- Swetnam, Thomas W.
University of Arizona
- Thompson, Marna Ares
- Sutherland, Elaine Kennedy
Budworms (C horistoneuro spp.) have repeatedly defoliated millions of hectares of conifers throughout North America. The seriousness of the problem is underscored by research findings in eastern Canada and the northern U.S. Rocky Mountains that the extent, severity, and frequency of budworm outbreaks have increased in the 20th century, primarily as a result of human-induced changes in forest structure and composition (Blais 1983, Carlson and others 1983, Fellin and others 1983).
Measurements of growth reduction and mortality may be the most important information in assessing damage caused by the budworm. Forest-pest managers can use this information to decide on appropriate responses and the timing of management actions. The extent of infestation, tree mortality, and height-growth reduction can usually be determined from aerial surveys and onsite observations, but measuring radial-growth reduction requires detailed analysis of tree-ring widths.
This paper describes dendrochronological techniques that can be applied to tree-ring samples to measure radial growth during past and present budworm outbreaks. The techniques will be illustrated with examples from a radial-growth study of trees infested by western spruce budworm (C. occidentalis Freeman) in northern New Mexico.
Most techniques described here have been standard for many years in dendroclimatic and dendroecologic studies (Anon. 1977, Fritts l97l) and have many advantages over other types of tree-ring analyses for evaluating effects of insects on radial growth. In particular, the rigorous use of cross-dating and standardizing can provide more precise measurements and improved understanding of the effects of insects, climate, and other environmental factors on tree growth.
Descriptions of dendrochronological techniques included here are not exhaustive because they are fully described elsewhere. Procedures are summarized in the context of the budworm problem so that they may be more accessible and useful to the interested researcher. The computer programs are described in general terms only. The programs (in FORTRAN 5), operating instructions, and limited technical advice are available from the Laboratory of Tree-Ring Research, Data Processing Section, University of Arizona, Tucson 85721. A small fee is charged for magnetic tape, handling, and shipping.