Evidence for plant traits driving specific drought resistance. A community field experiment

Drought is known to be a major bottleneck for woody-community recruitment. The species-specific resistance to drought and factors involved in its variation are of special interest to forecast community fate. We performed an experiment under natural field conditions from winter 2006 to end summer 2008 to investigate the structural responses of woody saplings to nine combinations of light (three habitats differing in plant cover: forest, shrubland, and open) and water (three climate scenarios: drier, current and wetter summers). Our working hypothesis is that plant strategies to cope with drought are determined by habitat characteristics and/or variation of plant traits, and that these different strategies may determine community composition and dynamics. Eight woody species with different life forms and successional stages (trees, mid-successional shrubs, and pioneer shrubs) were selected for the comparison, including therefore species representative of the entire woody community. We explored drought resistance at the population level (D_S), the relations of the different morphological traits to D_S, and the potential importance for plants of inter-specific trade-offs. D_S ranged from 0% to 99% for the different species, depending on the habitat. Some structural traits were found to be related to D_S: positively with total biomass and leaf mass ratio (LMR) and negatively with leaf area-root mass ratio (LARMR). Contrary to previous studies, the present work revealed no evidence of trade-offs, such as survival in forest vs. growth in open, or growth in forest vs. growth in open. Accordingly, some species with low D_S values (Acer opalus and Pinus sylvestris) would be threatened under the future climate conditions, while species having structural characteristics to increase their resistance under expected dry years in coming decades (i.e., high LMR and total biomass or low LARMR, such as Quercus ilex and the shrub Cytisus scoparius) might enhance their recruitment probabilities. Thus, species-specific plant traits, and their effect on D_S, may filter future community assemblages.     

Repercussions of Simulated Climate Change on the Diversity of Woody-Recruit Bank in a Mediterranean-type Ecosystem

Extreme climatic events have the potential to affect plant communities around the world, and especially in the Mediterranean basin, where the frequency of milder and drier summers is expected to be altered under a global-change scenario. We experimentally investigated the effect of three contrasting climatic scenarios on the diversity and abundance of the natural woody-recruit bank among three characteristic habitats in a Mediterranean-type ecosystem: forest, shrubland, and bare soil. The climatic scenarios were dry summers (30% summer rainfall reduction), wet summers (simulating summer storms), and current climatic conditions (control). Seedling emergence and survival after the first summer was recorded during 4 consecutive years. The wet summer boosted abundance and diversity at emergence and summer survival, rendering the highest Shannon H??index. By contrast, the dry summer had no effect on emergence, although survival tended to decline. Nonetheless, the habitat had a key role, bare soil showing almost null recruitment whatever the climatic scenario, and forest keeping the highest diversity in all of them. Our results show that recruit-bank density and diversity depends heavily on extreme climatic events. Community dynamics will depend not only on increased drought but also on the balance between dry and wet years.     

Oak seedling survival and growth along resource gradients in Mediterranean forests: implications for regeneration in current and future environmental scenarios

Understanding seedling performance across resource gradients is crucial for defining the regeneration niche of plant species under current environmental conditions and for predicting potential changes under a global change scenario. A 2‐year field experiment was conducted to determine how seedling survival and growth of two evergreen and two deciduous Quercus species vary along gradients of light and soil properties in two Mediterranean forests with contrasting soils and climatic conditions. Half the seedlings were subjected to an irrigation treatment during the first year to quantify the effects on performance of an alteration in the summer drought intensity. Linear and non‐linear models were parameterized and compared to identify major resources controlling seedling performance. We found both site‐specific and general patterns of regeneration. Strong site‐specificity was found in the identity of the best predictors of seedling survival: survival decreased linearly with increasing light (i.e. increasing desiccation risk) in the drier site, whereas it decreased logistically with increasing spring soil water content (i.e. increasing waterlogging risk) in the wetter site. We found strong empirical support for multiple resource limitation at the drier site, the response to light being modulated by the availability of soil resources (water and P). Evidence for regeneration niche partitioning among Quercus species was only found at the wetter site. However, at both sites Quercus species shared the same response to summer drought alleviation through water addition: increased first‐year survival but not final survival (i.e. after two years). This suggests that extremely dry summers (i.e. the second summer in the experiment) can cancel out the positive effects of previous wetter summers. Therefore, an increase in the intensity and frequency of summer drought with climate change might cause a double negative impact on Quercus regeneration, due to a general reduction in survival probability and the annulment of the positive effects of (infrequent) ‘wet’ years. Overall, results presented in this study are a major step towards the development of a mechanistic model of Mediterranean forest dynamics that incorporates the idiosyncrasies and generalities of tree regeneration in these systems, and that allow simulation and prediction of the ecological consequences of resource level alterations due to global change.     

