Planting native trees to restore riparian forests increases biotic resistance to nonnative plant invasions

Abstract Nonnative invasive plant species are a major cause of ecosystem degradation and impairment of ecosystem service benefits in the United States. Forested riparian areas provide many ecosystem service benefits and are vital to maintaining water quality of streams and rivers. These systems are also vulnerable to natural disturbances and invasion by nonnative plants. We assessed whether planting native trees on disturbed riparian sites may increase biotic resistance to invasive plant establishment in central Vermont in the northeastern United States. The density (stems per square meter) of invasive stems was higher in non-planted sites (x = 4.1 stems m-2) compared with planted sites (x = 1.3 stems m-2). More than 90% of the invasive plants were Japanese knotweed [Fallopia japonica (Houtt.) Ronse Decr.; syn. Polygonum cuspidatum Siebold & Zucc.]. There were no significant differences in total stem density of native vegetation between planted and non-planted sites. Other measured response variables such as native tree regeneration, species diversity, soil properties, and soil function showed no significant differences or trends in the paired riparian study sites. The results of this case study indicate that tree planting in disturbed riparian forest areas may assist conservation efforts by minimizing the risk of invasive plant colonization.