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

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 ( g s ) and crown ( g c ) conductance attained maximum values early during the day and then declined as both leaf‐to‐bulk air water vapour mole fraction difference ( V a ) 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 g c and individual variables. Normalization of g s and g c by I removed the hysteresis and revealed a strong stomatal response to humidity. At the crown scale, transpiration ( E ) increased linearly with net radiation ( R n ) and seemed to increase with increasing wind speed. Increasing wind speed imposed higher leaf interior to leaf surface water vapour mole fraction differences ( V s ) at given levels of V a . However, strong relationships between declining g c and E and increasing wind speed were obtained when g c and E were normalized by I and R n , respectively, without invoking additional potential interactions involving temperature or CO 2 concentration at the leaf surface. Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.