PARENT SESSION
Poster Session #73: Plant Demography.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC
138
Instability of dominant plants in the northern Vizcaíno region of the Sonoran Desert.
Bullock, Stephen*,1, Martijena, Nora1, Turner, Raymond2, Webb, Robert2, 1 Centro de Investigación Científica y de Educación Superior de Ensenada, Ensenada, Baja California2 U.S. Geological Survey, Tucson, AZ
ABSTRACT- Have populations of dominant woody plants changed in deserts lacking extensive human impacts? We studied mortality and recruitment in two giant succulents of the Sonoran Desert in Baja California, Fouquieria columnaris (cirio) and Pachycereus pringlei (cardón). The study covered time intervals ranging from 29 to 95 years, ending in 1996-2001, and included 73 sites distributed over >14000 km2 north of 28.4°N. The censuses used identified individuals (n= 3557 and 2112, respectively) and were based on repeat photography. Cirio occurred at more sites and was generally more common than cardón. However, mortality exceeded recruitment for cirio at 96% of the sites, compared to only 40% in cardón (n= 68 and 45 respectively). Median values of mortality were about 0.81% y-1 in cirio and 0.31% in cardón. The data suggested a widespread, major decline of cirio, while in cardón there have been fluctuations differing in direction as well as magnitude among local populations. For 14 sites, earlier repeat photographs (1963) permitted the separation of mid- and late-century changes: the qualitative trends were similar but rates varied. The trends in the two species were not locally complementary. Both species are exploited for wood, but this did not appear to be a major factor. Analysis of short and long-term variability of precipitation suggested that population trends were driven by climate history, spatial differences in seasonal precipitation, and the seasonality of recruitment.
KEY WORDS: population dynamics, vegetation change, Sonoran desert, long-term studies