Thirty-five-year growth of ponderosa pine saplings in response to thinning and understory removal.

Abstract

Diameter increments for individual trees increased curvilinearly and stand basal area increments decreased curvilinearly as spacing increased from 6.6 to 26.4 feet. Average height growth of all trees increased linearly, and stand cubic volume growth decreased linearly as spacing increased. Large differences in tree sizes developed over the 35 years of study with various spacing treatments. Plots without understory grew more during the first 20 years of study but soil quality decreased. During the last 15 years, growth rates on plots without understory were not superior to plots with understory when adjusted to common basal areas and volumes. Growth rates for the largest trees on the plots were decreased by competition from smaller trees. After 35 years, total cubic volume yield decreased linearly as spacing increased but Scribner board-foot yields increased curvilinearly as spacing increased, and spacings of 13.2, 18.7, and 26.4 feet produced about the same board-foot yield. Live crown ratios increased with increasing spacing, primarily because of increased height growth. Twenty years after thinning, crown width increased curvilinearly as spacing increased and was greater in the absence of understory. Crown cover appeared to be linearly related to stand density index. Mortality was so low that there was no practical difference in net and gross-year mean annual growth of cubic volume and basal area. Spacing for precommercial thinnings on similar sites should be at least 14 feet and much higher spacings could be warranted if managers wish to grow stands of large-diameter trees with low mortality from bark beetles.

Citation

Cochran, P.H.; Barrett, James W. 1999. Thirty-five-year growth of ponderosa pine saplings in response to thinning and understory removal. Res. Pap. PNW-RP-512. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 28 p