Reduced N-fixation in maritime chaparral under intense browsing

Fixation of atmospheric nitrogen by bacterial symbionts of plants affects not just plant growth, dynamics, and ecosystem properties, but also biotic interactions. However, direct impacts of biotic interactions on N-fixation are largely unknown, because most studies of variation in N-fixation focus on abiotic factors, including soil propertiess (nitrogen, phosphorus or water contents), light, and temperature. We assessed impacts of browsers (deer) on N-fixation by a shrub, Ceanothus cuneatus rigidus, and its actinorhizal symbiont, Frankia, to test the hypothesis that heavy browsing reduces N-fixation, presumably because loss of photosynthetic tissue reduces net carbon fixation and the plant’s contributions to the symbiosis. We examined stable N and C isotope compositions of leaves from Ceanothus and adjacent non-fixing shrubs, Arctostaphylos pumila, in a long-term field experiment that excluded browsers from some plants. Heavily browsed Ceanothus had the same δ¹⁵N values as adjacent non-fixing Arctostaphylos, and we conclude that Ceanothus was not fixing atmospheric N₂ under natural browsing levels. In contrast, within a year after deer browsing was prevented, δ¹⁵N values were close to 0‰, showing that the Ceanothus/Frankia symbiosis had begun to fix atmospheric N₂; these levels were maintained for at least 7 years without browsing. δ¹³C values did not change over time in browsed or unbrowsed Arctostaphylos or in browsed Ceanothus, but δ¹³C values increased in unbrowsed Ceanothus, which also grew faster than browsed controls. The higher δ¹³C values may reflect either higher photosynthetic rates or greater water limitation in N-fixing plants.