Effects of soil water conditions on the morphology, phenology, and photosynthesis of Betula ermanii in the boreal forest

Four 2-year watering treatments were applied to Betula ermanii seedlings to investigate their responses to soil water conditions: ID irrigated well (I) in the previous year and water deficit (D) in the current year], DD, DI, and II. RGR of the seedlings in current-year water deficit without experiencing previous-year water deficit (ID) was smaller than that of seedlings irrigated well in both years (II). Surprisingly, RGR did not differ between 2-year-water-deficited (DD) and -well-irrigated (II) treatments. There was no difference in the area-based photosynthetic rate of the late leaves, which are formed in the current-year environment, between the four water treatments, but their leaf longevity was shortened by water deficit. Area-based photosynthetic rate of the early leaves, whose buds are formed in the previous year environment, was smaller in ID than in II, but it did not differ between DD and II. The de-epoxidation state of the xanthophyll cycle was greater in ID than in DD. Leaf mass per area (leaf dry weight/leaf area) did not differ among the four water treatments. Specific root length (root total length/root dry weight) did not differ between ID and II. It was greater in DD than in II, probably resulting in increased water uptake efficiency. Photoprotective systems such as xanthophyll contents and antioxidant enzyme activities did not differ among the four treatments. Morphological responses of B. ermanii roots seem to be important as whole-plant-level responses to water deficit for maintaining RGR, in addition to leaf-level photosynthetic and phenological responses of two types of leaves. Our results partly explain how B. ermanii seedlings survive and maintain growth even under varying soil water conditions in the boreal forest.