Forest preserves constitute some of the most valued and complex green spaces in urban areas, but best management practices for conserving biodiversity within them are still developing and long-term impacts of management are unknown. Dormant season understory burns are a commonly used management practice in midwestern forests intended to mimic the natural disturbances that shaped the landscape prior to European arrival and are implemented to enhance native species richness while controlling invasive species.
We address three questions:1. Do controlled understory burns increase the number of native woodland herbs and decrease the number of invasive herbs in a management plot? 2. Does the efficacy of management depend on the pre-management condition at the plot? and 3. Have understory burns led to biotic homogenization of the forest understory across an urban landscape? We used a paired before-after control-impact (BACIP) framework to evaluate the impact of dormant season understory burns on both native and invasive species richness within 0.45-hectare management plots between 1986 and 2016 in DuPage County, Illinois. Changes in native and invasive species richness within the BACIP framework for each monitoring timestep where evaluated using Wilcoxon signed-rank tests. The impact of pre-management plot condition on management was evaluated using Fisher exact tests. Biotic homogenization was analyzed using a distance based test of homogeneity of multivariate dispersion with the ‘betadisper’ function in the R package ‘vegan’.
Results/Conclusions
Dormant season understory burns did not universally or significantly increase native herb species richness (p = 0.079) or decrease invasive species richness (p = 0.41) across unique monitoring timestep control impact plot pairs (N = 52) within our BACIP framework. Pre-management conditions of the plot, however, did significantly impact the effectiveness of burn management for both native herbs (p = 0.048) and invasive plant species (p = 0.001). Our results indicate that management is most effective in increasing native herb species richness in species poor plots and in reducing invasive richness in heavily invaded plots. Likewise, management may not be effective in especially species rich or lightly invaded sites. Dormant season understory burns also contributed to biotic homogenization within the herbaceous forest understory across DuPage County with burn managed plots (N = 23) decreasing in beta diversity (F = 33.249, p = 0.001) between 1986 and 2016, while beta diversity of unburned control plots (N = 11) did not change (F = 0.5527, p = 0.477).