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TITLE:
Controls on Transpiration in Cottonwood-Willow Forest
INVESTIGATORS:
Sean Schaeffer and Dave Williams (UA)
OBJECTIVES:
The objective of this work is to develop a mechanistic understanding of riparian tree transpiration with emphasis on scaling tree-level measurements to forest stands. The work will investigate:
1. Transpirational flux from riparian cottonwood-willow forests, and
2. Environmental control on leaf stomatal conductance and transpiration on diurnal and seasonal time frames.
APPROACH:
Sap flow will be measured using heat pulse and stem heat balance methods. Diurnal courses of tree water potential and leaf stomatal conductance will be assessed the day of sap flow and water source measurements. Predawn and midday water potential will be measured with a pressure bomb. Instantaneous measurements of stomatal conductance will be made using a portable infrared gas analyzer and leaf chamber. A micrometeorological station will supplement spot measurements of air temperature, humidity and solar radiation. A mobile lift with a 25 m boom will allow gas exchange to be measured on intact leaves high in the canopy of large trees.
DATA COLLECTION (Variable, Units, Method, Location, Frequency):
1. Variable: sap flux, transpiration. Units: g/hr (per tree), mm/d (whole stand). Method: heat pulse, heat balance. Location: 4 tree clumps. Frequency: over 48-hr period h (continuously) assessed at 30-min intervals on each synoptic period.
2. Variable: stomatal conductance. Units: mmol/m2/s. Method: porometry. Location: 4 tree clumps. Frequency over 48-h period (daytime only) assessed at 2-hr intervals on each synoptic period.
3. Variable: canopy leaf temperature. Method: hand-held IRT. Location: 4 tree clumps. Frequency: Frequency over 48-h period (daytime only) assessed at 2-hr intervals on each synoptic period.
4. Variable: std micromet (VPD, wind speed. Tair, solar radiation). Unit: kPa, m/s, deg C, W/m2. Method: Movable micrometeorology station. with data logger. Location: At LS, within canopy near representative clump of trees. Frequency: over 48-h period (daytime only) assessed at 2-hr intervals on each synoptic period.
DATA REQUIRED FROM OTHER INVESTIGATORS:
1. Russ: micrometeorological data.
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