Comparative measurements of transpiration and canopy conductance in two mixed deciduous woodlands differing in structure and
species composition
Mathias Herbst (1, 2, 3, 4), Paul T. W. Rosier (2), Michael D. Morecroft (2) and David J. Gowing (1)
1. The Open University, Department of Biological Sciences, Walton Hall, Milton Keynes MK7 6AA, U.K. / 2. Centre for Ecology and Hydrology, Crowmarsh Gifford, Wallingford OX10 8BB, U.K. / 3. Present address: Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark / 4. Corresponding author () / Received June 12, 2007; accepted November 16, 2007; published online April 1, 2008
Summary
Transpiration of two heterogeneous broad-leaved woodlands in southern England was monitored by the sap flux technique throughout
the 2006 growing season. Grimsbury Wood, which had a leaf area index (LAI) of 3.9, was dominated by oak (Quercus robur L.) and birch (Betula pubescens L.) and had a continuous hazel (Corylus avellana L.) understory. Wytham Woods, which had an LAI of 3.6, was dominated by ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.) and had only a sparse understory. Annual canopy transpiration was 367 mm for Grimsbury Wood and 397 mm for Wytham Woods.
These values were similar to those for beech (Fagus sylvatica L.) plantations in the same region, and differ from one another by less than the typical margin of uncertainty of the sap
flux technique. Canopy conductance (gc), calculated for both woodlands by inverting the Penman-Monteith equation, was related to incoming solar radiation (RG) and the vapor pressure deficit (D). The response of gc to RG was similar for both forests. Both reference conductance (gcref), defined as gc at D = 1 kPa, and stomatal sensitivity (–m), defined as the slope of the logarithmic response curve of gc to D, increased during the growing season at Wytham Woods but not at Grimsbury Wood. The –m/gcref ratio was significantly lower at Wytham Woods than at Grimsbury Wood and was insufficient to keep the difference between
leaf and soil water potentials constant, according to a simple hydraulic model. This meant that annual water consumption of
the two woodlands was similar despite different regulatory mechanisms and associated short-term variations in canopy transpiration.
The –m/gcref ratio depended on the range of D under which the measurements were made. This was shown to be particularly important for studies conducted under low and narrow
ranges of D.
