The association of two invasive shrubs, common buckthorn (Rhamnus cathartica) and Tartarian honeysuckle (Lonicera tatarica), with oak communities in the midwestern United States

Abstract

Oak forests throughout North America are declining due to changes in disturbance regimes that have led to increased competition from other tree and shrub species. We evaluated associations between oak regeneration, the occurrence of two common invasive shrubs (common buckthorn ( Rhamnus cathartica L.) and Tartarian honeysuckle ( Lonicera tatarica L.)), and forest edges in oak forests in a portion of the midwestern United States where bur ( Quercus macrocarpa Michx.), red ( Quercus rubra L.), and white oak ( Quercus alba L.) were historically dominant. We found poor recruitment of oaks in comparison to other, more shade-tolerant tree species. Results further revealed a strong stand-scale association between bur oak, open canopy conditions, high soil nutrient levels, and the presence of common buckthorn and Tartarian honeysuckle; these same site characteristics were disassociated with red and white oak. Within red and white oak stands, however, the presence of the invasive shrubs was more pronounced near forest edges. While oak recruitment is hampered throughout stands, our research suggests that predominant constraints may vary based on soil and light gradients found along forest edges.