This paper describes study results after 25 years of crop-tree thinning in a western larch (Larix occidentalis Nutt.) pole stand in western Montana. Five 1/2-acre plots were established in the Petty Creek drainage, Lolo National Forest, at an elevation of 4,200 feet above sea level. Western larch site index of the study area was 52 feet at 50 years. The stand was 50 years old when the study began in 1949. Two plots were thinned by the D+4 Rule, two by the Crown Rule, and one was left unthinned as a control. After 25 years, periodic diameter, basal area, and volume growth of crop trees on thinned plots were significantly greater (P=0.05) than designated crop trees in the unthinned plot, but differences are declining. Although statistically greater, the increase in crop-tree diameters - the major objective of the thinning - was not practically important, amounting to less than a 1 percent increase over crop trees in the unthinned control. Also, the crop-tree thinnings resulted in markedly lower basal areas and volumes of the total stand. From the results of this study, crop-tree thinnings by the D+4 or Crown Rules are not recommended for overstocked 50-year-old western larch stands, unless other resources benefit enough to offset the disappointing crop-tree growth response of the thinned stand. Instead, low thinning is recommended, preferably when the stand is younger.
United States Department of Agriculture, Forest Service, "Crop-Tree Thinning a 50-Year-Old Western Larch Stand: 25-Year Results" (1984). Forestry. Paper 14.