Combined effects of climate,habitat, and disturbance on seedling establishment of <Emphasis Type="Italic">Pinus pinaster</Emphasis> and <Emphasis Type="Italic">Eucalyptus globulus</Emphasis>

The natural expansion of forestry trees into habitats outside plantations is a concern for managers and conservationists. We studied seedling emergence and survival of the two main forestry species in Portugal: Eucalyptus globulus (exotic) and Pinus pinaster (native); using a seed addition experiment. Our main objective was to evaluate the combined effects of climate (mild-summer and warm-summer climate), habitat (oak forest and shrubland), and disturbance (vegetation removal and non-disturbance) on the seedling establishment of species in semi- and natural habitats. Furthermore, we tested the effect of the “sowing season” (autumn and spring) on seedling emergence and survival. Overall, seedling establishment of both species was enhanced by light and water. However, we found important interactions among climate, habitat, and disturbance on both species’ emergence and survival. The differences between habitats were more evident in the mild-summer climate than in the warm-summer climate. Our results also suggested that seedling survival may be enhanced by shrub cover in drier conditions (warm-summer climate). Eucalyptus globulus appears more sensitive to drought and disturbance changes than P. pinaster. In shrublands and mild-summer climate conditions, disturbance especially promoted E. globulus seedling establishment, while the forest canopy and the shade appeared to control it in both climatic conditions. After the first summer life, very low seedling survival was observed in both species, although the colonization of new areas appeared to be more limited for E. globulus. Our study suggests that climate conditions influence the effect (direction and intensity) of habitat and disturbance (plant–plant interactions) on seedling survival. Thus, the effect of light availability (forest canopy) and disturbance (vegetation removal) on these species establishment is climate context-dependent. This study presents very useful information to understand future shifts in these species distribution and has direct applications for the management of natural establishment outside the planted areas, and the management of the understorey to favor forest regeneration or limit forest colonization.     

Soil‐nutrient availability under a global‐change scenario in a Mediterranean mountain ecosystem

Changes in rainfall availability will alter soil‐nutrient availability under a climate‐change scenario. However, studies have usually analyzed the effect of either drier or wetter soil conditions, despite the fact that both possibilities will coexist in many climatic regions of the world. Furthermore, its effect may vary across the different habitats of the ecosystem. We experimentally investigated the effect of three contrasting climatic scenarios on different carbon (C), nitrogen (N), and phosphorus (P) fractions in soil and microbial compartments among three characteristic habitats in a Mediterranean‐type ecosystem: forest, shrubland, and open areas. The climatic scenarios were dry summers, according to the 30% summer rainfall reduction projected in the Mediterranean; wet summer, simulating summer storms to reach the maximum historical records in the study area; and current climatic conditions (control). Sampling was replicated during two seasons (spring and summer) and 2 years. The climatic scenario did not affect the nutrient content in the litter layer. However, soil and microbial nutrients varied among seasons, habitats, and climatic scenarios. Soil‐nutrient fractions increased with lower soil‐moisture conditions (dry scenario and summer), whereas microbial nutrients increased under the wet summer scenario and spring. This pattern was consistent both studied years, although it was modulated by habitat, differences being lower with denser plant cover. Holm oak seedlings, used as live control of the experiment, tended to increase their N and P content (although not significantly) with water availability. Thus, the results support the idea that higher rainfall boosts microbial and plant‐nutrient uptake, and hence nutrient cycling. By contrast, a rainfall reduction leads to an accumulation of nutrients in the soil, increasing the risk of nutrient loss by leaching or erosion. These results show that the projected climate change will have significant effects on nutrient cycles, and therefore will have important implications on the ecosystem functioning.     

