Drivers of tree species distribution across a tropical rainfall gradient

Abstract

Abstract. Understanding the factors shaping species distribution patterns along tropical rainfall gradients is necessary to predict the consequences of climate change for tropical tree communities. Direct effects of water availability exclude wet forest species from dry forests, but the exclusion of dry forest species from wet forests remains unexplained. We tested the hypothesis that high light and nutrient requirements exclude dry forest species from dark, infertile, wet forests. We transplanted seedlings of 26 woody species to six sites along a pronounced regional rainfall gradient across the Isthmus of Panama. We examined the effects of soil moisture, phosphorus and light availability, and species' drought resistance on seedling performance, and linked the results directly to known species distribution patterns. Surprisingly, seedlings of wet forest species did not exhibit a home advantage: All species survived better under moister conditions, and the effects of phosphorus availability and light on seedling performance did not differ among species from dry or wet forests. Instead, dry forest species had intrinsically slower growth rates than wet forest species, which may lead to their exclusion from wet forests at later life history stages. High phosphorus exacerbated susceptibility to drought, although the mechanism remains unknown. Overall, our results demonstrate that seedling performance across tropical rainfall gradients is determined primarily by variation in soil water availability across space and time, while variation in nutrient and light availability plays a lesser role. Future changes in rainfall patterns will therefore have direct and pervasive consequences for forest composition and ecosystem function.

An extensive suite of functional traits distinguishes Hawaiian wet and dry forests and enables prediction of species vital rates

“…The trait differences between forests aligned with their variation in vital rates. While the species of the two forests did not differ on average in RGR, the MWF species showed lower mortality rates than the LDF species, consistent with previous work showing higher mortality in drier forests elsewhere (Gaviria, Turner, & Engelbrecht, ; Laura Suarez & Kitzberger, ). The lower mortality of the MWF species is consistent with the greater supply of water and soil nutrients, related to greater accumulated weathering, organic material formation, N‐fixation, and nutrient retention capacity, and its richer microbial community.…”

Growth responses to soil water potential indirectly shape local species distributions of tropical forest seedlings

“…Surprisingly, three of the four species with significant growth responses to SWP grew slower at wetter sites. At wetter sites, higher pathogen pressure, anoxic conditions due to waterlogging in the wet season, or lower light conditions may limit growth (Brenes‐Arguedas, Roddy, Coley, & Kursar, ; Gaviria et al, ; Lopez & Kursar, ; Spear, Coley, & Kursar, ). Indeed, sites with higher SWP were more shaded ( r = 0.27, p < 0.001; Supporting Information Table , shade data from Condit, ).…”

“…With respect to water availability, reciprocal transplant experiments have implicitly tested these two hypotheses by comparing seedling growth and mortality of species common to dry and wet forests along a rainfall gradient in central Panama. Dry forest species tended to perform best in dry forests (i.e., “best at home”), and wet forest species had lower mortality in wet forests but showed no clear pattern for growth (Brenes‐Arguedas, Coley, & Kursar, ; Gaviria & Engelbrecht, ; Gaviria, Turner, & Engelbrecht, ). Wet forest species generally grew faster than dry forest species not only in wet forests (i.e., they had a “home advantage”) but also in dry forests, indicating that wet forest species have inherently higher growth rates.…”

“…We expect that species associated with wetter sites perform “best at home” (i.e., have higher growth and lower mortality rates under wetter conditions), whereas species associated with drier sites are drought tolerant and indifferent to SWP (Comita & Engelbrecht, ). Do species have a “home advantage,” that is, better performance at their end of the SWP gradient than species associated with the other end? In terms of growth, we expect that wet distributed species have a “home advantage” over dry distributed species due to inherently higher growth rates (Brenes‐Arguedas et al, ; Gaviria et al, ). In contrast, we expect that dry distributed species have a “home advantage” in terms of mortality, due to adaptations to cope with drought (Brenes‐Arguedas et al, ).…”

Effect of Soil Moisture on Composition and Diversity of Trees in Tropical dry Forests

“…The distribution, diversity and primary productivity of plant communities have been reported to be highly associated with rainfall gradients and the availability of soil moisture [1][2][3][4][5][6][7][8][9][10][11][12]. According to the theory of non-equilibrium ecology, the soil water availability is the most important factor in system dynamics [13].…”