INITIAL SIZE AND COMPETING VEGETATION EFFECTS ON WATER-STRESS AND GROWTH OF PICEA-MARIANA (MILL.) BSP SEEDLINGS PLANTED IN 3 DIFFERENT ENVIRONMENTS

Three experimental sites in Quebec were planted with four different si zes of containerized black spruce seedlings ( Picea mariana (Mill.) BS P). We examined the water stress experienced by each stock si:Ie of bl ade spruce seedlings in relation to different competing vegetation cov ers and also the effect of the water stress on spruce growth during th e first three growing seasons, The sites consisted of one abandoned ag ricultural field and two forest: locations. Containers of sizes 45-110 , 45-340, 15-700, and 12-1000 were employed to produce the four differ ent sizes of spruce seedlings. At each site, the experimental protocol used a split-plot in a randomized complete block design, in which the presence of a competing vegetation cover (weedy and bare plots) was a ssigned to the whole plot, while a specific seedling size was assigned to each subplot. At each experimental site both the predawn xylem wat er potential psi(xp), and the midday value psi(xm);, were measured thr ee times during each of the first three growing seasons. Data were ana lysed as a completely randomized split-split-plot design, where select ion of seedlings in time was considered as the whole plot. The competi ng vegetation tended to protect the spruce seedlings from excessive wa ter loss, without depressing the soil-water potential (SWP) to the poi nt of reducing the moisture available to the seedlings. Both psi(xp) a nd psi(xm) were found to decrease significantly with increasing initia l seedling size. The increased water stress experienced by the large s tock of spruce seedlings had an effect on the absolute growth rate (AG R) in height on only one experimental site, The AGR was impaired by th e presence of a competing vegetation cover, but more severely for the smaller stock-size than the larger, The short-term effect of a competi tion should be based on radial growth; height growth and mortality are not early indicators of such effect, These results emphasize the need to produce a large stock of spruce seedlings with well-developed root systems and root growth capacity, even though only moderate water str ess was observed during the first three years of plantation growth. (C ) 1998 Elsevier Science B.V.