|Abstract or Summary
- A field study was established to determine the impact of interfering vegetation on survival and growth of Engelmann spruce (Piceaeigelmannii Parry) and lodgepole pine (Pinus contorta var contorta Dougi.) at a site in the high elevation Engelmann spruce-Subalpine fir (ESSF) zone, of south-central British Columbia. The study examined (1) the influence of varying amounts of shrubs and herbs on microclimate and planted seedling performance, (2) relationships between various measures of vegetation interference and conifer seedling growth, and (3) the response of six major shrub and herb species to manual cutting and mechanical scarification. After two growing seasons survival of both conifers was greater than 97%, except in the presence of the highest levels of interfering vegetation, where survival was 82% to 84%. Diameter was the most responsive conifer growth measure to levels of interference. In the absence of interfering vegetation mean diameters of spruce and pine were 21.3% and 27.5% greater, respectively, than mean diameters of seedlings in the undisturbed brush. Height growth generally did not respond to interference levels. Soil water potential was ever lower than -0.01 MPa and was the same at all levels of interference roughout the growing season. Midday and predawn spruce xylem water tential also did not vary by vegetation removal treatment. Even though soil water was ample, moderate water stress was observed, most likely because of restricted uptake caused by cold soil temperatures. Light levels under undisturbed vegetation were low enough to impair photosynthesis. Results suggest that low soil and air temperatures and low light levels may be the most important primary factors inhibiting conifer seedling performance beneath the undisturbed brush community. The relationship between growth of individual spruce and pine and various measures of vegetation interference was always negative. Measures of percent vegetation cover were consistently the best predictors of seedling growth. A maximum of 25% of the variation in seedling growth was explained by measures of interference. The response of conifer seedlings to interference may be nonlinear, with a decreasing response observed at high amounts of interference. Tentative threshold points where growth can dramatically improve were identified. However, more than two growing seasons are probably required for the shape of the response curve or model to become clear in the slow-growing ESSF environment, Large variance in seedling growth at low levels of interference suggests either that microsite variability or genotype differences constrain seedling performance even in the absence of interference. Recovery of the four dominant shrubs, white-flowered rhododendron (Rhododendron albifloruni Hook.), black huckleberry (Vaccinium membranaceum Dougl.), oval-leaved blueberry (V. ovalifoliuin Smith.), and false azalea (Menziesia ferruginea Smith.) following manual cutting was slow. After two growing seasons, the height of the tallest stems averaged between 10 cm and 15 cm (or about 10-30% of precut levels), and had not overtopped the conifers. Shrub vigour appeared unaffected by one additional cutting. The two major herb species, Sitka valerian (Valeriana sirchensis Bong.) and Indian hellebore (Veratrum viride Ait.) had recovered to precut levels by early in the second growing season. Sitka valerian vigour decreased after multiple cutting. Mechanical scarification either severely damaged or completely killed the shrub species. Herb species esponded to mechanical scarification as they did to manual cutting.