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Questions: Interannual precipitation changes, both in amount and frequency, are expected to increase due to climate change, and these changes could dramatically affect the structure and function of ecosystems. However, how plant life‐forms and dominant species regulate the plant community structure after precipitation changes remains unclear. Location: Gaolan Experiment Station for Ecology and Agriculture Research of the Northwest Eco‐Environment and Resources Research Institute of the Chinese Academy of Sciences, located in the desert grassland of the western Loess Plateau, China. Methods: To investigate the effect of precipitation changes on the growth of communities, life‐forms, and species, we used rain shelters and irrigation to create a variety of precipitation gradients (i.e., −40%, −20%, 0% [= CK, i.e., control treatment], +20%, +40% of ambient precipitation) in desert grasslands from 2014 to 2015. Results: The plant community density, coverage, and height were significantly decreased in the −40% treatment compared to CK. In contrast, species richness, Shannon–Wiener index, and Pielous' evenness did not respond significantly to the precipitation treatment. In addition, at the level of life‐forms, the density of perennial herbaceous plants was significantly increased by the +40% treatment in 2015, while the richness and density of annual herb were significantly inhibited by extreme drought. The effect on the shrub growth index of increasing and decreasing precipitation compared to the CK was insignificant. Furthermore, at the level of species, the growth of dominant species was obviously suppressed by the −40% and +40% treatment in 2015. However, the positive effect of the +40% treatment on the growth of rare species reached significant levels in 2015. Also, the change in precipitation did not have a significant effect on the growth of companion species. Dominant species primarily drove the response of community‐level plant growth to precipitation changes over the three‐year study period. Conclusion: The response of plant community composition to changes in precipitation in desert grassland is largely regulated by changes in the composition of the dominant species. This finding could also provide a better prediction of the effects of extreme drought on desert grassland plant communities in the scenarios of future global climate change. [ABSTRACT FROM AUTHOR]

Questions: Knowledge of how extreme drought events induce plant dieback and, eventually, plant mortality, may improve our forecasting of ecosystem change according to future climate projections, especially in Mediterranean drylands. In them, shrublands are the main vegetation communities in transition areas from a subhumid to semi‐arid climate. This study analyzed differences in plant dieback after an unusual drought in 2014 and identified their main underlying factors in relation to three groups of explanatory variables: water availability, soil properties and vegetation structure attributes. Location: Four Mediterranean shrublands along a climatic gradient in SE Spain. Methods: At each experimental field site, we sampled a pool of vegetation structure characteristics, soil depth and soil surface properties, and we also determined water availability by continuously monitoring soil moisture and the microclimate conditions. Results: The climatic analysis showed that there was an extreme drought event in 2014, which was below the first percentile of the driest years. Under such conditions, vegetation dieback occurred at all the study sites. However, plant dieback differed between sites and plant biotypes. Subshrubs were the main affected biotype, with diebacks close to 60% at the driest sites, and up to 40% dieback for shrubs depending on their vertical development. Relative extractable water and bare soil surface cover were the best explanatory variables of plant community dieback but changed between plant biotypes. Vegetation structure variables related to plant vertical development (leaf area index [LAI], plant height, phytovolume) were significant explanatory variables of plant dieback in shrubs, subshrubs and grasses. Consecutive dry days fitted the best model to explain subshrub dieback. Conclusions: We found that rainfall pattern rather than total annual rainfall was the climatic factor that best determined water availability for plants in Mediterranean drylands. These results also pointed out the relevance of plant structure and soil properties for explaining ecosystem responses to extreme drought. [ABSTRACT FROM AUTHOR]

