TY - JOUR

T1 - Comparative measurements of transpiration an canopy conductance in two mixed deciduous woodlands differing in structure and species composition

AU - Herbst, Mathias

AU - Rosier, Paul T.W.

AU - Morecroft, Michael D.

AU - Gowing, David J.

PY - 2008

Y1 - 2008

N2 - Transpiration of two heterogeneous broadleaved 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, and was related to incoming solar radiation (RG) and the vapour 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 the 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.

AB - Transpiration of two heterogeneous broadleaved 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, and was related to incoming solar radiation (RG) and the vapour 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 the 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.

KW - Faculty of Science

KW - Broad-leaved woodland

KW - stomatal sensitivity

KW - seasonality

KW - sap flux

KW - potential evaporation

KW - heterogeneous forest

M3 - Journal article

VL - 28

SP - 959

EP - 970

JO - Tree Physiology

JF - Tree Physiology

SN - 0829-318X

ER -