Climate, trees, pests, and weeds: Change, uncertainty, and biotic stressors in eastern U.S. national park forests

The US National Park Service (NPS) manages over 8900 km2 of forest area in the eastern United States where climate change and nonnative species are altering forest structure, composition, and processes. Understanding potential forest change in response to climate, differences in habitat projections among models (uncertainty), and nonnative biotic stressors (tree pests and diseases and invasive plants) are vital for forward-looking land management. In this research, we examined potential changes in tree habitat suitability using two climate scenarios ('least change' and 'major change') to evaluate uncertainty in the magnitude of potential forest change. We further used nonnative tree pest and plant data to examine strengths and spatial patterns of these stressors and their correlations with projected changes in tree habitat. Analyses included 121 national parks, 134 tree species (from the US Forest Service Climate Change Atlas), 81 nonnative tree pests (from the US Forest Service Alien Forest Pest Explorer Database), and nonnative vascular plant presence data from each park. Lastly, for individual tree species in individual parks, we categorized potential habitat suitability change (from late 20th century baseline to 2100) into three change classes: large decrease (<50%), minor change (50-200%), and large increase (>200%). Results show that the potential magnitude of forest change (percentage of modeled tree species in the large decrease and large increase classes, combined) varies from 22% to 77% at individual parks. Uncertainty (the percentage of tree species in differing change classes across climate scenarios) varies from 18% to 84% at parks. Nonnative plant species comprise from <10% to about 50% of the flora at parks. The number of nonnative tree pest species ranges from 15 to 70 among parks. Potential forest change, uncertainty, and nonnative pests and plants have significant positive correlations, illustrating the broad scope of potential compounding effects and future changes in many eastern forests. The combination of rapid climate change and nonnative stressors may accelerate decline of some tree species and inhibit other species from occupying climatically suitable habitat. Stewarding forests for continuous change is a challenge for park managers. Understanding and anticipating projected rates and directions of forest change and nonnative biotic stressors should facilitate monitoring and management efforts on park lands and across the broader landscape.