Regulation of transpiration in coffee hedgerows: covariation of environmental variables and apparent responses of stomata to wind and humidity

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Abstract
Stomatal regulation of transpiration was studied in hedgerow coffee (Coffea arabica L.) at different stages of canopy development encompassing a range of leaf area indices (L) from 0·7 to 6·7. Stomatal (gs) and crown (gc) conductance attained maximum values early during the day and then declined as both leaf‐to‐bulk air water vapour mole fraction difference (Va) and photosynthetically active photon flux density (I) continued to increase. Covariation of environmental variables during the day, particularly V, I, and wind speed (u), obscured stomatal responses to individual variables. This also caused diurnal hysteresis in the relationship between gc and individual variables. Normalization of gs and gc by I removed the hysteresis and revealed a strong stomatal response to humidity. At the crown scale, transpiration (E) increased linearly with net radiation (Rn) and seemed to increase with increasing wind speed. Increasing wind speed imposed higher leaf interior to leaf surface water vapour mole fraction differences (Vs) at given levels of Va. However, strong relationships between declining gc and E and increasing wind speed were obtained when gc and E were normalized by I and Rn, respectively, without invoking additional potential interactions involving temperature or CO2 concentration at the leaf surface. Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.