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Forest dynamics are driven by micro‐disturbances leading to gap formation. Root rot pathogens can cause mortality to adult trees, which die forming gaps. However, little is known about the biotic and abiotic factors that govern gap creation and expansion such as tree infection or drought resilience, and what are the consequences for nutrient cycling and soil microbiota.We studied the dynamics of gaps created by Heterobasidion abietinum in mountain silver fir (Abies alba) forests from the Spanish Central Pyrenees. Tree‐ring width information was used to evaluate growth patterns of trees located within and at the edge of gaps and under closed canopy conditions. We analysed soil nutrient content and microbial structure at different positions with respect to the gap. Soil fungal community was also characterized by sequencing the ITS2 region.Gaps created by H. abietinum ranged 582–1072 m2 and gathered large amounts of standing and laying dead wood biomass amounting to up to 500 m3 ha−1 of coarse woody debris. This indicates that tree mortality had been occurring over decades, and it was preceded by a long‐term growth decline and impaired drought resilience. Root rot affected trees beyond the current limit of the gaps leading to growth decline, preventing canopy closure and allowing the release of suppressed trees within the gap. Gap formation did neither affect soil fertility nor fungal and bacterial biomass and diversity, but long‐distance ectomycorrhizal fungi were more abundant within gaps.Synthesis. Root rot pathogens play key ecological roles in forest dynamics at fine spatial scale. Heterobasidion abietinum infection, and its interaction with drought, creates and expands gaps by killing large trees and preventing surrounding trees from closing the gap. The survival of mycorrhizal networks to gap formation suggests little disruption of soil fungal communities. [ABSTRACT FROM AUTHOR]

Anticipating future impacts of climate warming and aridification on drylands requires understanding how coexisting woody plant species respond to climate variability. However, we lack knowledge of the growth resilience capacity of Mediterranean shrubs. Do coexisting trees and shrubs differ in their response to climate? Do coexisting shrub species have comparable post‐drought growth resilience?This study was conducted in two Mediterranean shrublands with sparse trees in semi‐arid north‐eastern Spain. We selected sites situated in formerly agricultural or grassland areas in two regions subjected to semi‐arid Mediterranean climate conditions. We sampled six shrubs' species (Juniperus phoenicea, Juniperus oxycedrus, Pistacia lentiscus, Pistacia terebinthus, Rhamnus lycioides and Rhamnus alaternus) and one tree species (Pinus halepensis) to measure their radial growth and to reconstruct their past growth patterns using dendrochronology. We quantified climate–growth relationships of trees and shrubs, as well as the growth resilience capacity after drought events of the six shrubs.Growth patterns differed between species but a prevalent trade‐off between growth and longevity was found. Growth responses to climate were comparable between species but differed between sites. Most species responded positively to precipitation and negatively to temperature. The most negative correlations of growth with drought severity were found at the driest Valcuerna site. Shrubs differed in their resilience capacity. Across sites, species were more resistant and resilient in Alcubierre, the wettest site. The shrubs P. terebinthus and R. lycioides were more resistant and resilient than the rest of species, which required longer growth recovery times and accumulated stronger growth reductions, particularly J. phoenicea.Synthesis. These results highlight the importance of drought as a driver of growth in Mediterranean scrublands. Growth resilience strategies differed between species with junipers being less resilient to drought than other coexisting shrubs. Further research should investigate how this difference in post‐drought resilience is related to functional traits, particularly those related with plant water‐use strategies. [ABSTRACT FROM AUTHOR]

Given the context of significant global warming and the intensification of extreme climate events in the last century, large‐scale reforestation and afforestation have been recognized as effective strategies to mitigate the climate crisis. Since the 1970s, China has launched several afforestation programs aimed at regional ecological protection, playing an important role in reaching carbon neutrality by 2060. This study provided a detailed analysis of the growth suitability of the main planted conifers (Pinus sylvestris var. mongolica and Pinus tabulaeformis) and broadleaves (Populus spp., Robinia pseudoacacia) in the semi‐arid northern China. We compared the radial growth trends of plantations and their responses to extreme droughts from 1980 to 2018. Growth of most plantations has significantly increased over time, but broadleaves showed recent growth reductions in the past decade, which may be related to tree age and reduced soil moisture. Nevertheless, under warmer climate scenarios, the growth of plantations is forecasted to continue increasing. Broadleaves showed a better post‐drought recovery, probably linked to their anisohydric behavior, than conifers, which presented a better resistance to drought. Growth of conifers depended more on warmer temperature and better precipitation conditions during the growing season, whereas broadleaves mainly reacted to warm temperature. Additionally, pre‐drought growth levels weakened resilience components, while post‐drought precipitation compensated for drought‐induced growth deficit. Growth and resilience were negatively related to tree age, while higher stand density reduced growth. This assessment and projections of growth and drought resilience indicate the sustainability of most plantations in semi‐arid regions, but future warmer and drier conditions may lead to an uncertain future regarding forest health and reduce their carbon sink potential. [ABSTRACT FROM AUTHOR]

Increased winter and early spring temperatures due to climate change can enhance forest productivity due to earlier growth onset in temperate regions. However, drought‐prone forests can be highly vulnerable to the combined effect of high summer temperatures and water stress. Understanding how water stress and rising temperatures along the growing season control tree growth in co‐occurring species with contrasting drought tolerances is key to project climate change effects on forest dynamics. Here, we evaluated the interactive effect of seasonal temperature and water availability on annual radial growth and climate–growth instability over six decades (1951–2015) in three pine species with contrasting drought tolerances (Pinus pinaster, Pinus nigra, and Pinus sylvestris). For this, we retrospectively observed radial tree growth using dendroecological methods and evaluated seasonal temperature and water availability effects by using linear mixed models. Early growing season temperature and water availability had a positive effect on tree growth, but the positive effect of late season temperature was modulated by water availability. Moving time‐window analyses revealed temporal instability in climate–growth relationships. Since the 1980s, pine species showed a higher growth sensitivity to both seasonal temperatures and annual water availability. Furthermore, growth reductions were more pronounced due to increased summer temperatures and reduced precipitation. Our results were similar for the three studied pine species despite their contrasting tolerance to drought. Overall, climate warming effects on pine growth are contingent upon water availability in Mediterranean continental forests. Synchronization among species, climate–growth instability, and negative growth trends suggests an increased vulnerability to drought of Mediterranean pine species in response to ongoing climate change. [ABSTRACT FROM AUTHOR]

