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• More frequent droughts and rising temperatures pose serious threats to tropical forests. When stomata are closed under dry and hot conditions, plants lose water through leaf cuticles, but little is known about cuticle conductance (g min) of tropical trees, how it varies among species and environments, and how it is affected by temperature. • We determined g min in relation to temperature for 24 tropical tree species across a steep rainfall gradient in Panama, by recording leaf drying curves at different temperatures in the laboratory. • In contrast with our hypotheses, g min did not differ systematically across the rainfall gradient; species differences did not reflect phylogenetic patterns; and in most species g min did not significantly increase between 25 and 50°C. g min was higher in deciduous than in evergreen species, in species with leaf trichomes than in species without, in sun leaves than in shade leaves, and tended to decrease with increasing leaf mass per area across species. There was no relationship between stomatal and cuticle conductance. • Large species differences in g min and its temperature response suggest that more frequent hot droughts may lead to differential survival among tropical tree species, regardless of species’ position on the rainfall gradient.

Summary Plant–plant interactions may critically modify the impact of climate change on plant communities. However, the magnitude and even direction of potential future interactions remains highly debated, especially for water‐limited ecosystems. Predictions range from increasing facilitation to increasing competition with future aridification. The different methodologies used for assessing plant–plant interactions under changing environmental conditions may affect the outcome but they are not equally represented in the literature. Mechanistic experimental manipulations are rare compared with correlative approaches that infer future patterns from current observations along spatial climatic gradients. Here, we utilize a unique climatic gradient in combination with a large‐scale, long‐term experiment to test whether predictions about plant–plant interactions yield similar results when using experimental manipulations, spatial gradients or temporal variation. We assessed shrub–annual interactions in three different sites along a natural rainfall gradient (spatial) during 9 years of varying rainfall (temporal) and 8 years of dry and wet manipulations of ambient rainfall (experimental) that closely mimicked regional climate scenarios. The results were fundamentally different among all three approaches. Experimental water manipulations hardly altered shrub effects on annual plant communities for the assessed fitness parameters biomass and survival. Along the spatial gradient, shrub effects shifted from clearly negative to mildly facilitative towards drier sites, whereas temporal variation showed the opposite trend: more negative shrub effects in drier years. Based on our experimental approach, we conclude that shrub–annual interaction will remain similar under climate change. In contrast, the commonly applied space‐for‐time approach based on spatial gradients would have suggested increasing facilitative effects with climate change. We discuss potential mechanisms governing the differences among the three approaches. Our study highlights the critical importance of long‐term experimental manipulations for evaluating climate change impacts. Correlative approaches, for example along spatial or temporal gradients, may be misleading and overestimate the response of plant–plant interactions to climate change. Lay Summary

1. The stress-gradient hypothesis (SGH) predicts an increasing importance of facilitative mechanisms relative to competition along gradients of increasing environmental stress. Although developed across a variety of ecosystems, the SGH's relevance to the dynamic tree—grass systems of global savannas remains unclear. Here, we present a meta-analysis of empirical studies to explore emergent patterns of tree—grass relationships in global savannas in the context of the SGH. 2. We quantified the net effect of trees on understorey grass production relative to production away from tree canopies along a rainfall gradient in tropical and temperate savannas and compared these findings to the predictions of the SGH. We also analysed soil and plant nutrient concentrations in subcanopy and open-grassland areas to investigate the potential role of nutrients in determining grass production in the presence and absence of trees. 3. Our meta-analysis revealed a shift from net competitive to net facilitative effects of trees on subcanopy grass production with decreasing annual precipitation, consistent with the SGH. We also found a significant difference between sites from Africa and North America, suggesting differences in tree—grass interactions in the savannas of tropical and temperate regions. 4. Nutrient analyses indicate no change in nutrient ratios along the rainfall gradient, but consistent nutrient enrichment under tree canopies. 5. Synthesis. Our results help to resolve questions about the SGH in semi-arid systems, demonstrating that in mixed tree—grass systems, trees facilitate grass growth in drier regions and suppress grass growth in wetter regions. Relationships differ, however, between African and North American sites representing tropical and temperate bioclimates, respectively. The results of this meta-analysis advance our understanding of tree—grass interactions in savannas and contribute a valuable data set to the developing theory behind the SGH.

