Diversity of plant nutrient-acquisition strategies increases during long-term ecosystem development

Plant species diversity increases as soil phosphorus availability declines during long-term ecosystem development(1,2). The increase in plant species diversity is associated with a decline in above-ground functional diversity, because leaf traits converge on a high phosphorus-use efficiency strategy on old and infertile soils(3,4). In contrast, the response of below-ground traits that directly influence nutrient acquisition remains poorly understood(3,5); yet it might be key to understanding how soil fertility drives patterns of plant species diversity(1). Here we show a marked increase in the richness and diversity of plant nutrient-acquisition strategies with declining soil phosphorus availability during long-term ecosystem development in a global biodiversity hotspot. Almost all nutrient-acquisition strategies currently known were found in plants from the most infertile soils, despite these being some of the most phosphorus-impoverished soils on Earth. Mycorrhizal plants declined in relative abundance by >30%, although the decline was compensated by an increase in non-mycorrhizal, carboxylate-exuding species that 'mine' phosphorus from the soil using different strategies. Plant species richness within individual nutrient-acquisition strategies also increased dramatically, with the species richness of many strategies more than doubling between the youngest and oldest soils. These results reveal increasing functional diversity of below-ground traits related to nutrient acquisition during ecosystem development, suggesting that no single combination of traits, including those related to nutrient-acquisition strategies, is superior to all others at extremely low soil fertility. Furthermore, the increasing diversity of nutrient-acquisition strategies with declining soil fertility, despite functional convergence of above-ground traits(4,6), suggests that fundamentally different plant community assembly processes operate above-and below-ground.