Spatiotemporal dynamics of recent mountain pine beetle and western spruce budworm outbreaks across the Pacific Northwest Region, USA

Abstract

Across the western US, the two most prevalent native forest insect pests are mountain pine beetle (MPB; Dendroctonus ponderosae; a bark beetle) and western spruce budworm (WSB; Choristoneura freemani; a defoliator). MPB outbreaks have received more forest management attention than WSB outbreaks, but studies to date have not compared their cumulative mortality impacts in an integrated, regional framework. The objectives of this study are to: (1) map tree mortality associated with MPB and WSB outbreaks by integrating forest health aerial detection surveys (ADS; 1970-2012), Landsat time series (1984-2012), and multi-date forest inventory data; (2) compare the timing, extent, and cumulative impacts of recent MPB and WSB outbreaks across forested ecoregions of the US Pacific Northwest Region (PNW; Oregon and Washington). Our Landsat-based insect atlas facilitates comparisons across space, time, and insect agents that have not been possible to date, complementing existing ADS maps in three important ways. The new maps (1) capture variation of insect impacts within ADS polygons at a finer spatial scale (30-m grain), substantially reducing estimated insect extent; (2) provide consistent estimates of change for multiple agents, particularly long-duration changes; (3) quantify change in terms of field-measured tree mortality (dead basal area). Despite high variation across the study region, spatiotemporal patterns are 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 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. At ecoregional and regional scales, WSB outbreaks have exceeded MPB outbreaks in extent as well as total tree mortality, suggesting that ongoing studies should account for both bark beetles and defoliators. Given projected increases of insect and fire activity in western forests, the accurate assessment and monitoring of these disturbances will be crucial for sustainable ecosystem management.

Does wildfire likelihood increase following insect outbreaks in conifer forests?

“…Indeed, WSB has the potential to influence Douglas-fir forests throughout the western PNW but has been limited to drier, interior forests to date (Meigs et al 2015).…”

“…1), converting the polygons to raster format (30-m grain) for further analysis. For each affected pixel, we identified the onset (first year of detection), cumulative magnitude (dead trees ha À1 for MPB, defoliation units on a 1-3 scale for WSB), cumulative extent (ha), and count of years detected after Meigs et al (2015).…”

“…Although integral for understanding mechanistic relationships among insects, fuels, and fire, this simulation approach assumes that an insect-affected stand actually burns. In reality, insects and wildfires affect only a small portion of forested landscapes in a given year, even in regional outbreak or fire years (Littell et al 2009, Meigs et al 2015. Other studies have assessed insect-fire interactions in actual wildfire events (e.g., Lynch et al 2006, Kulakowski and Veblen 2007, Prichard and Kennedy 2014, but these empirical assessments typically have been limited to individual fires or insect outbreaks (Bisrat 2010).…”

Assessing intra- and inter-regional climate effects on Douglas-fir biomass dynamics in Oregon and Washington, USA

“…3c and d) could reflect declines in P. menziesii in more open habitats generally associated with marginal forest habitat and increasing dominance in closedcanopy forests. Recent increases in the importance of insect attack in dry vegetation zones may contribute to these trends (Meigs et al, 2015). In addition, such patterns may highlight initial phases on species range contraction for which changes in tree recruitment have already been identified as an early indicator (Bell et al, 2014;Monleon and Lintz, 2015;Zhu et al, 2012).…”

Integrating remote sensing, GIS and dynamic models for landscape-level simulation of forest insect disturbance

“…An emerging new method is temporal trajectory analysis, which models temporal signatures of schemed land cover types from dense image stacks as a proxy for their phenological variations . This technique is particularly useful in assessing the extent and severity of subtle ecosystem changes (Meddens et al, 2013;Liang et al, 2014b;Meddens and Hicke, 2014;Meigs et al, 2015).…”