On retrogression and phosphorus limitation in diverse tropical forests

Abstract

Phosphorus availability declines during long-term ecosystem development, leading to a corresponding decline in the biomass and productivity of forests growing on old strongly weathered soils. This process - termed retrogression - occurs in forest ecosystems in a variety of landscapes worldwide, but not in species-rich tropical tree communities, where high growth rates and plant biomass occur on old soils with low phosphorus availability. We hypothesize that productivity is maintained in such forests by their high regional diversity, which means that they are more likely to contain species that can grow efficiently on infertile soils. This 'sampling' or 'selection' effect is supported by findings from growth rates of tree species across a strong phosphorus gradient in Panama, where widespread species-level phosphorus limitation in a diverse regional flora does not manifest at the community level due to an abundance of fast-growing species adapted to low phosphorus soils. Our hypothesis predicts that retrogression will be absent from species-rich ecosystems elsewhere and has broad implications for our understanding of nutrient limitation and the methods used to detect it, because diverse plant communities on low phosphorus soils are unlikely to respond strongly to abrupt experimental increases in soil phosphorus availability.