Western spruce budworm (Choristoneura occidentalis (Freeman)) is a native defoliator of the Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) forests of British Columbia, Canada. This thesis used dendrochronology and the software program OUTBREAK to reconstruct a defoliation history of Douglas-fir for nineteen forest sites near Kamloops in central British Columbia. By comparing the radial growth response of non-host ponderosa pine trees to Douglas-fir trees growing in nearby but separate stands, seven western spruce budworm outbreaks were distinguished over the past 300 years. Although there is considerable variation in the timing and duration of these western spruce budworm events at the stand level, synchronous outbreaks have occurred at approximately 43-year intervals. Climate variation appears to have been important to budworm outbreaks in the 20th century. Notable outbreaks tended to occur during years of early springs with average air temperature, following winters with lower than average precipitation. Based on this finding, it is proposed that with high overwintering survival, increased population growth rates, and a longer growing season, the extent of future outbreaks will shift northward and may increase in size.

Western spruce budworm (Choristoneura occidentalis (Freeman)) is a native defoliator of the Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) forests of British Columbia, Canada. This thesis used dendrochronology and the software program OUTBREAK to reconstruct a defoliation history of Douglas-fir for nineteen forest sites near Kamloops in central British Columbia. By comparing the radial growth response of non-host ponderosa pine trees to Douglas-fir trees growing in nearby but separate stands, seven western spruce budworm outbreaks were distinguished over the past 300 years. Although there is considerable variation in the timing and duration of these western spruce budworm events at the stand level, synchronous outbreaks have occurred at approximately 43-year intervals. Climate variation appears to have been important to budworm outbreaks in the 20th century. Notable outbreaks tended to occur during years of early springs with average air temperature, following winters with lower than average precipitation. Based on this finding, it is proposed that with high overwintering survival, increased population growth rates, and a longer growing season, the extent of future outbreaks will shift northward and may increase in size.

The western spruce budworm (Choristoneura occidentalis Freeman) is the most widespread and destructive defoliator of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests in British Columbia. Over the past two decades, western spruce budworm outbreaks have been sustained and widespread in the interior of British Columbia, leaving the forest industry and many forest-dependent communities increasingly vulnerable to the economic consequences of these outbreaks. While a great deal is known about the impact of western spruce budworm outbreaks on tree growth and form, substantial knowledge gaps remain as to the historic variability of western spruce budworm outbreaks and the consequences of defoliation on fundamental characteristics such as wood structure. This research focused on describing historic and contemporary western spruce budworm outbreaks across multiple spatial and temporal scales in south-central British Columbia using dendrochronology and wood anatomy techniques. Outbreak histories over the past 435 years were reconstructed using a network of tree-ring chronologies from central British Columbia, revealing that 12 western spruce budworm outbreaks have occurred since the early 1600s, with a mean return interval of 30 years. Further, the research illustrates that outbreaks observed over the last 40 years are not unprecedented, which does not support the perception that western spruce budworm is moving northward into central British Columbia. To evaluate the effects of a single western spruce budworm outbreak on the anatomical characteristics of Douglas-fir stemwood, tree ring data was collected from permanent sample plots that sustained both periodic and chronic western spruce budworm feeding. In mature even-aged stands of Douglas-fir, a documented outbreak occurred from 1976 to 1980 in the coastal transition zone of southern British Columbia. Based on microscopic wood anatomical measurements it was shown that the tree rings formed during this outbreak had significantly lower percentages of latewood, reduced mean cell wall thickness and smaller radial cell diameters relative to wood formed during periods without budworm feeding. Western spruce budworm defoliation temporarily modified cellular characteristics, which has implications for wood quality. In uneven-aged stands of mature Douglas-fir, located in the xeric southern interior of British Columbia, there has been a sustained western spruce budworm outbreak since 1997. Tree rings formed during this outbreak had progressively larger earlywood lumen area and radial cell diameter, reduced latewood cell wall thickness, latewood radial cell diameters, and lower percent latewood. Mixed-effects models revealed that climatic variables, defoliation severity, defoliation duration, and in limited cases canopy class were the best predictors of xylem features. The severity and duration of western spruce budworm defoliation, as well as site factors that influence moisture conditions effect the degree and direction of anatomical changes in the stemwood of Douglas-fir. This research fills a number of knowledge gaps by providing insights into the temporal and spatial dynamics of western spruce budworm outbreaks in central British Columbia over multiple centuries, and the plasticity of anatomical features in the stemwood of Douglas-fir during discrete western spruce budworm outbreaks. These research findings suggest that Douglas-fir forests are resilient to western spruce budworm outbreaks over space and time. / Graduate

Final Report / Contract on 43-8371-4-628 / For: USDA, Forest Service, Southwestern Region / Regional outbreaks of western spruce budworms (Choristoneura occidentalis Freeman) have recurred at least three times in northern New Mexico since the early 1920's when the U. S. Forest Service first began systematic forest-pest surveys and documentation (Lessard 1975, U. S. Forest Service documents). The current outbreak was first noticed in a small area on the Taos Indian Reservation in 1974, and since then the defoliated areas have increased in New Mexico and Arizona to more than 370,000 acres of Federal, Indian, State and private lands (Linnane 1984). Losses in timber values can generally be ascribed to radial growth loss, height growth loss, topkilling, reduced regeneration, and mortality (Carlson et al. 1983, Fellin et al. 1983). A damage assessment project was initiated in 1978 and was aimed at obtaining measurements of some of these losses in budworm infested stands on the Carson National Forest, New Mexico (Holland and Lessard 1979). A large data base has subsequently been developed, including yearly measurements on topkilling, mortality, defoliation, and insect population changes (Stein 1980, 1981, Stein and McDonnell 1982, Rogers 1984). A growth assessment study was undertaken in 1982 to determine the feasibility of using dendrochronological methods to identify the timing of past outbreaks and to quantify radial growth losses associated with budworm defoliation (Swetnam 1984). Results of this work showed that three major outbreaks during the twentieth century were clearly visible in the tree-ring samples obtained from currently infested trees. The radial growth of host trees was corrected for age, climate and other non-budworm environmental effects, and then growth losses were computed as a percentage of expected growth (Swetnam 1984). Additional collections were obtained in 1984 in order to expand the scope of the radial growth study. The objectives included 1) assessment of a larger number of tree -ring samples, 2) comparison of radial growth losses between the two primary host species - Douglas-fir (Pseudotsuga menziesii) and white fir (Abies concolor), 3) comparison of radial growth losses between age classes, and 4) analysis of the relationship between yearly measurements of defoliation, insect populations and radial growth. This report summarizes the findings of the above analyses. Increment core samples from the 1982 collections are included here, therefore this report supersedes the earlier report (Swetnam 1984). Information is also presented on observations derived from the dated tree-ring series on the timing of occurrence of known and inferred spruce budworm outbreaks for the past 284 years (1700- 1983). This is the longest record of spruce budworm occurrence yet developed for western North America.