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (d18O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower d18O than ND trees. The uncoupling between iWUE and d18O, together with the positive correlations between P and K leaf concentrations and d18O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies. [ABSTRACT FROM AUTHOR]

Tree‐ring data has been widely used to inform about tree growth responses to drought at the individual scale, but less is known about how tree growth sensitivity to drought scales up driving changes in forest dynamics. Here, we related tree‐ring growth chronologies and stand‐level forest changes in basal area from two independent data sets to test if tree‐ring responses to drought match stand forest dynamics (stand basal area growth, ingrowth, and mortality). We assessed if tree growth and changes in forest basal area covary as a function of spatial scale and tree taxa (gymnosperm or angiosperm). To this end, we compared a tree‐ring network with stand data from the Spanish National Forest Inventory. We focused on the cumulative impact of drought on tree growth and demography in the period 1981–2005. Drought years were identified by the Standardized Precipitation Evapotranspiration Index, and their impacts on tree growth by quantifying tree‐ring width reductions. We hypothesized that forests with greater drought impacts on tree growth will also show reduced stand basal area growth and ingrowth and enhanced mortality. This is expected to occur in forests dominated by gymnosperms on drought‐prone regions. Cumulative growth reductions during dry years were higher in forests dominated by gymnosperms and presented a greater magnitude and spatial autocorrelation than for angiosperms. Cumulative drought‐induced tree growth reductions and changes in forest basal area were related, but initial stand density and basal area were the main factors driving changes in basal area. In drought‐prone gymnosperm forests, we observed that sites with greater growth reductions had lower stand basal area growth and greater mortality. Consequently, stand basal area, forest growth, and ingrowth in regions with large drought impacts was significantly lower than in regions less impacted by drought. Tree growth sensitivity to drought can be used as a predictor of gymnosperm demographic rates in terms of stand basal area growth and ingrowth at regional scales, but further studies may try to disentangle how initial stand density modulates such relationships. Drought‐induced growth reductions and their cumulative impacts have strong potential to be used as early‐warning indicators of regional forest vulnerability. [ABSTRACT FROM AUTHOR]

Tree species display a wide variety of water-use strategies, growth rates and capacity to tolerate drought. However, if we want to forecast species capacity to cope with increasing aridity and drought, we need to identify which measurable traits confer resilience to drought across species. Here, we use a global tree ring network (65 species; 1931 site series of ring-width indices--RWI) to evaluate the relationship of long-term growth-drought sensitivity (RWI-SPEI drought index relationship) and short-term growth response to extreme drought episodes (resistance, recovery and resilience indices) with functional traits related to leaf, wood and hydraulic properties. Furthermore, we assess the influence of climate (temperature, precipitation and climatic water deficit) on these trait-growth relationships. We found a close correspondence between the long-term relationship between RWI and SPEI and resistance and recovery of tree growth to severe drought episodes. Species displaying a stronger RWI-SPEI relationship to drought and low resistance and high recovery to extreme drought episodes tended to have a higher wood density (WD) and more negative leaf minimum water potential (Ψmin). Such associations were largely maintained when accounting for direct climate effects. Our results indicate that, at a cross-species level and global scale, wood and hydraulic functional traits explain species' growth responses to drought at short-and long-term scales. These trait-growth response relationships can improve our understanding of the cross-species capacity to withstand climate change and inform models to better predict drought effects on forest ecosystem dynamics. [ABSTRACT FROM AUTHOR]

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Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree‐ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature–growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature–growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature–growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming. [ABSTRACT FROM AUTHOR]

Climate extremes, such as abnormally dry and wet conditions, generate abrupt shifts in tree growth, a situation which is expected to increase under predicted climate conditions. Thus, it is crucial to understand factors determining short‐ and long‐term tree performance in response to higher frequency and intensity of climate extremes.We evaluated how three successive droughts and wet years influenced short‐ and long‐term growth of six dominant Iberian tree species. Within species variation in growth response to repeated dry and wet years was evaluated as a function of individual traits related to resource and water use (diameter at breast height [DBH], wood density [WD] and specific leaf area [SLA]) and tree‐to‐tree competition across climatically contrasted populations. Furthermore, we assessed how short‐term accumulated impacts of the repeated dry and wet years influenced long‐term growth performance.All species showed strong short‐term growth decreases and enhancements due to repeated dry and wet years. However, patterns of accumulated growth decreases (AcGD) and enhancements (AcGE) across climatically contrasting populations were species‐specific. Furthermore, individual trait data were weakly associated with either AcGD or AcGE and the few relevant associations were found for conifers. Intraspecific variations in tree growth responses to repeated climates extremes were large, and not explained by intraspecific variability in SLA and WD. Accumulated impacts of repeated dry and wet years were related to long‐term growth trends, showing how the recurrence of climate extremes can determine growth trajectories. The relationships of AcGD and AcGE with long‐term growth trends were more common in conifers species.Synthesis. Repeated climate extremes do not only cause short‐term growth reductions and enhancements but also determine long‐term tree growth trajectories. This result shows how repeated droughts can lead to growth decline. Conifers were more susceptible to the accumulated effects of extreme weather events indicating that in the future, more intense and frequent climate extremes will alter growth performance in forests dominated by these species. [ABSTRACT FROM AUTHOR]

