Xeric Sites Research Articles

Aridity and cold temperatures drive divergent adjustments of European beech xylem anatomy, hydraulics and leaf physiological traits.

Understanding plant trait coordination and variance across climatic gradients is critical for assessing forests' adaptive potential to climate change. We measured 11 hydraulic, anatomical and leaf-level physiological traits in European beech (Fagus sylvatica L.) along a moisture and temperature gradient in the French Alps. We assessed how traits covaried, and how their population-level variances shifted along the gradient. The intrapopulation variances of vessel size and xylem-specific conductivity reduced in colder locations as narrow vessels were observed in response to low temperature. This decreased individual-level water transport capacity compared with the warmer and more xeric sites. Conversely, the maximum stomatal conductance and Huber value variances were constrained in the arid and warm locations, where trees showed restricted gas exchange and higher xylem-specific conductivity. The populations growing under drier and warmer conditions presented wide variance for the xylem anatomical and hydraulic traits. Our results suggest that short-term physiological acclimation to raising aridity and heat in southern beech populations may occur mainly at the leaf level. Furthermore, the wide variance of the xylem anatomical and hydraulic traits at these sites may be advantageous since more heterogeneous hydraulic conductivity could imply populations' greater tree-tree complementarity and resilience against climatic variability. Our study highlights that both intrapopulation trait variance and trait network analysis are key approaches for understanding species adaptation and the acclimation potential to a shifting environment.

Tree-Ring Analysis and Genetic Associations Help to Understand Drought Sensitivity in the Chilean Endemic Forest of Nothofagus macrocarpa

Extreme drought events have increased in frequency during the 20th century triggered by global change. Thus, understanding tree-growth resilience across different terrestrial biomes has become a key goal in forest ecology. Here, we evaluate the tree-growth resilience to severe drought in the only Mediterranean-type Ecosystems of South America, using five isolated populations ofNothofagus macrocarpa. For each tree, in each sampling site, we obtained wood cores and fresh leaves for dendrochronological and population genetic analysis, respectively. An evaluation was conducted on growth resilience components in response to the most extreme drought of the 20th century in central Chile (i.e., 1968, with ∼80% of rainfall deficit), and the influence of genetic variability, biogeography, and tree size. We hypothesize that even though current remnant populations ofN. macrocarpaare small and isolated, they have locally withstood changes in climate, and that they will be genetically diverse and have a high resilience to extreme droughts. We used nuclear microsatellite markers to estimate tree genetic variability inN. macrocarpaand investigate its correlation with phenotypic traits. We found a higher resistance in the two southernmost populations (mesic sites) than in the three northern populations (xeric sites), however those three xeric populations showed a higher recovery. In addition, a significant clear positive linear correlation between precipitation and resistance, and a negative recovery and relative resilience of tree growth to the extreme drought event of 1968 can be seen. High diversity for simple sequence repeats (SSR) markers was observed, although no population structure was inferred. Southern populations had a higher number of private alleles, which may be an indication of their long-lasting persistence under mesic conditions. Therefore, differences in resilience components are mainly explained by tree size and sites influences, but not genetic diversity. We concluded that observed differences in tree-growth resilience among sites can be explained by a great deal of phenotypic plasticity, fostered by genetically diverse gene pools. We advocate for a genome-wide analysis (i.e., SNP) so as to identify genomic regions correlated with phenotypic traits in order to improve the understanding of the evolutionary processes that shaped this forest resilience over time.

Marginal imprint of human land use upon fire history in a mire-dominated boreal landscape of the Veps Highland, North-West Russia

