Proximate factors affecting fruit set and seed mass of Styrax obassia in a masting year

Abstract

We studied the relative importance of pollen limitation and resource limitation as proximal factors affecting fruit set and seed mass in a sub-canopy tree, Styrax obassia Sieb. et Zucc. in a masting year. We selected four individual trees and three branches of each tree for pollination and artificial defoliation experiments. The pollination experiment contained natural, self and cross-pollination treatments for each branch. Artificial defoliation rates in branches were 0%, 30%, and 60%. Pollen limitation seems not to have affected fruit set, because cross-pollination did not result in greater fruit set than did natural pollination. S. obassia appears to be partially self-incompatible as fruit set obtained by self-pollination was decreased compared to natural fruit set. Fruit set in both 60% and 30%-defoliated inflorescences was not statistically different from that of non-defoliated ones. Mean seed mass varied among trees and was affected by the pollination experiment. Thus, we conclude pollen quantity did not limit fruit set although its quality may affect seed mass, but stored resources may be a factor because limitation of current resources did not affect fruit set and seed mass of S. obassia.