Regeneration dynamics of non-native northern red oak (Quercus rubra L.) populations as influenced by environmental factors: a case study in managed hardwood forests of southwestern Germany.
Quercus rubra L. (northern red oak), a tree species having moderate shade tolerance, is failing to regenerate across its native range in North America, largely due to its inability to compete with shade-tolerant species. Throughout central Europe, where it was introduced in the 18th and 19th centuries, Q. rubra exhibits prolific regeneration even when growing with shade-tolerant trees under closed canopy conditions. A better understanding of factors that allow the proliferation of Q. rubra in its adventive range may provide insights into the conditions necessary to promote Q. rubra in North America. Our study investigated the regeneration dynamics of Q. rubra in six hardwood stands near Freiburg, Germany in relation to site conditions and the relative abundance and growth of indigenous tree species in forest understoreys. Despite high (94-98%) canopy closure at all stands, the density of Q. rubra regeneration (<2 m in height) was greater than that of all other tree species combined, averaging 24 stems m-1. Density of Q. rubra seedlings reached 125 stems m-2 directly below seed trees; however, the lack of seedlings beyond 15 m from a seed tree suggested limited seed dispersal. Seedlings were less abundant at relatively fertile sites with lowest densities corresponding most closely to elevated soil calcium. The abundance of Q. rubra was most highly variable in the midstorey (trees and shrubs >2.0 m in height and <10 cm diameter at breast height) with densities ranging from 200 to 1500 stems ha-1. Periodic selective harvesting at all stands, appears to maintain a disturbed state of mid-succession that allows Q. rubra seedlings to persist and recruit into the midstorey as canopy gaps become available. Clearly, stands of this non-indigenous species are successfully regenerating and the dominance of Q. rubra appears to be sustainable. Despite its benign performance in North America, Q. rubra can be an effective competitor under suitable conditions. Our findings deemphasize the importance of canopy closure on Q. rubra regeneration and suggest that in North America, preliminary cuts performed prior to shelterwood harvests should focus on midstorey removal of competitor species especially following oak mast years.