Spatio-temporal patterns of forest disturbance in western North America: implications for forest resilience

Abstract

Globally, forest disturbance activity is changing in response to changing climate. As disturbance regimes change, concerns have been raised that the mechanisms of forest resilience (i.e., the capacity of forests to tolerate disturbance) may begin to break down. To successfully monitor, forecast, and manage for forest resilience in the context of changing disturbance regimes, quantifying indicators of forest resilience across spatial and temporal scales is critical. In this dissertation, I quantified facets of forest resilience to biotic disturbance (i.e., insects and plant pathogens) and wildfire in western North America. First, I evaluated compensatory responses of forests following severe bark beetle outbreak. I found that compensatory growth responses are strongly shaped by both the characteristics and spatial arrangement of surviving trees, and that increased post-disturbance growth acts as a key mechanism of forest resilience by providing continuity in forest function. Next, I characterized the patterns and drivers of biotic disturbance hotspots, an emerging phenomenon in the western United States (US) in which two or more distinct biotic disturbance agents co-occur in space and time. I found that while biotic disturbance hotspots are driven by forest composition and regionally important bioclimatic factors, they are also stochastic processes that cannot be predicted solely from deterministic landscape characteristics or other known drivers. Interactions among multiple disturbances such as these are important to understand, as they have the potential to erode compensatory responses and therefore mechanisms of forest resilience. Finally, I quantified the range of variation in burn severity patch structure characterizing Northwest US fire regimes. Despite changes in climate and fire activity in recent decades, I found that the range of variation in high-severity burn patches, conditional on fire size, has remained remarkably stationary in recent decades. Stationarity in the relationship between burn severity patterns and fire size offers a simple yet powerful means to anticipate the range of ecological effects of future fire activity at regional scales. Building on this finding, I conducted a simulation study demonstrating that shifts in fire size distributions towards larger fire events will lead to increasingly large high-severity burn patches with interior areas that are increasingly far from unburned seed sources following fire. Large high-severity patches directly affect rates of tree regeneration and forest recovery following fire, along with the potential for forests to transition to non-forest ecosystems. Collectively, this work provides insights into a range of mechanisms of forest resilience in western North America and has important implications for managing forests in the face of continued climate change and increasing disturbance activity.