Dendrochronological reconstructions inform us about historical climate-fire-human interactions, providing a means to calibrate projections of future fire hazard. Most of these reconstructions, however, have been developed in landscapes with a considerable proportion of xeric sites that could potentially inflate our estimates of the historic levels of fire activity. We provide a 420-year long reconstruction of fires in a mire-dominated landscape of the Veps Nature Park, North-West Russia. The area has mostly escaped large-scale forestry operations in the past and is an example of pristine mid-boreal vegetation with a high (approximately 30% for the area studied) proportion of waterlogged areas with ombrotropic mires. The historical fire cycle was 91.4 years (90% confidence intervals, CI 66.2–137.6 years) over the 1580–1720 period, decreasing to 35.9 (CI 28.1–47.6 years) between 1730 and 1770, and then increasing again to 122.7 years (CI 91.0–178.0 years) over the 1780–2000 period. Early season fires dominated over late season fires during the reconstruction period. We documented a higher fire activity period between 1730 and 1780, resulting from the increase in early season fires. This period coincided with one of the largest multi-decadal declines in the reconstructed spring precipitation since 1600 CE, although we found no significant relationship between fire and precipitation over the whole reconstructed period. The nine largest fire years were associated with negative summer precipitation and positive summer temperature anomalies over the study region. Land-use history of the area did not appear to have an effect on historical fire dynamics. Modern (1996–2016) fire records indicate a regional fire cycle of ∼ 1300 years, featuring a pronounced pattern with early (April–May) and late (July–September) season fires. The uniform fire cycle in the area since 1780, occurrence of nine largest fire years during years with spring-summer droughts, and low ignition frequencies over the last 420 years (0.005 to 0.037 ignitions per year and km2) suggest that the fire regime of the Veps Highland remained largely natural until the onset of the 20th century.

Oak forests and woodlands as Indigenous landscapes in the Eastern United States

Land use by Indigenous people (Native Americans) and climate are primary factors affecting the dynamics of oak (Quercus) forests and woodlands in the eastern United States. Prior to Euro-American settlement, much of the eastern deciduous forest was dominated by oak species. The role of periodic surface burning, agriculture, and other forms of land management by Indigenous peoples is frequently noted by cultural anthropologists and historical ecologists. However, these points are often debated by paleoecologists and climate scientists. Here we present a literature review, synthesis, and data summary to investigate the role of altered land use, season of burning, and climate change in relation to pre– and post–Euro-American changes in forest composition. Human-based ignitions, as reflected by dormant-season fires, prevailed over the oak- and pine-dominated forests, with intermediate fire frequency during Indigenous periods. From the 18th century on, Euro-American populations rapidly expanded, impacting much of the eastern United States through extensive timber harvesting, land clearing, and severe fires. Starting in the 20th century, a variety of ecological influences, including agricultural land abandonment, chestnut blight, fire suppression, mesophication, and urbanization, resulted in dramatic vegetation changes in eastern landscapes. These trends have culminated in recruitment failures of most oak species on all but the most xeric sites and an increase in mid- to late-successional mesic hardwoods, most notably red maple (Acer rubrum), a species with very low density in our analysis of the witness tree record. We conclude that prescribed burning, agriculture, and other land uses by Indigenous peoples created a mosaicked landscape of expansive oak savannas, woodlands, and forests. A warming world over the past century should have promoted warm-adapted, fire-tolerant, xerophytic genera such as oak, hickory (Carya), and pine (Pinus) and grassland communities but instead have promoted the invasion by cool-adapted, fire-sensitive, mesophytic trees due to the absence of burning, much to the detriment of major vegetation biomes. Understanding that eastern oak and other pyrogenic ecosystems represent an Indigenous landscape strengthens our ability to best manage vegetation against the expansion of less desirable species and restore historic fire cycles through prescribed burning.

Hurricane Michael Altered the Structure and Function of Longleaf Pine Woodlands

AbstractTropical cyclones can physically alter ecosystems, causing immediate and potentially long‐lasting effects on carbon dynamics. In 2018, Hurricane Michael hit the southeastern United States with category 5 winds at landfall and category 2 winds reaching over 100 miles inland, resulting in extensive damage. Longleaf pine woodlands in the path of the hurricane were damaged, but severity varied based on the storm track. We used a combination of eddy covariance measurements, airborne LiDAR, and forest inventory data to determine whether hurricane affects structure, function, and recovery of two longleaf pine woodlands at the ends of an edaphic gradient. We found that the carbon sink potentials in both sites were diminished following the storm, with reductions in net ecosystem exchange (NEE) primarily due to lower rates of photosynthesis, as respiration only increased marginally. The xeric site carbon losses and physiological reductions were smaller following the disturbance, which led to the recovery of ecosystem physiological activity to prestorm rates before that of the mesic site, as indicated by maximum ecosystem CO2 uptake rates. Two years following the hurricane both stands continued to have reduced NEE, which signaled altered function. We expect both locations to recover their lost carbon stocks in ∼10–35 years; however, long‐term studies are needed to examine how longleaf woodlands respond to compounding disturbances, such as drought, fire, or other wind storms, which vary significantly across the ecosystem's range. Additionally, hurricanes are intensifying due to climate change, potentially amplifying the degree to which they will alter this ecosystem in the future.

