Initial size and competing vegetation effects on water stress and growth of Picea mariana (Mill.) BSP seedlings planted in three different environments.

摘要

Three experimental sites in Quebec, Canada, were planted with 4 sizes of containerized black spruce seedlings (Picea mariana). The water stress experienced by each stock size of seedlings in relation to different competing vegetation covers, and theeffect of the water stress on spruce growth, were examined during the first 3 growing seasons. The sites consisted of one recently abandoned agricultural field and 2 forest locations (clear felled in summer 1991 and autumn 1992, and subsequently prepared for planting). Containers of sizes 45 (110 ml, the small, conventional stock size), 45 (340 ml, medium), 15 (700 ml, large) and 12 (1000 ml, extra large) were employed to produce the 4 sizes of seedlings. At each site, the experimental protocol used asplit-plot in a randomized complete block design, in which the presence of a competing vegetation cover (weedy and bare plots) was assigned to the whole plot, while a specific seedling size was assigned to each subplot. The seedlings were planted in May1993, and at each site both the predawn xylem water potential PSIxp and the midday value PSIxm were measured 3X during each of the first three growing seasons (1993-95). Data were analysed as a completely randomized split-split-plot design, where selection of seedlings in time was considered as the whole plot. The competing vegetation tended to protect the seedlings from excessive water loss, without depressing the soil water potential (SWP) to the point of reducing the moisture available to the seedlings. Both PSIxp and PSIxm decreased significantly with increasing initial seedling size. The increased water stress experienced by the large stock of seedlings had an effect on the absolute growth rate (AGR) in height on only one experimental site. The AGR was impaired by the presence of a competing vegetation cover, but more severely for the smaller stock-size than the larger. The results indicated that short-term effect of competition should be based on radial growth; height growth and mortality were not early indicators of such effects. These results also emphasize the need to produce a large spruce planting stock with well-developed root systems and root growth capacity, even though only moderate water stress was observed during the first 3 yr of plantation growth.