White-tailed deer (Odocoileus virginianus) can function as keystone herbivores that reduce the vital rates of plants they preferentially browse. Deer forage in “edge” habitats characterized by productive herbaceous vegetation, such as under canopy gaps. Chronic herbivory by O. virginianus reduces carbon assimilation in herbaceous perennials by removing photosynthetic organs. Carbon deficits, due to herbivory events, yield smaller plants in subsequent growing seasons. However, little is known about how plants subjected to chronic herbivory may recover following deer exclusion and gap creation. Here we examine the effects of deer exclusion and canopy gaps on the size and reproductive effort of four perennial herb species native to temperate forests: Hydrophyllum virginianum, Viola pubescens, Trillium grandiflorum and Uvularia grandiflora. Canopy gaps (200 m2 and 380 m2) and deer exclosures (0.64 ha) were created, respectively, in February and September 2007. During the summer of 2009, we observed the shoot length and reproductive effort (presence and number of flowers) of plants randomly located within experiment plots. We used ANOVA to test whether treatment combinations of gaps and deer exclosures had a significant effect on plant performance.
Deer exclosures improved plant performance, though the magnitude varied among species. H. virginianum and T. grandiflorum were significantly taller in deer exclosures than in unfenced plots (p < 0.05). H. virginianum plants showed the greatest increase in size, being on average 75% taller in exclosures compared to unfenced plants. H. virginianum and V. pubescens flowered at a rate of greater than 75% in exclosures and less than 75% in unfenced plots (p < 0.05). Furthermore, exclosed H. virginanum and V. pubescens produced twice as many flowers as unfenced plants (p<0.05). H. virginianum was the only species exhibiting significant increases in shoot length in canopy gaps relative to under a closed canopy (p < 0.05). We did not detect an interaction among deer exclosures and gaps. In summary, deer herbivory appears to be driving the performance of perennial herbs in this study while canopy gaps do not appear to greatly improve or disrupt the performance of these plants. Our results suggest that once deer are excluded, understory herbaceous perennials can quickly improve performance (i.e., within one – two growing seasons). The variation in the rate of recovery among plants may be related to differences in metabolic rates among taxa.