Successional Age, Not Functional Guild, Predicts Inter-Species Phosphorus Acquisition Strategies in a Lowland, Tropical Forest

Abstract

Trees capable of di-nitrogen fixation play a critical role in establishing the nitrogen content of early successional soils, yet the responses of this functional guild to nutrient co-limitation and nutrient cycling in tropical ecosystems remain largely unresolved. Notably, fixers in early successional forests grow more rapidly than non-fixers, despite supporting the high energetic costs of fixation. This eccentricity has led some to suggest fixation may provide an adaptive advantage to a fixer’s early development, possibly by assisting fixers in alleviating additional sources of nutrient limitation. We examined the interaction of nitrogen fixation and phosphorus acquisition strategies across a successional soil gradient in a lowland, Panamanian tropical forest. We quantified the extent of mycorrhizal colonization and root phosphatase activity for three species of fixer and three species of non-fixer. Statistical comparisons of relative differences in mycorrhizal colonization and root phosphatase activity among individuals allowed us to elucidate the extent and identity of relationships between phosphorus acquisition strategy, species, successional age, tree size, soil phosphorus content, and functional guild. Significantly, our findings indicate that a tree’s metabolic interest in phosphorus is not influenced by its propensity for di-nitrogen fixation, but rather by the local, macro-nutrient limitations relative to evolving successional soils. This study represents the first observational data set to combine both mycorrhizal and phosphatase data in the context of functional guild and successional age. Thusly, our results have broad implications for the study of nutrient limitation and biogeochemical cycling in tropical ecosystems.