The negative impacts of drought on forest growth and productivity last for several years generating legacies, although the factors that determine why such legacies vary across sites and tree species remain unclear.We used an extensive network of tree‐ring width (RWI, ring‐width index) records of 16 tree species from 567 forests, and high‐resolution climate and normalized difference vegetation index (NDVI) datasets across Spain during the common period 1982‒2008 to test the hypothesis that climate conditions and growth features modulate legacy effects of drought on forests. Legacy effects of drought were calculated as the differences between detrended‐only RWI and NDVI series (i.e. after removing long‐term growth trends) and pre‐whitened RWI and NDVI series predicted by a model including drought intensity. Superposed Epoch Analysis (SEA) was used to estimate whether legacy effects differed from random. Finally, legacy effects were related to water balance, growth persistence and variability, and tree species identity.We found a widespread occurrence of drought legacy effects on both RWI and NDVI, but they were seldom significant. According to SEA, first‐year drought legacies were negative and different from random in 9% and 5% of the RWI and NDVI series respectively. The number of significant second‐ and third‐year legacies was substantially lower. Differences between RWI and NDVI legacies indicate that canopy greenness and radial growth responses to drought are decoupled. We found variations in legacies between tree species with gymnosperms presenting larger first‐year drought legacies than angiosperms, which were exposed to less severe droughts. Greater growth variability can explain the presence of first‐year RWI legacies in gymnosperms from dry sites despite that the relationship between growth variability and legacies was complex.Synthesis. Accounting for species and site responses to drought provides a better understanding of the magnitude and duration of drought legacies on forest growth and productivity. Despite the widespread occurrence of growth reductions in the years during and after drought occurrence, significant legacies were not very common, mostly lasted one year, and were more widespread in gymnosperms. These are relevant factors to be considered in the future when studying the consequences of drought on forest productivity and tree growth. [ABSTRACT FROM AUTHOR]

Climate and forest structure are considered major drivers of forest demography and productivity. However, recent evidence suggests that the relationships between climate and tree growth are generally non‐stationary (i.e. non‐time stable), and it remains uncertain whether the relationships between climate, forest structure, demography and productivity are stationary or are being altered by recent climatic and structural changes. Here we analysed three surveys from the Spanish Forest Inventory covering c. 30 years of information and we applied mixed and structural equation models to assess temporal trends in forest structure (stand density, basal area, tree size and tree size inequality), forest demography (ingrowth, growth and mortality) and above‐ground forest productivity. We also quantified whether the interactive effects of climate and forest structure on forest demography and above‐ground forest productivity were stationary over two consecutive time periods. Since the 1980s, density, basal area and tree size increased in Iberian forests, and tree size inequality decreased. In addition, we observed reductions in ingrowth and growth, and increases in mortality. Initial forest structure and water availability mainly modulated the temporal trends in forest structure and demography. The magnitude and direction of the interactive effects of climate and forest structure on forest demography changed over the two time periods analysed indicating non‐stationary relationships between climate, forest structure and demography. Above‐ground forest productivity increased due to a positive balance between ingrowth, growth and mortality. Despite increasing productivity over time, we observed an aggravation of the negative effects of climate change and increased competition on forest demography, reducing ingrowth and growth, and increasing mortality. Interestingly, our results suggest that the negative effects of climate change on forest demography could be ameliorated through forest management, which has profound implications for forest adaptation to climate change. [ABSTRACT FROM AUTHOR]

Aims: Rising temperature and declining summer precipitation due to the 1970s‐climate shift in southern South America have reduced forest productivity at dry sites. Here, we worked with the most widespread Southern Hemisphere tree line species, Nothofagus pumilio, across contrasting climatic conditions and determined whether rising atmospheric CO2 concentrations as well as warmer and drier climatic conditions provoked by the 70s‐climatic shift have been causing systematic changes in tree line growth rates and intrinsic water‐use efficiency (iWUE). Location: 36–54°S, southern Andes. Time period: 1950–2010. Major taxa studied: Nothofagus pumilio. Methods: We worked at five disparate climatic tree line locations, spanning 18 degrees of latitude; at each location, we sampled trees at four different elevations, including tree line elevation. We quantified the variation in annual tree‐ring width (TRW) as a function of climate, elevation, tree age, size, annual CO2 concentrations and location, using linear mixed‐effects models and interpreted TRW trends in relation to iWUE and isotope (δ13C and δ18O) signalling. Results: Across locations, the patterns of tree line growth occurring in the 1980–2010 period exhibited a clear and significant negative trend, in contrast to the previous 1950–1980 period. We found an increase in iWUE and δ18O across time and locations. Given that an increase in δ18O indicates a decrease in stomatal conductance, we assert that drought‐induced stomatal closure appears to be causing the reduction in growth. Main conclusions: We show unequivocal evidence that warmer and drier summer conditions translated into a decrease in growth rates along the elevational tree line of the southern Andes, reverting previous growth improvements linked to climate warming. An improvement in iWUE at all locations is most likely explained by decreased stomatal conductance given the rising δ18O signal. An iWUE–growth decoupling may act as an ecological strategy to respond to drought. [ABSTRACT FROM AUTHOR]

Biotic stressors such as hemiparasites have a profound impact on forest functioning. However, predicting the future incidence of these stressors on forests remains challenging because climate‐based distribution does not consider tree–hemiparasite interactions or the impacts of extreme climate events on stressors' performance.We use species distribution models (SDMs) and ecophysiological and demographic field data to assess whether climatic suitability is a proxy for the performance of the hemiparasite mistletoe (Viscum album) in two forests with contrasting climatic conditions. Two host tree species representing wet‐cold (Scots pine) and dry‐warm (Aleppo pine) conditions were selected. We fitted SDMs based on climate variables, and measured different ecophysiological variables capturing cold‐ (photoinhibition) and drought tolerance (intrinsic water‐use efficiency, iWUE). We also assessed demographic variables related to seed germination and establishment rates of mistletoe through a translocation experiment.Species distribution models showed a high climatic suitability of mistletoe in both forests. Mistletoes living in the Scots pines site presented a higher cold tolerance, while those inhabiting the Aleppo pine site showed a higher iWUE. Seedlings coming from local seeds showed a lower mortality than seedlings coming from translocated seeds. Germination and seedling establishment showed temporal mismatches when comparing local and translocated seeds.Synthesis. Habitat suitability predicted by SDMs based on climate data and field performance were related in this mistletoe species. However, ecophysiological and demographic variables indicated a lower fitness of mistletoe in the dry‐warm site associated with drought stress. In conclusion, predicted climate suitability based on SDMs forecasts should be refined using field data on actual performance and considering plant‐to‐plant interactions and extreme climate events. Habitat suitability predicted by species distribution models based on climate data and field performance were related in this mistletoe species. However, ecophysiological and demographic variables indicated a lower fitness of mistletoe in the dry‐warm site associated with drought stress. In conclusion, predicted climate suitability based on species distribution models forecasts should be refined using field data on actual performance and considering plant‐to‐plant interactions and extreme climate events. [ABSTRACT FROM AUTHOR]