Questions: Drought timing during the growing season can profoundly influence plant diversity and cover, with subsequent impacts on ecosystem functions and services in terrestrial environments. However, the different responses of plant communities to drought in different periods of the growing season remain uncertain. Location: The research was conducted in a semi‐arid grassland at the Duolun Restoration Ecology Research and Demonstration Station, in Inner Mongolia, China. Methods: The responses of plant species richness and cover to early‐ and late‐season drought with a four‐year field experiment were examined in a semi‐arid grassland in northern China. Soil moisture at different depths was measured to explore the underlying mechanisms of the drought effects on plant richness and cover. Results: Over the four years, late‐ rather than early‐season drought suppressed plant richness. Decreased species richness (−1.65 species) under late‐season drought was mainly due to suppressed perennial forb richness (−1.88 species). Late‐season drought also exerted greater negative effects on plant cover than early‐season drought. Effects of late‐season drought on plant cover depended on annual precipitation. Reduced plant cover (−7.5%) under early‐season drought stemmed from declines in shrubs and semi‐shrubs, particularly Artemisia frigida cover (−4.9%), which is sensitive to water deficits because of its shallow root depth (14.01 cm). Declined plant cover (−39.0%) under late‐season drought was mainly caused by suppressed perennial forb (−21.5%) and shrub and semi‐shrubs cover (−17.4%). The combined effects of early‐ and late‐season drought on plant cover were greater than the treatment alone, likely due to aggravating water deficits under growing‐season drought. Conclusions: Our results suggest late‐season drought is more critical for shifting plant communities. than early‐season drought. Our findings highlight the regulatory role of drought timing on plant community composition and development in arid and semi‐arid regions. [ABSTRACT FROM AUTHOR]

Questions: Regeneration success of semi‐arid and arid regions worldwide is episodic and often linked to large rainfall events in consecutive years. It is also influenced by past and current land use, seed availability and herbivore grazing. This is particularly true for four dominant canopy trees — Allocasuarina luehmannii (Buloke), Casuarina pauper (Belah), Callitris gracilis (Slender Cypress Pine) and Myoporum platycarpum (Sugarwood) — within southeastern Australia semi‐arid woodlands. These species also apply hedge‐betting when responding to resource pulses with mass germination while low‐level background recruitment continues. We asked if drought‐breaking rainfall stimulated a germination pulse of these species and if it did, how did other external factors (grazing pressure from rabbits/hares, seed source, suitable seedbed) influence seedling abundance. We also asked if evidence of continuous recruitment had occurred during 14 years of drought by exploring trends in juvenile abundance. Location: Victorian Mallee semi‐arid woodlands, southeastern Australia. Methods: We recorded seedling and juvenile abundance of the four dominant canopy species at 310 1‐ha sites within semi‐arid woodlands. Seedling presence was explored in relation to rainfall, tree density, ground cover and herbivory. Juvenile presence was explored in relation to rainfall, seedling and tree density, total plant cover and herbivory. Results: Seedling and juvenile abundance were positively correlated with more mature trees, higher rainfall and no herbivory. Juvenile abundance was also positively correlated with more seedlings. There were more seedlings and juveniles at sites where rabbits/hares were present. Conclusions: Above‐average rainfall was positively correlated with initiation of recruitment; however, there was limited net recruitment of the four dominant canopy species indicating other factors are influencing germination (e.g. seed viability) or that seedlings are not surviving. The long‐term survival of these semi‐arid woodlands is in jeopardy without a concerted effort to maintain and improve their condition through revegetation works, weed removal and herbivore management. [ABSTRACT FROM AUTHOR]

Questions: Global climate change is predicted to cause widespread shifts in the distribution and composition of forests, particularly in mountain environments where climate exerts strong controls on tree community arrangement. The upslope movement of vegetation has been observed in association with warming temperatures and is especially evident in ecotones—the transition zones between vegetation types. We explored the role of drought and tree mortality on recent changes in high‐elevation forests. Location: Greater Yellowstone Ecosystem, USA. Methods: We established 19 forest demography plots along an elevational gradient spanning dominant high‐elevation vegetation types. Results: Tree establishment dates indicated the upslope movement of Pinus albicaulis (whitebark pine) treeline and ecotone shift from meadow to forest starting in the 1950s. An expansion of the growing season likely contributed to the upward expansion of the treeline. Comparisons between overstory and understory tree composition suggested ongoing succession in the absence of fire at lower elevations, namely the replacement of Pinus contorta (lodgepole pine) by Abies lasiocarpa (subalpine fir). P. contorta seedlings were distributed at higher elevations than overstory trees of the same species, suggesting some potential for upslope movement with warming conditions; P. albicaulis seedlings, conversely, were distributed throughout all elevations of the transect. Significant tree mortality occurred in Pinus spp. and disproportionately affected P. albicaulis, as a result of a regional Dendroctonus ponderosae (mountain pine beetle) outbreak (2008–2012). Mortality events were strongly associated with drier than average conditions 2–3 years prior to tree death. Conclusion: Rising sensitivity to arid conditions in the mid‐20th century amid already dense, aging forests appears to have increased susceptibility to beetle‐induced mortality during the most recent drought. Tree species in the study area responded individually to global change stressors, which acted on these forests in complex ways and led to both ecotone shifts and stability. This work highlights the interplay between succession, forest disturbances and climate‐related growth responses in driving forest compositional change in subalpine and treeline environments. [ABSTRACT FROM AUTHOR]

