-ring chronologies. The general aim was to assess whether Q. ilex trees growing in pure or mixed stands have a different wood production through time, depending on climatic conditions and stand structure. The occurrence of dry climatic conditions triggered opposite complementarity interactions for Q. ilex growing with P. pinea trees at the two sites. Competitive reduction was experienced at the T site characterized by higher soil water holding capacity (WHC), lower stand density and less steep slope than the S site; on the opposite, high competition occurred at S site. The observed difference in wood growth was accompanied by a higher WUEi due to a higher photosynthetic rate at the T site, while by a tighter stomatal control in mixed stand of S site. IADF frequency in Q. ilex tree rings was linked to higher WUEi, thus to stressful conditions and could be interpreted as strategy to cope with dry periods, independently from the different wood growth. Considering the forecasted water shortage, inter-specific competition should be reduced in denser stands of Q. ilex mixed with P. pinea. Such findings have important implications for forest management of mixed and pure Q. ilex forests.

Evaluation of the effect of the 2011 Tsunami on coastal forests by means of multiple isotopic analyses of tree-rings

The March 2011 Mega-Tsunami in eastern Japan damaged at different degrees the black pine (Pinus thunbergii) forests along the coast. In order to evaluate the recovery of black pine four years later, tree-ring samples from 9 trees for the period 2002-2014 were analyzed for ring growth and stable isotopes (δ¹³C, δ¹⁵N and δ¹⁸O). The results showed that annual tree-ring width decreased approximately 70 % from the year 2011 to 2014 compared to the period previous to the tsunami (2002-2010). The multiple isotopic analyses showed that the reduction in growth was caused by soil salinity that prompted stomatal closure and an abrupt increase of tree-ring δ¹³C. Sea water deposition in the soil did not affect tree-ring δ¹⁸O values. Two years after the tsunami, decreasing tree-ring δ¹³C values caused by apparently photosynthetic recovery did not translate into radial tree-growth, indicating a possible shift in carbon allocation to foliage and mainly roots as a defense mechanism to sodium toxicity. The dual δ¹³C-δ¹⁸O model explains neither the limited growth nor the subsequent recovery in δ¹³C. Similarly tree-ring δ¹⁵N indicated that there was no difference in nitrogen availability before and after the tsunami, suggesting that nutrients were not a limitation but rather soil salinity.

Increased water-use efficiency translates into contrasting growth patterns of Scots pine and sessile oak at their southern distribution limits

In forests, the increase in atmospheric CO₂ concentrations (C_a ) has been related to enhanced tree growth and intrinsic water-use efficiency (iWUE). However, in drought-prone areas such as the Mediterranean Basin, it is not yet clear to what extent this "fertilizing" effect may compensate for drought-induced growth reduction. We investigated tree growth and physiological responses at five Scots pine (Pinus sylvestris L.) and five sessile oak (Quercus petraea (Matt.) Liebl.) sites located at their southernmost distribution limits in Europe for the period 1960-2012 using annually resolved tree-ring width and δ¹³ C data to track ecophysiological processes. Results indicated that all 10 natural stands significantly increased their leaf intercellular CO₂ concentration (C_i ), and consequently iWUE. Different trends in the theoretical gas-exchange scenarios as a response to increasing C_a were found: generally, C_i tended to increase proportionally to C_a , except for trees at the driest sites in which C_i remained constant. C_i from the oak sites displaying higher water availability tended to increase at a comparable rate to C_a . Multiple linear models fitted at site level to predict basal area increment (BAI) using iWUE and climatic variables better explained tree growth in pines (31.9%-71.4%) than in oak stands (15.8%-46.8%). iWUE was negatively linked to pine growth, whereas its effect on growth of oak differed across sites. Tree growth in the western and central oak stands was negatively related to iWUE, whereas BAI from the easternmost stand was positively associated with iWUE. Thus, some Q. petraea stands might have partially benefited from the "fertilizing" effect of rising C_a , whereas P. sylvestris stands due to their strict closure of stomata did not profit from increased iWUE and consequently showed in general growth reductions across sites. Additionally, the inter-annual variability of BAI and iWUE displayed a geographical polarity in the Mediterranean.

