10. Browsing by White-Tailed Deer on Invasive Oriental Bittersweet Spreading into Restored Grasslands

Non-native oriental bittersweet (Celastrus orbiculatus Thunb.) was first detected in southeastern Minnesota in 2010 and has spread from woodlands into adjacent grasslands. Anecdotal evidence suggests that browsing by white-tailed deer (Odocoileus virginianus) on young oriental bittersweet slows the growth and spread of this plant. This study assessed the population characteristics of density, age structure, growth rate, and branching morphology of oriental bittersweet in small, restored grasslands (mixed native grasses and forbs) in Winona County, Minnesota, and quantified the browsing damage inflicted by white-tailed deer on bittersweet and native red osier dogwood (Cornus sericea) at the end of winter. Bittersweet densities averaged 20 plants/m2 in grasslands near infested woodlands. Plants ranged in age from 1 to 7 yr, and growth rates (stem length) in plants greater than or equal to 2 yr old averaged only 9 cm/yr but were highly variable. Spread of bittersweet into grasslands from woodland edges averaged 48 m across all transects, but densities declined with increasing distance from woodlands. Winter browsing damage was present on 100% of all bittersweet and dogwood plants, with terminal buds removed from 70% (2,183 of 3,118) of all bittersweet branches and 99% (391 of 392) of all dogwood branches. Browsing seems to have suppressed fruiting in greater than or equal to 35% of plants greater than or equal to 2 yr old in the grasslands examined. Overwinter browsing by deer heavily damaged terminal buds and greatly reduced the growth rates of oriental bittersweet in restored grasslands in southeastern Minnesota, but not enough to prevent most plants in the population from maturing and producing fruit and seed