Effects of resource availability on plant recruitment at the community level in a Mediterranean mountain ecosystem

Coexisting plant species usually differ in resource requirements, which may also vary within species at successive demographic stages. Such differences become extremely important during the early life stages, since these are the most critical phases in woody-species recruitment, they depend heavily on resources, and they may determine future community composition. Under a global-change scenario, where climatic conditions, nutrient availability, and habitat characteristics are expected to be altered, it is difficult to predict the way in which plant recruitment will be affected. To understand the impact of different global-change drivers on community recruitment, we sowed a set of species representative of the different successional groups of a complete Mediterranean woody community under field conditions, and studied their emergence, growth, and survival along the main resource gradients of light, water, and nutrients. The light and nutrient gradients followed the natural range of conditions in the study area, but water availability was manipulated to simulate three contrasting climatic scenarios: wetter, drier, and current conditions. Structural equation modelling was used to provide a comprehensive analysis of the factors and relations governing plant recruitment. Overall, seedling emergence was determined directly by light; growth was determined by light and summer soil moisture; and survival was determined by summer soil moisture. Light was the main factor indirectly affecting the demographic stages of all species. However, the magnitude of the direct and indirect relationships varied among species. Particularly, species differed in their response to the expected drier climatic conditions, some (e.g. Pinus sylvestris, Acer opalus) being much more vulnerable than others (e.g. Cytisus scoparius, Salvia lavandulifolia). These differential responses could translate as major shifts in the structure of the overall plant community. Our results support the idea that the analysis of complex relations among essential resources is critical for accurate forecasts of the impact of climate change on community dynamics.     

Preference of wet dune species for waterlogged conditions can be explained by adaptations and specific recruitment requirements

The preference of wetland angiosperms for waterlogged soils has been explained by several hypotheses: (1) wetland species are adapted to waterlogging and sensitive to drought; (2) wetland species are tolerant to drought, but inferior competitors at drier conditions; (3) wetland species have narrow moisture optima for recruitment. We tested these hypotheses by the application of permanent and fluctuating water levels to experimental wet dune plant communities with four species that frequently occur in wet dune slacks (Carex flacca, Molinia caerulea, Samolus valerandi and Schoenus nigricans) and one competitive species of drier conditions (Calamagrostis epigejos). After 3 years, fluctuating water levels had led to lower total biomass production than permanent water levels, indicating that switching from aerated to anoxic soil conditions involved physiological costs. The collective biomass of wetland species was highest at permanently waterlogged conditions with interspecific variation in the biomass optima. At the nutrient-poor growth conditions of this long-term experiment, biomass of Calamagrostis was independent of water level treatment, thus the hypothesized competitive superiority of this species at drier conditions could not explain the biomass responses of the wetland species in this study. Instead, this is evidence for an ecological preference of adult wet dune plants for waterlogged to moist conditions. Recruitment of most wet dune slack species occurred in a narrower range of water levels than adult growth, indicating that recruitment requirements also pose a limitation to the distribution of these wet dune species. Incorporation of recruitment into nature management support models may improve their predictions.     

Phenotypic plasticity facilitates resistance to climate change in a highly variable environment

Increased summer drought will exacerbate the regeneration of many tree species at their lower latitudinal and altitudinal distribution limits. In vulnerable habitats, introduction of more drought-tolerant provenances or species is currently considered to accelerate tree species migration and facilitate forest persistence. Trade-offs between drought adaptation and growth plasticity might, however, limit the effectiveness of assisted migration, especially if introductions focus on provenances or species from different climatic regions. We tested in a common garden experiment the performance of Pinus sylvestris seedlings from the continental Central Alps under increased temperatures and extended spring and/or summer drought, and compared seedling emergence, survival and biomass allocation to that of P. sylvestris and closely related Pinus nigra from a Mediterranean seed source. Soil heating had only minor effects on seedling performance but high spring precipitation doubled the number of continental P. sylvestris seedlings present after the summer drought. At the same time, twice as many seedlings of the Mediterranean than the continental P. sylvestris provenance were present, which was due to both higher emergence and lower mortality under dry conditions. Both P. sylvestris provenances allocated similar amounts of biomass to roots when grown under low summer precipitation. Mediterranean seedlings, however, revealed lower phenotypic plasticity than continental seedlings under high precipitation, which might limit their competitive ability in continental Alpine forests in non-drought years. By contrast, high variability in the response of individual seedlings to summer drought indicates the potential of continental P. sylvestris provenances to adapt to changing environmental conditions.     

