Linkages among canopy tree neighbourhoods, small mammal herbivores and herbaceous communities in temperate forests

QUESTION: Small mammals are important consumers of tree seeds in forests worldwide. Few studies, however, have addressed the impacts of small mammals as consumers of herbaceous species in the forest understorey. Local neighbourhood‐scale variation in canopy tree composition has strong effects on resource availability for understorey plants, but can also influence the distribution, abundance and foraging behaviour of small mammals, primarily through spatial and temporal variation in seed rain. In this study we examine the direct and indirect effects of canopy tree neighbourhoods and herbivory by small mammals on the diversity, abundance and flowering of herbaceous species in a temperate forest. LOCATION AND METHODS: In 1994 we established 36 1 m × 2 m fenced small mammal exclosures, paired with similar‐sized control plots, under the major canopy tree species found at Great Mountain Forest, in Norfolk, CT, US. RESULTS: When re‐sampled in 2005, after 10 yrs of small mammal exclusion, the abundance of herbaceous plants had increased inside the exclosures (F = 5.005, df = 1,38, P = 0.031), as did the proportion of plants flowering (t = −2.11, df = 60, P = 0.039). However, species richness and Shannon–Wiener diversity (H′) were similar inside and outside of exclosures. At one site the identity of the canopy tree species did impact species richness (F = 4.494, df = 5,18, P = 0.008), independent of exclosure. Plots under Fraxinus americana (white ash) had the highest species richness (average number of species = 7.5) while the lowest diversity was found in plots under Tsuga canadensis (Eastern hemlock; average number of species = 2.25) and Quercus rubra (northern red oak; average number of species = 1.5). CONCLUSIONS: The results demonstrate that small mammal herbivores can directly affect the abundance and reproduction of herbaceous species, and that local conditions (such as the surrounding canopy tree neighbourhood) can also play direct and indirect roles in the maintenance of the herbaceous layer in temperate deciduous forests.