Phenology matters: Extended spring and autumn canopy cover increases biotic resistance of forests to invasion by common buckthorn (Rhamnus cathartica)

Forest light availability strongly regulates understory community composition, and low availability may confer resistance to invasion by exotic species, yet common buckthorn (Rhamnus cathartica L.) invades North American temperate forests with a broad range of light habitats. It is unclear to what extent buckthorn's success is due to high mid-season shade tolerance versus shade avoidance permitted by early leaf out and late senescence. We used buckthorn seedlings planted into a forest diversity experiment in Cloquet, Minnesota, USA and a combined buckthorn physiology-canopy light model to test (1) how buckthorn germination, growth, and survival depend on canopy shading and (2) how canopy species richness and phenology affect light availability and buckthorn performance. Based on the mean of May, August, and October light measurements, we found that canopies that permitted ≤3% transmission of incoming light had almost complete mortality of buckthorn and that growth of surviving buckthorn was strongly tied to light availability, but not canopy richness. Compared to deciduous canopies or deciduous-evergreen mixtures, evergreen canopies restricted light availability the most and led to the smallest and least likely to survive buckthorn. Our canopy models further indicated a tight linkage between buckthorn performance and spring and autumn light availability, but not summer light availability. We conclude that spring and autumn light availability are key regulators of buckthorn performance and that buckthorn relies on shade avoidance via an extended phenology to succeed in temperate forests. Consequently, we suggest species with extended spring or autumn canopy cover offer the greatest resistance to invasion, and communities which are often leafless during such periods are most vulnerable to invasion by buckthorn and similar invasive shrubs.