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Complementarity of resource utilization is crucial to the success of intercropping and agroforestry systems. Increased capture and/or more efficient use of limiting resources, such as light, water and nutrients, are key factors. Field techniques for measuring light interception and conversion efficiency in mixed communities are well established, but techniques for partitioning water use between the components of mixed communities have only recently become available. A major advance has been the heat balance approach for measuring sap flux through the stems of individual plants. Several studies indicate that this approach can measure transpiration with an accuracy of±10% or better in relatively large plants (Baker and van Bavel, 1987; Steinberg, van Bavel and McFarland, 1989). The technique is sensitive to small changes in sap flux and has a fast response time.

A heat balance system similar to that described by Ishida, Campbell and Calissendorff (1991) was used in a field trial at ICRISAT, India in which the water use, light interception and productivity of groundnut-perennial pigeonpea agroforestry systems were examined. Line-planted (5.4 m pigeonpea alleys) and dispersed arrangements (1.8 x 1.2 m pigeonpea spacing) were compared. The pigeonpea population was 0.5 plants m-2 in both arrangements, while groundnut was sown at 10 cm intervals in rows 30 cm apart. Sole pigeonpea and groundnut were also grown for comparison. The heat balance was used to measure transpiration by the dispersed and line planted pigeonpea between February 1990 and January 1991, when the pigeonpea were 7-18 months old. Typical diurnal …