Persisting soil drought reduces leaf specific conductivity in Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens)

Summary Leaf specific conductivity (LSC; the ratio of stem conductivity (KP) to leaf area (AL)), a measure of the hydraulic capacity of the stem to supply leaves with water, varies with soil water content. Empirical evidence for LSC responses to drought is ambiguous, because previously published results were subject to many confounding factors.We tested howLSC of similar-sized trees of the same population, under similar climatic conditions, responds to persistently wet or dry soil. Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescensWilld.) trees were compared between a dry site and a wet site in theValais, an inner alpine valley in Switzerland. Soilwater strongly influenced AL and KP and the plant components affecting KP, such as conduit radius, conduit density and functional sapwood area. Trees at the dry site had lower LSC than trees with the same stem diameter at the wet site. Low LSC in trees at the dry site was associated with a smaller functional sapwood area and narrower conduits, resulting in a stronger reduction in KP than in AL. These observations support the hypothesis that trees maintain a homeostatic water pressure gradient. An alternative hypothesis is that relatively high investments in leaves compared with sapwood contribute to carbon gain over an entire season by enabling rapid whole-plant photosynthesis during periods of high water availability (e.g., in spring, after rain events and during morning hours when leafto- air vapor pressure deficit is small). Dynamic data and a hydraulic plant growth model are needed to test how investments in leaves versus sapwood and roots contribute to transpiration and to maximizing carbon gain throughout entire growth seasons. Keywords: Huber value, leaf area, soil water potential, stem conductivity.