COS 124-6
Changing paradigms in post-fire chaparral communities: interactions of herbivory, competition and facilitation continue affecting chaparral dynamics of >70 years after fire
Most studies of chaparral communities focus on the first decade after fires, when herbivory and interactions among plants drive rapid changes in plant composition. The few studies of chaparral many decades after fire conclude that recruitment and biotic interactions are rare and unlikely to influence community composition before and after the next fire. However, our field experiments and demographic analyses, at the Fort Ord Natural Reserve in coastal California, found substantial plant establishment and intense herbivore/plant and plant/plant interactions in chaparral that has not burned for >70 years. We focus here on the age- and spatial-structures of young Ceanothus rigidus and Quercus agrifolia (oaks) growing within vs. outside dense manzanita mounds to test the hypotheses that: 1) Ceanothus and Quercus outside mounds are heavily browsed by deer; 2) Ceanothus and Quercus within the mounds are protected from deer browsing although they may be exposed to intense competition from the taller, surrounding manzanita; and 3) successful Ceanothus and Quercus eventually overtop and outcompete surrounding manzanita and reach a size-refuge from deer browsing.
Results/Conclusions
We show that dense manzanita mounds provide both Ceanothus and oak with safe sites for germination, growth and reproduction despite the potential for competition. Plants emerging near the middle of manzanita mounds had limited deer browsing; but, Ceanothus and oak saplings (or parts of saplings) growing near mound edges or in the open were heavily browsed. Ceanothus just emerging from mounds set >6 times the number of seeds (per unit surface area) as unprotected plants, are ~20-30 years old, and ~6 years younger than surrounding manzanita stems; the pattern for oaks is similar. The protected Ceanothus and Quercus grow and eventually overtop surrounding manzanita, which dies back, possibly from direct competition with these taller plants. These complex interactions have major implications for the dynamics of broader landscape mosaics of chaparral and oak woodlands; for instance, plants within manzanita mounds may contribute disproportionately to the seed bank. We hypothesize that interactions of Ceanothus and oak with the surrounding manzanita even in old chaparral stands are critical to future community composition both before and after the next fire.