Increasing carbon discrimination rates and depth of water uptake favor the growth of Mediterranean evergreen trees in the ecotone with temperate deciduous forests

Tree populations at the low-altitudinal or -latitudinal limits of species' distributional ranges are predicted to retreat toward higher altitudes and latitudes to track the ongoing changes in climate. Studies have focused on the climatic sensitivity of the retreating species, whereas little is known about the potential replacements. Competition between tree species in forest ecotones will likely be strongly influenced by the ecophysiological responses to heat and drought. We used tree-ring widths and δ¹³ C and δ¹⁸ O chronologies to compare the growth rates and long-term ecophysiological responses to climate in the temperate-Mediterranean ecotone formed by the deciduous Fagus sylvatica and the evergreen Quercus ilex at the low altitudinal and southern latitudinal limit of F. sylvatica (NE Iberian Peninsula). F. sylvatica growth rates were similar to those of other southern populations and were surprisingly not higher than those of Q. ilex, which were an order of magnitude higher than those in nearby drier sites. Higher Q. ilex growth rates were associated with high temperatures, which have increased carbon discrimination rates in the last 25 years. In contrast, stomatal regulation in F. sylvatica was proportional to the increase in atmospheric CO₂ . Tree-ring δ¹⁸ O for both species were mostly correlated with δ¹⁸ O in the source water. In contrast to many previous studies, relative humidity was not negatively correlated with tree-ring δ¹⁸ O but had a positive effect on Q. ilex tree-ring δ¹⁸ O. Furthermore, tree-ring δ¹⁸ O decreased in Q. ilex over time. The sensitivity of Q. ilex to climate likely reflects the uptake of deep water that allowed it to benefit from the effect of CO₂ fertilization, in contrast to the water-limited F. sylvatica. Consequently, Q. ilex is a strong competitor at sites currently dominated by F. sylvatica and could be favored by increasingly warmer conditions.

Intra-specific association between carbon isotope composition and productivity in woody plants: A meta-analysis

The study of intra-specific variations in growth and plant physiological response to drought is crucial to understand the potential for plant adaptation to global change. Carbon isotope composition (δ(13)C) in plant tissues offers an integrated measure of intrinsic water-use efficiency (WUEi). The intra-specific association between δ(13)C and productivity has been extensively studied in herbaceous crops, but species-specific information on woody plants is still limited and has so far provided contradictory results. In this work we explored the general patterns of the relationship between δ(13)C and growth traits (height, diameter and biomass) using a meta-analysis. We compiled information from 49 articles, including 176 studies performed on 34 species from 16 genera. We found a positive global intra-specific correlation between δ(13)C and growth (Gr=0.28, P<0.0001), stronger for biomass than for height, and non-significant for diameter. The extent of this intra-specific association increased from Mediterranean to subtropical, temperate and boreal biomes, i.e. from water-limited to energy-limited environments. Conifers and shrubs, but not broadleaves, showed consistent positive intra-specific correlations. The meta-analysis also revealed that the relationship between δ(13)C and growth is better characterized at juvenile stages, under near-optimal and controlled conditions, and by analyzing δ(13)C in leaves rather than in wood.

Increased water use efficiency does not prevent growth decline of Pinus canariensis in a semi-arid treeline ecotone in Tenerife, Canary Islands (Spain)

KEY MESSAGE

Intrinsic water-use efficiency of Pinus canariensis (Sweet ex Spreng.) growing at a semi-arid treeline has increased during the past 37 years. Tree-ring width by contrast has declined, likely caused by reduced stomatal conductance due to increasing aridity.

