FACTORS LIMITING SEED PRODUCTION OF TAXUS-BREVIFOLIA (TAXACEAE) IN WESTERN OREGON

Seed production of Pacific yew (Taxus brevifolia), an understory conif er, was studied at four sites in western Oregon over 2 yr. The effects of pollen supplementation, overstory canopy, and predator exclusion o n ovule attrition were examined. Supplemental hand-pollination of ten trees at two sites resulted in significantly increased rates of ovule development and a doubling of seed efficiency (ratio of seeds to ovule s). However, seed efficiency still averaged <15% on branches receiving supplemental pollen, so pollination was not a primary factor limiting seed production. The number of developing ovules was positively assoc iated with overstory openness, but seed production was not. Seed effic iency was negatively associated with overstory openness. Branches bagg ed to exclude vertebrate seed predators had higher seed production tha n unbagged branches at three of four sites for 2 yr. Ln contrast to un bagged branches, seed production on bagged branches was positively ass ociated with overstory openness, as was the effectiveness of bagging. Therefore. both vertebrate predation and overstory were important ill limiting seed production, and these factors interacted. Factors limiti ng seed production varied in importance among the four sites and betwe en years, illustrating the importance of examining multiple limiting f actors over several sites and years.