Aim: Foliar pH, an emerging plant functional trait, plays a key role in plant ecophysiological processes. However, it remains unclear how foliar pH varies among plants with different preferences concerning soil salinity, and how it responds to drought stress in desert ecosystems. Location: Xinjiang, northwestern China (40.24° N to 46.24° N, 80.66° E to 91.33° E). Methods: We addressed variation in foliar pH in four desert shrub groups (i.e., euhalophytes, secretohalophytes, pseudohalophytes and glycophytes) with differing adaptive strategies to soil salinity in 29 sites in Xinjiang, China, and explored the effects of drought and saline stress on foliar pH variability. Results: We found that average foliar pH of desert shrubs was 5.54 ± 0.02, higher than that of shrubs from humid areas. At the functional group level, euhalophytes (5.65 ± 0.03) and glycophytes (5.61 ± 0.04) had significantly higher foliar pH than secretohalophytes (5.47 ± 0.03) and pseudohalophytes (5.39 ± 0.05). Coverageweighted mean foliar pH of glycophytes-dominated communities was significantly higher (5.80 ± 0.18) than those of the halophytes-dominated communities (5.34 ± 0.07 ~ 5.49 ± 0.07). Only in pseudohalophytes there was a significant relationship between foliar pH and both drought stress (i.e., low soil water stress coefficient, aridity index, and mean annual precipitation) and saline stress (shown by high soil electrical conductivity and soil pH). In contrast, secretohalophytes displayed opposite or non-significant relationships between foliar pH and drought and saline stress. Compared with salinity indicators, drought exerted a much larger impact on foliar pH, particularly in pseudohalophytes and secretohalophytes. Conclusions: Our work documented a comprehensive analysis of foliar pH across halophyte and glycophyte shrub species, and found contrasting responses of foliar pH to drought and salinity in desert ecosystems, which can provide an alternative perspective for understanding the survival strategies of desert plants, and may also offer new insights into the population dynamics of desert shrub communities under global climate change. [ABSTRACT FROM AUTHOR]

Increased drought combined with extreme episodes of heatwaves is triggering severe impacts on vegetation growth, particularly for plant communities in arid and semiarid ecosystems. Although there is an abundance of short‐term field drought experiments in natural ecosystems, remaining knowledge gaps limit the understanding and prediction of vegetation growth to ongoing and future climate scenarios. Here, we assessed the impacts of long‐term (1999–2016) experimental drought (ca. −30% rainfall) on the vegetation growth of a Mediterranean high (H) and low (L)‐canopy forests and an early‐successional shrubland, as indicated by above‐ground biomass increment (ABI) and standing density, respectively. We found habitat context (impact of historical climate change, soil depth and successional status) of the study sites significantly affected the magnitude of climate impacts; there were synergistic effects of experimental drought and meteorological drought (Standardised Precipitation–Evapotranspiration Index, SPEI) as well as extreme dry years on vegetation growth. Long‐term experimental drought decreased the ABI for the two forest canopy types and the standing density for the shrubland. Water availabilities in winter–spring (SPEIs) were positively correlated with the ABI and standing density. Moreover, experimental drought decreased the vegetation growth in extreme dry years for the shrubland. We propose that future work not only study the vegetation dynamics with physiological, phenological and demographical changes in long‐term processes and across climate gradients, but also should explore the changes of multiple functions simultaneously (e.g. multifunctionality) under long‐term processes and extremes. This type of analysis of long‐term data is essential to understand and predict biodiversity loss, composition shifts, declines in ecosystem function and carbon budgets at temporal and spatial scales, to enable policy makers to design and implement strategies for the maintenance of sustainable ecosystem function under future climate change scenarios. [ABSTRACT FROM AUTHOR]

