STRUCTURE AND YIELD OF 2-AGED STANDS ON THE WILLAMETTE-NATIONAL-FOREST, OREGON - IMPLICATIONS FOR GREEN TREE RETENTION

Green tree retention, a practice based on ecosystem science, has been integrated into forest management as a working hypothesis, requiring r esearch and monitoring to quantify its effects. We undertook a retrosp ective study of natural, two-aged forest stands on the Willamette Nati onal Forest to provide preliminary estimates of the effects of green t ree retention on forest structure and yield. Fourteen stands in the we stern hemlock (Tsuga hetepophylla (Raf.) Sarg.) zone (mostly dominated by Douglas-fir, Pseudotsuga menziesii (Mirb.) France) were sampled us ing adjacent, paired plots: one with residual trees over a younger coh ort (median age of younger cohort, 97 years) and one with only the you nger cohort. The younger cohorts all had much higher densities of tree s than typical of local Douglas-fir plantations. Basal area, volume, a nd mean annual increment (MAI) of the younger cohort all declined with increasing residual tree basal area. These relationships were best de scribed by curvilinear models; the greatest effect per unit of residua l tree basal area occurred at low residual tree levels (5-10 m(2)/ha). For 10 m(2)/ha residual tree basal area (equivalent to about 12 avera ge (diameter at breast height 105 cm) residual trees per hectare), we predicted a 26% decline in younger cohort MAI (95% confidence interval : -30% to -22%).