Spatial variability in tree-ring carbon isotope discrimination in response to local drought across the entire loblolly pine natural range

Considering the temporal responses of carbon isotope discrimination (Δ13C) to local water availability in the spatial analysis of Δ13C is essential for evaluating the contribution of environmental and genetic facets of plant Δ13C. Using tree-ring Δ13C from years with contrasting water availability at 76 locations across the natural range of loblolly pine, we decomposed site-level Δ13C signals to maximum Δ13C in well-watered conditions (Δ13Cmax) and isotopic drought sensitivity (m) as a change in Δ13C per unit change of Palmer's Drought Severity Index (PDSI). Site water status, especially the tree lifetime average PDSI, was the primary factor affecting Δ13Cmax. The strong spatial correlation exhibited by m was related to both genetic and environmental factors. The long-term average water availability during the period relevant to trees as indicated by lifetime average PDSI correlated with Δ13Cmax, suggesting acclimation in tree gas-exchange traits, independent of incident water availability. The positive correlation between lifetime average PDSI and m indicated that loblolly pines were more sensitive to drought at mesic than xeric sites. The m was found to relate to a plant's stomatal control and may be employed as a genetic indicator of efficient water use strategies. Partitioning Δ13C to Δ13Cmax and m provided a new angle for understanding sources of variation in plant Δ13C, with several fundamental and applied implications.

Modeling Climate Impacts on Tree Growth to Assess Tree Vulnerability to Drought During Forest Dieback.

Forest dieback because of drought is a global phenomenon threatening particular tree populations. Particularly vulnerable stands are usually located in climatically stressing locations such as xeric sites subjected to seasonal drought. These tree populations show a pronounced loss of vitality, growth decline, and high mortality in response to extreme climate events such as heat waves and droughts. However, dieback events do not uniformly affect stands, with some trees showing higher symptoms of drought vulnerability than other neighboring conspecifics. In this study, we investigated if trees showing different vulnerabilities to dieback showed lower growth rates (Grs) and higher sensitivities to the climate in the past using dendroecology and the Vaganov-Shashkin (VS) process-based growth model. We studied two Pinus pinaster stands with contrasting Grs showing recent dieback in the Iberian System, north-eastern Spain. We compared coexisting declining (D) and non-declining (ND) trees with crown defoliation values above and below the 50% threshold, respectively. The mean growth rate was lower in D than in ND trees in the two stands. The two vigor classes showed a growth divergence prior to the dieback onset and different responsiveness to climate. The ND trees were more responsive to changes in spring water balance and soil moisture than D trees, indicating a loss of growth responsiveness to the climate in stressed trees. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. The presented comparisons indicated different stand vulnerabilities to drought contingent on-site conditions. Further research should investigate the role played by environmental conditions and individual features such as access to soil water or hydraulic traits and implement them in process-based growth models to better forecast dieback.

Climate windows of intra-annual growth and post-drought recovery in Mediterranean trees

Forests store large amounts of carbon as wood, but this storage potential depends on seasonal radial growth and how it is driven by climate. Therefore, we need a better understanding on how intra-annual radial increment rates determine growth responses to climate variability. There is a large gap of knowledge on the climate windows, i.e. the effective climatic time-window for tree growth, and predictors of intra-annual growth, particularly in seasonally dry biomes such as Mediterranean forests where growth may show several seasonal peaks. We described the radial increment rates of five tree species inhabiting mesic (Quercus ilex, Quercus faginea, Arbutus unedo) and xeric (Pinus halepensis, Juniperus thurifera) sites using dendrometers. Then, we related those rates and tree-ring width to climate data using correlations and model selection. All species presented bimodal patterns of radial increment rates with a major peak in spring and a minor peak in autumn, being this pattern most marked in Q. ilex, A. unedo and J. thurifera. Differences in climate-rates relationships were greater between species than between sites. Wet and cool conditions in winter and spring increased spring rates, but the climatic time-windows were longer in some species (Q. ilex, J. thurifera) and shorter in others (Q. faginea, P. halepensis). Similarly, wet and cool conditions also enhanced autumn rates in the xeric site, but warm conditions did it in the mesic site. Rainy autumn conditions reduced A. unedo rates. Climate-growth associations at intra- and inter-annual scales were consistent in species showing long intra-annual climatic time-windows such as Q. ilex. Our findings confirm the importance of wet winters and cool springs for radial growth in some species, and the relevance of autumn growth in A. unedo. Similar studies on intra-annual radial increment rates and climatic time-windows of forest growth and productivity can reveal which will be the species most impacted by drought.