Questions: (a) How did seedling numbers and species composition change in the first year after a wildfire during drought, relative to pre‐fire variation? (b) Has the community returned to pre‐fire composition after five years? (c) Has the degree of dominance by exotic annual grasses changed? (d) Is there any evidence that drought conditions affected community cover, before or after fire? Location: Exotic‐dominated annual grassland in southern California, USA. Methods: We monitored community cover and native annual forb densities for four years before and four (cover) to five (densities) years after an unintentional fire (fall 2013) coinciding with the spring 2012–2019 California drought. We also measured seedling emergence both before and during the first year post‐fire. We assessed post‐fire changes in cover and density relative to pre‐fire variation, and tested correlations between community cover and annual rainfall measures. Results: Seedling emergence declined strongly after fire for exotic grasses, but remained stable for exotic forbs. Seedling densities of the most common native forbs declined, but several previously‐rare natives increased. Community cover initially shifted towards the exotic forbs Erodium spp., then returned to higher exotic grass densities. Yet the previously dominant Bromus diandrus declined steeply, even as other exotic grasses and some native forbs increased. Up to five years after fire, relative cover and abundance of the most common exotic and native species still differed from pre‐fire composition. Common species were uncorrelated with annual precipitation, but several may have responded to shorter growing seasons. Conclusions: Immediate post‐fire conditions favoured exotic and native forbs over grasses, as predicted. Yet in contrast to many previous studies, the community did not return quickly to pre‐fire composition but showed persistent changes that favoured neither natives nor exotics. Our results suggest post‐fire recovery in this habitat may be contingent on abiotic conditions, with drought one potential explanation for changes. [ABSTRACT FROM AUTHOR]

Questions: What are the effects of extreme drought on above‐ground and below‐ground net primary production (ANPP and BNPP, respectively) of an annual plant community in a Mediterranean shrubland? How do annual plant communities in Mediterranean shrublands respond to extreme drought, in terms of changes in their species composition, species richness, and species diversity? Location: A Mediterranean shrubland in the Judean Hills, Israel. Method: Rainout shelters were used to simulate a 66% reduction in annual rainfall, preceded by a baseline year when no treatments were applied. Above‐ground and below‐ground (i.e., root) biomass was harvested and species surveys were performed following one growing season. Results: Above‐ground primary production was significantly reduced by drought, while below‐ground production remained mostly stable. Consequently, the shoot:root ratio was reduced by drought, indicating either increased community resource allocation to root production at the expense of shoots or stable root biomass production in the face of drought. Community diversity increased in droughted plots, and significant changes were detected in species composition and dominance patterns. Conclusions: Extreme drought had a profound effect at the ecosystem and community level on the studied Mediterranean shrubland. Extreme drought seems to increase species diversity and evenness in the short term, likely due to release from competition. We predict this effect to be countered by low biomass production and individual species abundance if extreme drought persists a sufficiently long time, leading to significant changes in long‐term plant dominance patterns and community structure. [ABSTRACT FROM AUTHOR]

