Redistribution of soil water by mature trees towards dry surface soils and uptake by seedlings in a temperate forest

Hydraulic redistribution is considered a crucial dryland mechanism that may be important in temperate environments facing increased soil drying–wetting cycles. We investigated redistribution of soil water from deeper, moist to surface, dry soils in a mature mixed European beech forest and whether redistributed water was used by neighbouring native seedlings. In two experiments, we tracked hydraulic redistribution via (1) ²H labeling and (2) ¹⁸O natural abundance. In a throughfall exclusion experiment, ²H water was applied to 30–50 cm soil depth around mature beech trees and traced in soils, in coarse and fine roots, and in the rhizosphere. On five additional natural plots, the ¹⁸O signal was measured in seedlings of European beech, Douglas fir, silver fir, sycamore maple, and Norway spruce at dawn and noon after a rain-free period. We found a significant enrichment in ²H in surface soil fine roots of mature beech, and an indication for transfer of this water into their rhizosphere, suggesting hydraulic redistribution from deeper, moist to drier surface soils. On four of the five additional plots, δ¹⁸O of seedlings' root water was lower at dawn than at noon. This indicated that dawn root water originated from soil layers deeper than the seedlings' rooting depth, suggesting hydraulic redistribution by neighbouring mature trees. Hydraulic redistribution equated to about 10% of daily transpiration in mature beech trees, and contributed to root water in understory seedlings, emphasizing hydraulic redistribution as a notable mechanism in temperate forests. Transport mechanisms and potential of different tree species to redistribute water should be further addressed.