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Articles

Leaf Temperature Effects on Measurements of Diffusive Resistance to Water Vapor Transfer ¹

^a Department of Biological Sciences, Stanford University, Stanford, California 94305, ^b Department of Environmental Biology, Research School of Biological Sciences, Australian National University, Canberra, Australia

Substantial errors can be introduced to estimates of leaf resistance (r_l) obtained from diffusion porometers unless precautions are taken to reduce the leaf-cup temperature difference (T_leaf — T_cup) to acceptable levels prior to measurement. When T_leaf > T_cup, underestimation of leaf resistance occurs; the reverse applies when T_leaf < T_cup.

The effect is most pronounced under open-stomata conditions and declines as stomatal resistance increases. Under typical measurement conditions, T_leaf — T_cup values of the order of 1 C induce a reduction in the ratio of the apparent to true leaf resistance to about 0.8 when the leaf resistance is low (r_l = 1-2 sec cm^–1). When T_leaf — T_cup = 5 C, the ratio drops below 0.5. Under high leaf resistance conditions (r_l = 10-50 sec cm^–1) the comparable ratios are approximately 0.9 and 0.7, respectively.