Species loss is often associated with a decline in ecosystem functions. Globally, digging mammals (or ecosystem engineers) are functionally important, altering soil processes at local scales. However, their effects on the process of decomposition are poorly understood, particularly at larger scales, where the environment may moderate the magnitude of effects. We tested the landscape‐scale effects of reintroducing ecologically extinct digging mammals on two aspects of nutrient cycling over a large environmental gradient in Australia, where many digging mammals became extinct or ecologically extinct following the arrival of Europeans. We measured the impacts of digging mammals on soil organic matter content and plant litter decomposition over a 3,000 km transect, where annual average rainfall varied between 166 and 877 mm. We set up paired study plots (n = 8–10) inside and outside five reintroduction reserves. We took soil samples to assess soil organic matter content and set up litter bags to measure plant litter decomposition over 4 and 12 months. We used macroinvertebrate exclusion and macroinvertebrate access treatments to determine the relative importance of macroinvertebrates in decomposition with and without digging mammals. Soil organic matter was greater in reintroduction areas, but the magnitude of the effect was driven by productivity (average annual rainfall as a proxy), with little effect of digging activity at the wettest sites. Short‐term plant matter decomposition was greater in the presence of digging mammals, and their effect was dependent on the amount of rain that fell during the study period. Long‐term litter decomposition increased with annual rainfall, independent of digging mammals. Unexpectedly, macroinvertebrate exclusion increased decomposition rates over 12 months. Reintroduction of digging mammals substantially alters soil processes and organic matter decomposition, but impacts are rainfall‐dependent. Restoring native digging mammals to their historical distribution is likely to reverse degradation of ecosystem processes, but the magnitude of this effect depends on the environment. A free Plain Language Summary can be found within the Supporting Information of this article. A free Plain Language Summary can be found within the Supporting Information of this article.

Rainfalls with heavy intensity and long duration are the main factors that trigger landslide. A large-scale test of physical model of landslide induced by gradient rainfall is conducted to investigate the mechanism of landslide caused by rainfalls with an indoor large-scale physical model test system. By monitoring the variation of pore water pressure, soil pressure, slope surface displacement, and groundwater level, the deformation and failure mechanism of deposit landslide under gradient rainfall are studied. The results show that the pore water pressure and soil pressure increase with the increase of the duration of rainfall, and the greater the rainfall intensity is, the greater the peak values of pore water pressure and soil pressure are. The intensity and duration of rainfall are the main influencing factors of landslide deformation. The decrease of effective stress and the rise of groundwater level are the root causes of landslide deformation. During a rainfall, excessive pore water pressure appears in the slope, which can be used as an early warning indicator of landslide deformation and failure. The deformation of slope body is nonlinear and shows a stepped pattern under gradient rainfall. During a rainfall, the toe of the slope is usually damaged first. Tension cracks appear in the rear edge and the middle part of the slope successively, showing the characteristics of traction failure.

Bedrock landslides are the primary agent of hillslope erosion, and mass wasting, and an essential source of sediment flux to the fluvial network in the mountainous terrain, in particular, in the Himalayan mountain belt. To understand the characteristics of the landscape, we calculate geomorphic matrices including the topographic variables, longitudinal and topographic swath profile, channel steepness index, and stream length gradient index to analyze the spatial distribution of landslide occurrences over landscape evolution. The intensity of rainfall gradient and topographic variables were spatially correlated with the erosion and exhumation rates of the studied catchment. Our analysis suggests that the zones with slope ranges of 24°–28°, relief ranges of 800–1000 m, and elevation ranges of 1500–1700 m, which coincide with the rainfall intensity range of 2500–2700 mm/year in the Teesta river catchment, have the highest probability of frequently occurring landslides. Higher tectonic activity is principally responsible for the landslide over the Higher Himalaya to the north of the Main Central Thrust (MCT)–Main Boundary Thrust (MBT) along the orographic barrier. In contrast, litho-tectonics regulates and mostly triggers landslides adjacent to the MCT–MBT structural affinity dominated by rainfall intensity. Our observation suggests that erosion rates frequently exceed long-term exhumation rates and are spatially more variable. Moreover, they exhibit significantly divergent spatial patterns, which suggests that the processes governing these rates are independent. Exhumation rates have been shown to decrease from south to north over geological periods, rising in the southwest region at ~ 1.2 mm/year and decreasing to ~ 0.5 mm/year in the northernmost region of the Teesta catchment. Long-term exhumation rates are not correlated with geomorphic or climatic variables. The highest apparent erosion rates (5 mm/year) are seen in the catchment that crosses the MCT Zone, however, these rates appear to have been severely impacted by recent landslides. Conversely, changes in rainfall rate do not appear to significantly impact either rate of long-term exhumation or erosion in the Teesta catchment.

Ecotypic differentiation, reflected in substantial trait differences across populations, has been observed in various plant species distributed across aridity gradients. Nevertheless, ecotypic differentiation in leaf silicon concentration, known to alleviate drought stress in plants, remained hardly explored. Here, we provide a systematic test for ecotypic differentiation in leaf silicon concentration along two aridity gradients in the grass Brachypodium hybridum in Israel. Seed material was sampled in 15 sites along a macroclimatic aridity gradient (89 – 926 mm mean annual rainfall) and from corresponding north (moister) and south (more arid) exposed slopes (microclimatic gradient) at similar altitudes (mean north: 381 m a.s.l., mean south: 385 m a.s.l.). Plants were subsequently grown under common conditions and their leaf silicon concentration was analysed. Leaf silicon concentration increased with increasing aridity across the macroclimatic gradient, but did not differ between north and south slopes. The higher leaf silicon concentrations under more arid conditions can enhance the ability of plants to cope with more arid conditions by two mutually not exclusive mechanisms: (i) withstanding drought by reducing water loss and increasing water uptake or (ii) escaping drought by facilitating fast growth. Our study highlights that leaf silicon concentration contributes to ecotypic differentiation in annual grasses along macroclimatic aridity gradients.

