Water relations, survival and growth of conifer seedlings planted on a high elevation south-facing clearcut

Equal numbers of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and Pacific silver fir (Abies amabilis (Doug.) Forbes) seedlings,,were spring planted, in 1981 and 1982, as 1-0 styroplugs on a south-facing high elevation clearcut located on Mount Arrowsmith, Vancouver Island, British Columbia. A number of treatments were applied to determine whether modification of seedling microclimate would increase survival and growth. These treatments included inclining the seedlings to the southwest, provision of shade cards, irrigation, and irrigation and shade cards combined. The highest survival rates, regardless of treatment, were exhibited by Douglas-fir. Approximately 72% and 82% of untreated seedlings, planted in 1981 and 1982 respectively, survived. Survival of seedlings that received treatment ranged from 81% to 95%. Douglas-fir seedlings were able to survive and grow because they were highly drought tolerant. Unirrigated seedlings lowered their osmotic potential by over 1.1 MPa in the course of a growing season in response to declining soil water potentials and consequently were able to maintain turgor. The transpiration rates of these seedlings were never less than 50% of those that were irrigated. Western hemlock, and to an even greater extent, Pacific silver fir seedlings lacked stress avoidance and tolerance mechanisms. Consequently there was very poor survival of both species. Survival rates were significantly increased by shade cards and irrigation but never exceeded 64% in either species. The extent and type of growth in all species was markedly affected by changes in microclimate. Irrigated seedlings developed the largest shoots and the smallest root systems. Shaded seedlings had larger shoots, but generally less roots, than untreated seedlings which had the lowest shoot to root dry mass ratios. A boundary-line analysis model that related seedling stomatal conductance (g[sub s]) to measurements of hourly average solar irradiance, air temperature, vapour pressure deficit and average root zone soil water potential, accounted for over 70% of the variability in g[sub s]. When the number of hours from sunrise was included as an independent variable, over 85% of the variability in g[sub s]could be explained. Measurements of seedling seasonal dry matter accumulation were better correlated with estimates of seedling growing season transpiration than with estimates of average growing season total seedling g[sub s].Land and Food Systems, Faculty ofGraduat