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Determinants of the distribution and reproductive success of Dirca occidentalis

Bill Graves (Iowa State)


One of the botanical treasures of Jasper Ridge is its large population of western leatherwood (Dirca occidentalis), a San Francisco Bay area endemic whose yellow flowers brighten the start of winter. Dirca is one of the best-mapped plant species at Jasper Ridge, thanks to work by several JRBP docents. Both of these factors drew Bill Graves to JRBP as the center of a study of Dirca's regional distribution, reproduction, and similarity to Dirca palustris of eastern North America. Bill found significant genetic differentiation among geographically separated populations of Dirca within the Bay area but relatively little variation within populations, and he confirmed the capacity of Dirca to reproduce asexually as well as by seeds. 

Following his initial studies, Bill began an experiment to test the hypothesis that low temperatures during winter limit the sexual reproductive success of western leatherwoods at certain sites. For the 2005-06 winter, he placed "Tyvec" tents over three individual plants to try to raise night-time temperatures (without excessively raising daytime temperatures). Because the Tyvec will exclude pollinators from the plants, Bill has caged three other plants in fiberglass screening, which is intended to block pollinators without altering temperature. Three additional plants are unmanipulated controls. Bill is monitoring the temperature at all nine plants to see if his treatments meet his goals. If so, he will compare seed set among 1) self-pollinating, warmed plants, 2) self-pollinating, un-warmed plants, and 3) open-pollinating, un-warmed plants. 

Dirca presents a rare opportunity to study a species that is strongly restricted in distribution yet abundant enough at Jasper Ridge that it can be investigated in detail without putting it at risk. Bill's results will serve as a basis for continued efforts to understand this rare and beautiful member of the Bay area flora.

One of the Tyvec-tented plants, as seen from the fire road between Escobar and Goya.

Project Location (Sector 34)

Visible from Trail/Road

F - Grassland Fire Road