Variation in abiotic conditions along altitudinal gradients may sort plant species from regional species pools according to their seed mass. 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 owing to energy constraints. Using a large trait database involving 1355 species from the northeastern verge of the Tibetan Plateau, we found that, overall, these two opposing mechanisms balanced out one another, resulting in non-significant seed mass-elevation relationship across all species after controlling for phylogeny. At the same time, we found that the influence of energy constraints on seed mass was indirect and mediated by the variation in plant height. Moreover, our results revealed a mass-dependent seed mass variation along elevation gradients: with increasing elevation small seeds tended to increase (supporting stress-tolerance mechanism) but large seeds tended to decrease (supporting energy-constraints mechanism). Finally, the seed mass-elevation relationships were significantly different among species with different life forms or different dispersal modes, but statistically similar for anemophilous and entomophilous species. This implies that life-history cycle, resource allocation pattern and availability of dispersals agents, rather than pollination efficiency, can affect the responses of seed mass to elevation. Together our results suggest that a comprehensive perspective is necessary when interpreting geographic distribution of even a single trait. Synthesis With increasing elevation, seed mass may be either larger for its advantage during seedling establishment ('stress-tolerance' force), or smaller owing to energy constraints. Our paper shows some novel and importance results in the seed mass-elevation relationship in a northeastern Tibetan flora. Firstly, these two opposing forces operate simultaneously but overall balance out one another. Secondly, the balance tends to shift toward increased energy-constraints (stress-tolerance) with the increase (decreased) in average seed mass. Thirdly, energy constraints on seed mass is indirect and mediated by the variation in plant height. Finally, plant resource allocation pattern, life-history cycle, and availability of dispersal agents can affect the responses of seed mass to elevation. © 2013 The Authors.