Response of Sierra Nevada forests to projected climate–wildfire interactions

Climate influences forests directly and indirectly through disturbance. The interaction of climate change and increasing area burned has the potential to alter forest composition and community assembly. However, the overall forest response is likely to be influenced by species‐specific responses to environmental change and the scale of change in overstory species cover. In this study, we sought to quantify how projected changes in climate and large wildfire size would alter forest communities and carbon (C) dynamics, irrespective of competition from nontree species and potential changes in other fire regimes, across the Sierra Nevada, USA. We used a species‐specific, spatially explicit forest landscape model (LANDIS‐II) to evaluate forest response to climate–wildfire interactions under historical (baseline) climate and climate projections from three climate models (GFDL, CCSM3, and CNRM) forced by a medium–high emission scenario (A2) in combination with corresponding climate‐specific large wildfire projections. By late century, we found modest changes in the spatial distribution of dominant species by biomass relative to baseline, but extensive changes in recruitment distribution. Although forest recruitment declined across much of the Sierra, we found that projected climate and wildfire favored the recruitment of more drought‐tolerant species over less drought‐tolerant species relative to baseline, and this change was greatest at mid‐elevations. We also found that projected climate and wildfire decreased tree species richness across a large proportion of the study area and transitioned more area to a C source, which reduced landscape‐level C sequestration potential. Our study, although a conservative estimate, suggests that by late century, forest community distributions may not change as intact units as predicted by biome‐based modeling, but are likely to trend toward simplified community composition as communities gradually disaggregate and the least tolerant species are no longer able to establish. The potential exists for substantial community composition change and forest simplification beyond this century.     

Effects of Drought,Pest Pressure and Light Availability on Seedling Establishment and Growth: Their Role for Distribution of Tree Species across a Tropical Rainfall Gradient

Tree species distributions associated with rainfall are among the most prominent patterns in tropical forests. Understanding the mechanisms shaping these patterns is important to project impacts of global climate change on tree distributions and diversity in the tropics. Beside direct effects of water availability, additional factors co-varying with rainfall have been hypothesized to play an important role, including pest pressure and light availability. While low water availability is expected to exclude drought-intolerant wet forest species from drier forests (physiological tolerance hypothesis), high pest pressure or low light availability are hypothesized to exclude dry forest species from wetter forests (pest pressure gradient and light availability hypothesis, respectively). To test these hypotheses at the seed-to-seedling transition, the potentially most critical stage for species discrimination, we conducted a reciprocal transplant experiment combined with a pest exclosure treatment at a wet and a dry forest site in Panama with seeds of 26 species with contrasting origin. Establishment success after one year did not reflect species distribution patterns. However, in the wet forest, wet origin species had a home advantage over dry forest species through higher growth rates. At the same time, drought limited survival of wet origin species in the dry forest, supporting the physiological tolerance hypothesis. Together these processes sort species over longer time frames, and exclude species outside their respective home range. Although we found pronounced effects of pests and some effects of light availability on the seedlings, they did not corroborate the pest pressure nor light availability hypotheses at the seed-to-seedling transition. Our results underline that changes in water availability due to climate change will have direct consequences on tree regeneration and distributions along tropical rainfall gradients, while indirect effects of light and pests are less important.     

Patterns of drought tolerance in major European temperate forest trees: climatic drivers and levels of variability

The future performance of native tree species under climate change conditions is frequently discussed, since increasingly severe and more frequent drought events are expected to become a major risk for forest ecosystems. To improve our understanding of the drought tolerance of the three common European temperate forest tree species Norway spruce, silver fir and common beech, we tested the influence of climate and tree‐specific traits on the inter and intrasite variability in drought responses of these species. Basal area increment data from a large tree‐ring network in Southern Germany and Alpine Austria along a climatic cline from warm‐dry to cool‐wet conditions were used to calculate indices of tolerance to drought events and their variability at the level of individual trees and populations. General patterns of tolerance indicated a high vulnerability of Norway spruce in comparison to fir and beech and a strong influence of bioclimatic conditions on drought response for all species. On the level of individual trees, low‐growth rates prior to drought events, high competitive status and low age favored resilience in growth response to drought. Consequently, drought events led to heterogeneous and variable response patterns in forests stands. These findings may support the idea of deliberately using spontaneous selection and adaption effects as a passive strategy of forest management under climate change conditions, especially a strong directional selection for more tolerant individuals when frequency and intensity of summer droughts will increase in the course of global climate change.     