Questions: How have tree growth and growth trends varied in structurally complex old‐growth forests? What determines this variation and how this differs among species? Location: Natural boreal old‐growth forests, northern Finland. Methods: We sampled trees over 10‐cm diameter on 48 randomly located 0.1‐ha plots in three landscapes and compiled a data set of 1,565 trees including all three main species, and a full range of sizes and competitive states in northern boreal old‐growth forests. We extracted tree ring samples from every tree and used ring widths to reconstruct tree sizes and their neighbourhood for the past 50 years for Pinus sylvestris and Picea abies, and 30 years for Betula pubescens. We used mixed‐effects models to study variation in tree growth. In the models, we used annually varying environmental variables, forest and tree structural variables and site variables as predictors. We also measured soil hydraulic properties for a water balance model to specifically include soil moisture deficits as predictors. Results: Tree growth rates had increased during the analysis period for all three species, with the highest increases for P. sylvestris and B. pubescens. All three species grew well during warm summers, but P. sylvestris and P. abies growth was also negatively influenced by soil moisture deficits. Competition in these sparse forests had a small influence on growth. However, its influence increased over time. Conclusions: The trees showed positive growth trends, but the increasing competition means that such strong positive trends are unlikely to continue over the long term. For year‐to‐year variability, negative influence of droughts implied that increasing temperatures are not only beneficial to tree growth even in these high‐latitude forests. Overall, the findings demonstrate the site‐ and species‐dependent complexity in tree responses to environmental and structural changes we may expect in the future. Tree growth rates in the northern boreal old‐growth forests in Finland showed increasing trends in the recent decades. However, at annual resolution, droughts negatively influenced the growth of Norway spruce and Scots pine, implying that even in these high latitude (67°N) forests the benefits of increasing temperatures to tree growth are limited. [ABSTRACT FROM AUTHOR]

Aims Analyse the controversial relationship existing between wood density, precipitation, temperature and aridity, and its importance in shaping forest communities facing drought. Location New Caledonia (SW Pacific). Methods We sampled wood density in 1580 trees belonging to 175 species across nine sites (eight 1-ha plots) located in dry, mesic and humid forests. We tested whether wood density varied across species, communities and habitats, and depended on mean annual precipitation (MAP), mean annual temperature (MAT) and an aridity index (AI). We also tested whether the variations observed in community mean wood density (WDmean) and in wood density standard deviation differed from those obtained by a randomized species distribution across communities. We constructed a phylogenetic supertree at genus level and tested for the non-random distribution of WDmean. Results We found that WDmean tended to increase with increasing aridity (i.e. decreasing MAP and increasing MAT) and was significantly higher than expected under randomized species distribution at the dry forest site only. However, some species with low wood density grew well at these dry sites. At genus level, wood density was not random within the phylogenetic supertree. Some lineages exhibiting higher wood density than expected by randomization were also the most represented at the dry sites. Conclusions Although we did not observe an environmental filter sensu stricto, aridity associated with a low MAP and a high MAT at the dry and mesic forest sites was likely to favour species with a high wood density. Our phylogenetic results suggested that species belonging to a few lineages exhibiting high wood density were likely to be adapted to drought, hence favoured by increasing drought. [ABSTRACT FROM AUTHOR]

Questions: Climate change-type drought (the combination of drought and heatwave) has become a widely documented driver of forest dieback yet, to date, limited measurement of post-event forest dynamics has been reported. Can climate change-type drought trigger structural and/or compositional changes in a forest type which is usually highly resilient to other disturbances? Location: Mixed Eucalyptus forest in southwestern Australia. Methods: Forest areas that were severely and minimally affected by drought were measured repeatedly at 3, 6, 16, 26 and 49 mo post-event for changes in forest structure and composition of the two dominant tree species (Eucalyptus marginata/ Corymbia calophylla). Means and dispersal from pre-drought conditions were analysed for each structural variable among drought severity classes and between measurement periods. Resprouting, the predominant resilience mechanism in Eucalyptus, was assessed at 6 and 16 mo, while regeneration type and density were determined at 16 mo post-drought. Results: Structural changes were observed after 49 mo on plots severely but not minimally affected by drought, including a 30% increase in stem density (P < 0.0001) and reduction in tree diameter (23%; P < 0.01), basal area (33%; P < 0.0001), canopy height (40%; P < 0.0001) and live biomass (36%; P = 0.01). On severely affected plots, all structural variables plateaued at levels different from pre-drought conditions. Large, old trees on severely affected plots were replaced by high densities of small stems (1-10 cm DBH). Resprouting among drought affected trees (P < 0.001) and tree regeneration (P = 0.02) were higher on severely affected plots. No significant shifts in the proportional abundance of the two dominant species, E. marginata and C. calophylla, were observed for structural attributes or regeneration after 49 mo. Conclusion: Climate change-type drought can cause structural shifts in a resproutingmediterranean-type forest, providing evidence for a shift to an alternate state, particularly with repeated disturbance. While the study area showed low structural resistance and resilience, tree species composition was resilient to change, as E. marginata is likely to remain dominant in the future due to its resprouting capacity. Findings support the view that climate change-type drought will drive replacement of large trees with short,multi-stemmed individuals, transforming ecosystem structure. [ABSTRACT FROM AUTHOR]

