Harvesting impacts on understory regeneration in two-storied Douglas-Fir stands

Conifers were underplanted beneath thinned 50-year-old Douglas-fir stands in western Oregon. Overstories had been thinned to four densities ranging from 18 to 31 m(2)/ha (78-134 ft(2)/ac) from stands originally stocked at about 23 to 51 m(2)/ha (100-220 ft(2)/ac). Seedlings beneath some densities showed declining vigor in the first 8 years after planting, resulting from overstory growth of about 5.7 m(2)/ha (25 ft(2)/ac) in that period. Some of the stands were rethinned to remove the basal area accrued since the first thinning. Timber was harvested by ground and cable equipment, with log length averaging over 10 m (32 ft) and averaging 52 m(3)/ha (4.7 mbf/ac, Scribner rule) on 29 ha (72 ac). Overall, 40% of planted seedlings were damaged by logging. Among the four species planted, between 18% and 30% of seedlings alive at the time of harvest were removed or lodged/buried under debris. An additional 13% to 16% had broken or dead tops or were bent at a lean greater than 45 degrees. Two years after thinning, damaged seedlings grew an average of 10 cm less than those not damaged. Unless densities are initially very low at the time of planting, e.g., < 18 m(2)/ha (78 ft(2)/ac), rapidly growing overstories are likely to suppress understory plantings. Thinning will inflict losses unless logging is carefully laid out and executed., and long log lengths are minimized.