Seasonal drought limits tree species across the Neotropics

Abstract

Within the tropics, the species richness of tree communities is strongly and positively associated with precipitation. Previous research has suggested that this macroecological pattern is driven by the negative effect of water‐stress on the physiological processes of most tree species. This implies that the range limits of taxa are defined by their ability to occur under dry conditions, and thus in terms of species distributions predicts a nested pattern of taxa distribution from wet to dry areas. However, this ‘dry‐tolerance’ hypothesis has yet to be adequately tested at large spatial and taxonomic scales. Here, using a dataset of 531 inventory plots of closed canopy forest distributed across the western Neotropics we investigated how precipitation, evaluated both as mean annual precipitation and as the maximum climatological water deficit, influences the distribution of tropical tree species, genera and families. We find that the distributions of tree taxa are indeed nested along precipitation gradients in the western Neotropics. Taxa tolerant to seasonal drought are disproportionally widespread across the precipitation gradient, with most reaching even the wettest climates sampled; however, most taxa analysed are restricted to wet areas. Our results suggest that the ‘dry tolerance' hypothesis has broad applicability in the world's most species‐rich forests. In addition, the large number of species restricted to wetter conditions strongly indicates that an increased frequency of drought could severely threaten biodiversity in this region. Overall, this study establishes a baseline for exploring how tropical forest tree composition may change in response to current and future environmental changes in this region.

Convergence in growth responses of tropical trees to climate driven by water stress

“…It is well known that wetter sites with less seasonal climates support the greatest biodiversity in the Neotropics (ter Steege et al 2006, Esquivel-Muelbert et al 2017. Water stress is the most important physiological threats to tropical tree species (Brenes-Arguedas et al 2011), and precipitation gradients correlate with species richness on macroecological scales (ter Steege et al 2003).…”

“…Since tree richness in tropical forest communities is directly relate to total rainfall (Esquivel-Muelbert et al 2017), it is of interest to look for ecological patterns in growth responses of tropical trees to climate across precipitation gradients. Since tree richness in tropical forest communities is directly relate to total rainfall (Esquivel-Muelbert et al 2017), it is of interest to look for ecological patterns in growth responses of tropical trees to climate across precipitation gradients.…”

“…Since tree richness in tropical forest communities is directly relate to total rainfall (Esquivel-Muelbert et al 2017), it is of interest to look for ecological patterns in growth responses of tropical trees to climate across precipitation gradients. Total precipitation and its seasonality not only modulate macroecological patterns in the richness of tree communities (ter Steege et al 2006, Esquivel-Muelbert et al 2017), but may also influence the diversity of growth responses to climate variations in tropical trees. The sampling sites are located in contrasting environments of the Bolivian Cerrado, ranging in elevations from 200 to 500 m and recording different total precipitation amounts and dry season lengths.…”

Compositional response of Amazon forests to climate change

“…Note that in this case we would not be interested in the growth of trees surviving from t 0 to t 1 , and so the net flux would be represented as R t1 − L t1 inventory plots across the Amazon Basin (Fauset et al, 2015); Water deficit affiliation (WDA; mm): derived from relative abundances across 513 inventory plots distributed along a large gradient of MCWD across the Western Neotropics (Esquivel-Muelbert, Baker, et al, 2017). This figure represents dynamics in basal area terms; similar logic can be applied for stem-based analyses.…”

Filter‐dispersal assembly of lowland Neotropical rainforests across the Andes

“…The Huancabamba Depression in northern Peru might have been an alternative route for dispersal, but we consider this route less likely because it has a more ancient uplift history (Gregory-Wodzicki 2000, Hoorn et al 2010 and higher elevation than the lowest mountain passes of the Eastern Cordillera near the Colombian-Venezuelan border (2145 m vs ca 1300 m, respectively; Weigend 2002). In addition to elevation, precipitation is another key environmental determinant of the distribution of lowland rainforest plants at broad spatial scales (Engelbrecht et al 2007, Condit et al 2013, Esquivel-Muelbert et al 2017 and only a subset of rainforest trees is also able to inhabit dry areas (Esquivel-Muelbert et al 2017). Dispersal through the Llanos and other lowland dry habitats around the northernmost Andes, especially along habitat corridors such as gallery forests, represents an alternative to highelevation dispersal, but may have been available to only the most drought tolerant species.…”