GAS-EXCHANGE AND GROWTH OF 3 ARCTIC TREE-LINE TREE SPECIES UNDER DIFFERENT SOIL-TEMPERATURE AND DROUGHT PRECONDITIONING REGIMES

Low soil temperatures and water availability are thought to be major f actors determining the distribution of tree species at the arctic tree line. A comparative study examined the response of Betula papyrifera, Populus balsamifera, and Picea mariana seedlings to different soil te mperatures and drought regimes in a growth chamber experiment. Morphol ogical and ecophysiological responses (net assimilation rate, stomatal conductance to water vapour, and residual conductance) of these tree line tree species were measured and compared. Mean biomass accumulatio n of the deciduous species was greater than that of Picea mariana with increasing soil temperatures. Root biomass showed an increase of 30% in the three species between the soil temperatures of 3 and 15 degrees C. Response of ecophysiological variables to increased soil temperatu re was greater in B. papyrifera and Populus balsamifera than in Picea mariana. In a second experiment, drought-preconditioned B. papyrifera and Populus balsamifera seedlings were subjected to a B-day water-with holding treatment. Drought decreased shoot mass and increased the root to shoot ratio equally in B. papyrifera and Populus balsamifera. Drou ght-preconditioned B. papyrifera and Populus balsamifera seedlings res ponded differently to the 6-day water-withholding treatment. Betula pa pyrifera used a water-conserving strategy and maintained low net assim ilation rates and low water use after drought preconditioning, whereas in Populus balsamifera greater net assimilation rates were associated with drought preconditioning. These results are consistent with the d istribution of these three tree species at the arctic tree line.