Ecosystems worldwide are changing rapidly from the individual and combined effects of climate warming and associated ecological disturbances. Key mechanisms of resilience, such as biological legacies (i.e., live and dead organic material remaining after a disturbance) and compensatory responses (e.g., post-disturbance regeneration or growth), may be altered substantially by cumulative impacts from disturbance interactions and shifting environmental conditions. In cases where mechanisms of resilience are eroded substantially, forest ecosystems can potentially be converted to alternative ecosystems such as grasslands or shrublands. Temperate conifer forests in western North America are facing unprecedented increases in biotic disturbances (e.g., insect and pathogen outbreaks), many of which are co-occurring in space and time and with unknown ecological consequences. Understanding the drivers, characteristics, and effects of interacting disturbances is important as these phenomenon become more common in forests across the globe.

To address this gap in knowledge, this project integrated existing permanent-plot forest demography data with remote sensing products and spatial analyses to examine the spatiotemporal interactions among biotic disturbance agents. With an extensive field dataset of forest inventory plots and aerial survey forest health data across forests of western North America, our findings suggest widespread occurrence of biotic disturbance hotspots (i.e., forested locations affected by two or more biotic disturbances in short succession) in the first two decades of the 21st century. Using these datasets, we also found that bark beetle outbreaks are becoming more spatially autocorrelated over larger spatial ranges, whereas the dynamics of defoliating insect outbreaks have remained relatively spatially static. We found that at broad spatial scales, the most consistent driver of disturbance hotspot occurrence is forest structure and composition, and that outbreak contagion and other dynamic factors strongly shape patterns of biotic disturbance hotspots. At fine scales, our analyses of spatially explicit permanent forest plot data highlight that tree neighborhoods mediate the likelihood of tree mortality and the effects of tree neighborhood on mortality likelihood vary by tree size. Thus, the pre-outbreak spatial pattern of trees within a forest can result in fine scale hotspots where mortality is greater than expected from random chance alone.

Further, we examined the effects of post-disturbance biological legacies and compensatory responses on ecosystem resilience to interacting / overlapping biotic disturbances. Using a combination of forest inventory plots and remotely sensed data, our findings suggest that when biotic agents prefer different host species, the effects of overlapping biotic disturbance depend strongly upon the identity of disturbance agents present. Further, in some forest ecosystems (e.g., subalpine forests), disturbance hotspots are rarely more severe in terms of tree mortality than in areas affected by severe outbreaks of a single bark beetle species, and hotspots can exhibit similar levels of post-disturbance recovery. Findings suggest that greater diversity of tree species that can coincide with biotic hotspot occurrence is also associated with greater potential for post-outbreak compensatory growth responses. At fine and broad spatial scales, our results highlight that post-disturbance responses are widely variable and dependent on the spatial structure of forest stands prior to biotic disturbance and the spatial pattern of tree mortality at fine and broad spatial scales, whereas spatial variability in climate is also important for driving post-disturbance responses at broad spatial scales. Collectively, findings from this project deepen understanding of interacting biotic disturbances in western US coniferous forests and have important implications for forest resilience as the climate warms and forest disturbance activity increases.

This project contributed new knowledge and provided insights that are highly relevant for scientific and societal understanding of environmental change and ecological resilience, and findings were disseminated broadly to a wide diversity of audiences through a range of formats. This project also provided valuable professional development and training opportunities for postdoctoral researchers, PhD students, MS students, and undergraduate students across multiple universities, and for early-career researchers in academia and natural resource agencies.

Last Modified: 10/30/2023
Modified by: Brian Harvey