Overstory canopy in second-growth coast redwoods: thinning a surrogate watercourse and lake protection zone

An 80-year coast redwood and Douglas-fir forest was used to estimate overstory canopy across successive treatments. Seven ground-based techniques were assessed in this surrogate Watercourse and Lake Protection Zone (WLPZ, California's FPRs) with the successive treatments as follows: 1) no operations, 2) cut to retain 200-foot radius circles, 3) cut to retain 150-foot radius circles, and 4) thin within 150-foot radius circles to retain 85% canopy overlay. Variability between cover and closure was greatest at a zenith angle of 2.5° (standard deviation=27.8) then gradually decreased until a maximum zenith angle of 87.5° (standard deviation=8.29). Mann-Whitney U Rank-Sum test indicated all medians to be equal if the zenith angle was ≤2.5° (p=0.1186).Direct site factor shared the strongest correlation with Solar Pathfinder (r2=0.8966), and sight-tube shared the second strongest correlation with gap fraction 175° (r2=0.8589). Solar Pathfinder and gap fraction 75° (r2=0.4460) shared the weakest correlation with all others. After removing 33% of the basal area (157 ft2•ac-1) canopy cover was reduced by 9% and canopy closure was reduced by 6%, both resulting in the targeted value of ~85% canopy overlay. Second-growth redwood stands can be thinned in a surrogate WLPZ while retaining legally sufficient overstory canopy.