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Study of stomatal diffusion resistance in a Douglas Fir Forest Tan, Chin-Sheng

Abstract

This thesis reports the results of a study of stomatal diffusion resistance in a Douglas fir forest near Courtenay, in the dry east-coastal belt of Vancouver Island. Stomatal diffusion resistance was measured by a ventilated diffusion porometer designed by Turner et al. The theory of the operation of this porometer as proposed by Turner and Parlange is reviewed. The problems of using the theory to interpret ventilated porometer data are discussed and the porometer evaluated for use in the measurement of the stomatal diffusion resistance of Douglas fir needles. As part of a two-year study of the effect of thinning on evapotranspiration in Douglas fir, the relationship between stomatal diffusion resistance and environmental parameters were studied. Research was conducted in an unthinned stand (1840 stems ha⁻¹) with negligible undergrowth and a thinned stand (840 stems ha⁻¹) with substantial salal undergrowth. During the daytime, stomatal resistance was mainly related to the soil water potential and the vapour pressure deficit of the canopy air. Daytime values of stomatal resistance for Douglas fir ranged from 2 to 60 s cm⁻¹ for values of vapour pressure deficit between 4 and 24 mb and values of soil water potential between 0 and -12.5 bars. Although increasing stomatal resistance was usually associated with decreasing twig water potential, increasing stomatal resistance appeared to be associated with increasing twig water potential when the vapour pressure deficit was high. Stress history was found to cause a shift in the relationship of stomatal resistance to twig water potential, but had little effect on the relationship of stomatal resistance to vapour pressure deficit and soil water potential. Daytime values of stomatal resistance for salal ranged from 2 to 45 s cm⁻¹. The physiological nature of the forest canopy resistance was studied by comparing the stomatal and canopy resistance of the unthinned stand. Canopy resistance was calculated using energy balance/Bowen ratio measurements of evapotranspiration. The typical steady increase in both canopy and stomatal resistance during daytime hours, even at high soil water potentials indicated that the increasing canopy resistance was caused by gradually closing stomata. During a dry period, the mean daytime value of canopy resistance increased in proportion to the mean daytime value of the stomatal resistance. Values of canopy resistance calculated from stomatal resistance and leaf area index measurements agreed well with those calculated from energy balance measurements. Values of transpiration rate in the thinned stand calculated from a simple vapour diffusion model that uses the vapour pressure deficit of the canopy air, and measurements of the stomatal resistance and leaf area index of the canopy agreed well with those obtained from energy balance/ Bowen ratio measurements. Stomatal resistance characteristics were also used in the diffusion model instead of actual stomatal resistance measurements to calculate transpiration rate.. There was reasonable agreement between these transpiration values and energy balance measurements. Both the model and energy balance measurements showed that transpiration rate increased with increasing vapour pressure deficit until a certain vapour pressure deficit was reached after which the rate declined. Calculations from the model indicated that the fraction of transpiration from the thinned stand transpired by the salal during daytime hours increased from approximately 45 to 70% during a four-week drying period.

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