Effects of forest canopy gaps on the ground-layer plant community depend on deer: Evidence from a controlled experiment

Questions: How do deer mediate the response of forest ground-layer plant communities to canopy gap size? Location: Flambeau River State Forest in north central Wisconsin, USA (47 degrees 37.4MODIFIER LETTER PRIME N, 90 degrees 47.8MODIFIER LETTER PRIME W). Method: We examined responses of resources, growth forms and temporal guilds to factorial combinations of canopy gap treatments consisting of a range of gap sizes (80, 200 and 380 m(2)), and deer (Odocoileus virginianus) exclusion (0.64 ha fenced exclosures) 4 years following gap creation (3 years following deer exclusion). We contrasted responses in central and transition zones. Results: In the absence of deer, forb productivity increased from pre-treatment levels, particularly where light transmittance and soil moisture were high. In contrast, forb productivity decreased in the presence of deer. These effects were stronger among forbs with early- and late-summer leafing phenology than spring ephemeral, and evergreen species. In contrast, deer amplified increases in graminoid productivity, which were related primarily to light transmittance. Deer did not alter responses of ferns, shrubs, trees seedlings or vines. Canopy gaps increased community heterogeneity among gap sizes and zones, but only in the absence of deer. This was related to the effects of deer on the composition and diversity of gap specialists. Conclusions: The maintenance of biodiversity in northern temperate forest ground-layer plant communities by canopy gap dynamics appears to hinge on deer herbivory, even at relatively modest populations of deer and a productive site. Conversely, increases in resource availability within gaps may function to mitigate some negative effects of deer on forb productivity. Effects may be transient, as species turnover to shrub- and sapling-dominated communities and canopy closure may cause communities to converge in the future. However, populations of undefended gap specialists remain particularly sensitive.

Automated summary

Deer play a significant role in mediating the response of forest ground-layer plant communities to canopy gap size, particularly impacting forb productivity. Canopy gaps increase community heterogeneity, but this effect is only observed in the absence of deer. The study suggests that the maintenance of biodiversity in forest plant communities may hinge on deer herbivory, with implications for community dynamics and species turnover.