Question: What components of drought influence the drought limit of Fagus sylvatica forests? This study contributes to the ongoing discussion regarding the future of Fagus as a major component of central European forests. Location: The drought limit of F. sylvatica at its ecotone with forest dominated by Quercus pubescens, Q. petraea and their hybrids in two limestone regions (Klettgau, SchwäbischeAlb) in southwestern Germany was compared. Methods: Vegetation relevés were classified and a gradient analysis was performed. The vegetation pattern was analysed with several drought relevant variables. Classification trees were used to determine the drought limits of the Fagus forest. Results: The Fagus, Quercus and the ecotone forests were floristically characterized. The lower humidity in the submontane Klettgau, compared to the montane Schwäbische AIb, was compensated for by greater soil moisture (ASWSC). Therefore, Fagus forest in the SchwäbischeAlb grew on sites with ASWSC values similar to those of ecotone forest in Klettgau. Conclusions: The interaction between climatic and edaphic drought related factors demonstrates that drought is a complex edaphic-climatic factor. Both components contribute to limiting the distribution of Fagus. For the two regions in southwestern Germany, and under the existing climatic conditions, it could be shown that Fagus is able to dominate forests on soils with very low ASWSC (≥68 l.m-2). [ABSTRACT FROM AUTHOR]

Question: This study evaluates how fire regimes influence stand structure and dynamics in old-growth mixed conifer forests across a range of environmental settings. Location: A 2000-ha area of mixed conifer forest on the west shore of Lake Tahoe in the northern Sierra Nevada, California. Methods: We quantified the age, size, and spatial structure of trees in 12 mixed conifer stands distributed across major topographic gradients. Fire history was reconstructed in each stand using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing the fire history with the age, size, and spatial structure of trees in a stand. Results: There was significant variation in species composition among stands, but not in the size, age and spatial patterning of trees. Stands had multiple size and age classes with clusters of similar aged trees occurring at scales of 113 – 254 m². The frequency and severity of fires was also similar, and stands burned with low to moderate severity in the dormant season on average every 9 – 17 years. Most fires were not synchronized among stands except in very dry years. No fires have burned since ca. 1880. Conclusions: Fire and forest structure interact to perpetuate similar stand characteristics across a range of environmental settings. Fire occurrence is controlled primarily by spatial variation in fuel mosaics (e.g. patterns of abundance, fuel moisture, forest structure), but regional drought synchronizes fire in some years. Fire exclusion over the last 120 years has caused compositional and structural shifts in these mixed conifer forests. [ABSTRACT FROM AUTHOR]

Question: What was the role of fire during the establishment of the current overstory (ca. 1870-1940) in mixed-oak forests of eastern North America?Location: Nine sites representing a 240-km latitudinal gradient on the Allegheny and Cumberland Plateaus of eastern North America. Methods: Basal cross-sections were collected from 225 trees. Samples were surfaced, and fire scars were dated. Fire history diagrams were constructed and fire return intervals were calculated for each site. Geographic patterns of fire occurrence, and fire-climate relationships were assessed. Results: Fire was a frequent and widespread occurrence during the formation of mixed-oak forests, which initiated after large-scale land clearing in the region ca. 1870. Fire return ranged from 1.7 to 11.1 years during a period of frequent burning from 1875 to 1936. Fires were widespread during this period, sometimes occurring across the study region in the same year. Fires occurred in a variety of climate conditions, including both drought and non-drought years. Fires were rare from 1936 to the present. Conclusions: A variety of fire regime characteristics were discerned. First, a period of frequent fire lasted approximately 60 years during the establishment of the current oak overstory. Second, fire occurred during a variety of climate conditions, including wet climates and extreme drought. Finally, there was within-site temporal variability in fire occurrence. These reference conditions could be mimicked in ongoing oak restoration activities, improving the likelihood of restoration success. [ABSTRACT FROM AUTHOR]