Shallow rooted understory plants of different species use hydraulically redistributed water by mature oak trees during natural drought periods

Both facilitative and competitive interactions between trees affect water relations and fluxes in temperate forests. During drought, hydraulic redistribution (HR) by deep-rooting species such as oak (Quercus robur L.) can provide water from deeper soil layers to shallow-rooted understorey plants. Assuming that shallow-rooted plants take up HR water from mature oaks during drought, we tested two hypotheses: (1) mature oaks share more HR water with oak seedlings than with other understorey plants, and (2) seedlings accumulate most HR water in their roots at sunrise because HR occurs over night. We also quantified how much HR water seedlings used in daily transpiration.

These hypotheses were tested in two experiments. 1) Over a period of six days in July 2023, a total of 7.2 L of ²H-labelled water (5 atom%) was added to a depth of 50-70 cm around mature oak trees in a forest in Brandenburg, Germany. We sampled soils, stem xylem of mature oaks, and roots of two tree species seedlings (oak and black cherry, Prunus serotina EHRH.) and a herbaceous plant (small balsam, Impatiens parviflora DC.) near the labelled trees. From all samples in the water was extracted via cryogenic water extraction and the isotopic composition of the water was analysed. After six days, recovery of δ²H in 0-10 cm soil depths indicated HR via oak roots. Also, seedling roots were enriched in δ²H, confirming HR water uptake with 16 ± 8 % (oak), 13 ± 7 % (black cherry) and 8 ± 4 % (small balsam) of root water originating from HR. Oak seedlings initially had more HR water in root tissues than other species, suggesting faster transport of HR water to oak seedlings, possibly due to shared mycorrhizae or root contact. However, after 60 days, the HR water content of all shallow-rooted understorey plants equalised (~20 %), rejecting our hypothesis 1 that HR water is preferentially found in seedlings of the same species (here: oak).

2) In August 2024, in a Bavarian forest (Germany), soil and stem xylem samples of mature oaks were collected together with root samples from oak seedlings at five daily intervals. Water was again extracted from all samples as in experiment 1. Following the natural gradient of stable water isotope composition in the soil profile, we considered HR water uptake by seedlings, if δ¹⁸O values in seedlings did not match δ¹⁸O in the surrounding soil, but reflected deeper soil values. At each time interval, seedling transpiration was measured before root excavation. The highest HR water content was found at midday, not at sunrise, in seedlings’ root water, rejecting hypothesis 2. Nevertheless, 29 ± 6 % of the oak seedlings’ daily transpired water originated from HR, emphasising the importance of HR for shallow-rooted understorey plants during drought. Future research should focus on the transport pathways of HR water from mature trees to shallow-rooted understorey plants to improve mechanistic understanding.