Stomatal resistance, photosynthesis and water relations in mountain birch in the subarctic.

The role of water in regulating stomatal aperture and carbon assimilation of mountain birch (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) was studied in the Swedish Subarctic. Stomata exhibited a clear response to changes in humidity and water potential. High evaporative demand caused stomatal closure at midday. The diurnal curve of net photosynthesis, measured on a warm day in early July, had two peaks, and photosynthetic rate was reduced by 25-30% at midday.