Document: RAM-3-19-9

Controls on ecosystem water use.

OREN, R.* ¹, K.V.R.SCHÄFER ¹, G.G.KATUL ¹, K.LIU ¹ and J.S.SPERRY ²

Duke University, Durham, NC USA ¹
University of Utah, Salt Lake City, UT USA ²

Abstract:
Variation in stand transpiration (E_C), a major part of ecosystem water use, is related to the surface area of leaves per unit ground (leaf area index, L), which reaches a maximum depending on growing season water balance and site quality. Departures from the relationship between E_C and L can be expected when hydraulic constraints and environmental conditions reduce stomatal conductance. Hydraulic constraints, such as flow path-length that increases with tree height, and specific hydraulic conductivity of xylem, may set a limit to maximum stomatal conductance, in order to avoid unregulated decreases in water potential and hydraulic conductance. Further reductions in stomatal conductance are imposed by constraints on uptake of soil moisture, light and temperature. Stomatal conductance is also sensitive to vapor pressure deficit. It seems that where hydraulic conductivity is high and when environmental conditions are favorable for high stomatal conductance, the sensitivity of stomatal conductance to vapor pressure deficit increases, again consistent with the role of stomata in regulating plant water potential. As a result, the variation in E_C driven by the diurnal pattern of vapor pressure deficit should be greater in absolute terms in forests and times in which maximum stomatal conductance is high, but the relative change with changing vapor pressure deficit should be similar to that when maximum stomatal conductance is low.

Keywords: stand transpiration, canopy stomatal conductance, hydraulic constraints, vapor pressure deficit

This abstract is being presented at: 11:35 AM in session:
Symposium # 9: The Water Limitation: Issues in Plant, Community, and Ecosystem Water Use.