Reproductive phenology determines the linkages between radial growth, fruit production and climate in four Mediterranean tree species

The influence of climate on seed or fruit production and tree growth is a central question in forest ecology, with a key role on forest dynamics. However, the mechanisms linking inter- and intra-annual climate variability, fruiting and growth remain poorly understood, although they seem to be largely species-specific. The resource-matching hypothesis posits that fruit production responds to annual environmental variability, predicting a positive relationship between reproduction and growth, whereas other hypotheses such as resource switching or resource budget imply allocation trade-offs between reproduction and growth. Climate control over fruiting is thought to be more likely in variable, limiting environments such as those existing in seasonally dry Mediterranean forests. However, the studies about these forests are much scarcer than on temperate ecosystems, probably due to the sparseness and shortness of fruit production records. Here we analysed the relationships between monthly climate variables, tree-ring widths and fruit production at stand level in four tree species under different climatic conditions in Spain (12-27 trees and 24-54 radii were sampled in each site). For the species from xeric sites such as Pinus pinea, Castanea sativa and Quercus ilex, tree-ring width positively correlated with fruit production, as envisaged by the resource-matching hypothesis. This association was driven by prior wet winter climate conditions, which enhanced growth. Some of the significant correlations between climate variables and fruit yield corresponded to key reproductive phenophases. On the other hand, for the species from mesic sites (Abies alba) no positive growth–fruit production correlation was found, likely due to the mismatch in the effect of climatic factors on growth and fruiting. These results support the hypothesis that climatic conditions, particularly a positive water balance, play a major role as a proximate cause of fruit production in seasonally dry Mediterranean forests and underscore the importance of species-specific reproductive phenology traits.

Zajímavé nálezy teplomilných plžů v Karlovarském kraji (nejzápadnější Čechy) [Interesting records of thermophilic snails in the Karlovy Vary Region (westernmost Bohemia)

Several xeric sites in the Karlovy Vary Region, mainly along railways, were surveyed for thermophilic snails of open habitats in 2019 and 2020. Two localities of Euomphalia strigella (Draparnaud, 1801), 13 localities of Monacha cartusiana (O. F. Müller, 1774) and two localities of Xerolenta obvia (Menke, 1828) were found. These three species are all new for the region. The locality of Tři Sekery (665 m a.s.l.) represents the altitudinal maximum of M. cartusiana in Czechia. Although the two sites with Xerolenta obvia were secondary; relatively high densities may suggest that these populations were established in this regions some time ago.

How future-proof is Sweet chestnut (Castanea sativa) in a global change context?

In the debate about tree species suited to cope with the ongoing global changes, Sweet chestnut (Castanea sativa Mill.) is frequently discussed as a potentially future-proof tree species for Central Europe. This is mainly due to its current southern distribution range, which climatically corresponds to what is expected for Central Europe in the near future. Present chestnut forests are, however, the result of historic long-term intensive management, which has extended and partially obscured the species’ natural ecological range and autecology. Therefore, knowledge about the competitiveness and suitability of chestnut for future conditions is still poor. Here, we analysed data of the Swiss National Forest Inventory (NFI) for Southern Switzerland to compare the survival probability of chestnut to other co-occurring tree species and detect drivers of chestnut mortality under natural dynamics including the Asian Chestnut Gall Wasp (Dryocosmus kuriphilus) using survival models, such as the Kaplan-Meier estimator and the Cox-Proportional Hazards model.Despite the potential longevity of cultivated chestnuts, under natural conditions the survival curves rank the chetsnut among short-lived and light-demanding tree species, especially for individuals smaller than 50 cm diameter at breast height. Moreover, the survival models confirm the pioneer-like characteristics of the chestnut tree such as its low shade tolerance and poor competitiveness in comparison to late-successional species. The mortality probability of chestnut increases when growing under dry conditions and under grazing pressure from goats, while fire disturbances have a positive effect on survival. Severe damage due to repeated D. kuriphilus attacks represents an additional significant factor of increased chestnut mortality risk.Our results suggest that chestnut is not per se a future-proof tree species. Rather, forest managers should be aware that chestnut might not be the best option to face expected rises in summer temperatures and dry periods on xeric site conditions, whereas the species can be considered on good site conditions when regular silvicultural management is provided.

