The effect of temperature differences between porometer head and leaf surface on stomatal conductance measurements

Abstract

It is generally known that instantaneous values of leaf conductance as measured with a dynamic porometer need to be corrected for the temperature difference, ΔT, between the porometer cup and the sampled leaf. Leaf conductances, obtained with a Delta-T AP4 dynamic porometer, with and without correction for AT are compared for a hush species (Guiera senegalensis) and two forb species (Jacquemontia tamnifolia and Mitracarpus scaber). With temperature differences predominantly varying within the ±2.5 °C recommended by the manufacturer, it appears that the differences between uncorrected and corrected conductances are very large, up to 100% on average, especially fur the two forbs. Furthermore, it is shown that, using the Mitracarpus data, a relatively small error of ±0.5 °C in ΔT can cause a difference of 25-50% in the final conductance value, in particular for the high conductance range. An error of ±0.5 °C may easily occur: the accuracy of ΔT as measured by the thermistors in the porometer is 0.2 °C and the temperature variation within a leaf can be much larger. This result will have implications for upscaling of leaf conductances to canopy values or may explain why upscaled values appear not to correspond with down-scaled values, obtained from eddy correlation measurements and an inverted canopy transpiration model.