CONTEXT

Rising atmospheric CO₂ concentration (C_a ) has been related to tree growth enhancement accompanied by increasing intrinsic water-use-efficiency (iWUE). Nevertheless, the extent of rising C_a on long-term changes in iWUE and growth has remained poorly understood to date in Mediterranean treeline ecosystems.

AIMS

This study aimed to examine radial growth and physiological responses of P. canariensis in relation to rising C_a and increasing aridity at treeline in Tenerife, Canary Islands, Spain.

METHODS

We evaluated temporal changes in secondary growth (tree-ring width; TRW) and tree ring stable C isotope signature for assessing iWUE from 1975 through 2011.

RESULTS

Precipitation was the main factor controlling secondary growth. Over the last 36 years P. canariensis showed a decline in TRW at enhanced iWUE, likely caused by reduced stomatal conductance due to increasing aridity.

CONCLUSION

Our results indicate that increasing aridity has overridden the potential CO₂ fertilization on tree growth of P. canariensis at its upper distribution limit.

Oak decline analyzed using intraannual radial growth indices, δ(13)C series and climate data from a rural hemiboreal landscape in southwesternmost Finland

Decline of pedunculate oak (Quercus robur L.) was studied in SW Finland. This is a region where the species is growing near its northern distributional limit globally and a recent decline of mature trees has been described regionally. Tree rings of declining oaks were compared to the chronologies of healthy and oaks that died, climate series and stable isotope discrimination of carbon (δ(13)C) of comparable mature trees. The radial growth (earlywood, latewood, and annual ring width) of declining oaks was clearly deteriorated in comparison to healthy oaks, but recuperated, compared to oaks that died, through all index types. Comparison of climate relationships between growth and δ(13)C, expected to reflect oaks' intrinsic water use efficiency, indicated enhancing resistance to droughts through the growing season. The growth and the climatic growth response was differentiated in declining oaks as compared with the healthy and oaks that died revealing that: (1) declining oaks exhibited decreasing competitive strength as indicated by reduced overall growth relative to healthy oaks, (2) the growth of declining oaks was more sensitive to winter conditions, but less restricted by summer droughts than the growth of other oaks, and (3) healthy oaks were seen having benefitted from the ongoing lengthening of the growing season. Lack of correlativity between growth and δ(13)C became evident as their responses to temperature and precipitation variations deviated drastically during the other but summer months. Our results indicate that several different ecological factors, rather than a single climatic factor (e.g., drought), are controlling the oak decline in the studied environment.

Drought-induced mortality selectively affects Scots pine trees that show limited intrinsic water-use efficiency responsiveness to raising atmospheric CO2

Widespread drought-induced tree mortality has been documented around the world, and could increase in frequency and intensity under warmer and drier conditions. Ecophysiological differences between dying and surviving trees might underlie predispositions to mortality, but are poorly documented. Here we report a study of Scots pines (Pinus sylvestris L.) from two sites located in north-eastern Iberian Peninsula where drought-associated mortality episodes were registered during the last few decades. Time trends of discrimination against 13C (Δ13C) and intrinsic water-use efficiency (WUEi) in tree rings at an annual resolution and for a 34 year period were used to compare co-occurring now-dead and surviving pines. Results indicate that both surviving and now-dead pines significantly increased their WUEi over time, although this increase was significantly lower for now-dead individuals. These differential WUEi trends corresponded to different scenarios describing how plant gas exchange responds to increasing atmospheric CO2 (Ca): the estimated intercellular CO2 concentration was nearly constant in surviving pines but tended to increase proportionally to Ca in now-dead trees. Concurrently, the WUEi increase was not paralleled by a growth enhancement, regardless of tree state, suggesting that in water-limited areas like the Mediterranean, it cannot overcome the impact of an increasingly warmer and drier climate on tree growth.

Drought impact on water use efficiency and intra-annual density fluctuations in Erica arborea on Elba (Italy)

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco-physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood-anatomical analyses and stable isotopes measurements. Intra-annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site-specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the (13) C-derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ(18) O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree-ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.

