Untangling interacting mechanisms of seed mass variation with elevation: insights from the comparison of inter-specific and intra-specific studies on eastern Tibetan angiosperm species

Abstract

With increasing elevation, seed mass is expected to be either larger for its advantage during seedling establishment in stressful high-elevation environments ("stress-tolerance" mechanism) or smaller due to energy constraints. Based on the combination of inter- and intra-specific analyses on 4,023 populations of 320 Tibetan plant species, we found an overall positive within species but negative among species seed mass-elevation relationship, suggesting that regional seed mass distribution with elevation is affected mainly by the energy-constraint mechanism at among-species level, but by the stress-tolerance mechanism at within-species level. Moreover, both intra- and inter-specific analyses revealed a mass-dependent seed mass variation along elevation gradients: small seeds tended to increase but large seeds tended to decrease with increasing elevation, indicating that the stress-tolerance (respectively energy-constraint) mechanism may exert a stronger effect on elevational distribution of small-seeded (respectively large-seeded) populations or species. The mass-dependent seed mass variation along elevation gradients, however, was absent within woody and zoochorous species, implying substantial available resources and long time for seed development, and the covariation or coevolution between the mass of zoochorous seeds and their dispersers may allow for a (partial) decoupling of species' seed mass and their intra-specific variation with elevation. Together our results call for more comparative analyses at different taxonomic levels in detecting geographic variation in a trait.