Arid environments select for larger seeds in pines (Pinus spp.)

Seed mass is a main determinant of seedling establishment, particularly under stressful conditions, as bigger seeds offer more resources to the emerging seedling. This has led to the expectation of increased seed mass under harsh conditions. Here, we studied the role of different components of environmental harshness on the evolution of seed size in Pinus. We assessed whether differences in cold and arid conditions between subgenera Strobus and Pinus matched differences in seed mass and tested the existence of positive trends between seed mass and cold and aridity, by controlling for evolutionary patterns that might derive from shared ancestry. Our results corroborate that Strobus pines inhabit drier and/or colder environments and have heavier seeds than subgenus Pinus. Variation in seed mass was largely determined by co-ancestry, and this strong phylogenetic signal cancelled the association between cold and seed size. Conversely, the effect of aridity on the size of pine seeds was unambiguous and independent of ancestry. These results strongly support that aridity exerts directional selection on seed size, which might trigger a selective feedback with vertebrate dispersal. Coupled selection on recruitment and dispersal traits will ultimately determine seed evolution.