Underplanting and competition in thinned Douglas-fir

Graduation date: 1999Understory conifer regeneration needs to occur beneath conifer-dominated\ud canopies if two-storied or uneven-aged structures are to be considered for western\ud Oregon Coast Range stand management. To ensure adequate numbers of seedlings\ud to meet stocking or habitat structure objectives, planting may be a solution. We\ud undertook a multi-level study to determine limits to underplanted conifer seedling\ud growth.\ud This experiment was installed in the McDonald-Dunn Research Forest.\ud Plots were assigned four residual basal areas ranging from 16 to 31 m²/ha and split\ud into three site preparation treatment subplots; broadcast herbicide application,\ud release herbicide treatment and no site preparation. Subplots were planted with\ud Douglas-fir, grand fir, western red cedar, and western hemlock.\ud Seedling survival, absolute volume and relative growth rate were negatively\ud correlated with overstory density. Broadcast herbicide application was beneficial\ud to grand fir and western hemlock seedling growth. Release spraying did not have a\ud beneficial effect on seedling volume growth.\ud Soil moisture depletion was greatest near the soil surface. Individual\ud vegetation variables were only weakly correlated with soil moisture depletion.\ud Overstory tree density had the best overall prediction value for soil water depletion\ud but was still weak.\ud Western redcedar moisture stress increased with increasing overstory\ud density and neighboring vegetation. Douglas-fir seedlings exhibited water stress\ud throughout the season under all treatments. Early in the summer some Douglas-fir\ud seedlings had pre-dawn xylem potentials that indicated they were not fully\ud recovering from diurnal water stress. Grand fir xylem potentials decreased in\ud unsprayed subplots. Western hemlock xylem potentials decreased steadily and\ud were within the range that indicated stomatal closure by late July. Understory\ud vegetation did not significantly affect western hemlock water relations.\ud Black-tailed deer browsed 46% of Douglas-fir and 30% of western redcedar\ud seedlings. Thinning spatial pattern and residual overstory density did not have an\ud appreciable effect on browsing frequency. There was a slight increase in browsing\ud frequency in sprayed subplots. Vexar tubing gave a slight reduction in browsing\ud frequency, but did not affect fourth year volume.\ud Stand basal area, photographically estimated canopy coverage and visually\ud estimated canopy coverage were used to predict seedling stem volume. Predictions\ud based on visually estimated canopy coverage were the most accurate and efficient