Influences of Mechanized Thinning and Prescribed Underburning on Radial Growth of Jeffrey Pine

Abstract

Forest thinning utilizing cut-to-length and whole-tree harvesting systems with subsequent underburning were assessed for their impacts on several components of radial increment in uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) on the east slope of the Sierra Nevada during a period of extended drought. At the conclusion of three growing seasons following thinning and two seasons after burning, increment cores were extracted from dominant and codominant crown class trees over a century old as well as from intermediate crown class trees that were approximately 20 years younger. Separate measurements of early wood and late wood ring width during each of the 10 growing seasons preceding treatment and three seasons subsequent to thinning were summed for each year to obtain total ring width and permitted the calculation of proportions of total increment of the respective components. Comparisons of posttreatment values or averages of each component as well as of the magnitude of change, in both absolute and relative terms, in their values following treatment from 10-year pretreatment means were made among treatments for three posttreatment phases. Thinning treatment differences in direct measurements of the several components were marginal and uncommon. However, disparities in the magnitude of shifts in radial increment resulting from thinning were highly significant. Total ring width in dominant and codominant trees averaged 14% below to 13% above the pretreatment level in thinned portions of the stand over the three postthinning phases examined, while in the unthinned control total increment was 30-53% lower than the pretreatment mean, with the lowest values in all treatments occurring during the driest period of the study and highest observed after annual precipitation increased nearer to normal. In intermediate crown class trees, changes in ring width among the thinning treatments were similar to those in the dominant and codominant trees. Differences between the cut-to-length and whole-tree treatments were more pronounced in the dominant/codominant trees and comparisons generally revealed higher relative growth in the latter than in the former, while in the intermediate crown class, trees in the cut-to-length treatment performed marginally better in this regard than those in the whole-tree treatment. Influences of prescribed fire were limited to only a few components of radial growth in both sets of trees considered. The magnitude of change in several components of radial growth following thinning was found to be negatively correlated with residual basal area, but only in dominant and codominant trees. Live crown percentage influenced radial growth preceding thinning in both the dominant/ codominant and intermediate crown classes, in which it was a positive factor, as well as changes in increment subsequent to thinning in the former alone, when it was negative. Overall, results of this study demonstrate that considerable improvement in stand productivity can be realized with density management in an older, dry site forest type, which at minimum is not diminished by subsequent underburning.