Use of Tropical Dry Forests and Agricultural Areas by Neotropical Bird Communities

Agricultural activities represent a global threat to biodiversity, yet little is known about the relative importance of different agricultural land uses in relation to their wildlife communities. We explored bird community structure, diversity, and composition in a landscape dominated by primary tropical dry forest, and in three agricultural systems (i.e., tree orchards, herbaceous cropfields, cattle pastures) to evaluate the way in which birds use different habitats within the landscape. Tropical dry forests had the highest species richness and community evenness, although the bird community in tree orchards was also relatively species rich and even. Cattle pastures had more bird species than cropfields but both habitats were comparatively species poor with low evenness. Our results are related to habitat structure and the spatial location of sites within the landscape matrix. Based on our observations, we recommend including native tree species within agricultural systems and surrounding areas to provide additional resources for birds. Finally, we suggest promoting natural recruitment of native trees and shrubs within cattle pastures to provide suitable habitat for species that use tropical dry forest plants. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

Species‐Specific Barriers to Tree Regeneration in High Elevation Habitats of West Virginia

Herbaceous competition and herbivory have been identified as critical barriers to restoration of native tree species in degraded landscapes around the world; however, the combined effects of competition and herbivory are poorly understood. We experimentally manipulated levels of herbivory and herbaceous competition and analyzed the response of tree seedling performance over three growing seasons as a function of species and habitat in north‐central West Virginia. Four native tree species were planted in old field and forest experimental plots: Castanea dentata (American chestnut), Quercus rubra (red oak), Acer saccharum (sugar maple), and Picea rubens (red spruce). Red spruce demonstrated the highest growth increment and greatest survival (64%) and most consistent results among treatments and habitats. Red spruce survival was not reduced in the presence of Odocoileus virginianus (white‐tailed deer) browse and herbaceous competition; however, growth was improved by suppression of herbaceous competition. We suspect that this deciduous forest landscape would regenerate to a red spruce dominated forest if seed source was available. In contrast, the other three species tested had very low survival when exposed to deer and were more responsive to competing vegetation and habitat type. American chestnut had low survival and growth across all treatments, suggesting basic climate limitations. Vigorous natural regeneration of Prunus serotina (black cherry) occurred in forest plots where both competing herbs and deer were excluded. Our results demonstrated the importance of testing multiple potential recruitment barriers and species at once and the need for species and habitat‐specific restoration treatments.     

Canopy vs. soil effects of shrubs facilitating tree seedlings in Mediterranean montane ecosystems

Abstract. Question: Is the facilitative effect of nurse shrubs on early recruitment of trees mediated by a ‘canopy effect’(microclimate amelioration and protection from herbivores), a ‘soil effect’(modification of soil properties), or both? Location: Two successional montane shrublands at the Sierra Nevada Protected Area, SE Spain. Method: Seedlings of Quercus and Pinus species were planted in four experimental treatments: (1) under shrubs; (2) in open interspaces without vegetation; (3) under shrubs where the canopies were removed; (4) in open interspaces but covering seedlings with branches, mimicking a shrub canopy. Results: Both effects benefited seedling performance. However, microclimatic amelioration due to canopy shading had the strongest effect, which was particularly pronounced in the drier site. Below‐ground, shrubs did not modify soil physical characteristics, organic matter, total N and P, or water content, but significantly increased available K, which has been shown to improve seedling water‐use efficiency under drought conditions. Conclusions: We propose that in Mediterranean montane ecosystems, characterised by a severe summer drought, pioneer shrubs represent a major safe site for tree early recruitment during secondary succession, improving seedling survival during summer by the modification of both the above‐and below‐ground environment.     

Woodland recovery following drought‐induced tree mortality across an environmental stress gradient

