Does excluding small mammals for 20 years change tree species establishment?
Small mammals are well known consumers of tree seeds and seedlings in temperate deciduous forests and can affect the successful recruitment of seedlings over the short term. However, less is known about how these impacts may change forest dynamics over long time scales. In 1994 we established 36 1x2m small mammal exclosures, paired with similar sized control plots, at 2 different sites at Great Mountain Forest in Norfolk, Connecticut. We counted all tree seedlings after 1, 2, 3, 10, 18 and 20 years of exclusion. We analyzed seedling density separately for each of the major tree species using linear mixed models in R, with site and treatment (exclosure vs. control) as fixed effects, and year as a random effect. The full model with site, treatment and year was then compared (using AIC) to null models that omitted the treatment effect to determine whether there was support for effects of excluding small mammals on seedling density for a given species.
We found that excluding seed predators for 20 years increased seedling recruitment for three of the six common tree species: Acer rubrum, Acer saccharum, and Prunus serotina. Quercus rubra showed no response to fencing for the first 3 years (because of the almost complete absence of new seedlings in either fenced or unfenced plots), but did show a response to the exclosures after 10 years. There were no effects on the other two major tree species (Fraxinus americana or Tsuga canadensis). Therefore, after 20 years of exclusion from small mammal consumers, the medium to large-seeded tree species – those that are preferred by small mammal seed predators - had increased seedling recruitment. This suggests that small mammals can play an important role in the long-term dynamics of temperate deciduous forests.