Impact of the invasive shrub glossy buckthorn (Rhamnus frangula L.) on juvenile recruitment by canopy trees

The invasion of forests in the northeastern USA by glossy buckthorn (Rhamnus frangula L.) has resulted in a dense, non-native shrub layer that frequently dominates the understory. We investigated the effects of buckthorn on the survival and growth of juvenile canopy trees spanning a wide range of shade tolerance (sugar maple, Acer saccharum Marsh.; red maple, Acer rubrum L.; white ash, Fraxinus americana L.; and white pine, Pinus strobus L.), in a stand dominated by white pine. First, we measured the effect of buckthorn on sapling growth in a field study. Second, we inferred effects on sapling survivorship from age data and from published relationships between radial growth and mortality rate. Third, we evaluated the effects of buckthorn on seedling growth and survival in canopy openings, by felling trees to create experimental gaps. Buckthorn reduced the growth and survival of saplings of all species, and altered the relative abundance of seedlings in favor of shade-tolerant species. Estimates of sapling survival implied that <10% of tree saplings can survive to grow through high density buckthorn under closed canopies. This reduces the probability that understory saplings will survive to recruit into all newly formed canopy gaps. The experimental results suggest that tree seedlings are most likely to recruit in canopy gaps, despite the generally high buckthorn cover in gaps. Thus, recruitment of tree seedlings in gaps (even under buckthorn) may become the main source of canopy recruits. The increasing dominance of glossy buckthorn in New England pine forests is likely to change the relative abundance of tree species in the forest canopy, and may delay the filling of canopy gaps.