Graduation date: 2008
This study examined patterns and controls on 35-years of forest succession following logging in the 236 ha South Umpqua Experimental Forest within the Umpqua National Forest in southwestern Oregon. Prior to logging, the overstory in all three watersheds (~50% cover) was composed of Douglas-fir (30-40% cover), grand fir (2 to 10% cover), incense cedar (2-4% cover), western hemlock (1-2 % cover), sugar pine (1-2% cover), ponderosa pine (0-1% cover), and madrone (0-1% cover). The sapling cover in all three watersheds (30-35% cover) was dominated by grand fir (15-20%), western hemlock (6-8% cover) incense cedar (3-5% cover), and Douglas-fir (3-5% cover), with <1% cover by the other tree species.
In 1971, the overstory of the four watersheds received: WS-1 (69.2 ha) a 50% basal area removal (shelterwood); WS-2 (68.4 ha) 30% in 20 small clearcuts; WS-3 (49.8 ha) 100% clearcut; WS-4 (48.6 ha) uncut control. Prior to treatment, tree, shrub, and herb cover in WS 1-3 (n = 50 plots) was visually estimated using the reconnaissance survey method, but only WS 2-3 annually through 1981 (n = 39 plots). In 2006, plots were relocated and new plots added, and resurveyed (n = 77 plots). Plots were classified by environmental (aspect, slope, soil series, vegetation community) and disturbance factors (treatment, logging method, in-watershed location, and soil disturbance). Vegetation was classed into three groups: (1) vegetation structures; (2) overstory species; and (3) residual and invader understory species.
Total vegetation cover decreased slightly over the 35-year period, with a slight decrease in shrubs, saplings, and herbs, and an increase in overstory tree cover in 2006 compared to 1971. Few statistically significant differences were observed in vegetation cover change according to environmental variables. However heavily disturbed sites were characterized by low shrub cover and high overstory cover, compared to lightly disturbed sites. These differences were still apparent 35 years after logging.
Between 1971 and 2006 in WS-2 and WS-3, Douglas fir, incense cedar and ponderosa pine cover significantly increased, while other overstory species declined. In contrast, over the same period in WS-1, there was a significant change in grand fir and western hemlock cover. Sapling cover of all species stayed constant or declined in all watersheds, except Douglas-fir and western hemlock, which increased in WS-1 only. In the post-logging period, cover of ponderosa pine and Douglas fir cover was significantly higher on moderately and heavily disturbed plots compared to lightly disturbed plots.
Before logging, residual species cover was significantly higher than invader species cover. Disturbance appeared to have the greatest effect upon the residual-invader species relationship. Residual species maintained higher cover on lightly disturbed plots, compared to invader species cover, which was highest on moderately and heavily disturbed plots. All species declined in WS-2 and WS-3 following stem exclusion.
Overall patterns of tree, shrub, and herb cover succession over the 35-year post-logging period were consistent with the expected effects of clearcutting versus thinning on light levels and their interactions with known shade tolerances of tree species.
Additionally, results were consistent with previously documented site disturbance effects on regeneration of residual and invader species, and shrub-tree competition and regeneration. These results permit prediction of the likely consequences for these 35-yr-old forest stands, and the thinned mature forest, of future, planned thinning treatments.