I assessed long-term changes in the community interaction web of long-lived, slow growing shrubs in response to deer browsing, using a field experiment that has been maintained since 1996 in chaparral vegetation near the central California coast. Natural levels of deer browsing were high on Ceanothus cuneatus rigidus¸ a woody shrub with symbiotic bacteria (Frankia) that can fix atmospheric N2, while the other two main shrubs, Arctostaphylos pumila and Ericameria ericoides, are non-fixers and are not heavily browsed. The goal of this experiment was to identify mechanisms underlying dynamic interactions in this community.
Results/Conclusions
Ceanothus responded rapidly in exclosures that reduced deer browsing: growth and seed production increased and plant morphology began to change within a year, while new Ceanothus seedlings were found almost entirely in plots without deer. Over time, Ceanothus has out-competed the other shrubs. However, the seed bank beneath unbrowsed Ceanothus did not change, despite the large increase in seed output compared to control plants with ambient levels of browsing, because of increased activity from seed-eating rodents. Woodrats, Neotoma fuscipes, began removing branches on many of the unbrowsed Ceanothus with the new morphology, whereas previously they clipped branches only on Arctostaphylos. Nitrogen-fixation, which may have been limited in heavily browsed Ceanothus, appeared to increase in plants no longer browsed. However, overall, soil N levels declined in plots without deer. Finally, the net growth of shrubs in the experimental exclosures reduced the amount of open space between them, which negatively affected annual plants that grow and reproduce successfully only in these open areas. In summary, community interaction webs were very sensitive to deer browse despite the slow growth of the dominant plants in this system.