Seed mass and seed nutrient content as predictors of seed output variation between species

Abstract

In patch‐occupancy models for vegetation, propagule output per area occupied is a key species trait, influencing the potential to colonize vacant patches, and hence species dynamics and coexistence. We estimated seed output across a range of species and quantified its relationship to seed dry mass, seed N and P content, and accessory costs in fruiting structures. Fruiting and seed production data were obtained for 47 woody perennial species, spanning an almost 3000‐fold range of seed mass, over a period of one year in Ku‐ring‐gai Chase National Park, New South Wales, Australia. Seed output was measured as numbers per m 2 canopy outline and per m 2 leaf area. Of cross‐species variation in seed output per m 2 canopy outline per year, 72% could be predicted from seed mass alone, with a directly inverse relationship (log‐log slope not significantly different from −1). Seed output per m 2 leaf area could be predicted somewhat more tightly (75%), indicating leaf area per canopy outline area accounted for some cross‐species variation. Reproductive production per m 2 occupied per year varied much less than seed mass and accounted for the remaining variation in seed output. Although accessory costs were about equal in magnitude to seed mass as a component of aggregate investment per seed, they were strongly correlated with seed mass, and consequently did not add substantially to the predictive power. Total mass of N or P per seed were found to be slightly but significantly better predictors of seed output variation than dry seed mass (83% and 78%, respectively). This supports the idea that mineral nutrients are a more fundamental currency for seed production than dry mass. Seed mass, whether measured as dry mass or as N or P, appears to be the principal driver of variation in seed output per m 2 occupied, and consequently is among the most important dimensions of ecological variation across coexisting species.