Physiological strategies of co-occurring oaks in a water- and nutrient-limited ecosystem

Oak species are well suited to water-limited conditions by either avoiding water stress through deep rooting or tolerating water stress through tight stomatal control. In co-occurring species where resources are limited, species may either partition resources in space and/or time or exhibit differing efficiencies in the use of limited resources. Therefore, this study seeks to determine whether two co-occurring oak species (Quercus prinus L. and Quercus velutina Lam.) differ in physiological parameters including photosynthesis, stomatal conductance, water-use (WUE) and nitrogen-use efficiency (NUE), as well as to characterize transpiration and average canopy stomatal responses to climatic variables in a sandy, well-drained and nutrient-limited ecosystem. The study was conducted in the New Jersey Pinelands and we measured sap flux over a 3-year period, as well as leaf gas exchange, leaf nitrogen and carbon isotope concentrations. Both oak species showed relatively steep increases in leaf-specific transpiration at low vapor pressure deficit (VPD) values before maximum transpiration rates were achieved, which were sustained over a broad range in VPD. This suggests tight stomatal control over transpiration in both species, although Q. velutina showed significantly higher leaf-level and canopy-level stomatal conductance than Q. prinus. Average daytime stomatal conductance was positively correlated with soil moisture and both oak species maintained at least 75% of their maximum canopy stomatal conductance at soil moistures in the upper soil layer (0-0.3 m) as low as 0.03 m(3) m(3)(-3). Quercus velutina had significantly higher photosynthetic rates, maximum Rubisco-limited and electron-transport-limited carboxylation rates, dark respiration rates and nitrogen concentration per unit leaf area than Q. prinus. However, both species exhibited similar WUEs and NUEs. Therefore, Q. prinus has a more conservative resource-use strategy, while Q. velutina may need to exploit niches that are locally higher in nutrients and water. Likewise, both species appear to tap deep, stable water sources, highlighting the importance of rooting depth in modeling transpiration and stomatal conductance in many oak ecosystems.

Growth and carbon isotopes of Mediterranean trees reveal contrasting responses to increased carbon dioxide and drought

Forest dynamics will depend upon the physiological performance of individual tree species under more stressful conditions caused by climate change. In order to compare the idiosyncratic responses of Mediterranean tree species (Quercus faginea, Pinus nigra, Juniperus thurifera) coexisting in forests of central Spain, we evaluated the temporal changes in secondary growth (basal area increment; BAI) and intrinsic water-use efficiency (iWUE) during the last four decades, determined how coexisting species are responding to increases in atmospheric CO2 concentrations (C(a)) and drought stress, and assessed the relationship among iWUE and growth during climatically contrasting years. All species increased their iWUE (ca. +15 to +21%) between the 1970s and the 2000s. This increase was positively related to C(a) for J. thurifera and to higher C(a) and drought for Q. faginea and P. nigra. During climatically favourable years the study species either increased or maintained their growth at rising iWUE, suggesting a higher CO2 uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C(a) even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions.

Stand structure modulates the long-term vulnerability of Pinus halepensis to climatic drought in a semiarid Mediterranean ecosystem

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ(13)C(LW) and δ(18)O(LW), respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ(13)C(LW) and δ(18)O(LW) were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher δ(18)O(LW) and similar δ(13)C(LW) values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands.

Leaf δ18O of remaining trees is affected by thinning intensity in a semiarid pine forest

Silvicultural thinning usually improves the water status of remaining trees in water-limited forests. We evaluated the usefulness of a dual stable isotope approach (δ¹³C, δ¹⁸O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60-year-old Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf δ¹³C and δ¹⁸O, foliar elemental concentrations, stem water content, stem water δ¹⁸O (δ¹⁸O(stem water)), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low-density stands (heavily thinned) showed lower leaf δ¹⁸O, and higher stomatal conductance (g(s)), photosynthetic rate and radial growth than those in moderate-density stands (moderately thinned). By contrast, leaf δ¹³C, intrinsic water-use efficiency, foliar elemental concentrations and δ¹⁸O(stem water) were unaffected by stand density. Lower foliar δ¹⁸O in heavily thinned stands reflected higher g(s) of remaining trees due to decreased inter-tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO₂ uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.

