CONIFER-SEEDLING SUCCESS AND MICROCLIMATE AT DIFFERENT LEVELS OF HERB AND SHRUB COVER IN A RHODODENDRON-VACCINIUM-MENZIESIA COMMUNITY OF SOUTH CENTRAL BRITISH-COLUMBIA

The influence of various amounts of shrub and herb cover on microclimate and on survival and growth of Engelmann spruce (Picea engelmannii Parry) and lodgepole pine (Pinus contorta Dougl.) was examined in plots established in the Engelmann Spruce - Subalpine Fir zone of south central British Columbia. The planting sites varied from those with undisturbed vegetation to bare plots with exposed mineral soil. After three growing seasons, survival of both conifer species was greater than 95%, except at the highest levels of vegetation cover, where survival was 76 to 80%. Diameter was the conifer growth measure most responsive to reduction in adjacent vegetation. In the absence of adjacent vegetation, mean diameters of spruce and pine seedlings were 44 and 84% greater, respectively, than those of seedlings planted in undisturbed vegetation, and mean diameters increased still more when mineral soil was exposed. Height growth was less responsive to reduction in adjacent vegetation. Soil water potential, never more negative than -0.01 MPa, was the same at all levels of vegetation cover. Midday and predawn xylem pressure potential of spruce did not vary with the amount of vegetation. Although soil water was ample, moderate water stress occurred, most likely because uptake was restricted by low soil temperatures. Results suggest that soil temperature, air temperature, and light level are the primary factors controlling conifer seedling performance in an undisturbed vegetation community.