Delayed uptake and intra-tree distribution of 2H-labeled irrigation water after repeated experimental summer drought in mature spruce compared with beech

Water uptake and distribution are critical for drought recovery, yet previous drought conditions have been shown to impair water transport by affecting soil-root contact and xylem conductivity. In order to investigate these dynamics, the approach of applying δ2H-labeled water as a controlled irrigation was adopted, with this irrigation being administered to a mixed stand of mature European beech (Fagus sylvatica (L.)) and Norway spruce (Picea abies Karst. (L)) trees in control (CO) and throughfall exclusion (TE) plots following 5 years of experimental summer drought. The δ2H concentrations were measured in soil, stem, twig and leaf water before and after rewetting to assess water pool turnover. The labeled water infiltrated the upper 70 cm of soil in both treatments within 48 h. However, a notable delay in water uptake and distribution was exhibited by TE trees in comparison with CO trees, where the label was detected in stems and leaves within 24 h. The TE beech demonstrated water uptake after 4 days, while TE spruce exhibited a more pronounced delay of 7 days. Despite this delay, TE trees exhibited a higher turnover of stem water pools (>75%) compared with CO trees (<50%), while leaf water turnover remained similar between treatments. The delayed uptake in TE trees may be attributed to fine root loss in both species and the suberization of surviving fine roots in spruce, which likely reduced water absorption efficiency. Additionally, the depleted stem water reserves in TE spruce may have delayed internal redistribution. These findings underscore the importance of considering species-specific recovery dynamics and provide valuable insights into the long-term impacts of drought on tree water relations.