Abstract: Forecasted increase drought frequency and severity may drive worldwide declines in forest productivity. Species‐level responses to a drier world are likely to be influenced by their functional traits. Here, we analyse forest resilience to drought using an extensive network of tree‐ring width data and satellite imagery. We compiled proxies of forest growth and productivity (TRWi, absolutely dated ring‐width indices; NDVI, Normalized Difference Vegetation Index) for 11 tree species and 502 forests in Spain corresponding to Mediterranean, temperate, and continental biomes. Four different components of forest resilience to drought were calculated based on TRWi and NDVI data before, during, and after four major droughts (1986, 1994–1995, 1999, and 2005), and pointed out that TRWi data were more sensitive metrics of forest resilience to drought than NDVI data. Resilience was related to both drought severity and forest composition. Evergreen gymnosperms dominating semi‐arid Mediterranean forests showed the lowest resistance to drought, but higher recovery than deciduous angiosperms dominating humid temperate forests. Moreover, semi‐arid gymnosperm forests presented a negative temporal trend in the resistance to drought, but this pattern was absent in continental and temperate forests. Although gymnosperms in dry Mediterranean forests showed a faster recovery after drought, their recovery potential could be constrained if droughts become more frequent. Conversely, angiosperms and gymnosperms inhabiting temperate and continental sites might have problems to recover after more intense droughts since they resist drought but are less able to recover afterwards. [ABSTRACT FROM AUTHOR]

Abstract: How the patterns of niche occupancy vary from species‐poor to species‐rich communities is a fundamental question in ecology that has a central bearing on the processes that drive patterns of biodiversity. As species richness increases, habitat filtering should constrain the expansion of total niche volume, while limiting similarity should restrict the degree of niche overlap between species. Here, by explicitly incorporating intraspecific trait variability, we investigate the relationship between functional niche occupancy and species richness at the global scale. We assembled 21 datasets worldwide, spanning tropical to temperate biomes and consisting of 313 plant communities representing different growth forms. We quantified three key niche occupancy components (the total functional volume, the functional overlap between species and the average functional volume per species) for each community, related each component to species richness, and compared each component to the null expectations. As species richness increased, communities were more functionally diverse (an increase in total functional volume), and species overlapped more within the community (an increase in functional overlap) but did not more finely divide the functional space (no decline in average functional volume). Null model analyses provided evidence for habitat filtering (smaller total functional volume than expectation), but not for limiting similarity (larger functional overlap and larger average functional volume than expectation) as a process driving the pattern of functional niche occupancy. Synthesis. Habitat filtering is a widespread process driving the pattern of functional niche occupancy across plant communities and coexisting species tend to be more functionally similar rather than more functionally specialized. Our results indicate that including intraspecific trait variability will contribute to a better understanding of the processes driving patterns of functional niche occupancy. [ABSTRACT FROM AUTHOR]

Abstract: Aim: We investigate the effects of the environmental and geographical processes driving growth resilience and recovery in response to drought in Mediterranean Pinus pinaster forests. We explicitly consider how intraspecific variability modulates growth resilience to drought. Location: Western Mediterranean basin. Methods: We analysed tree rings from a large network of 48 forests (836 trees) encompassing wide ecological and climatic gradients, including six provenances. To characterize the major constraints of P. pinaster growth under extremely dry conditions, we simulated growth responses to temperature and soil moisture using a process‐based growth model coupled with the quantification of climate–growth relationships. Then, we related growth–resilience indices to provenance and site variables considering different drought events. Results: Pinus pinaster displayed strong variation in growth resilience across its distributional range, but common patterns were found within each provenance. Post‐drought resilience increased with elevation and drier conditions but decreased with spring precipitation. Trees from dry sites were less resistant to drought but recovered faster than trees from wet sites. Main conclusions: Resilience strategies differed among tree provenances: wet forests showed higher growth resistance to drought, while dry forests presented faster growth recovery, suggesting different impacts of climate warming on forest productivity. We detected geographically structured resilience patterns corresponding to different provenances, confirming high intraspecific variability in response to drought. This information should be included in species distribution models to simulate forest responses to climate warming and forecasted aridification. [ABSTRACT FROM AUTHOR]