Increase in water-use efficiency and underlying processes in pine forests across a precipitation gradient in the dry Mediterranean region over the past 30 years

Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (W(i)) with environmental conditions in Pinus halepensis. We used 30-year (1974-2003) tree-ring records of basal area increment (BAI) and cellulose (13)C and (18)O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280-700 mm) in Israel. The results show a clear trend of increasing W(i) in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These W(i) trends were better correlated with the increase in atmospheric CO(2) concentration, C(a), than with the local increase in temperature (~0.04°C year(-1)), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but W(i) from EW and BAI were dependent on the interannual variations in P. From reconstructed C(i) values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W(i) sensitivity to C(a) between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C(i)/C(a) towards a homeostatic set-point observed at the most mesic site, with a decrease in the W(i) response to C(i) with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W(i) driven, and there was an increase in the W(i) sensitivity to C(a) with aridity and a decreasing sensitivity of C(i) to C(a). The apparent C(a)-driven increases in W(i) can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.

Tree-ring C-H-O isotope variability and sampling

In light of the proliferation of tree-ring isotope studies, the magnitude and cause of variability of tree-ring δ(13)C, δ(18)O and δ(2)H within individual trees (circumferential) and among trees at a site is examined in reference to field and laboratory sampling requirements and strategies. Within this framework, this paper provides a state-of-knowledge summary of the influence of "juvenile" isotope effects, ageing effects, and genetic effects, as well as the interchangeability of species, choice of ring segment to analyze (whole ring, earlywood or latewood), and the option of sample pooling. The range of isotopic composition of the same ring among trees at a site is ca. 1-3‰ for δ(13)C, 1-4‰ δ(18)O, and 5-30‰ for δ(2)H, whereas the circumferential variability within a tree is lower. A standard prescription for sampling and analysis does not exist because of differences in field environmental circumstances and mixed findings represented in relevant published literature. Decisions in this regard will usually be tightly constrained by goals of the study and project resources. Sampling 4-6 trees at a site while avoiding juvenile effects in rings near the pith seems to be the most commonly used methodology, and although there are some reasoned arguments for analyzing only latewood and developing separate isotope records from each tree, the existence of some contradictory findings together with efforts to reduce cost and effort have prompted alternate strategies (e.g., most years pooled with occasional analysis of rings in the sequence separately for each tree) that have produced useful results in many studies.

A conceptual framework: redefining forest soil's critical acid loads under a changing climate

Federal agencies of several nations have or are currently developing guidelines for critical forest soil acid loads. These guidelines are used to establish regulations designed to maintain atmospheric acid inputs below levels shown to damage forests and streams. Traditionally, when the critical soil acid load exceeds the amount of acid that the ecosystem can absorb, it is believed to potentially impair forest health. The excess over the critical soil acid load is termed the exceedance, and the larger the exceedance, the greater the risk of ecosystem damage. This definition of critical soil acid load applies to exposure of the soil to a single, long-term pollutant (i.e., acidic deposition). However, ecosystems can be simultaneously under multiple ecosystem stresses and a single critical soil acid load level may not accurately reflect ecosystem health risk when subjected to multiple, episodic environmental stress. For example, the Appalachian Mountains of western North Carolina receive some of the highest rates of acidic deposition in the eastern United States, but these levels are considered to be below the critical acid load (CAL) that would cause forest damage. However, the area experienced a moderate three-year drought from 1999 to 2002, and in 2001 red spruce (Picea rubens Sarg.) trees in the area began to die in large numbers. The initial survey indicated that the affected trees were killed by the southern pine beetle (Dendroctonus frontalis Zimm.). This insect is not normally successful at colonizing these tree species because the trees produce large amounts of oleoresin that exclude the boring beetles. Subsequent investigations revealed that long-term acid deposition may have altered red spruce forest structure and function. There is some evidence that elevated acid deposition (particularly nitrogen) reduced tree water uptake potential, oleoresin production, and caused the trees to become more susceptible to insect colonization during the drought period. While the ecosystem was not in exceedance of the CAL, long-term nitrogen deposition pre-disposed the forest to other ecological stress. In combination, insects, drought, and nitrogen ultimately combined to cause the observed forest mortality. If any one of these factors were not present, the trees would likely not have died. This paper presents a conceptual framework of the ecosystem consequences of these interactions as well as limited plot level data to support this concept. Future assessments of the use of CAL studies need to account for multiple stress impacts to better understand ecosystem response.