Root Growth Potential and Microsite Effects on Conifer Seedling Establishment in Northern Idaho

Environmental conditions and seedling quality interact to produce complex patterns of seedling survival and growth. Root growth potential (RGP) is one metric of seedling quality that can be rapidly measured prior to planting, but the correlation of RGP and seedling performance is not consistent across studies. Site factors including microsite objects that cast shade and competing vegetation can also influence seedling performance. We examined the effects of RGP, presence/absence of a microsite object, and competition cover on the survival and growth of three native conifers to the Inland Northwest, USA, over 5 years. We found that RGP had no effect on the survival or growth of western larch (Larix occidentalis), Douglas fir (Pseudotsuga menziesii var. glauca), and grand fir (Abies grandis) at a mesic north aspect site and a xeric south aspect site. Comparatively, the presence of a microsite increased the odds of survival by 37% for western larch and 158% for grand fir, while the absence of forb cover increased the odds of survival of western larch by 72% and of grand fir by 26%. Douglas fir was less sensitive to microsites and competition. The strong effects of neighborhood conditions around seedlings help inform silvicultural practices to enhance the establishment of western larch and grand fir, including planting seedlings near shading objects and competition control, while these practices may not be as important for Douglas fir.

A candidate gene association analysis identifies SNPs potentially involved in drought tolerance in European beech (Fagus sylvatica L.)

Studies of genetic variation underlying traits related to drought tolerance in forest trees are of great importance for understanding their adaptive potential under a climate change scenario. In this study, using a candidate gene approach, associations between SNPs and drought related traits were assessed in saplings of European beech (Fagus sylvatica L.) representing trees growing along steep precipitation gradients. The saplings were subjected to experimentally controlled drought treatments. Response of the saplings was assessed by the evaluation of stem diameter growth (SDG) and the chlorophyll fluorescence parameters FV/FM, PIabs, and PItot. The evaluation showed that saplings from xeric sites were less affected by the drought treatment. Five SNPs (7.14%) in three candidate genes were significantly associated with the evaluated traits; saplings with particular genotypes at these SNPs showed better performance under the drought treatment. The SNPs were located in the cytosolic class I small heat-shock protein, CTR/DRE binding transcription factor, and isocitrate dehydrogenase genes and explained 5.8–13.4% of the phenotypic variance. These findings provide insight into the genetic basis of traits related to drought tolerance in European beech and could support the development of forest conservation management strategies under future climatic conditions.

Forest structure and composition drive differences in metabolic energy and entropy dynamics during temperature extremes in longleaf pine savannas

The southeastern US has experienced an increase in the number of extreme heat events since the 1970s, due in part to global change. Despite rising temperatures, greater variability in weather has also led to more freeze events across parts of the Southeast, particularly during El Niño winters. Structural variation in forest stands and plant functional diversity can lead to localized micrometeorological differences that may alter the recovery of different forests from these temperature-induced disturbances. This variation can lead to differences in energy and entropy dynamics, which drive the metabolic response of these systems.Using thermodynamic metrics, we quantified metabolic energy and entropy production in response to extreme heat and freeze events at three longleaf pine savanna sites spanning an edaphic moisture gradient (i.e., xeric, intermediate, mesic). The sites also differed in anthropogenic legacy, with soil tillage occurring at the intermediate site, resulting in greater woody species abundance in the understory and overstory. We found that energy reserves in this ecosystem were built during low precipitation periods and when temperatures were below 20°C. The mesic site, which had the highest plant functional diversity, exhibited an adaptive capacity to temperature extremes by maintaining low metabolic activity throughout temperature disturbances, while the intermediate and xeric sites started with high metabolic activity that gradually declined by ~15% with prolonged temperature extremes. Response to these temperature extremes was a function of hydrological drivers, as lower water availability reduced energy reserves during cold periods and photosynthetic activity during heatwaves – especially at the intermediate and xeric sites as a result of the high energy demand of oaks in the overstory. As climate change continues to alter weather patterns across the globe, it becomes increasingly important to assess metabolic resilience to greater weather variability as a function of ecosystem structure.

