- The influence of overstory openness on the reproductive
ecology of Pacific yew (Taxus brevifolia Nutt.) was
investigated on 4 sites in western Oregon over 2 years. The
breeding system of T. brevifolia was found to deviate from
pure dioecy under a broad range of canopy and site
conditions. Production of female strobili was observed on
17 of 58 predominantly male trees, while no male strobili
were observed on 57 female trees. Genet sex ratios were
significantly biased in only 1 population, where male genets
outnumbered female genets by almost 2 to 1. Mean floral sex
ratios were significantly male-biased in all populations and
ranged from 5 to 12. Pollen-ovule ratios were in excess of
1,000,000 for all populations. In contrast, reproductive
effort based on masses of mature strobili were female-biased
by a factor of 1.1 to 5 for all sites. Seed masses also
varied inversely with elevation.
Pollination phenology varied with elevation and
overstory openness. Pollen first began shedding at the
lowest sites, and earlier in trees under open conditions
than in trees with overstory canopy cover. The duration of
pollen shedding varied from 3 to 20 days, and tended to be
more protracted at lower sites and under open canopy
Most of the variation in reproductive potential, as
indexed by strobilus production, occurred within sites and
within trees. Little variation between years was observed
in male strobilus production during the three years of this
study. Also, while female strobilus production was
significantly greater in 1993 than in 1994, seed production
did not differ between years.
Overstory openness was positively associated with
growth and reproductive potential of T. brevifolia.
Specific leaf area was inversely correlated with overstory
openness, and branching was positively correlated with
overstory openness, suggesting that T. brevifolia adapts to
overstory removal by producing denser foliar tissue and
increased self-shading. In contrast to reproductive
potential, seed production was not significantly associated
with overstory openness during the two years of this study.
Also, seed efficiency (the ratio of seed production to ovule
production) was negatively associated with overstory
Seed efficiency ranged from 5 to 34%, and attrition
occurred in two phases. The early phase occurred during the
pollination period and was probably due in part to
pollination failure. Supplemental hand-pollination resulted
in a doubling of seed efficiency on two of the sites, but
average seed efficiency was still less than 15% on branches
receiving supplemental pollen. Other potential sources of
early attrition included damage from phytophagous mites,
pathogens, frost and genetic incompatibility.
Attrition in the later stages of seed development was
due in part to predation by vertebrate seed consumers.
Predator exclusion significantly increased seed development
efficiency (the ratio of seed production to developing
ovules) on 3 of 4 sites over 2 years.
Seed production was positively correlated with
overstory openness on branches bagged to exclude
vertebrates, suggesting that resource availability was
important for seed production in the absence of predation.
However, evidence for resource limitation of seed production
was not consistent. Seed efficiency was not significantly
associated with overstory openness in 1993, and no
associations were detected between vegetative growth or
previous reproduction and seed efficiency in 1994.
Possible evolutionary explanations for low seed
efficiency in T. brevifolia include the effects of sexual
selection, stochasticity in pollination and predation, and
the importance of excess ovaries as reserves that compensate
for constant sources of mortality. Sources of seed
attrition varied considerably among years and sites,
emphasizing the importance of spatial and temporal
variability in the reproductive ecology of T. brevifolia.