Summer and winter drought drive the initiation and spread of spruce beetle outbreak

Abstract

This study used Landsat-based detection of spruce beetle (Dendroctonus rufipennis) outbreak over the years 2000-2014 across the Southern Rocky Mountain Ecoregion to examine the spatiotemporal patterns of outbreak and assess the influence of temperature, drought, forest characteristics, and previous spruce beetle activity on outbreak development. During the 1999-2013 period, time series of spruce beetle activity were highly spatially correlated (r > 0.5) at distances <5 km, but remained weakly correlated (r = 0.08) at distances >400 km. Furthermore, cluster analysis on time series of outbreak activity revealed the outbreak developed at multiple incipient locations and spread to unaffected forest, highlighting the importance of both local-scale dispersal and regional-scale drivers in synchronizing spruce beetle outbreak. Spatial overlay analysis and Random Forest modeling of outbreak development show that outbreaks initiate in areas characterized by summer, winter, and multi-year drought and that outbreak spread is strongly linked to the proximity and extent of nearby outbreak, but remains associated with drought. Notably, we find that spruce beetle outbreak is associated with low peak snow water equivalent, not just summer drought. As such, future alterations to both winter and summer precipitation regimes are likely to drive important changes in subalpine forests.

Anomalous outbreaks of an invasive defoliator and native bark beetle facilitated by warm temperatures, changes in precipitation and interspecific interactions

“…In addition to temperature, drought may act alone as a landscape scale disturbance (Hanson and Weltzin ) and/or facilitate disturbances by insects (Mattson and Haack ). While drought is positively correlated with outbreaks of endophytic feeding guilds such as bark beetles and wood wasps (Hart et al , Lantschner et al ), our finding that defoliation increased with spring precipitation (MayPPT, Table 4a) suggests that drought negatively impacts larch casebearer populations. Larvae may perform poorly on desiccating needles and/or may suffer from premature needle drop by hosts in response to water stress.…”

“…Beneficial relationships between precipitation and insect outbreaks by understudied species or feeding guilds may be an area of concern as precipitation regimes shift with a changing climate (Trenberth , Jamieson et al ). Likewise, the relationship between drought and eastern larch beetle outbreaks is not well understood (Crocker et al ), although the initiation and spread of outbreaks by spruce beetle, a congener that also attacks trees early in spring, are facilitated by drought (Hart et al , ). Total annual precipitation in the previous three years is negatively correlated with mortality by eastern larch beetle (Table 4b), indicating that drought may indeed facilitate outbreaks of eastern larch beetle in Minnesota.…”

“…1). The year of observed mortality by eastern larch beetle was lagged by one year to the previous year to reflect the ecology of eastern larch beetle, as there is a one year delay between beetle colonization of the tree and visually apparent crown fading noted in aerial surveys (Meddens and Hicke 2014, Mckee 2015, Hart et al ). As a baseline estimate of spatial association between these two disturbance agents, percent overlap of spatial polygons attributed to the two agents was calculated via the gArea() command in the GISTools package (Brunsdon and Chen 2014) in R statistical software (R Core Team).…”

“…The dispersal capacity of eastern larch beetle is unknown (Mckee 2015), but comparable resolutions (1–100 km 2 cells) have been used in similar analyses of other Dendroctonus spp. (Chapman et al , Preisler et al , Sambaraju et al , Hart et al ). For each year, the rasterization process was conducted separately for each agent such that all cells had a 17‐yr (2000–2016) and 16‐yr (2001–2016) time‐series for larch casebearer and eastern larch beetle, respectively.…”

Disturbance–climate relationships between wildfire and western spruce budworm in interior British Columbia

“…These analyses suggest that anomalously warm, dry conditions associated with drought are an important prerequisite for initiation of WSB outbreaks . Research documenting the association of climate and bark beetle outbreaks in conifer-dominated forests (Hart et al 2014(Hart et al , 2017 indicates that drought conditions reduce photosynthesis (Pallardy and Kozlowski 2008), leaf area (Mattson and Haack 1987), and terpene production (Bohlmann 2012). However, for a defoliating insect such as the WSB, these drought-triggered physiological process changes would likely have limited impact.…”