Plant diversity is linked to nutrient limitation of dominant species in a world biodiversity hotspot

Questions What is the role of nutrient limitation of dominant species in maintaining plant species diversity at small spatial scales, particularly in biodiversity hotspots? Location Southwest of Western Australia. Methods The extent of nutrient limitation was determined by measuring variation in the foliar nutrients of dominant plant species and total above‐ground biomass in relation to soil N and P across 16 plots. Plant species richness and evenness for the same plots were then regressed against foliar N:P and δ 15 N of the dominant species. Results Foliar N and P content as well as above‐ground biomass increased, while foliar N:P decreased with increasing soil N and P, suggesting that dominant species are limited by both N and P in southwest Australia. In contrast, foliar δ 15 N values only increased with decreasing soil P, indicating that δ 15 N enrichment reflects low P availability. Species richness increased at sites where foliar N:P was higher and δ 15 N more enriched, suggesting that a relatively greater number of rare plant species co‐exist where the growth of dominant vegetation is more nutrient‐limited. Conclusions The mechanisms that allow an exceptionally high number of plant species to co‐exist at small spatial scales in a southwest Australian biodiversity hotspot are linked to the extent to which the dominant plant species are limited in N and P. In addition, on nutrient‐impoverished soils, a foliar N:P ratio >16 can reflect N as well as P limitation. While foliar δ 15 N also reflects availability of soil P, it is not necessarily a reliable indicator of nutrient limitation in southwest Australia, and perhaps in other environments where plants are highly adapted to low‐nutrient conditions.