Growth of lodgepole pine thinned to various densities on two sites with differing productivities in central Oregon

Plots in two natural lodgepole pine (Pinus conforta Dougl. ex Loud.) stands with differing productivities were repeatedly thinned to one of five growi ng-stock levels (GSLs). Dole area was used to define GSLs. A linear relatio n between stand density index (SDI) and bole area was found after each thin ning on the highly productive site, but the slope of this relation decrease d with successive thinnings as trees grew larger. On the site with intermed iate productivity, the upper limit for bole area was higher and a curviline ar SDI-bole area relation occurred. A constant bole area level probably doe s not represent the same competition level across a range of tree sizes. Lo w incidence of mortality caused by mountain pine beetle (Dendroctonus ponde rosae Hopkins) occurred at SDIs below 170 for both sites. Concave curviline ar decreases in diameter growth occurred with increasing GSLs. Significant decreases in height growth with increasing GSLs were not detected. A convex curvilinear increase in gross basal-area growth and cubic-volume growth to ok place with increasing GSLs. Gross total cubic-volume PAIs increased with increasing SDIs for both sites until stand densities reached 95 percent of the normal stand SDI. These cubic-volume PAI-SDI curves then flattened wit h increasing SDIs. Maximum cumulative net cubic-volume (total and merchanta ble) and board-foot yields were produced at the intermediate growing-stock level at the high site. Little apparent differences in these yields occurre d among the four highest GSLs at the intermediate site. Net total cubic-vol ume yield was higher for the three highest GSLs than net yields for unmanag ed stands from yield tables at comparable sites and ages. These studies hav e not continued long enough to determine the approximate age of culmination of net mean annual cubic- or board-foot volume increments. Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) outgrew lodgepole pine for the range of s tand ages on the highly productive site where the growth of both species wa s examined (33 to 58 years). Ponderosa pine should not be planted on lodgep ole pine sites on flats and basins, however, because ponderosa pine is subj ect to radiation frost damage. Early spacing control coupled with later com mercial thinnings to keep stand densities between SDI 114 and SDI 170 shoul d reduce mortality considerably, allow most of the wood produced to be capt ured by merchantable trees, and greatly increase quadratic mean diameters a nd live crown ratios over unmanaged stands at the same age. These stands wo uld be more pleasing visually, and their rotation ages may be longer.