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

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 ml 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.