Reproductive dynamics of Dirca occidentalis, a rare, endemic shrub.
Graves, William*,1, Ackerly, David2, 1 Iowa State University, Ames, IA2 University of California, Berkeley, Berkeley, CA
ABSTRACT- Dirca occidentalis A. Gray (western leatherwood) is a rare shrub endemic to the San Francisco Bay Area, occurring in patchy populations within communities ranging from chaparral to moist woodlands. Conflicting reports regarding whether D. occidentalis reproduces asexually led us to document the rhizomatous spread of the species. We then used DNA analyses of two groups of ten adjacent plants to estimate the frequency of clones. Averaged over groups, eight of ten plants were putative clones with at least one other group member. Because little was known about recruitment through sexual reproduction of this winter-blooming species, we also documented the success of fruit formation on 50 plants, 25 in each of two populations that differed in elevation. Flowers at the higher elevation led to fruits at a comparatively high rate (32% vs. 5% at lower elevation). Anthesis was earlier at the lower elevation. This may have reduced fruit set by exposing flowers to low temperatures or by decreasing the frequency of pollinators. We hypothesize the former, because fruits matured on branches that we bagged to preclude pollinators, and because the possible pollinator that we observed visiting flowers most often, Calypte anna Lesson (Anna's hummingbird), is active near D. occidentalis throughout the winter. The highest rates of fruit set (up to 71%) were among late-blooming plants at relatively unshaded positions in the landscape. While D. occidentalis in certain niches may reproduce sexually at high frequency, the population dynamics of this typically understory species appear strongly linked to its capacity to spread asexually.
Key words: Dirca occidentalis, plant reproduction, endemism
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