COS 38-6 - Do soil nutrients determine a dramatic shift in biodiversity and ecosystem properties in Amazonian forests?

Abstract: A central question in ecology is what determines sharp boundaries in adjacent ecological systems. In the Amazon basin, two dramatically different plant communities persist side by side: the biodiverse, highly productive, and carbon-rich terra firme forest vs. the low-diversity, diminished, and carbon-poor white sand forest. Traditional explanations of this difference have centered on the influence of soil texture — notably, sand vs. clay — and its potential effect on soil nutrient availability. However, it is unclear if and how soil resources govern the differences in ecosystem properties between white sand and terra firme forests, and why plant species have not more successfully traversed this sharp ecosystem boundary. We here evaluate the hypothesis that soil nutrients (nitrogen [N] or phosphorus [P]) can act as a barrier to the ability of the more productive terra firme tree species to invade and persist within the white sand plant community. To explore this idea, we established an in-situ garden experiment in white sand forests where we transplanted both white sand and congener terra firme seedlings in 40 gardens. We administered 5 treatments (+N, +P, +N+P, control, and exclusion of belowground competition) and quantified seedling growth, defoliation, and mortality.

Our results after 34 months of growth indicate that soil nutrient availability could play a fundamental role in the separation of white sand and terra firme plant communities. When transplanted into white sand habitat under control conditions, terra firme species consistently under-performed congener white sand species in relative leaf area growth rate and survivorship. However, both N and P addition decreased the growth of white-sand species relative to control, and terra firme species were able to grow at the same rate as their white sand congeners under P addition. White sand species, which are likely adapted to nutrient-poor conditions, seem to demonstrate a physiological intolerance to increased N and P availability. On the other hand, P enrichment may allow the highly competitive terra firme species to invade and persist in white sand habitats. This study lends insight to how soil resources, which may be influenced by the underlying geology and the functional traits of the plant community itself, can control dramatic shifts in biodiversity, productivity, and carbon storage in tropical forests.