Multi-factor control of seed mass of species on the eastern part of the Qinghai-Tibetan Plateau: Integration of environmental filters, local adaptation and correlated evolution

•We examined seed mass variation of 1340 Qinghai-Tibetan species.•Dispersal mode contributed most to seed mass variation and evolution.•Species’ light/moisture/disturbance niche position also drove seed mass variation.•Proposing phylogenetic regression trees to examine nonlinear correlated evolution among traits.•Revealing seed mass’ regional distribution, local adaptation and evolutionary pattern. Seed mass is a central plant life history trait that varies and evolves due to selection by biotic or abiotic factors. To determine the factors affecting seed mass variation among species and phylogeny, we used linear and regression tree analyses of three biotic traits and four niche traits to quantify the relative importance of each trait on seed mass for 1340 angiosperm species from the eastern part of the Qinghai-Tibetan Plateau. Dispersal mode was the strongest predictor of both species and phylogenetic seed mass variation, whereas life form only had a small impact. These results suggest that seed mass has responded to dispersers rather than the life history or available stored resources. Moreover, in both inter-specific and phylogenetic comparisons, species with similar light, moisture and disturbance niche positions tended to have similar seed mass, indicating that adaptation to local environments is a second force driving species-level seed mass variation. The lack of any relationship between thermal niche and seed mass in the phylogenetic analysis implies that seed mass variation along thermal gradients may be a result of regional species sorting, or alternatively, a plastic response to the variation in plant size. All species and phylogenetic multivariate models showed that both dispersal mode and species light/moisture niche explain seed mass variation, suggesting the importance of coevolution or covariation between seeds and vertebrates and the selection or filter effect of habitats on early life-history stages of plants in affecting individual seed development. Lastly, our results imply that phylogenetic constraints and niche conservatism may operate simultaneously in determining regional patterns of seed mass variation.