Growth and yield of western larch under controlled levels of stocking in the Blue Mountains of Oregon.

Repeated thinning to five growing-stock levels resulted in widely differing tree sizes and volumes per acre after 30 years. Largest trees but the leas t cubic-volume yield per acre were produced in the heaviest thinning level, whereas highest board-foot yields were found in intermediate thinning leve ls. Partial defoliation by larch casebearer (Coleophora laricella Hubner), drought, and top damage from ice occurred, and site trees grew less in heig ht than expected during the 30-year study. Curvilinear increases in periodi c annual increments of both basal area and cubic volume generally occurred with increasing stand density, but increments dropped off at the highest st and densities for some periods. Anticipated patterns for these increments w ere found after fitting a model that included stand density index, height i ncrements of site trees, and dummy variables for periods as independent var iables. Heavy thinning did not increase the age of culmination of cubic-vol ume mean annual increment as expected. Thinning stands of larch to densitie s as low as 50 percent of "normal" results in little loss of basal-area gro wth, a moderate loss in volume production, and a large increase in tree dia meter. Thinning is necessary in many larch stands to maintain vigorous, rap idly growing trees. Thinning levels will greatly affect the appearance of f uture stands.