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

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 s tand cubic volume growth decreased linearly as spacing increased. Large dif ferences in tree sizes developed over the 35 years of study with various sp acing treatments. Plots without understory grew more during the first 20 ye ars of study but soil quality decreased. During the last 15 years, growth r ates on plots without understory were not superior to plots with understory when adjusted to common basal areas and volumes. Growth rates for the larg est trees on the plots were decreased by competition from smaller trees. Af ter 35 years, total cubic volume yield decreased linearly as spacing increa sed but Scribner board-foot yields increased curvilinearly as spacing incre ased, and spacings of 13.2, 18.7, and 26.4 feet produced about the same boa rd-foot yield. Live crown ratios increased with increasing spacing, primari ly because of increased height growth. Twenty years after thinning, crown w idth increased curvilinearly as spacing increased and was greater in the ab sence of understory. Crown cover appeared to be linearly related to stand d ensity index. Mortality was so low that there was no practical difference i n net and gross 35-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 s tands of large-diameter trees with low mortality from bark beetles.