Abstract: Questions: What are the differences in tree growth and resilience in response to extreme droughts between three co‐existing pine tree species? Are growth–resilience components more influenced by intrinsic (age, tree height) or extrinsic factors (mass effects or functional diversity of the neighbourhood)? Is the tree's vulnerability to drought buffered by the functional diversity of the surrounding plant community? Location: Mediterranean forest, northeast Spain. Methods: Trees from three co‐existing pine species with different drought tolerances (Aleppo pine – Pinus halepensis Mill.; Black pine – Pinus nigra subsp. salzmannii (Dunal) Franco; Scots pine – Pinus sylvestris L.) were sampled. The average age of the studed pines was ~30 years. Radial growth from 1980 to 2015 was measured and the pines’ responses to the most severe recent droughts (2005 and 2012) were studied by relating different growth resilience components (Rt, resistance; Rc, recovery; RRs, relative resilience) with intrinsic (age and height) and extrinsic factors of the neighbouring community around each tree. The considered extrinsic factors were CWM and niche complementarity of tree (height), leaf (specific leaf area, leaf dry matter content) and wood (wood specific gravity) traits. The way in which these factors affected growth resilience to drought was assessed. Results: For the whole study period we found lower growth of Scots pine than Aleppo pine, but similar resilience components (except for higher Rt in 2005 for the former species). Despite each drought year resulting in distinct responses, the growth resilience to drought was mostly dependent on tree age for Aleppo pine, while functional diversity modulated a high percentage of the resilience variance of Scots pine. Black pine had an intermediate strategy. CWM explained, in general, less variance of the resilience components than functional diversity. Conclusions: The positive effect of surrounding functional diversity on Scots pine could be related to a greater resistance to drought, likely due to improved resource acquisition favouring its performance at the dry limit of its distribution area. Intrinsic tree features, mass effects and functional diversity differently shape growth resilience to different droughts for co‐existing tree species. [ABSTRACT FROM AUTHOR]

Climate warming is expected to enhance productivity and growth of woody plants, particularly in temperature-limited environments at the northernmost or uppermost limits of their distribution. However, this warming is spatially uneven and temporally variable, and the rise in temperatures differently affects biomes and growth forms. Here, applying a dendroecological approach with generalized additive mixed models, we analysed how the growth of shrubby junipers and coexisting trees (larch and pine species) responds to rising temperatures along a 5000-km latitudinal range including sites from the Polar, Alpine to the Mediterranean biomes. We hypothesize that, being more coupled to ground microclimate, junipers will be less influenced by atmospheric conditions and will less respond to the post-1950 climate warming than coexisting standing trees. Unexpectedly, shrub and tree growth forms revealed divergent growth trends in all the three biomes, with juniper performing better than trees at Mediterranean than at Polar and Alpine sites. The post-1980s decline of tree growth in Mediterranean sites might be induced by drought stress amplified by climate warming and did not affect junipers. We conclude that different but coexisting long-living growth forms can respond differently to the same climate factor and that, even in temperature-limited area, other drivers like the duration of snow cover might locally play a fundamental role on woody plants growth across Europe. [ABSTRACT FROM AUTHOR]

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought-prone areas, tree populations located at the driest and southernmost distribution limits (rear-edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear-edges of the continuous distributions of these tree species. We used tree-ring width data from a network of 110 forests in combination with the process-based Vaganov-Shashkin-Lite growth model and climate-growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO2 emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear-edge. By contrast, growth of high-elevation mountain pine forests is enhanced by climate warming. The emission scenario ( RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of −10.7% and −16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear-edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear-edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions. [ABSTRACT FROM AUTHOR]

Aims Spatial environmental heterogeneity has been considered an important co-existence mechanism because environmental variation enables different species to co-occur. We predict that if functional differences are important for co-existence, then both species and functional diversity should be positively related to environmental heterogeneity. Location Thirty three dry calcareous grassland sites in Estonia. Methods In each site, we established a transect (10.0 m × 0.1 m), consisting of 100 quadrats (10 cm × 10 cm). In each quadrat, we recorded species richness and composition, and measured soil depth, moisture and light availability. We collected data on eight traits from most of the species found across the sites. We calculated functional diversity ( FD) at the quadrat scale, and compared the observed FD to that expected at random using two null models. The first null model used all the species that occurred in the transect in the randomizations to determine environmental filtering from the transect to the quadrat. The second null model restricted the species used in the randomizations to those species whose trait range was within the range of the observed values in the quadrat, to detect evidence for niche partitioning. Quadrat mean trait values and variability in functional composition were then related to small-scale mean environmental conditions and heterogeneity, respectively. Results We found convergent patterns in biomass, specific leaf area, specific root length and clonality compared to randomized communities that included all species occurring in the transect, and found divergence in height and leaf area using the range-restricted null model. Hence, we found patterns consistent with environmental filtering and niche partitioning depending on the traits considered. Quadrat mean traits were significantly correlated with the measured environmental variables, with the main trend being a positive relationship between size-related traits and soil resources, and a negative relationship between these traits and light availability. Trait variability in height, leaf area, leaf dry matter content, specific root length and clonality was positively related to soil depth heterogeneity. There was a trend for species richness to be negatively related to environmental heterogeneity. Conclusions Co-occurring species were more similar in several traits partly due to micro-environmental filtering in response to variability in soil depth. While environmental heterogeneity increased trait variability, negative relationships with species richness were observed. Hence, niche partitioning does not appear to be important for the maintenance of high small-scale species richness in these grasslands. [ABSTRACT FROM AUTHOR]

Questions How does fine-scale soil heterogeneity impact on co-occurring species? Which species are advantaged in heterogeneous soils? Location Greenhouse experiment using European grassland species, University of Tartu, Estonia. Methods We grew plant assemblages consisting of 15 species in five soil treatments - comprising three spatially uniform fertility levels (low, medium or high) and two heterogeneous conditions created using checkerboard combinations of low- and high-fertility patches at two spatial scales (6.25 × 6.25 cm or 12.5 × 12.5 cm patches, overall medium fertility). We compared species responses (above-ground biomass) between heterogeneous and homogeneous treatments. Additionally, we compared species responses within low-fertility patches in heterogeneous treatments to the homogeneous treatment of the same fertility. Results Larger, dominant species were advantaged in heterogeneous compared to homogeneous conditions (with the same or lower overall fertility), whereas the growth and survival of smaller, subordinate species was reduced. Larger, dominant species also had increased above-ground biomass within the low-fertility patches in heterogeneous compared to homogeneous low-fertility conditions, but the opposite was true for smaller, subordinate species. In general, species responses in heterogeneous conditions did not differ from the homogeneous high-fertility treatment, although the heterogeneous conditions had lower overall fertility. Conclusions In our experimental grasslands, species differed in their responses to fine-scale soil heterogeneity. Patchy resource distribution directly benefits larger, dominant species that can forage among patches and produce more above-ground biomass compared to conditions where the same amount of resources is distributed homogeneously. Smaller, subordinate species that are more likely confined to a uniform soil patch are disadvantaged by heterogeneity due to increased root and shoot competition from neighbouring species. These species-specific responses to fine-scale soil heterogeneity and altered competitive interactions have important implications for plant community structure and productivity. [ABSTRACT FROM AUTHOR]