Species extinctions alter ecosystem services, and the magnitude of this impact is likely to change across environmental gradients. In Australia, soil‐disturbing mammals that are now considered ecologically extinct are thought to be important ecosystem engineers. Previous studies have demonstrated microsite‐level impacts of reintroduced soil‐disturbing mammals on soil functions, but effects are yet to be tested across larger scales. Further, it is unclear how impacts vary across environmental gradients and if the restoration potential of reintroductions changes with climate. We examined the effects of soil‐disturbing mammal reintroductions across a large rainfall gradient in Australia to test the hypothesis that ecosystem engineering effects on soil function depend on climate. We compared soil labile carbon, available nitrogen and the activity of four enzymes associated with nutrient cycling in three microsite types with and without soil‐disturbing mammals in five sites along a large rainfall gradient (166–870 mm). Soil enzyme activity was greatest in the presence of soil‐disturbing mammals and increased with rainfall, but soil available carbon and nitrogen varied across the gradient and among microsites. Microsite effects were often stronger than any effects of soil‐disturbing mammals, with soil beneath vegetated patches (shrubs and trees) having greater enzyme activity, carbon and nitrogen than bare soils. However, soil‐disturbing mammals homogenised nutrient distributions across microsites. The impacts of soil‐disturbing mammals on soil function previously detected at micro‐scales was detected at a landscape‐scale. However, the overall effects of soil‐disturbing mammals on soil functions varied with productivity (rainfall). The context of soil‐disturbing mammal reintroductions is thus likely to be critical in determining their effectiveness in restoring soil function.

Mycorrhizal fungi have considerable effects on soil carbon (C) storage, as they control the decomposition of soil organic matter (SOM), by modifying the amount of soil nitrogen (N) available for free-living microbes. Through their access to organic N, ectomycorrhizal (ECM) fungi compete with free-living soil microbes; this competition is thought to slow down SOM decomposition. However, arbuscular mycorrhizal (AM) fungi cannot decompose SOM, and therefore must wait for N to first be processed by free-living microbes. It is unclear what form of N the ECM fungi and free-living microbes compete for, or which microbial groups compete for N with ECM fungi. To investigate this, we focused on the N transformation steps (i.e., the degradation of high-molecular-weight organic matter, mineralization, and nitrification) and the microbes driving each step. Simple comparisons between AM forests and ECM forests are not sufficient to assert that mycorrhizal types would determine the N transformation steps in soil, because soil physiochemistry, which strongly affects N transformation steps, differs between the forests. We used an aridity gradient with large differences in soil moisture, pH, and SOM quantity and quality, to distinguish the mycorrhizal and physicochemical effects on N transformation. Soil samples (0–10 cm depth) were collected from AM-symbiotic black locust forests under three aridity levels, and from ECM-symbiotic oak forests under two aridity levels. Soil physicochemical properties, extractable N dynamics and abundance, composition, and function of soil microbial communities were measured. In ECM forests, the ammonia-oxidizing prokaryotic abundance was low, whereas that of ECM fungi was high, resulting in lower nitrate N content than in AM forests. Since ECM forests did not have lower saprotrophic fungal abundance and prokaryotic decompositional activity than the AM forests, the hypothesis that ECM fungi could reduce SOM decay and ammonification by free-living microbes, might not hold in ECM forests. However, the limitation of ECM fungi on nitrate N production would result in a feedback that will accelerate plant dependence on these fungi, thereby raising soil C storage through an increase in the ECM biomass and plant C investment in soils.

Climate gradient-focused ecological research can provide a foundation for better understanding critical ecological transition points and nonlinear climate-ecological relationships, which is information that can be used to better understand, predict, and manage ecological responses to climate change. In this study, we examined the influence of freshwater availability upon the coverage of foundation plant species in coastal wetlands along a northwestern Gulf of Mexico rainfall gradient. Our research addresses the following three questions: (1) What are the regional-scale relationships between measures of freshwater availability (e.g., rainfall, aridity, freshwater inflow, salinity) and the relative abundance of foundation plant species in tidal wetlands; (2) how vulnerable are foundation plant species in tidal wetlands to future changes in freshwater availability; and (3) what is the potential future relative abundance of tidal wetland foundation plant species under alternative climate change scenarios? We developed simple freshwater availability-based models to predict the relative abundance (i.e., coverage) of tidal wetland foundation plant species using climate data (1970-2000), estuarine freshwater inflow-focused data, and coastal wetland habitat data. Our results identify regional ecological thresholds and nonlinear relationships between measures of freshwater availability and the relative abundance of foundation plant species in tidal wetlands. In drier coastal zones, relatively small changes in rainfall could produce comparatively large landscape-scale changes in foundation plant species abundance that would affect some ecosystem good and services. Whereas a drier future would result in a decrease in the coverage of foundation plant species, a wetter future would result in an increase in foundation plant species coverage. In many ways, the freshwater-dependent coastal wetland ecological transitions we observed are analogous to those present in dryland terrestrial ecosystems.