COS 246-3 - Seasonal variation in small mammal granivory of native tree species reveals an optimal window for seed additions in the absence of invasive woody shrubs

Abstract: Invasive shrubs dramatically reduce the biodiversity of native plants, making invaded areas important targets of restoration efforts. While a potentially promising means for restoration of native plant communities is to add seeds of native plants, the success of seeding efforts for restoration in invaded habitats may be reduced because dense habitats created by invasive shrubs can alter the activity of native seed-eating rodents, fueling significant increases in seed consumption, and these increases in consumption may vary depending on the time of year. The habitat structure provided by invasive shrubs is seasonally variable, and this seasonal structure could be an important factor in predicting seasonal trends in small mammal granivory. Although invasive shrubs may generate strong seasonal changes in granivory, we lack experimental studies to evaluate whether seasonal changes in granivory caused by invasive plants amplify granivory at times of year when seeds are most vulnerable. By understanding seasonal variation in granivory, land managers can better implement seed additions to coincide with when rodent granivory is lowest. We manipulated the presence of the widespread invasive species common buckthorn (Rhamnus cathartica) in a deciduous oak-maple dominated forest to track rodent granivory of three native tree species, basswood (Tilia americana), black cherry (Prunus serotina), and sugar maple (Acer saccharum) and the invasive woody shrub R. cathartica across early and late summer, autumn, winter, and spring. Our results reveal that the effect of invasive shrubs on rodent granivory changed across seasons with invaded habitat experiencing, on average, 25.9% higher seed removal, with the largest difference in winter. Our findings reveal that rodent granivores displayed no preference in seed species across seasons and that native tree seeds were consumed at similar rates to R. cathartica seeds. Our results suggest that, following removal of invasive shrubs, sowing seeds in winter may be the optimal window for seed additions to increase seedling establishment. These findings further reinforce the importance of invasive shrub removal as a necessary restoration tool and reveal previously unexplored impacts of invasive plants on seasonal patterns of granivory. Understanding the mechanisms that could be affecting seasonal granivory within invaded systems are critical for continual restoration efforts to ensure future forest health. By examining the critical role of timing in R. cathartica removal habitats, our results illuminate the value of understanding seasonal patterns in granivory for forest ecologists and seasonal patterns of seed additions for restoration managers.