Responses of stomata to humidity, light intensity, and leaf water status were investigated throughout a growth season under field conditions for five competing woody species dominating in various stages of a Central European hedgerow. Humidity sensitivity of stomata varied between species. Leaf conductance to H2O, g, as measured in steady-state humidity response curves under constant climatic conditions, remained on high summer levels in Prunus and Crataegus, and on a lower level in Acer, but fell from a high spring level to a low summer level in Ribes and Rubus. The effect of partial stomatal closure in dry air on CO2 uptake, A, varied seasonally and between species. Responses of stomata to light, measured by tracking gas exchange in the field but for controlled climatic conditions, were hyperbolic, similar to CO2 assimilation. The relationship between g and A at variable irradiance was always linear and depended on the species investigated and on the leaf age. In no case did stomata respond to short-term (hourly) changes in leaf water potential (up to 13.5 bar).
A low leaf conductance level appeared to be correlated with low hydraulic conductivity in the plant, G, whereas high G in most cases coincided with high stomatal conductances. In Ribes and Rubus G and stomatal opening at maximum CO2 uptake varied in parallel during the season at high soil water content, suggesting a root/stomata interaction independent of plant water status. Water use efficiency (WUE, at certain leaf/air water vapour concentration differences), as determined from light and humidity responses of stomata and CO2 assimilation, was high in species of low G (Ribes, Rubus), and low in species of high G (Prunus, Crataegus). Surprisingly, species of low WUE optimized gas exchange in the sense of Cowan and Farquhar's (1977) theory, where stomata maximized CO2 uptake at a certain water loss within a certain period, while species of high WUE did not optimize in this sense. Other control mechanisms at the leaf level independent from optimization may be involved.
It appeared that low G and loss of humidity sensitivity of stomata in Ribes limited distribution of this species outside the half shade of the hedgerow, as exposed plant parts desiccated on dry days. However, water use efficiency per se did not determine species-specific competitive ability, as it was highest in species of low competitive strength.