Shallow rooted understory plants use hydraulically redistributed water by mature oak trees during drought

Hydraulic redistribution (HR) by deep-rooting trees can provide water from deeper soil layers to shallow-rooted understory plants, yet species-specific and temporal patterns of HR water uptake remain poorly understood. We investigated whether HR water from mature oaks ( Quercus robur L./ Quercus petraea (Matt.) Liebl.) is preferentially taken up by oak seedlings compared to co-occurring understory species, and how HR water use varies over the course of the day. During drought periods in mature oak stands in Germany, two field experiments were conducted. In a deep-soil 2 H-labeling experiment of mature oak trees, HR water was detected in the roots of three different understory plants. Six days after labeling, HR water amounted to 16 ± 8% (oak), 13 ± 7% (black cherry, Prunus serotina Ehrh.), and 8 ± 4% (small balsam, Impatiens parviflora DC.) of total root water content. Although intraspecific oak–oak HR was initially the highest, after 60 days all species contained approximately 20% HR water, indicating no persistent species-specific advantage. A second experiment analyzing natural δ 18 O abundance revealed pronounced diurnal dynamics of HR water use in oak seedlings. Contrary to expectations, the HR water fraction peaked at midday, when transpiration was highest. Overall, 29 ± 6% of the daily transpired water in oak seedlings originated from HR. These results demonstrate that HR can substantially contribute to seedling water use during dry periods, highlighting the ecological importance of HR for understory plant water supply in temperate forests. Future studies should analyze specific pathways through which HR water is transported from redistributing plants to the roots of receiving plants to assess the impact of plant species, degree and type of mycorrhization or soil physical properties on HR.