Question Fertilization, i.e. nutrient addition, and changes in grazing intensity are important factors associated with global change that threaten grasslands worldwide. How does the plant community assembly in Mediterranean grasslands respond after 11 yr to a single fertilization event? Location Species-rich Mediterranean grassland, Navarra, northern Spain. Methods In 2003, 11 permanent plots were established in a Mediterranean grassland: six plots were fertilized and five were used as controls. Fertilization consisted in the addition of sewage sludge on the soil surface. Grazing was excluded during the course of the experiment. Changes in vegetation abundance were monitored from 2004 to 2014. In 2012, key plant functional traits (height, leaf area, SLA, LDMC) of the most frequent species were collected. We measured taxonomic and functional species composition and diversity. Niche overlap compared to null models was used to assess the mechanisms influencing community assembly. Results Species composition changed markedly in the fertilized plots. These changes were accompanied by a reduction in taxonomic diversity. One yr after starting the experiment there were no differences in functional composition (i.e. CWM trait values) between fertilized and control plots (excluding SLA), but 11 yr later fertilized plots were composed of taller plants with larger SLA and lower LDMC. We found a major reduction in niche overlap in plant height and SLA after fertilizing associated with a replacement in several species that could be indicative of competitive exclusion (principle of limiting similarity). Conversely, we found an increase in niche overlap in leaf area indicating that competition increases similarity among species (weaker competitor exclusion or abiotic filtering). At the end of the experiment, we found large differences in SLA and LDMC between fertilized and control plots. Conclusions A single fertilization event that occurred 11 yr ago triggered a wave of changes in grassland community assembly that are still on-going. Fertilization enhanced the establishment of taller species with larger leaf areas, which displaced, by competitive exclusion, the species that had previously been present. Community assembly patterns in Mediterranean grassland are highly sensitive to abrupt changes in resource availability. [ABSTRACT FROM AUTHOR]

It is expected that extreme climate events such as droughts will increase in both severity and intensity as a consequence of forecasted climate change. Complementarity among tree species in resource acquisition strategy may reduce interspecific competition and increase the occurrence of facilitative interactions, resulting in an improved tree growth and resilience to extreme climatic events. However, the response of individual trees growing in more functionally diverse stands to extreme events is still under debate., We investigate the growth response of silver fir ( Abies alba) to an extreme drought event which occurred in 1985-1986 in the Central Spanish Pyrenees and how the growth and resilience of trees relate to their neighbourhood functional diversity, stand heterogeneity and intraspecific and interspecific competition. Dendroecological methods were used to reconstruct radial growth. We recorded all tree species living around each sampled tree and calculated indices of intra- and interspecific competition among coexisting trees. Functional diversity around each focal tree was calculated by using the Rao's quadratic entropy., Silver fir growth decreased significantly in response to the drought in the vast majority of trees, but most of them showed a growth recovery 3 years later. Despite most trees showed negative growth trends following the drought event (70%), those trees growing in more functionally diverse stands were more resilient, recovered more quickly and displayed greater growth and growth trends., Synthesis. The presented findings support the growing body of evidence that shows a positive impact of diversity on forest function. In addition, we provide a direct linkage between tree growth response to climate at the individual tree scale and a direct quantification of neighbourhood functional diversity and competition. An enhanced functional diversity facilitating a more efficient use of resources at the interspecific level and potentially increasing facilitative interactions contributes to lessening the negative impacts of extreme events as droughts on forest growth. [ABSTRACT FROM AUTHOR]

Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought-induced forest dieback and tree death. Here, we evaluate the relative role played by these two mechanisms in the long-term by quantifying wood-anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water-use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long-lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2-5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long-term capacity of trees to withstand drought-induced dieback. [ABSTRACT FROM AUTHOR]

Aim Forest responses to global-change drivers such as rising atmospheric CO2 concentrations ( Ca), warming temperatures and increased aridification will depend on tree species and site characteristics. We aim to determine if rising Ca enhances growth of coexisting pine species along broad ecological gradients in a drought-prone area. Location Iberian Range, Spain. Methods We sampled 557 trees of five pine species encompassing a wide climatic gradient and measured their radial growth. We used nonlinear flexible statistics (generalized additive mixed models) to characterize growth trends and relate them to Ca, temperature and water balance. Results The sites most responsive to the growing-season water balance were dominated by Pinus pinaster and Pinus nigra at low elevations, whereas those most responsive to temperatures were high-elevation Pinus sylvestris and Pinus uncinata stands. From 1950 onwards, most sites and species showed decreasing radial growth trends. Growth trends were coherent with a CO2-related fertilization effect only in one P. sylvestris site. Main conclusions We found little evidence of growth stimulation of Iberian pine forests due to rising Ca. The results indicated that any positive effect of a Ca-induced growth increase was unlikely to reverse or cancel out the drought-driven trends of reduced growth in most Mediterranean pine forests. Further assessments of CO2-fertilization effects on forest growth should be carried out in sites where climatic stressors such as drought do not override the effects of rising Ca on forest growth. [ABSTRACT FROM AUTHOR]

