PS 43-132
Interactions between manzanitas and coast live oak and their effect on herbivory by deer and woodrats in maritime chaparral
Biotic interactions affect the outcome of ecological succession, but studies in older chaparral communities conclude that interactions are no longer important and will not affect community structure before the next fire. However, work at our site in maritime chaparral at Fort Ord, near the coast of central California, a community >70 years post-fire, shows that biotic interactions, including herbivory, competition and facilitation affect the community. We investigated how interactions between manzanita shrubs (Arctostaphylos pumila and A. tomentosa) and coast live oak (Quercus agrifolia) affected browsing by black-tailed deer (Odocoileus hemionus columbianus) and Monterey dusky-footed woodrats (Neotoma fuscipes luciana). We hypothesized that 1) oak saplings are less browsed by black-tailed deer when they grow inside a dense manzanita mound compared to growing on their own; 2) oak saplings are negatively affected by competition from the manzanitas; and 3) young oaks growing inside a manzanita clump may eventually outcompete them when they become taller than the manzanitas. To assess these interactions we measured the structure of manzanita clumps (e.g., height, intactness, distance from oak to manzanitas, age of manzanita stems growing next to the oaks), and assessed browsing damage as well as the age of oaks from wood samples.
Results/Conclusions
Deer browsing is much higher in oaks growing outside compared to inside manzanita clumps, but woodrat damage did not change at all. In addition, the average height with respect to age was higher in oaks growing inside a clump, showing that dense clumps of manzanita can benefit oak growth by making a “natural fence” that shelters young oaks from browsing, despite likely strong competition from the manzanita. Moreover, the browsing intensity in oaks growing within a clump increases as the “natural fence” becomes less intact and as the distance between the oak vegetation and the “inner” edge of the manzanita increases; in both cases, these allow easier deer access. Finally, oaks within a clump are several years younger than nearby manzanita, but manzanita growing immediately beneath taller, older oaks tend to die. As these interactions further develop they can potentially determine the community composition of the marine chaparral where oak trees could eventually become the dominant species.