A Dendrochronological Assessment of Western Spruce Budworm, Choristoneura Occidentalis Freeman, in the Southern Rocky Mountains

TitleA Dendrochronological Assessment of Western Spruce Budworm, Choristoneura Occidentalis Freeman, in the Southern Rocky Mountains
Publication TypeThesis
Year of Publication1987
AuthorsSwetnam, T
AdvisorZwolwski, M
Academic DepartmentSchool of Renewable Natural Resources
UniversityUniversity of Arizona
KeywordsWatershed Management

Tree-ring chronologies from ten mixed conifer stands in the Colorado Front Ranges and New Mexico Sangre de Cristo Mountains were used to reconstruct timing, duration, and radial growth impacts of past outbreaks of western spruce budworm, Choristoneura occidentalis Freeman. Graphical and statistical comparisons of tree-ring chronologies from host and non-host tree species, in conjunction with Forest Service records of outbreaks during the twentieth century, revealed that outbreaks were identifiable only in the host chronologies as sharply reduced growth periods. These comparisons also showed that host and non-host tree-ring chronologies were generally similar between outbreaks and that both were responding in a similar manner to climatic variation. A study of defoliation and insect population data that was available for the New Mexico stands demonstrated that host radial growth from 1978 to 1983 was highly correlated with budworm activity. The non-host chronologies from each stand were used to correct the host chronologies for climatic and other non-budworm environmental variations by a differencing procedure. The corrected chronologies were then used to estimate the dates and radial growth effects of past budworm outbreaks. Tree-ring characteristics of twentieth century documented outbreaks were used as criteria for inferring the occurrence of outbreaks in previous centuries. At least nine periods of increased budworm activity were identified in the region from 1700 to 1983. The mean duration of reduced growth periods caused by known and inferred budworm outbreaks was 12.6 years, and the mean interval between initial years of successive outbreaks was 34.9 years. The mean maximum radial growth loss was 50 percent of expected growth, and the mean periodic growth loss was 21.6 percent. There was an unusually long period of reduced budworm activity in the first few decades of the twentieth century, and since that time outbreaks have been markedly more synchronous between stands. Increased synchroneity of outbreaks in the latter half of the twentieth century suggests that areal extent of outbreaks has increased. This phenomenon may be due to changes in the age structure and species composition of forests following harvesting and fire suppression in the late nineteenth and early twentieth centuries.