Aim Climate change is expected to modify growth trends of forests around the world. However, this modification may vary in strength and intensity across a species' biogeographical range. Here, we study European populations of silver fir ( Abies alba) across its southern distribution limits in Spain, Italy and Romania. We hypothesized that growth trends of silver fir will differ across its distribution range, with a marked decline in growth in drought-prone regions near the species' southernmost biogeographical limits. Location Europe (Spain, Italy, Romania). Methods We collected tree-ring data from at least 1300 silver fir trees located in 111 sites. The dataset was used to assess and model growth trends, quantified as changes in basal area increment, and to determine how growth responds to climate. Results We found contrasting patterns of basal area increments among countries and sites. Populations of silver fir located outside the Mediterranean area (e.g. northern Italy, Romania) have shown a clear increase in growth over the last two decades, whereas most populations in Spain and southern Italy have displayed a marked decline in growth since the 1980s. The growth of silver fir forests at the south-western distribution limit is severely constrained by low spring-summer water availability, whereas growth of silver fir forests in non-Mediterranean areas is limited by cold conditions in late winter to early spring. Main conclusions Climate warming is distinctly modifying growth patterns and responses to climate in silver fir across most of the species' European distribution area. In south-western Europe the reduction in growth of many populations is related to an observed increase in aridity, whereas in more temperate areas warming is enhancing growth. Our results confirm a decline in the growth of silver fir at its south-western distribution limits as a consequence of climate warming. [ABSTRACT FROM AUTHOR]

Theory predicts that the postindustrial rise in the concentration of CO2 in the atmosphere ( ca) should enhance tree growth either through a direct fertilization effect or indirectly by improving water use efficiency in dry areas. However, this hypothesis has received little support in cold-limited and subalpine forests where positive growth responses to either rising ca or warmer temperatures are still under debate. In this study, we address this issue by analyzing an extensive dendrochronological network of high-elevation Pinus uncinata forests in Spain (28 sites, 544 trees) encompassing the whole biogeographical extent of the species. We determine if the basal area increment ( BAI) trends are linked to climate warming and increased ca by focusing on region- and age-dependent responses. The largest improvement in BAI over the past six centuries occurred during the last 150 years affecting young trees and being driven by recent warming. Indeed, most studied regions and age classes presented BAI patterns mainly controlled by temperature trends, while growing-season precipitation was only relevant in the driest sites. Growth enhancement was linked to rising ca in mature (151-300 year-old trees) and old-mature trees (301-450 year-old trees) from the wettest sites only. This finding implies that any potential fertilization effect of elevated ca on forest growth is contingent on tree features that vary with ontogeny and it depends on site conditions (for instance water availability). Furthermore, we found widespread growth decline in drought-prone sites probably indicating that the rise in ca did not compensate for the reduction in water availability. Thus, warming-triggered drought stress may become a more important direct driver of growth than rising ca in similar subalpine forests. We argue that broad approaches in biogeographical and temporal terms are required to adequately evaluate any effect of rising ca on forest growth. [ABSTRACT FROM AUTHOR]

1. Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought-induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought-related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir (Abies alba), Scots pine (Pinus sylvestris) and Aleppo pine (Pinus halepensis), to a severe drought event. 2. We compared basal area increment (BAI) trends and responses to climate and drought in declining (very defoliated and dying) vs. non-declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non-structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood-inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death. 3. Declining silver fir and Scots pine trees showed less growth than non-declining trees one to three decades, respectively, before the drought event, whereas Aleppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood-inhabiting fungi were found in post-drought declining trees apart from blue-stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death. 4. Synthesis. Early warning signals of drought-triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for Scots and Aleppo pines. Longer growth and defoliation series, additional vigour parameters and multi-species comparisons are required to understand and predict drought-induced tree death. [ABSTRACT FROM AUTHOR]

If isotherms shift upslope or poleward due to climate warming, treeline trees will be exposed to more or less thermal constraints of growth depending on treeline shift rates. Climate warming is expected to enhance tree growth at alpine treelines. Isotherms may move upslope, but treeline trees may not; our study focused on trees growing at the altitudinal or latitudinal limits of tree existence today regardless of where the treeline will be in the future. [Extracted from the article]

* Premise of the study: Plants respond to the prevailing conditions in the surrounding environment, but since they are dynamic systems this response may vary during their life. Thus, the identification of key aspects for the maintenance of plant populations requires the consideration of plant performance across environmental gradients and along life stages. This study examines how abiotic conditions and biotic interactions and processes determine the spatial distribution of two life-story stages that play a key role in the functioning of a representative population of Carex remota. * Methods: We used structural equation modeling (SEM) to test for direct and indirect influences of abiotic and biotic factors on seedlings and adults of Carex remota. The variables used in the analysis were number of seedlings, cover of adults, soil moisture, leaf litter cover, relative light, and topographic position. * Key results: Population patterns partially depend on direct and indirect effects of abiotic conditions. Whereas adult individuals were only affected by topsoil moisture, seedling emergence was largely affected by multiple environmental conditions. The number of seedlings increased with high topsoil moisture, low leaf-litter values, high light values as well as in low parts of the study area. The importance of adult individuals in determining seedling success is also highlighted: higher abundance provides seed rain in the surroundings and modifies the microenvironmental conditions favoring high seedling establishment. * Conclusions: As hypothesized, adults and seedlings responded to the environmental conditions differently. Seedling emergence was a critical aspect in C. remota performance, and abrupt changes in the environment during this stage may strongly influence population performance. [ABSTRACT FROM AUTHOR]

