Influence of rate of change in stomatal conductance to fluctuating irradiance on estimates of daily water use by Pinus taeda leaves

Pinus taeda L. stomata respond slowly to changes in irradiance. Because incident irradiance on a leaf varies constantly, the rate of change in stomatal conductance to fluctuating irradiance may have a large effect on plant water use. We estimated total daily water use of Pinus taeda foliage for 10 days with very different irradiance patterns, assuming that rates of stomatal opening and closing were similar. To determine how the most extreme imbalance in rates of stomatal opening and closing affects estimates of water use, we also estimated total daily water use assuming instantaneous stomatal opening and a realistic rate of stomatal closing. Total daily water use was calculated by summing estimates of transpiration based on irradiance and vapor pressure deficit measured every minute at locations atop and within a canopy. Estimates of total daily water use calculated on the basis of realistic rates of stomatal opening and closing were similar to estimates calculated assuming instantaneous stomatal change (mean difference between methods of calculation was less than 0.2%). Estimates of total daily water use assuming instantaneous stomatal opening and a realistic rate of closing differed from estimates of total daily water use based on similar rates of stomatal opening and closing. The discrepancy was greater within the canopy (mean difference 6%) than at the top of the canopy (mean difference 1%). Calculation of mean daily conductance from mean daily irradiance, without accounting for minute-by-minute variations in irradiance, resulted in overestimations of daily stomatal conductance (13% mean error) and the magnitude of the error was directly related to the variation in irradiance for that day. We conclude that, provided variation in irradiance is accounted for, rates of stomatal opening and closing have little effect on estimates of daily water use.