Small mammal granivory as a biotic filter for tree establishment beyond elevation range boundaries

Trees often experience migration lags in their response to rapidly changing climate. Preferential granivory by nocturnal small mammals has the potential to create lags by reducing germination success beyond range edges. To determine how granivory may limit establishment of trees beyond their range margins, we conducted a seed choice experiment which offered seeds of five regionally dominant tree species to small mammals within distinct forest communities across a 400 m elevational gradient on four mountains in the northeastern United States. Multinomial logistic mixed-effects models were used to (a) quantify seed preference of each species across the elevational gradient and (b) assess relationships between seed preference and abiotic variables. A separate seed dispersal experiment was used to compare the probability of seed consumption versus seed caching. The low elevation temperate tree species Fagus grandifolia and Acer saccharum had an equally high probability of granivory within and beyond their range margins (similar to 40% and similar to 20%, respectively). Generally, seed preference was positively correlated with seed mass and nutrient content regardless of elevation. Our seed dispersal experiment revealed that seeds were 3 x more likely to be consumed than cached, suggesting that small mammals can potentially decrease germination success. Overall, temperate tree species with either high seed mass or nutritional value may experience substantial granivory beyond their range margin, partially explaining the observed lag between tree dispersal and climate change. Thus, granivory is vital to consider when modeling future tree species distributions under various climate change scenarios.