Belowground effects of enhanced tropospheric ozone and drought in a beech/spruce forest (Fagus sylvatica L./Picea abies [L.] Karst)

The effects of experimentally elevated O(3) on soil respiration rates, standing fine-root biomass, fine-root production and delta(13)C signature of newly produced fine roots were investigated in an adult European beech/Norway spruce forest in Germany during two subsequent years with contrasting rainfall patterns. During humid 2002, soil respiration rate was enhanced under elevated O(3) under beech and spruce, and was related to O(3)-stimulated fine-root production only in beech. During dry 2003, the stimulating effect of O(3) on soil respiration rate vanished under spruce, which was correlated with decreased fine-root production in spruce under drought, irrespective of the O(3) regime. delta(13)C signature of newly formed fine-roots was consistent with the differing g(s) of beech and spruce, and indicated stomatal limitation by O(3) in beech and by drought in spruce. Our study showed that drought can override the stimulating O(3) effects on fine-root dynamics and soil respiration in mature beech and spruce forests.

Seasonal carbon storage and growth in Mediterranean tree seedlings under different water conditions

In all Mediterranean-type ecosystems, evergreen and deciduous trees differing in wood anatomy, growth pattern and leaf habit coexist, suggesting distinct adaptative responses to environmental constraints. This study examined the effects of summer water stress on carbon (C) storage and growth in seedlings of three coexisting Mediterranean trees that differed in phenology and wood anatomy characteristics: Quercus ilex subsp. ballota (Desf.) Samp., Quercus faginea Lam. and Pinus halepensis L. Seedlings were subjected to two levels of watering during two consecutive summers and achieved a minimum of -0.5 and -2.5 MPa of predawn water potential in the control and water stress treatment, respectively. Both Quercus species concentrated their growth in the early growing season, demanding higher C in early spring but replenishing C-stores in autumn. These species allocated more biomass to roots, having larger belowground starch and lipid reserves. Quercus species differed in seasonal storage dynamics from P. halepensis. This species allocated most of its C to aboveground growth, which occurred gradually during the growing season, leading to fewer C-reserves. Soluble sugar and starch concentrations sharply declined in August in P. halepensis, probably because reserves support respiration demands as this species closed stomata earlier under water stress. Drought reduced growth of the three species, mainly in Q. faginea and P. halepensis, but not C-reserves, suggesting that growth under water stress conditions is not limited by C-availability.

Intra-annual variability of anatomical structure and delta(13)C values within tree rings of spruce and pine in alpine, temperate and boreal Europe

