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Indirect effects of deer herbivory on local nitrogen availability in a coastal dune ecosystem

Herbivores can have indirect effects on local nutrient availability if their direct effects on plants lead to changes in the amount or chemical composition of litter reaching the soil surface. Using two exclosure experiments, we evaluated this possibility for blacktailed deer (Odocoileus hemionus columbianus ) feeding on silver bush lupine (Lupinus chamissonis ) in a coastal dune system in northern California. Our first experiment assessed the effects of deer herbivory on 360 lupines that were tracked from newly germinated seedlings in 1996 and 1997 until 2000, when most surviving individuals had reached reproductive age. Results from this experiment showed that browsing by deer significantly reduced growth rates and seed production of lupines and delayed the onset of reproductive maturity. Although deer had no effect on the density of litter accumulating underneath shrubs (g m 2 ), browsing significantly decreased C:N ratios of leaf tissue, due primarily to increased nitrogen content of leaves. Deer browsing also caused significantly increased net nitrogen mineralization rates in the soil under shrubs, although pools of ammonium and nitrate were uninfluenced. The second experiment examined 72 established bush lupines from 1997 to 2000, and showed that deer browsing significantly decreased seed production but had no effect on shrub growth. We also detected trends for browsed shrubs to have reduced pools of ammonium and nitrate underneath their canopies, although no such patterns emerged for nitrogen mineralization rates. Collectively, these data suggest that changes in litter quality, more so than litter quantity, contributed to the indirect effects of deer herbivory on local nitrogen dynamics. We hypothesize that deer browsing induced chemical changes in lupine plant tissue, which increased the nitrogen content of litter reaching the soil surface and subsequently increased rates of nitrogen mineralization.