Western spruce budworm effects on throughfall N, P, and C fluxes and soil nutrient status in the Pacific Northwest

Western spruce budworm (Choristoneura freemani Razowski, 2008) is the most widely distributed insect herbivore in western North American coniferous forests. By partially or completely defoliating tree crowns, budworms influence fluxes of water, nutrients, and organic carbon from forest canopies to soils and, in turn, soil chemistry. To quantify these effects, throughfall water, inorganic nitrogen (N), phosphorus (P), and dissolved organic carbon (DOC) concentrations, as well as fluxes and soil N and P concentrations, were measured in coniferous forest sites with high and background levels of budworm herbivory. Throughfall N and P concentrations and fluxes increased at sites with high budworm levels during and (or) immediately after larval-stage budworm feeding, indicating reduced uptake and (or) greater leaching from canopies as a result of budworm activity. Annual throughfall N fluxes (<67–71 g N·ha−1·year−1) and soil N concentrations were low regardless of herbivory level. In contrast, throughfall P was considerably greater at sites with high herbivory levels (2174 g P·ha−1·year−1) compared with those with background levels (1357 g P·ha−1·year−1), and this was reflected in nearly threefold higher soil P concentrations at sites with high budworm levels. Our findings suggest that by altering throughfall chemistry and soil N:P, budworms could influence elemental export from watersheds.

Drought and Moisture Availability and Recent Western Spruce Budworm Outbreaks in the Western United States

Western spruce budworm (WSBW) is a common defoliating insect that has caused extensive damage and mortality to a number of tree species across the western United States (US). Past studies have linked outbreaks of WSBW to increased moisture stress of host trees in the Northwest and decreased moisture stress in the Southwest. Our study analyzed seasonal drought stress metrics with WSBW outbreaks within Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests in the western US during 1997–2015. Superposed epoch analysis and defoliation area growth rates (representing insect population growth rates) were assessed to quantify the drought conditions associated with the initiation and continuation of outbreaks, respectively. We found that multiple years of drought occurred prior to and during outbreak initiation in the Northwest, and that outbreak initiation in the Southwest was associated with only weak drought or neutral conditions. During the outbreak continuation stage, there was a weak positive correlation between May moisture availability and defoliation area growth rates in the Southwest (R2 = 0.12), but no clear relationship was identified in the Northwest. Increased frequency of summer droughts such as these expected from climate change may increase WSBW outbreaks and promote tree dieoff. Improved understanding of the role of different influences of drought and moisture availability across landscapes will lead to improved predictions and management of future outbreaks of WSBW.

Characterization of western spruce budworm outbreak regions in the British Columbia Interior

The western spruce budworm (WSB; Choristoneura freemani Razowski) shapes Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests throughout western North America with periodic, severe landscape-level defoliation events. The largest and most continuous recorded defoliation occurred in the 2000s, largely centered in the Williams Lake and 100 Mile House WSB outbreak regions, peaking in 2007 at 847 000 ha defoliated in British Columbia (B.C.). Unique WSB outbreak regions in south-central B.C. are described using biogeoclimatic ecosystem classification, geography, 106 years of documented defoliation, and 46 stand-level Douglas-fir host tree-ring chronologies. Since the 1980s, recorded defoliation in B.C. has shifted from coastal ecosystems and become a dominant disturbance in drier, colder, interior Douglas-fir ecosystems. Defoliation records demarcate four outbreaks from 1950–2012 and up to three growth suppression events from 1937–2012. Outbreak duration was shorter in the north and far south of B.C., with recovery periods (no trees showing growth suppression) shorter over all WSB outbreak regions in the 2000s, suggesting that trees may be increasingly susceptible to each successive defoliation event. Knowing the regional outbreak periodicity may facilitate early detection of incipient WSB populations, which is critical for management as many of our low-elevation Douglas-fir forests become more stressed with changing and unpredictable climate regimes.