Effects of blackberry (Rubus discolor) invasion on oak population dynamics in a California savanna

We conducted studies to assess the effects of Rubus discolor encroachment on native oak recruitment on a cattle ranch in northern California. R. discolor (Himalayan blackberry) is an invasive exotic plant species, especially in more mesic areas of California rangelands. Our results demonstrated that saplings of both Quercus douglasii (blue oak) and Quercus lobata (valley oak) were preferentially associated with thickets of R. discolor. Analysis of browsing damage, growth forms, and reconstructed histories of height growth of small oaks suggested that berry thickets promoted oak seedling recruitment into the sapling size class by protecting them from browsing. Experimental plantings of acorns across the grassland/thicket interface, however, revealed that the interiors of the berry thickets were very poor locations for oak seedling establishment. The overall effect of blackberry encroachment appeared to be a transient effect, producing a pulse of recruitment into the sapling size class from a smaller size class.Long-term effects on oak recruitment will be negative, due to the elimination of oak seedling establishment inside berry thickets. Even if berry thickets had not suppressed seedling establishment, however, a simple model suggested that even temporary occupancy of a site by berry thickets would reduce recruitment of oaks into the sapling size class for many decades. In the model, the pulse of sapling recruitment, caused by protection from browsing, depleted the pool of short, suppressed oaks that gave rise to the saplings, thereby reducing rates of further sapling recruitment. Recognition of the importance of a pool of small, browsed oaks and its impact on population dynamics in savanna systems has wide-ranging implications for interpretations of current and historical oak population structures and for predicting long-term impacts of management practices in oak savanna.