SEED MASS IN KOBRESIA-DOMINATED COMMUNITIES IN ALPINE MEADOWS AT TWO DIFFERENT ELEVATIONS

Many different selection stresses could affect seed mass. We collected evidence in support of the hypothesis that there exists a significant positive effect of altitude on both intraspecific and interspecific changes in seed mass. We weighed 71 seed species, including 44 species that occurred in both low-altitude and high-altitude Kobresia communities in alpine meadows of the Qinghai-Tibetan plateau. We calculated average seed mass, seed mass variance within community, and seed mass range for the Kobresia community at two altitudes. In 50% of the 44 common seed species, mass increased with altitude, but in the other 50%, mass decreased. Average seed mass within communities decreased with altitude. There was no single significant relationship, positive or negative, between seed mass and altitude for all species in the two Kobresia communities. The unique species in the low-altitude site presented a larger mean seed than the unique species in the high-altitude site. However, the higher-altitude Kobresia community had a lower average community seed mass and seed mass range than the low-altitude community. Our results indicated that mean seed mass of each species responded to altitude within community, but did not show a uniform trend for seed mass change between communities along altitudinal gradients.