Soil moisture regime and canopy closure structure subalpine understory development during the first three decades following fire

Subalpine coniferous forests are adapted to cycles of fire and successional development, but increasing fire frequency and severity are altering historical stand structure, composition, and plant diversity. For instance, conifer regeneration has become increasingly variable as a result of prolonged aridity following fire, but the potential cascading effects on understory community development remain virtually unexplored. We used a natural experiment to investigate the relationships among understory plant succession, soil moisture regime, and variable tree canopy development over 30 years following the 1988 fires in the Greater Yellowstone Ecosystem, U.S.A. In 1990, at each of two study areas, plots were established in different site types defined by burn status (burned or unburned) and soil moisture regime (mesic or xeric), determined using biophysical indicators. We asked: (1) Are differences in soil moisture regime associated with differences in understory community composition (species richness, diversity, and functional group representation) during the first three decades after fire? (2) Does the relationship between soil moisture regime and community composition change over time as subcanopy (height >137 cm) tree densities increase? We confirmed our original designations of soil moisture regime using in situ measurements of soil moisture. Over the first decade of succession, species richness and diversity were lower in xeric-burned than in mesic-burned plots. Nearly 30 years after fire at the south-aspect study area, where subcanopy tree densities were low, seral understories diverged by soil moisture regime. There, graminoid cover was 23-fold higher, and forb cover was 3-fold lower, in xeric-burned than in mesic-burned plots. In contrast, in the mixed aspect study area, subcanopy tree densities were markedly higher. There, the understory did not vary by soil moisture regime, converging successionally and becoming more similar to unburned forest communities. Our results suggest that site-specific soil moisture regimes structure the early trajectory of post-fire understory recovery, but the relationship diminishes as tree canopies develop over time. However, under a warming climate, variable tree canopy development may compound increasing aridity, particularly on xeric sites, resulting in decreases in plant diversity and forb cover, increases in graminoid cover, and the potential for altered successional dynamics.

Frameworks on Patterns of Grasslands’ Sensitivity to Forecast Extreme Drought

Climate models have predicted the future occurrence of extreme drought (ED). The management, conservation, or restoration of grasslands following ED requires a robust prior knowledge of the patterns and mechanisms of sensitivity—declining rate of ecosystem functions due to ED. Yet, the global-scale pattern of grasslands’ sensitivity to any ED event remains unresolved. Here, frameworks were built to predict the sensitivity patterns of above-ground net primary productivity (ANPP) spanning the global precipitation gradient under ED. The frameworks particularly present three sensitivity patterns that could manipulate (weaken, strengthen, or erode) the orthodox positive precipitation–productivity relationship which exists under non-drought (ambient) condition. First, the slope of the relationship could become steeper via higher sensitivity at xeric sites than mesic and hydric ones. Second, if the sensitivity emerges highest in hydric, followed by mesic, then xeric, a weakened slope, flat line, or negative slope would emerge. Lastly, if the sensitivity emerges unexpectedly similar across the precipitation gradient, the slope of the relationship would remain similar to that of the ambient condition. Overall, the frameworks provide background knowledge on possible differences or similarities in responses of grasslands to forecast ED, and could stimulate increase in conduct of experiments to unravel the impacts of ED on grasslands. More importantly, the frameworks indicate the need for reconciliation of conflicting hypotheses of grasslands’ sensitivity to ED through global-scale experiments.

High resilience, but low viability, of pine plantations in the face of a shift towards a drier climate

Plantations of Mediterranean pines will face warmer and drier climate conditions during the 21st century. However, we lack field assessments of how severe droughts could threaten the viability of many of these plantations, often located at xeric sites and lacking adaptive management measures such as thinning. In this work, we related radial-growth variability to recent defoliation, to evaluate the resilience and vulnerability of Maritime pine (Pinus pinaster) plantations from xeric sites in south-eastern Spain to three recent droughts (1995, 2005, and 2012). Tree diameter and growth resistance after the most severe drought, which occurred in 1995, were positively related to growth. In addition, the current defoliation level, a proxy for tree vigor, was related to growth, decreasing as tree-to-tree competition increased. These findings indicate that increasing drought will reduce growth and increase the needle loss and mortality rate across many dense pine plantations in the Mediterranean Basin, suggesting that they are more prone to die. Thus, limited resilience of plantations may explain their higher defoliation and indicate that future severe droughts could lead to widespread dieback and high mortality at marginal sites. Therefore, if effective management is not implemented, warmer and drier conditions will likely threaten the viability of many of these drought-prone plantations and their ability to deliver key ecosystem services.

