First-year growth of planted Douglas-fir and white fir seedling under different shelterwood regimes in California

A 17-ha area in the central California Sierra was harvested in 1979 to provide three shelterwoods with residual basal areas of 10, 15 and 20 m²/ha. An adjacent uncut stand of 75 m²/ha was used as a control. During the summer of 1980, seedling performance was assessed along with five microenvironmental characteristics. Maximum air temperature was similar among shelterwoods; minimum air temperature varied by about 3°C. Daily potential evaporation in all shelterwoods was similar and doubledfrom June to July and remained constant through September. Evaporation in the control was half of the shelterwoods. Subsurface soil temperature reached a maximum in August of 24°C in the least-dense shelterwood and 16°C in the control. Subsurface soil moisture was 35% for all treatments in May, but in September was 18% in the least-dense shelterwood and 12% in the control. Daily incident light in June ranged from 76% to 54% of maximum in the least-dense and most-dense shelterwood respectively, and 12% of maximum in the control.Survival of Douglas-fir was greater than 93% in all treatments. However, survival of white fir ranged from 63% to 85% as the overstory density increased; these differences were not significant. The period of rapid white fir mortality occurred later in the summer with increasing canopy cover. Height growth of both species was about 2.3 cm in the control to 3.1 cm under the least-dense shelterwood; however, treatment differences were significant only for white fir, Stem diameter growth of about 1.3 mm in treatments 10 and 15 was significantly greater than the growth of 0.3 mm in treatments 20 and the control. In Douglas-fir shoot: root ratios and biomass tended to increase with decreasing shelterwood. In white fir, the reverse trend was observed for shoot:root ratio but biomass production was similar in each shelterwood. Seedling sshowed the least overall growth in the uncut stand where plant moisture stress reached -19 bar (-1.9 MPa) in Douglas-fir and -22 bar (-2.2 MPa) in white fir.Results indicate that a 10 m²/ha shelterwood provides conditions for successful 1st-year survival and growth of container-grown Douglas-fir, but a 15 m²/ha density is preferable for bare-root white fir. Shoot growth initiation does not seem to be delayed under shelterwoods of up to 20 m²/ha.