Niche differences among species are considered key drivers of coexistence (limiting similarity), and increased heterogeneity of resources should promote species diversity through niche partitioning (niche theory). Two predictions follow - co-occurring species should be more different than expected by chance, especially in productive communities due to increased competition, and heterogeneity should increase both species and functional diversity. However, evidence for these predictions in plant communities is limited., We examined niche overlap in functional traits among co-occurring grassland species in relation to fertility and resource heterogeneity to determine whether species are more different than expected and reveal dominant assembly processes. We conducted a mesocosm experiment with five treatments, with three levels of soil nutrient availability (low, medium, high) and scale of resource heterogeneity (at medium fertility, large-scale heterogeneous - patch size 156 cm2, small-scale heterogeneous - patch size 39 cm2)., We measured community weighted means and niche overlap for three functional traits related to resource acquisition and competitive ability (specific leaf area, leaf size and plant height), and compared our observed values to those expected when abundance was randomly allocated to species in the experimental treatments., For leaf size and plant height, we found more niche overlap with increased fertility, and a positive relationship between shoot biomass and niche overlap, whereas the opposite pattern was found for specific leaf area. For all traits, the heterogeneity treatments generally had similar niche overlap values as the high-fertility treatment and differed significantly from the homogeneous treatment of the same overall fertility. Patch type was important in most cases; significant differences were detected in functional composition and niche overlap between the low- and high-fertility patches in the heterogeneity treatments., Increased niche overlap with increased fertility and heterogeneity can be due to both habitat filters restricting the range of trait values and competition - when resources are either homogenously high or high-fertility patches are available. The small-scale distribution of soil resources impacts on community assembly processes and should be incorporated into future studies examining functional diversity, especially when the aim is to determine the influence of productivity gradients. [ABSTRACT FROM AUTHOR]

Question To what extent do small-scale biotic interactions and local variations determine the spatial pattern and plant performance of three Mediterranean subshrubs growing under contrasting climatic conditions? Location Three sites subjected to climatically contrasting conditions in Aragón (Javalambre, cold site; Bernués, mesic site; Monegros, xeric site), NE Spain. Methods The three species studied were Hormathophylla spinosa, Ononis fruticosa and Linum suffruticosum in the cold, mesic and xeric sites, respectively. Two transects (100 × 5 m) were located in each site, one in the upper part and another in the north-oriented slope of a hill. All adult individuals located within each transect of each species were tagged, spatially located and their size measured (height and crown diameter). We also estimated visually the cover of other woody species around each subshrub individual to study inter-specific interactions. We estimated the age of all subshrub individuals by counting annual rings in basal wood sections. To investigate the spatial patterns and plant performance of the three species we used spatial point pattern analyses and structural equation modelling. Results The density of individuals and the spatial pattern of the species varied greatly among and within sites. H. spinosa and L. suffruticosum showed higher variability in density among transects and a more aggregated spatial pattern than O. fruticosa. Similarly, the three subshrubs presented different architecture and performed differently in the two transects studied for each species. All three species presented higher growth rates in the upper part of the hill than in the north-oriented slope. Inter-specific interactions were more important than intra-specific ones in determining the performance of the three subshrubs, and were more intense in the case of H. spinosa and L. suffruticosum than in the case of O. fruticosa. Conclusions Our results demonstrate that the spatial patterns of the three species studied varied within sites as a function of local environmental conditions. Moreover, plant-plant interactions can play an important role in explaining subshrub spatial patterns and performance in Mediterranean ecosystems. Inter- and intra-specific interactions acting at local scales should be considered for understanding the responses of Mediterranean subshrubs to regional climate. [ABSTRACT FROM AUTHOR]

Species richness is influenced both by mechanisms occurring at landscape scales, such as habitat availability, and local-scale processes, that are related to abiotic conditions and plant-plant interactions. However, it is rarely tested to what extent local species richness can be explained by the combined effect of factors measured at multiple spatial scales. In this study, we quantified the simultaneous influence of historical landscape-scale factors (past human population density, and past habitat availability - an index combining area and connectivity) and small-scale environmental conditions (shrub cover, and heterogeneity of light, soil depth, and other soil environmental variables) on plant species richness in dry calcareous grasslands (alvars). By applying structural equation modelling (SEM) we found that both landscape conditions and local environmental factors had significant direct and indirect (i.e. through the modification of another factor), effects on species richness. At the landscape scale, we found a direct positive influence of historical habitat availability on species richness, and indirect positive influence of past human population (via its effects on historical habitat availability). At small scales, we found a positive direct influence of light heterogeneity and shrub cover on species richness. Conversely, we found that small-scale soil environmental heterogeneity, which was mainly determined by soil depth heterogeneity, had a negative effect on species richness. Our study indicates that patterns of species richness in alvar grasslands are positively influenced by the anthropogenic management regime that maintained the landscape habitat conditions in the past. However, the abandonment of management, leading to shrub invasion and increased competition for light resources also influenced species richness. In contrast to the positive heterogeneity-diversity relationship we found that soil heterogeneity reduced species richness. Environmental heterogeneity, occurring at the plant neighbourhood scale (i.e. centimetres), can increase the isolation among suitable soil patches and thus hinder the normal functioning of populations. The combination of previous knowledge of the system with new ecological theories facilitates disentangling how species richness responds to complex relationships among factors operating at multiple scales. [ABSTRACT FROM AUTHOR]

Temperate forests are one of the most important ecosystems in the world, and thus disentangling the factors that drive diversity within these ecosystems is of major concern. However, due to the complex interactions among forests layers, topography and soil factors, discovering the drivers of diversity is often complicated. In this study, we tested three a priori hypotheses about the effect of the dominant competitor ( Pinus koraiensis) on the different forest layers in a 25 ha full mapped plot of temperate forest in the Changbai Mountain of northeastern China. Structural equation modelling (SEM) was used to study the direct and indirect interactions between four vertical forest layers (dominant competitor, canopy composition, sub-canopy diversity and shrub diversity), topographic factors, edaphic factors to discover sub-canopy and shrub diversity drivers. Our results suggest that the dominant competitor ( Pinus koraiensis) is a key factor explaining canopy variation, and sub-canopy diversity and shrub diversity, and that this competitor can act directly (through shading) and indirectly (through the modification of the soil). Topographic heterogeneity also had significant effects on the soil variation and the diversity of the sub-canopy and shrub layers. Finally our results indicate that the influence of canopy composition on the diversity of the rest of forest layers is indirect and positive, suggesting that the dominant competitor is the main factor limiting diversity. In conclusion, we have found strong evidence that the dominant species of the canopy can influence, both directly and indirectly, the diversity of the different vertical forest layers. Patterns of diversity in forests are driven by a multiplicity of factors that are inherently related. [ABSTRACT FROM AUTHOR]