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Abstract
Abstract. Leaf transpiration and energy exchange are coupled processes that operate at small scales yet exert a significant influence on the terrestrial hydrological cycle and climate. Surprisingly, experimental capabilities required to quantify the energy–transpiration coupling at the leaf scale are lacking, challenging our ability to test basic questions of importance for resolving large-scale processes. The present study describes an experimental set-up for the simultaneous observation of transpiration rates and all leaf energy balance components under controlled conditions, using an insulated closed loop miniature wind tunnel and artificial leaves with pre-defined and constant diffusive conductance for water vapour. A range of tests documents the above capabilities of the experimental set-up and points to potential improvements. The tests reveal a conceptual flaw in the assumption that leaf temperature can be characterized by a single value, suggesting that even for thin, planar leaves, a temperature gradient between the irradiated and shaded or transpiring and non-transpiring leaf side can lead to bias when using observed leaf temperatures and fluxes to deduce effective conductances to sensible heat or water vapour transfer. However, comparison of experimental results with an explicit leaf energy balance model revealed only minor effects on simulated leaf energy exchange rates by the neglect of cross-sectional leaf temperature gradients, lending experimental support to our current understanding of leaf gas and energy exchange processes.
Highlights
Most of the precipitation falling on land returns to the atmosphere by the process of transpiration, i.e. passing through the plant vascular system, undergoing phase change in leaves, and diffusing through stomata
More detailed analysis of pore geometries was performed on duplicate foils and suggests that the laser perforations were done from the shiny side, resulting in irregular surfaces around the pores and slightly conical pore geometries with smaller diameters on the matte side compared to the shiny side
When we compared estimations of pore sizes and conductances based on images taken on either side of the aluminium foil, we found higher conductance values by up to 50 % if images were taken on the shiny side compared to the matte side (Fig. A1, Table A1)
Summary
Most of the precipitation falling on land returns to the atmosphere by the process of transpiration, i.e. passing through the plant vascular system, undergoing phase change in leaves, and diffusing through stomata. Plant transpiration rates and CO2 uptake are controlled by stomata and by the leaf energy balance, i.e. the partitioning of the absorbed solar irradiance into radiative, sensible, and latent heat fluxes. Present understanding of leaf gas and energy exchange is based on controlled experiments with real and artificial leaves, where the individual components of the energy balance and their sensitivities to environmental forcing were assessed separately. The transfer of heat between a leaf and the surrounding air is less commonly measured. Studies exist where this heat flux was estimated from cooling curves after a sudden reduction in absorbed radiation (Kumar and Barthakur, 1971), but in order to test our understanding of the leaf energy balance, we need a way to monitor leaf heat and gas exchange simultaneously under controlled steady-state conditions
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