Water availability drives gradients of tree diversity,structure and functional traits in the Atlantic-Cerrado-Caatinga transition,Brazil

Aims Climate and soil are among the most important factors determin-ing variation in tree communities,but their effects have not been thoroughly elucidated to date for many vegetation features.In this study,we evaluate how climate and soil gradients affect gradients of vegetation composition,species diversity and dominance,structure and functional traits(seed mass and wood density)using over 327000 trees in 158 sites distributed along environmental gradients in the transitions among the Atlantic forest,Cerrado and Caatinga in Minas Gerais State(MG),Brazil(nearly 600000 km2).Methods Gradients in species,genus and family abundance in addition to basal area,stem density,species diversity(Fisher’s alpha),domin-ance percentage,seed mass and wood density were correlated using multiple regressions with environmental variables,as summarized in four principal component analysis axes(two climatic-precipi-tation seasonality and temperature range-and two edaphic-soil fertility and soil moisture).Additionally,ordinary kriging maps were used to better illustrate the gradients.Important Findings Multiple regression models indicate that all variables but domin-ance percentage were affected by one or more of the environmental gradients,but the average R2 was low(26.25%).Kriging maps rein-forced the patterns observed in the regression models.Precipitation seasonality and soil moisture gradients were the most important gra-dients affecting vegetation features.This finding suggests that water availability is an important determinant of vegetation features in these vegetation transitions.

Functional traits shape size-dependent growth and mortality rates of dry forest tree species

Aims Functional traits have emerged as an important tool to evaluate plant performance.However,the environmental conditions and ecological pressures that plants face change with their size,and the relationship between traits and plant performance should therefore be size-dependent,which has rarely been tested.Methods Here,we evaluated over a broad range of tree sizes the interspe-cific relationship between tree growth and mortality and eight func-tional stem,leaf and seed traits.We did so across 59 tree species in Brazilian dry forests and evaluated whether the relationships found for wet forest types in the literature also hold for dry forests,where water rather than light might limit tree performance.Important Findings We indeed found a strong size-dependent relationship between demographic rates and functional traits.At small sizes,when trees are in the shaded understory,species with functional trait values that enhance light capture or shade tolerance(i.e.higher maximum adult stature,taller heights,wider crowns,higher seed mass)have higher growth and/or lower mortality rates(MR).This relationship disappears at larger sizes when trees attain better light conditions in the canopy.Drought adaptations play only a role at larger tree sizes;once trees are in the dry and exposed canopy,species with higher wood density(an indicator of cavitation resistance)have lower MRs.Our study shows that both drought and shade adaptations are important in this dry forest,and that the relationships between func-tional traits and plant performance changes with plant size.Plant size should therefore explicitly be included as an axis of variation in functional analyses,to better understand the relationship between functional traits and demographic rates.