Weeds, worms, and deer: positive relationships among common forest understory stressors
Biotic global change agents, such as non-native plants (‘weeds’), non-native earthworms (‘worms’), and overabundant herbivores (white-tailed ‘deer’), can be major stressors in the forest understory. The status and relationships among these global change stressors across large spatial extents and under naturally varying conditions are poorly understood. Here, through an observational study using a network of U.S. National Park Service forest health monitoring plots (n = 350) from eight parks in seven northeastern states, we modeled causal pathways among global change stressors through model selection in a structural equation (SEM) framework. Weeds, worms, and, deer were common across all parks in the study—46% of plots had non-native plants, 42% of plots had evidence of earthworms, and all parks had plots with high deer browse damage. All biotic global change stressors were significantly and positively correlated with one another (all Spearman rank correlations ≥ 0.44). Consequently, 28% of plots had a combination of earthworms absent, low deer browse, and no non-native plants, and 29% of plots included earthworms, non-native plants, and moderate or greater browse damage. Through SEM, we found strong support for pathways among global change stressors, e.g., deer browse positively influenced earthworm presence and both deer and earthworms promoted non-native plants. Warmer air temperatures and higher soil pH also facilitated non-natives. This research highlights the tremendous multipronged management challenge for areas already experiencing the combined effects of weeds, worms, and deer and the future vulnerability of other areas as temperatures warm and conditions become more amenable to biotic global change stressors.
KeywordsClimate Earthworms Forest soils Herbivory Global change Non-native plants
This research was supported by the Maine Timberlands Charitable Trust. We thank the National Park Service Northeast Temperate Network staff for their tremendous work which makes such studies possible.
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