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Aim: The climate variability hypothesis proposes that species subjected to wide variation in climatic conditions will evolve wider niches, resulting in larger distributions. We test this hypothesis in tropical plants across a broad elevational gradient; specifically, we use a species‐level approach to evaluate whether elevational range sizes are explained by the levels of thermal variability experienced by species. Location: Central Andes. Time Period: Present day. Taxon: Woody plants. Methods: Combining data from 479 forest plots, we determined the elevational distributions of nearly 2300 species along an elevational gradient (~209–3800 m). For each species, we calculated the maximum annual variation in temperature experienced across its elevational distribution. We used phylogenetic generalized least square models to evaluate the effect of thermal variability on range size. Our models included additional covariates that might affect range size: body size, local abundance, mean temperature and total precipitation. We also considered interactions between thermal variability and mean temperature or precipitation. To account for geometric constraints, we repeated our analyses with a standardized measure of range size, calculated by comparing observed range sizes with values obtained from a null model. Results: Our results supported the main prediction of the climate variability hypothesis. Thermal variability had a strong positive effect on the range size, with species exposed to higher thermal variability having broader elevational distributions. Body size and local abundance also had positive, yet weak effects, on elevational range size. Furthermore, there was a strong positive interaction between thermal variability and mean annual temperature. Main Conclusions: Thermal variability had an overriding importance in driving elevational range sizes of woody plants in the Central Andes. Moreover, the relationship between thermal variability and range size might be even stronger in warmer regions, underlining the potential vulnerability of tropical montane floras to the effects of global warming. [ABSTRACT FROM AUTHOR]

Aim: We examined tree beta diversity in four biogeographical regions with contrasting environmental conditions, latitude, and diversity. We tested: (a) the influence of the species pool on beta diversity; (b) the relative contribution of niche-based and dispersal-based assembly to beta diversity; and (c) differences in the importance of these two assembly mechanisms in regions with differing productivity and species richness. Location: Lowland and montane tropical forests in the Madidi region (Bolivia), lowland temperate forests in the Ozarks (USA), and montane temperate forests in the Cantabrian Mountains (Spain). Methods: We surveyed woody plants with a diameter ≥2.5 cm following a standardized protocol in 236 0.1-ha forest plots in four different biogeographical regions. We estimated the species pool at each region and used it to recreate null communities determined entirely by the species pool. Observed patterns of beta diversity smaller or greater than the null-expected patterns of beta diversity implies the presence of local assembly mechanisms beyond the influence of the species pool. We used variation partitioning analyses to compare the contribution of niche-based and dispersal-based assembly to patterns of observed beta diversity and their deviations from null models among the four regions. Results: (a) Differences in species pools alone did not explain observed differences in beta diversity among biogeographic regions. (b) In 3/4 regions, the environment explained more of the variation in beta diversity than spatial variables. (c) Spatial variables explained more of the beta diversity in more diverse and more productive regions with more rare species (tropical and lower-elevation regions) compared to less diverse and less productive regions (temperate and higher-elevation regions). (d) Greater alpha or gamma diversity did not result in higher beta diversity or stronger correlations with the environment. Conclusion: Overall, the observed differences in beta diversity are better explained by differences in community assembly mechanism than by biogeographical processes that shaped the species pool. [ABSTRACT FROM AUTHOR]

Aim Oligarchic patterns can vary from weak (i.e. little difference between rare and common species) to strong (i.e. a set of dominant species is immediately evident). Our aim was to understand the relationships between the strength of the oligarchic patterns, diversities (alpha, beta and gamma), and five potential causes (elevational variability, soil heterogeneity, elevation, soil conditions and geographical extent). Location The Amazon-Andes transition in the Madidi region (Bolivia). Methods We established 398 plots of 0.1 ha each, containing 121,183 individual woody plants belonging to 2390 species. Then we defined 500 sub-regions (= unique overlapping subsets of 50 plots from the pool of 398 plots) so they varied in extent from 220 to 17,700 km2 within the study area. We employed two independent path analyses to relate environmental characteristics and geographical extent of sub-regions to (1) oligarchic strength and (2) alpha, beta and gamma diversities. We used generalized linear models to relate diversities to different measures of oligarchic strength. Results Oligarchies at larger extents were weaker, a trend strongly driven by the pure effect of area and, secondarily, by environmental heterogeneity. Oligarchies at higher elevations were weaker than expected, and oligarchies in acidic and nutrients-poor soils were not stronger than those in less stressful soils. Trends in oligarchic strength were inversely correlated with those of gamma and beta diversity: weaker oligarchies were found in species-rich and heterogeneous communities. Main conclusions Environmental heterogeneity and low landscape connectivity limit the strength of the oligarchic pattern. Although diversities (particularly beta diversity) and oligarchic strength are closely related, they are somewhat differently driven by external factors. In particular, oligarchic strength is more sensitive to spatial extent and less sensitive to environmental heterogeneity than beta diversity. Finally, the study of oligarchic patterns should consider a priori expectations based on species richness and turnover. [ABSTRACT FROM AUTHOR]