Effects of canopy cover and neighboring vegetation on the early development of planted Austrocedrus chilensis and Nothofagus dombeyi in north Patagonian degraded forests

Austrocedrus chilensis and Nothofagus dombeyi, are endemic forest species of ecological and productive importance in the Andean-Patagonian region of Argentina. Both species grow in pure or mixed stands. In the last decades, several A. chilensis stands have been degraded by different disturbances that have compromised their natural regeneration. Plantation of seedlings of both species may be an alternative to help restore degraded A. chilensis stands. In this study we determined, in degraded A. chilensis stands grown in a xeric and a mesic site, the effects of canopy cover, the presence/absence of neighboring herbaceous vegetation and the interaction between these factors, on the performance (i.e. survival and growth) of planted N. dombeyi and A. chilensis seedlings, during three growing seasons. Results showed that in both xeric and mesic sites, survival and growth of either species were reduced when canopy cover was below 30%. Removal of neighboring vegetation improved survival of both species at the xeric site, and improved growth of both species at the mesic site. Canopy cover and removal of neighboring vegetation seem to interact by improving survival and growth of A. chilensis, while for N. dombeyi the effects of these factors, when present, are independent, both improving survival and growth. Austrocedrus chilensis has greater capacity than N. dombeyi to tolerate the high radiation and low soil moisture combination, so this species is more recommendable for planting at most exposed conditions (no canopy cover at xeric sites), but both species could be successfully used for restoring degraded A. chilensis stands.

Plant profit maximization improves predictions of European forest responses to drought.

Knowledge of how water stress impacts the carbon and water cycles is a key uncertainty in terrestrial biosphere models. We tested a new profit maximization model, where photosynthetic uptake of CO2 is optimally traded against plant hydraulic function, as an alternative to the empirical functions commonly used in models to regulate gas exchange during periods of water stress. We conducted a multi-site evaluation of this model at the ecosystem scale, before and during major droughts in Europe. Additionally, we asked whether the maximum hydraulic conductance in the soil-plant continuum kmax (a key model parameter which is not commonly measured) could be predicted from long-term site climate. Compared with a control model with an empirical soil moisture function, the profit maximization model improved the simulation of evapotranspiration during the growing season, reducing the normalized mean square error by c. 63%, across mesic and xeric sites. We also showed that kmax could be estimated from long-term climate, with improvements in the simulation of evapotranspiration at eight out of the 10 forest sites during drought. Although the generalization of this approach is contingent upon determining kmax , it presents a mechanistic trait-based alternative to regulate canopy gas exchange in global models.

Incorporating a model for ground lichens into multi-functional forest planning for boreal forests in Finland

The quantitative assessment of forest management impacts on ecosystem services calls for ecological models to be linked to forest planning systems. In these systems, such models are expected to produce valid and accurate predictions to quantify the effects of alternative forest management strategies on ecosystem services. As an example, we present an evaluation of the model for the percent cover of ground lichens in forests on mineral soils in Finland. First, the performance of the lichen cover model fitted to the data of forest vegetation surveyed in 1985–1986 was comprehensively validated with the data resurveyed on the same plots in 1995. The differences between the observed and predicted values in lichen cover were calculated and analysed. Second, stand-level analyses were conducted to validate the predicted development of lichen cover in accordance with the rotation length development and management of forest stands. Third, the impacts of forest management scenarios on the average lichen cover at regional level were calculated to validate the results over larger areas. Lichen cover was predicted by site fertility, growing stock, forest harvests, soil preparation and geographic location. Our evaluation suggested that the model logically predicted the 10-year change in lichen cover at the stand level. A negative bias was found in predictions especially on xeric sites in northern Finland, although this was not related to stand management, but probably to global greening and reindeer grazing. The model based on the cross-sectional vegetation survey data showed its ability to describe the long-term changes in lichen cover along with the development and management of stands in different site fertility classes. On xeric and barren sites, the analyses showed decreasing trends in the average lichen cover at the regional level during the 30-year simulation period. However, the development of the average lichen cover varied only slightly among the alternative forest management scenarios. This study provides a modelling approach to evaluate the impacts of forest management on ecosystem services, here the cover of ground lichens, as an indicator of forest biodiversity and quality of reindeer pastures.