Conditional migration and foraging patch selection by a generalist herbivore results in a spatial resource subsidy in relict eastern hemlock forest

Background/Question/Methods

Ungulates interact with ecosystem processes through a number of direct and indirect mechanisms, across a wide range of spatial and temporal scales. In forested ecosystems, ungulates may affect nitrogen availability directly through nitrogenous waste deposition and indirectly via selective herbivory. The former mechanism tends to increase available nitrogen, whereas the latter mechanism results in a decrease if ungulates are selecting nitrogen-rich vegetation. Isolating these mechanisms is challenging, and most work thus far has focused on indirect consumptive effects. We draw upon two long-term ungulate habitat monitoring studies paired with exclosure experiments to address the hypothesis that forest ungulates can directly increase nitrogen availability. In one study, white-tailed deer (Odocoileus virginianus) habitat use was monitored through annual fecal pellet counts for eight consecutive years in 39 eastern hemlock (Tsuga canadensis) stands across the western upper peninsula of Michigan. Fecal pellets were collected and analyzed for nitrogen content to develop allometric relationships between pellet abundance and nitrogenous waste deposition. In the second study, deer use of twelve canopy gaps was assessed in 2007 and 2011. Each study contains a subset of deer exclosures. We compare the findings of each study to explore potential contingencies for forest ungulate impacts on nitrogen availability.

Results/Conclusions

In the eight-year, 39-site study, estimated deer-excreted nitrogen deposition often approached and occasionally exceeded that of atmospheric deposition at the site level (up to 12.3 kg N ha^-1). Within-site deer use patterns were clumped in space and time, suggesting that repeated deer use results in concentrated hotspots of nitrogen availability. In the canopy gap study, greater nitrogen availability was observed in deer access plots compared to exclosure plots within the same gap.  While deer use was highest in small gaps, the greatest differences between exclosures and controls were observed in large gaps shortly after snowmelt. This is likely the result of a greater initial draw-down of available nitrogen in small gaps by more proximate overstory trees.  Additionally, warmer conditions in large gaps relative to small gaps early in the growing season may enhance mineralization rates. By May no difference was observed between exclosures and controls suggesting concentrated winter/dormant season use is likely driving ungulate related nitrogen availability in these openings. In summary, these studies suggest that forest ungulates may increase nitrogen availability via nitrogenous waste deposition, but the effect is contingent upon spatial scale, the seasonal timing of deposition, and habitat type.