The relationship between climate and Rothrock sagebrush colonization patterns

In montane meadows of the southern Sierra Nevada mountains (Calif., USA), Rothrock sagebrush (Artemisia rothrockii G.) has expanded into sites once dominated by herbaceous species. We explored the relationship between climate and shrub establishment by estimating Rothrock sagebrush age distributions from growth rings. We compared these age distributions with annual records of spring snowpack and summer precipitation across 4 vegetation types that differed in water table depth, soil moisture, and vegetation cover. In the 2 vegetation types where the water table is consistently deeper than I m, Rothrock sagebrush stands were up to 40 years old and had relatively even age structures that showed no strong relationship to climate. In the 2 vegetation types with a shallow water table - but with contrasting soil moisture and herbaceous cover - the majority of shrubs colonized synchronously between 1984 and 1994, a relatively dry period that followed the wet 1982 to 1983 El Nino. These and other published data suggest that initial shrub colonization of new sites is facilitated by wet years, which may increase seed production, germination, and seedling survival. However, once sagebrush stands are established and local seed supply is abundant, its continued recruitment seems independent of climate.