Browsing by mammalian herbivores like white-tailed deer is known to dramatically alter patterns of forest regeneration, but how different levels of top-down pressure cascade back up the food chain over the long-term remains untested. We investigate this question using data from a 30-year study wherein deer density was manipulated experimentally for the first 10 yr (1979-1990) in northwest Pennsylvania. Ten-year average deer density ranged from 3.6 to 28.0 deer/km2 in 16 experimental stands across four study sites. Canopy tree composition was sampled in 3-5 400-m2 square plots per stand. To investigate how deer-induced changes in vegetation affected foliage density, we estimated leaf area index in each stand using 262 hemispherical canopy photographs analyzed with Gap Light Analyzer software. Data presented elsewhere (ESA posters by Wheatall et al. 2010 and Nuttle et al. 2009) complement these data to develop a more complete understanding of the long-term effects of deer browsing on forest ecosystems.
Results/Conclusions
Deer density during stand initiation has lasting impacts on the structure of plant, insect, and bird communities inhabiting forest canopies 20 yr after deer density treatments ended. Increasing deer density caused significant (P = 0.0166, R2 = 0.3922) reductions in canopy foliage density even though tree density and basal area are no longer correlated with former deer density (P = 0.8894 and P = 0.1277, respectively). Canopy foliage density was influenced largely by proportion of black cherry in the canopy (P = 0.0066, R2 = 0.4726), which was positively related to deer density during stand initiation (P < 0.0001, R2 = 0.7352). Combined with the result that species of trees preferred by deer as browse support higher densities of caterpillars than species avoided by deer (results presented by Wheatall et al., this conference), these results extrapolate to the stand scale to produce a significant negative relationship between former deer density and current caterpillar density in the forest canopy (P = 0.0341, R2 = 0.3472). Lower foliage and caterpillar densities may explain why deer density during stand initiation was negatively related to bird density after 29 yr (Nuttle et al., 2009 ESA poster). These results suggest that relatively short-term (10 yr) perturbations in top-down control over mammalian herbivores ricochet all the way back up the food chain over the long-term (20 yr later). Overpopulation of mammalian herbivores can thus shift an ecosystem from top-down to bottom-up regulation of ecosystem function.