Tree-ring width, wood density, anatomical structure and ¹³C/¹²C ratios expressed as δ¹³C-values of whole wood of Picea abies were investigated for trees growing in closed canopy forest stands. Samples were collected from the alpine Renon site in North Italy, the lowland Hainich site in Central Germany and the boreal Flakaliden site in North Sweden. In addition, Pinus cembra was studied at the alpine site and Pinus sylvestris at the boreal site. The density profiles of tree rings were measured using the DENDRO-2003 densitometer, δ¹³C was measured using high-resolution laser-ablation-combustion-gas chromatography-infra-red mass spectrometry and anatomical characteristics of tree rings (tracheid diameter, cell-wall thickness, cell-wall area and cell-lumen area) were measured using an image analyzer. Based on long-term statistics, climatic variables, such as temperature, precipitation, solar radiation and vapor pressure deficit, explained <20% of the variation in tree-ring width and wood density over consecutive years, while 29–58% of the variation in tree-ring width were explained by autocorrelation between tree rings. An intensive study of tree rings between 1999 and 2003 revealed that tree ring width and δ¹³C-values of whole wood were significantly correlated with length of the growing season, net radiation and vapor pressure deficit. The δ¹³C-values were not correlated with precipitation or temperature. A highly significant correlation was also found between δ¹³C of the early wood of one year and the late wood of the previous year, indicating a carry-over effect of the growing conditions of the previous season on current wood production. This latter effect may explain the high autocorrelation of long-term tree-ring statistics. The pattern, however, was complex, showing stepwise decreases as well as stepwise increases in the δ¹³C between late wood and early wood. The results are interpreted in the context of the biochemistry of wood formation and its linkage to storage products. It is clear that the relations between δ¹³C and tree-ring width and climate are multi-factorial in seasonal climates.

Physiology-phenology interactions in a productive semi-arid pine forest

This study explored possible advantages conferred by the phase shift between leaf phenology and photosynthesis seasonality in a semi-arid Pinus halepensis forest system, not seen in temperate sites. Leaf-scale measurements of gas exchange, nitrogen and phenology were used on daily, seasonal and annual time-scales. Peak photosynthesis was in late winter, when high soil moisture, mild temperatures and low leaf vapour pressure deficit (D(L)) allowed high rates associated with high water- and nitrogen-use efficiencies. Self-sustained new needle growth through the dry and hot summer maximized photosynthesis in the following wet season, without straining carbon storage. Low rates of water loss were associated with increasing sensitivity of stomatal conductance (g(s)) to soil moisture below a relative extractable water (REW) of 0.4, and decreased g(s )sensitivity to D(L) below REW of approx. 0.2. This response was captured by the modified Ball-Berry (Leuning) model. While most physiological parameters and responses measured were typical of temperate pines, the photosynthesis-phenological phasing contributed to high productivity under warm-dry conditions. This contrasts with reported effects of short-term periodical droughts and could lead to different predictions of the effect of warming and drying climate on pine forest productivity.

Ecophysiological traits associated with drought in Mediterranean tree seedlings: individual responses versus interspecific trends in eleven species

Species differ regarding their drought tolerance and individuals of a given species can modify their morphology and physiology in response to drought. However, since evolutionary and ecological selective pressures differ, individual and interspecific responses to drought might not match. We determined summer survival and a number of ecophysiological variables in two factorial experiments with seedlings of eleven tree species present in Mediterranean ecosystems, grown under slowly imposed water stress and control conditions. Plants experiencing drought exhibited reduced growth, low specific leaf area, chlorophyll content, and photosynthetic rate when compared to the controls, and species-specific drought tolerance was associated with an analogous set of trait values. However, while species with high leaf area ratio and shoot-root ratio exhibited greater drought tolerance, drought induced the reversed response within species. Contrary to expectations, water use efficiency was lower in drought-tolerant species and decreased in water-stressed individuals compared to the control plants. There was a distinctive phylogenetic signal in the functional grouping of species, with oaks, pines, and other genera being clearly different from each other in their drought tolerance and in their functional responses to drought. However, all relationships between ecophysiological variables and drought tolerance were significant after accounting for phylogenetic effects, with the exception of the relationship between drought tolerance and photochemical efficiency. Our results show that drought tolerance is not achieved by a single combination of trait values, and that even though evolutionary processes and individual responses tend to render similar results in terms of functional traits associated with drought, they do not necessarily match.