Exotic plant responses to large-scale commercial logging in ponderosa pine forests of the Black Hills National Forest, South Dakota, USA

Abstract

Current and long-term historic commercial logging in the Black Hills National Forest provided a cost-effective opportunity to investigate exotic plant responses to a wide range of thinning intensities on a large scale. We asked two questions. First, after over 125 years of post-settlement commercial logging, what role does the pre-harvest exotic plant community play in post-harvest responses to logging. Second, how does a broad range of harvest intensities influence exotic plant responses on a large spatial scale. We averaged exotic species relative cover and relative richness to create a synthetic pre-harvest Exotic Vegetation Index (EVI) that was used as one of 15 predictor variables. The response variable was the difference between the pre-harvest and post-harvest EVI. We evaluated plots in ten widely scattered timber sales at pre-harvest and 1-, 2-, 3-, and 7-years post-harvest. In individual plots where the pre-harvest EVI was greater than 20 %, post-harvest exotic species responses were highly variable that, when averaged, the difference in EVI between pre-harvest and 7-years post-harvest was zero. In plots where the pre-harvest EVI was < 20 %, the difference in EVI between pre-harvest and 7-years post-harvest was significantly and positively correlated with harvest intensity (r = 0.65, p < 0.001). Plots with the highest levels of harvest intensity showed the greatest increases above pre-harvest values, which included several noxious weed species. In contrast, plots with the lowest levels of harvest intensity showed low levels of exotic increases with some plots exhibiting reductions in EVI below pre-harvest levels at 7-years post-harvest.