Recent droughts and increasing temperatures have resulted in extensive tree mortality across the globe. Understanding the environmental controls on tree regeneration following these drought events will allow for better predictions of how these ecosystems may shift under a warmer, drier climate. Within the widely distributed piñon–juniper woodlands of the southwestern USA, a multiyear drought in 2002–2004 resulted in extensive adult piñon mortality and shifted adult woodland composition to a juniper‐dominated, more savannah‐type ecosystem. Here, we used pre‐ (1998–2001) and 10‐year post‐ (2014) drought stand structure data of individually mapped trees at 42 sites to assess the effects of this drought on tree regeneration across a gradient of environmental stress. We found declines in piñon juvenile densities since the multiyear drought due to limited new recruitment and high (>50%) juvenile mortality. This is in contrast to juniper juvenile densities, which increased over this time period. Across the landscape, piñon recruitment was positively associated with live adult piñon densities and soil available water capacity, likely due to their respective effects on seed and water availability. Juvenile piñon survival was strongly facilitated by certain types of nurse trees and shrubs. These nurse plants also moderated the effects of environmental stress on piñon survival: Survival of interspace piñon juveniles was positively associated with soil available water capacity, whereas survival of nursed piñon juveniles was negatively associated with perennial grass cover. Thus, nurse plants had a greater facilitative effect on survival at sites with higher soil available water capacity and perennial grass cover. Notably, mean annual climatic water deficit and elevation were not associated with piñon recruitment or survival across the landscape. Our findings reveal a clear shift in successional trajectories toward a more juniper‐dominated woodland and highlight the importance of incorporating biotic interactions and soil properties into species distribution modeling approaches.     

Differential effect of drought regimes on the seedling performance of six floodplain grassland species

• The performance of seedlings is crucial for the survival and persistence of plant populations. Although drought frequently occurs in floodplains and can cause seedling mortality, studies on the effects of drought on seedlings of floodplain grasslands are scarce. We tested the hypotheses that drought reduces aboveground biomass, total biomass, plant height, number of leaves, leaf area and specific leaf area (SLA), and increases root biomass and root‐mass fraction (RMF) and that seedlings from species of wet floodplain grasslands are more affected by drought than species of dry grasslands.
• In a greenhouse study, we exposed seedlings of three confamilial pairs of species (Pimpinella saxifraga, Selinum carvifolia, Veronica teucrium, Veronica maritima, Sanguisorba minor, Sanguisorba officinalis) to increasing drought treatments. Within each plant family, one species is characteristic of wet and one of dry floodplain grasslands, confamilial in order to avoid phylogenetic bias of the results.
• In accordance with our hypotheses, drought conditions reduced aboveground biomass, total biomass, plant height, number of leaves and leaf area. Contrary to our hypotheses, drought conditions increased SLA and decreased root biomass and RMF of seedlings. Beyond the effects of the families, the results were species‐specific (V. maritima being the most sensitive species) and habitat‐specific. Species indicative of wet floodplain grasslands appear to be more sensitive to drought than species indicative of dry grasslands.
• Because of species‐ and habitat‐specific responses to reduced water availability, future drought periods due to climate change may severely affect some species from dry and wet habitats, while others may be unaffected.

     

Summer drought exposure,stand structure,and soil properties jointly control the growth of European beech along a steep precipitation gradient in northern Germany

Increasing exposure to climate warming-related drought and heat threatens forest vitality in many regions on earth, with the trees' vulnerability likely depending on local climatic aridity, recent climate trends, edaphic conditions, and the drought acclimatization and adaptation of populations. Studies exploring tree species' vulnerability to climate change often have a local focus or model the species' entire distribution range, which hampers the separation of climatic and edaphic drivers of drought and heat vulnerability. We compared recent radial growth trends and the sensitivity of growth to drought and heat in central populations of a widespread and naturally dominant tree species in Europe, European beech (Fagus sylvatica), at 30 forest sites across a steep precipitation gradient (500–850 mm year⁻¹) of short length to assess the species' adaptive potential. Size-standardized basal area increment remained more constant during the period of accelerated warming since the early 1980s in populations with >360 mm growing season precipitation (April–September), while growth trends were negative at sites with <360 mm. Climatic drought in June appeared as the most influential climatic factor affecting radial growth, with a stronger effect at drier sites. A decadal decrease in the climatic water balance of the summer was identified as the most important factor leading to growth decline, which is amplified by higher stem densities. Inter-annual growth variability has increased since the early 1980s, and variability is generally higher at drier and sandier sites. Similarly, within-population growth synchrony is higher at sandier sites and has increased with a decrease in the June climatic water balance. Our results caution against predicting the drought vulnerability of trees solely from climate projections, as soil properties emerged as an important modulating factor. We conclude that beech is facing recent growth decline at drier sites in the centre of its distribution range, driven by climate change-related climate aridification.     

