Subsurface structure regulates water use strategies of economic tree species in karst areas

Subsurface structure is becoming a key link in understanding terrestrial ecosystem water cycles and plant adaptation mechanisms. Previous studies have mostly focused on homogeneous soil habitats, it remains unclear how the complex subsurface structures formed by alternating rock and soil, together with root distribution, collaboratively regulate plant water use strategies. In this study, we selected rock‑dominated (RD) and soil‑dominated (SD) habitats in a karst area of Southwest China, and used deep‑rooted Juglans regia and shallow‑rooted Eriobotrya japonica as target economic tree species. By integrating stable isotope analyses (δ 2 H, δ 18 O and δ 13 C), electrical resistivity tomography, geological drilling and root investigation, we quantitatively analyzed the seasonal water source dynamics and intrinsic water use efficiency (iWUE) of plants in different habitats. The results showed that plants in RD exhibited a clear seasonal water source shift: they mainly used soil water in the rainy season (>47.5%) but shifted to rock fissure water in the dry season (>55.3%). In contrast, plants in SD relied continuously on soil water throughout the year (>69.4%) with relatively limited use of rock fissure water (<23.0%). This difference in strategies was co‑regulated by subsurface structure and root distribution: in RD, well‑developed fissure networks stored deep water sources, driving roots to extend along fissures to obtain water, but the higher hydraulic cost led to a marked increase in iWUE; in SD, plants accessed soil water more easily and had a lower iWUE, yet faced a higher risk of water shortage during the dry season. Research indicates that the degree of bedrock fragmentation controls the convergence of plant water use strategies, primarily by regulating fissure water storage and root distribution, two factors that collectively determine plant water source selection. Therefore, when evaluating vegetation responses or planning species configuration, subsurface structure should be prioritized over the intrinsic water preferences of individual species.