Persisting soil drought reduces leaf specific conductivity in Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens)
Frank J. Sterck (1, 2), Roman Zweifel (3), Ute Sass-Klaassen (1) and Qumruzzaman Chowdhury (1)
1. Forest Ecology and Forest Management Group, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands / 2. Corresponding author () / 3. Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, Switzerland / Received January 31, 2007; accepted July 6, 2007; published online February 1, 2008
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 how LSC 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 pubescens Willd.) trees were compared between a dry site and a wet site in the Valais, an inner alpine valley in Switzerland. Soil
water 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 leaf-to-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.