Species-specific climate sensitivity of tree growth in Central-West Germany

Growth responses to twentieth century climate variability of the three main European tree species Fagus sylvatica, Quercus petraea, and Pinus sylvestris within two temperate low mountain forest sites were analyzed, with particular emphasis on their dependence upon ecological factors and temporal stability in the obtained relationships. While site conditions in Central (~51°N, 9°E, KEL) and West (50.5°N, 6.5°E, EIF) Germany are similar, annual precipitation totals of ≅700 mm and ≅1,000 mm describe a maritime-continental gradient. Ring-width samples from 228 trees were collected and PCA used to identify common growth patterns. Chronologies were developed and redundancy analysis and simple correlation coefficients calculated to detect twentieth century temperature, precipitation, and drought fingerprints in the tree-ring data. Summer drought is the dominant driver of forest productivity, but regional and species-specific differences indicate more complex influences upon tree growth. F. sylvatica reveals the highest climate sensitivity, whereas Q. petraea is most drought tolerant. Drier growth conditions in KEL result in climate sensitivity of all species, and Q. petraea shifted from non-significant to significant drought sensitivity during recent decades at EIF. Drought sensitivity dynamics of all species vary over time. An increase of drought sensitivity in tree growth was found in the wetter forest area EIF, whereas a decrease occurred in the middle of the last century for all species in the drier KEL region. Species-specific and regional differences in long-term climate sensitivities, as evidenced by temporal variability in drought sensitivity, are potential indicators for a changing climate that effects Central-West German forest growth, but meanwhile hampers a general assessment of these effects.     

Observed and projected impacts of climate on radial growth of three endangered conifers in northern Mexico indicate high vulnerability of drought-sensitive species from mesic habitats

A decline in productivity and radial growth in conifer forests from mesic areas has been associated with increased drought stress induced by climate warming. Nevertheless, studies showing how vulnerable tree species will be in response to forecasted warming conditions are scarce in such mesic habitats. Here we address this issue by analyzing how growth responds to drought and to observed and projected climate conditions in a conifer forest from northern Mexico, which is a hotspot of conifer diversity. We quantify the trends in radial growth (quantified as basal area increment, BAI) of three species (Abies durangensis, Picea chihuahuana, Cupressus lusitanica) using dendrochronology and a process-based model of tree growth. Growth decreased in A. durangensis and P. chihuahuana from the late 1980s onwards in response to warmer and drier conditions, whereas C. lusitanica growth showed very low sensitivity to precipitation and increased as temperature did. Winter-spring dry conditions adversely affected the growth of A. durangensis and P. chihuahuana. Our modeling approach anticipates growth reductions and an increase in the vulnerability of A. durangensis and the endangered P. chihuahuana against the warmer and more arid conditions predicted after the 2050s. Future warmer and drier climatic conditions could reduce the productivity and lead to growth decline of these mesic conifer forests triggering dieback episodes in highly drought-sensitive species as A. durangensis and P. chihuahuana.     

Water availability,fundamental niches and realized niches: A case study from the Cape flora

The ability of plants to survive drought or waterlogging constitutes an important niche parameter, which might be particularly significant in explaining species coexistence in the species‐rich and seasonally dry Cape Floristic Region of South Africa. However, the degree of physiological adaptation and specialization to these eco‐hydrological parameters (the fundamental niche) cannot be readily inferred from correlative studies based on species distributions and spatial variation in environmental parameters (the realized niche). We used an ex situ greenhouse experiment to compare the fundamental hydrological niches (different mean annual precipitation, rainfall seasonality and soil drainage) of six eco‐hydrologically divergent African Restionaceae species. Juvenile plants were subjected to six different watering treatments, ranging from no watering to waterlogging, to determine drought and waterlogging susceptibility and optimal growth conditions. We used the rate of biomass accumulation and survival rate as response measures. We found that species from dry and mesic (but well‐drained) habitats had optimal or near‐optimal growth at benign conditions (under which most restio species grow well). All species performed worse when droughted and died when not watered. Species from dry habitats tended to perform better (assessed in growth) than species from wet habitats under droughting. Species from wet habitats performed best when waterlogged, whereas species from dry habitats performed very poorly when waterlogged – thus showing that realized and fundamental niches covaried at the wet end of the hydrological gradient. We conclude that eco‐hydrological parameters are part of the fundamental niche, and fundamental and realized species niches are approximately correlated along them. The distribution of wet habitat species appears not to reflect their drought tolerance, suggesting that it may not be predicted by bioclimatic variables, but rather by soil drainage characteristics.