Larger seeds in tropical floras: consistent patterns independent of growth form and dispersal mode

Many species of tropical moist forests have large seeds compared to those found in temperate floras. This could be attributed to a prevalence of woody growth forms, or adaptation to dispersal by vertebrates, or the dense shade of a closed canopy, rather than to an intrinsic tropical v. temperate difference. We compared tropical v. temperate seed mass data at two geographic scales; firstly within Australia between tropical, subtropical and temperate open woodlands, then between six tropical and temperate datasets from five continents. Within Australia we found that seed mass increased with decreasing latitude in ten out of eleven growth form-dispersal mode combinations: only wind-dispersed graminoids showed no significant effect. While the pattern of generally larger seeds in the tropical flora was associated with tropical families tending to have larger seeds than temperate families, we also found that tropical species had larger seeds than temperate cofamilials. As a preliminary test of the generality of these findings we included a further four tropical and temperate data sets in a second analysis. Average tropical seed masses were larger than temperate seed masses in all testable growth form-dispersal mode combinations, and statistically larger in five out of seven cases: unassisted, vertebrate- and wind-dispersed woody species, and unassisted and vertebrate-dispersed forbs. No difference was found for wind-dispersed forbs or for graminoids lacking dispersal structures. That an effect was found in these combinations rules out explanations based solely on characteristics of tropical v. temperate dispersers, although this may be a contributing factor. Instead we suggest that some aspect of tropical systems, for example higher metabolic costs of seedling production, has selected for a general increase in seed mass among tropical taxa.