Browsing impacts on the stable isotope composition of chaparral plants

We assessed the effects of herbivory on competition, water stress, and potentially biological nitrogen fixation on three species of endemic shrubs using variations in the stable isotope ratios ( 13 C/ 12 C and 15 N/ 14 N) in leaves of chaparral shrubs in a long‐term field experiment. While variations in isotopic ratios of plants are often attributed to abiotic stresses, impacts of biotic interactions are rarely evaluated. Our site was a low‐nutrient, chaparral community on the central California coast. In this system, deer browsing on Ceanothus rigidus , which have symbiotic N‐fixing bacteria ( Frankia ), was intense and suppressed growth, while the two non‐fixing shrubs ( Arctostaphylos pumila and Ericameria ericoides ) were not browsed heavily. For Ceanothus , excluding deer increased both plant size and the δ 15 N value to ~0‰; δ 13 C values also increased as the plants increased in mass. In Arctostaphylos and Ericameria , stable isotope values did not change, while plant sizes remained the same or even declined when deer were excluded. We interpret the change in Ceanothus δ 15 N values as due to increased N fixation after evaluating possible alternative explanations. The increase in Ceanothus δ 13 C values may be due to increased water stress with substantial shrub growth. More broadly, herbivore suppression of N fixation may impact ecosystem processes such as productivity and N cycling, as well as an ecosystem's ability to respond to increased CO 2 .