The Distribution, Variation And Evolution Of Seed Size On The Eastern Qinghai-Tibet Plateau

In this paper,1268 angiosperm species, being derived from 427 genera and 109 families (Davies 2004), were gathered up from the eastern Qinghai-Tibetan flora to analyze the relationships between seed size and 17 related attributes, including altitude, microhabitat light, microhabitat moisture, soil PH, soil organic matter, seed dispersal mode, growth form, plant height, seed development time, seeds per fruit, specific leaf area, sexual system, floral symmetry, pollination pattern, competitive strategy, ruderal strategy and areal-types. Statistical methods included one-way and nested ANOVA, linear regression, regression tree, and phylogenetically independent contrast. The results showed:1. Seed size of the Qinghai-Tibetan flora ranged over 6-7 orders of magnitude, from 10-3mg to 104 mg. However, seed size of 75.9% species ranged from 10"1 mg to 101 mg, suggesting the flora was characterized by small seeds.2. Order, family and genus accounted for 19.5%,48.9%and 90.2%of total variation in seed size, respectively. Thus, it was evident that seed size was strongly related to phylogeny.3. The cross-species and evolutional relationships between seed size and most related attributes, except for floral symmetry, were significant. Seed dispersal mode was the main factor affecting seed size variation, following by seed development time, seeds per fruit, plant height and growth form.4. The optimal model about the cross-species relationship between seed size and altitude was a 3-step regression, which could account for 8.8%seed size variation. In the model, the slope of the relationship was steep in the altitudinal group< 2200 m asl., about 1.4-fold decrease in seed size with every 100 m increase in altitude, but was moderately steep in the altitudinal group from 2200 m to 3450 m asl., about 1.1-fold decrease in seed size with every 100 m increase in altitude. Moreover, there was no significant seed size-altitude relationship in the altitudinal group> 3450 m asl.5. The influences of microhabitat light, microhabitat moisture, seed dispersal mode, growth form and plant height on seed size decreased, but the influences of seeds per fruit increased, with increasing altitude.6. There was a size-dependent seed size variation along altitudinal gradients. Based on phylogenetically independent contrast (PIC), the relationship between seed size contrasts and generic seed size averages was significantly negative. If generic average seed size was less than 1mg, it would increase along altitudinal gradients, otherwise, would decrease along altitudinal gradients.7. There was an altitude-dependent seed size variation along altitudinal gradients. Based on PIC, the relationship between seed size contrasts and generic altitudinal averages was significantly positive. When generic average altitude was less than 3000m asl., seed size contrast would be less than zero, otherwise, would be more than zero.8. Seed size was tightly related with the areal-types of angiosperm. World widespread species had the smallest seeds, while the seeds of tropical species were biggest.