Thinning improves growth of crop trees in natural shortleaf pine stands

Dense, previously unthinned, 24- to 28-yr-old natural shortleaf pine (Pinus echinata Mill.) stands in the Ouachita Mountains of eastern Oklahoma showed good growth responses during the first 5 yr after thinning to three stocking levels -- 30, 50 and 70% of full stocking. Residual trees in stands thinned to minimum full stocking (60%) or less appeared to rapidly utilize the additional growing space. Net periodic annual basal area growth averaged 6.7, 7.9, and 8.5 ft2/ac/yr, respectively, for plots thinned to 30, 50, and 70% stocking (PS), but only 4.5 ft2/ac/yr on the unthinned controls due to mortality. Periodic annual diameter growth for trees comprising a dominant stand component averaged 0.42, 0.35, and 0.29 in. on the 30, 50, and 70 PS plots, respectively, and 0.24 in. on the unthinned controls. Periodic annual merchantable volume (3 in. top dob) growth of trees larger than 3.5 in. dbh was not significantly different among the 50 PS, 70 PS, and unthinned control plots, and ranged from 183 to 213 ft3/ac/yr. The excellent growth rates observed during the 5 yr study period exceeded expectations for these sites (SI50 = 57) and stand ages, and might be due to the above-normal precipitation received during 4 of the 5 yr