Defining an effective forest management strategy that deters invasion from exotic plants: Invasive plant response to five forest management regimes
Oak regeneration in Eastern forests is dependent on increases in light to the forest understory using harvesting and/or fire. Unfortunately, these disturbances may promote invasions by non-native invasive plants (NNIS). Adequate oak growth in the field has been achieved at light levels 200-300 umol m-2 s-1 (10-15% PAR). The purpose of this study was to define forest management that allows for oak regeneration but deters growth of NNIS. Germination, survival, and growth of three planted NNIS (Ailanthus altissima, Alliaria petiolata, Microstegium vimineum) were evaluated and existing populations of exotic invasive plants were surveyed at 56 sites under five management regimes (control, single burn, repeat burn, diameter limit cut, and shelterwood) located on northeast or southwest slopes and within two physiographic regions (Allegheny Plateau and Ridge and Valley) in West Virginia, Ohio, and Virginia. Secondly, we determined the spread rate of the three NNIS using seed traps and vegetation plots. Thirdly, we grew these NNIS and Quercus rubra under eight light levels, representing typical light conditions of managed and undisturbed forests. Height and leaf number were determined as well as biochemical indicators of stress and growth. These results were used to calculate an Invasive Potential Value (IPV) for each combined management and environment type.
All three NNIS showed more growth at light levels above 180 umolm-2s-1 (9% PAR). IPV was calculated using vulnerabiltiy to invasion data (likelihood of germination, survival, and growth) multiplied by the averaged spread rate (1.72 m/yr), and one minus the lowest light level showing NNIS growth (180 umolm-2 s-1). All four shelterwood types had among the highest IPVs, followed by all four diameter limit cut types, but northeastern slopes within each of these categories had the largest IPVs. These results indicate that management resulting in light levels below 10% PAR may prevent invasion but enable oak regeneration. Using control stands as a starting point and our IPVs as a weighted NNIS cost, we ran growth models in Forest Vegetation Simulator under different management scenarios in order to evaluate if cutting at the recommended level is more cost effective than a more aggressive harvest including the cost of invasion. The results indicate that the potential cost invasion after a heavy shelterwood is more than the lost revenue associated with removing less wood. The models also predict that stands starting with equal native species regeneration (i.e., no impact from deer) compared to invasive species invasion may eventually recover from an invasion.