Seed predation by small mammals could inhibit upslope range shifts of deciduous hardwood tree species in montane forests of the northeastern United States

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Jordon Tourville and Martin Dovciak, Department of Environmental Biology, State University of New York College of Environmental Science and Forestry, Syracuse, NY

Background/Question/Methods
Recent evidence suggests that tree species are experiencing a migration lag in response to rapid changes in temperature and precipitation associated with climate change. Biotic interactions are often invoked to explain the existence of these lags. Mammalian herbivores are known to be major players in ecosystem-level processes including nutrient cycling and forest succession. Preferential granivory by small mammals has the potential to slow or inhibit range expansions in many tree species. The purpose of this study was to assess the impact of small mammal seed predation on tree species dynamics within and beyond their range margin. We hypothesized that low elevation tree species would experience high levels of seed predation and limited seed caching compared to other tree species. We designed a seed choice field experiment which offered seeds of five dominant tree species (Fagus grandifolia, Acer saccharum, Betula alleghaniensis, Abies balsamea, and Picea rubens) to small mammals across four elevation gradients in montane forests of the northeastern United States. Multinomial logistic mixed effects models were used to quantify seed preference by small mammals. A separate seed dispersal experiment was used to compare the probability of seed consumption against seed caching via a logistic mixed effects model.
Results/Conclusions
Mammal seed preference was high for two low-elevation tree species (Fagus grandifolia, Acer saccharum) and one high elevation species (Picea rubens). Seed preference was low for two small seeded species (Betula alleghaniensis, Abies balsamea). Seed preferences did not differ between species across the elevation gradient. Multinomial models indicated the importance of nighttime lunar illumination and temperature on choice of seed. Our seed dispersal experiment revealed that seeds were more likely to be consumed than cached (p=0.001). Caching behavior of small mammals is often beneficial to seeds, providing increased dispersal distance and safe sites for germination. Our findings suggested that most mammal interactions with seeds acted to decrease germination success at these sites. Overall choice of seed seemed to be influenced by both seed mass and seed nutrient content. Taken together, these results indicate that low elevation tree species with either high seed mass or nutritional value may experience heightened seed predation beyond their range edge. This could act to slow or inhibit range expansions of trees and may partly explain the observed lag between dispersal and climate change. We suggest that herbivore interactions with trees are vital to consider when modeling future forest species distributions under various climate change scenarios.