- Given the vital role of forest ecosystems in landscape pattern and process, it is important to quantify the effects, feedbacks, and uncertainties associated with forest disturbance dynamics. In western North America, insects and wildfires are both native disturbances that have influenced forests for millennia, and both are projected to increase with anthropogenic climate change. Although there is acute concern that insect-caused tree mortality increases the likelihood or severity of subsequent wildfire, previous research has been mixed, with results often based on individual fire or insect events. Much of the ambivalence in the literature can be attributed to differences in the particular insect of interest, forest type, and fire event, but it is also related to the spatiotemporal scale of analysis and a general lack of geospatial datasets spanning enough time and space to capture multiple forest disturbances consistently and accurately.
This dissertation presents a regional-scale framework to map, quantify, and understand insect-wildfire interactions across numerous insect and fire events across the Pacific Northwest region (PNW). Through three related studies, I worked with many collaborators to develop regionally extensive but fine-grained maps to assess the spatiotemporal patterns of wildfires and the two most pervasive, damaging forest insects in the PNW – mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins [Coleoptera: Scolytidae]; a bark beetle) and western spruce budworm (WSB; Choristoneura freemani Razowksi [Lepidoptera: Tortricidae]; a defoliator). The proximate objectives of developing new maps and summarizing where and when insects have occurred before wildfires enable us to address the ultimate question: How does forest insect activity influence the likelihood of subsequent wildfire?
In a pilot study focused on the forest stand scale (Chapter Two), we leveraged a Landsat time series change detection algorithm (LandTrendr), annual forest health aerial detection surveys (ADS), and field measurements to investigate MPB and WSB effects on spectral trajectories, tree mortality, and fuel profiles at 38 plots in the Cascade Range of Oregon. Insect effects were evident in the Landsat time series as combinations of both short- and long-duration changes. WSB trajectories appeared to show a consistent temporal evolution of long-duration spectral decline followed by recovery, whereas MPB trajectories exhibited both short- and long-duration spectral declines and variable recovery rates. When comparing remote sensing data with field measurements of insect impacts, we found that spectral changes were related to cover-based estimates (e.g., tree basal area mortality and down coarse woody detritus). In contrast, ADS changes were related to count-based estimates (e.g., dead tree density). Fine woody detritus and forest floor depth were not well correlated with Landsat- or aerial survey-based change metrics. This study demonstrated the utility of insect mapping methods that capture a wide range of spectral trajectories, setting the stage for regional-scale mapping and analysis.
In a regional assessment of MPB and WSB effects on tree mortality (Chapter Three), we developed Landsat-based insect maps and presented comparisons across space, time, and insect agents that have not been possible to date, complementing existing ADS maps by: (1) quantifying change in terms of field-measured tree mortality; (2) providing consistent estimates of change for multiple agents, particularly long-duration changes; (3) capturing variation of insect impacts at a finer spatial scale within ADS polygons, substantially reducing estimated insect extent. Despite high variation across the study region, spatiotemporal patterns were evident in both the ADS- and Landsat-based maps of insect activity. MPB outbreaks occurred in two phases -- first during the 1970s and 1980s in eastern and central Oregon and then more synchronously during the 2000s throughout the dry interior conifer forests of the PNW. Reflecting differences in habitat susceptibility and epidemiology, WSB outbreaks exhibited early activity in northern Washington and an apparent spread from the eastern to central PNW during the 1980s, returning to northern Washington during the 1990s and 2000s. Across the region, WSB exceeded MPB in extent and tree mortality impacts in all ecoregions except for one, suggesting that ongoing studies should account for both bark beetles and defoliators, particularly given recent and projected increases in wildfire extent.
By combining these insect maps with an independent wildfire database (Chapter Four), we investigated wildfire likelihood following recent MPB and WSB outbreaks at ecoregional and regional scales. We computed wildfire likelihood with two-way binary matrices between fire and insects, testing for paired differences between percent burned with and without prior insect activity. All three disturbance agents occurred primarily in the drier, interior conifer forests east of the Cascade Range, with recent wildfires extending through the southern West Cascades and Klamath Mountains. In general, insect extent exceeded wildfire extent, and each disturbance typically affected less than 2% annually of a given ecoregion. In recent decades across the PNW, wildfire likelihood is not consistently higher in forests with prior insect outbreaks, but there is evidence of linked interactions that vary across insect agent (MPB and WSB), space (ecoregions), and time (interval since insect onset). For example, fire likelihood is higher following MPB activity in the North Cascades and West Cascades, particularly within the past 10 years, whereas fire likelihood is lower at various time lags following MPB in the Northern Rockies, East Cascades, and Blue Mountains. In contrast, fire likelihood is lower following WSB outbreaks at multiple time lags across all ecoregions. In addition, there are no consistent relationships between insect-fire likelihood and interannual fire extent, suggesting that other factors (such as climate) control the disproportionately large fire years accounting for the majority of regional fire extent. Although insects and wildfires do not appear to overlap enough to facilitate consistently positive linked disturbance interactions, specific fire events and years – such as 2003 and 2006 in the North Cascades – demonstrate high insect-fire co-occurrence and potential compound disturbance effects at the landscape scale.
The results from this dissertation highlight the key ecological roles that native disturbances play in PNW forests. WSB, MPB, and wildfire have been relatively rare at the regional scale, but all three have had and will continue to have profound effects on particular forest stands and landscapes. Because scale is such an important aspect of both the disturbance phenomena themselves as well as our ability to detect the ecological changes they render, our results also underscore the importance of geospatial datasets that span multiple scales in space and time. Given concerns about forest health in a rapidly changing climate, long-term monitoring will enable forest managers to quantify and anticipate the independent and interactive effects of insects, wildfires, and other disturbances.