To manage or not? Successful native tree seedling restoration despite a dense, invasive shrub, Berberis thunbergii

Abstract

Invasive plant species suppress native trees through a variety of mechanisms. A non-native shrub, Berberis thunbergii, has been shown to depress native tree seedling densities in eastern North American deciduous forests, but the mechanisms remain unclear. We attempted to identify the mechanisms leading to decreased native tree seedling densities in Berberis-invaded understories by experimentally measuring survivorship and growth of three common eastern deciduous tree seedlings, Prunus serotina, Quercus alba, and Q. velutina. First, we reared native tree seedlings in soil samples extracted from Berberis-invaded and control forest plots to determine if early growth and survival varied between medias. Then, we introduced surviving seedlings into three classes of in situ field subplots: control (outside Berberis invasion), invaded (Berberis present), and managed subplots where Berberis removal occurred following non-chemical best practice guidelines. Slight decrease of early-stage seedling survivorship and growth occurred in extracted soils from invaded plots. Seedling survival differed between field subplots, with seedlings either showing no differences between invaded subplots (Q. alba) or faring better (P. serotina and Q. velutina) compared to managed subplots. Invaded subplots were about 1.18–1.30 °C cooler with 583–709 lumens m−2 less light exposure compared to control or managed subplots. Additionally, managed and invaded subplots had increased moisture levels (12.0–14.9%) compared to control subplots. Seedling compromise was due to a legacy effect from Berberis disruption via soil sample extraction and/or management. Our findings contrast with other studies that show dense, invasive species outcompeting native tree seedlings and unsuccessful native restoration in Berberis stands.