A computer simulation model of seasonal transpiration in Douglas-fir based on a model of stomatal resistance

Graduation date: 1972Four study sites were selected along an altitudinal gradient on Mt. Ashland in the Siskiyou Mountains of southern Oregon. The altitudinal gradient roughly corresponded to a gradient in temperature and water availability ranging from very hot and dry to cool and moist. Air and soil temperatures were continuously measured on each of the plots by recording thermographs. Periodic measurements of atmospheric humidity were taken on each plot, and daily humidity data taken by the U. S. Forest Service were also used. The environmental data were used to develop a mathematical model of daily changes in vapor concentration gradient. Measurements of diurnal patterns of xylem water potential (plant moisture stress) in young Douglas-fir trees (Pseudotsuga menziesii (Mirb.) Franco) were made at intervals throughout the summers of 1969 and 1970, using the pressure bomb technique. Xylem water potential was found to be at a maximum value before dawn, and would decrease to approximately -15 atm during the day, then recover to a maximum level the following night. This maximum value of xylem water potential is believed to roughly correspond to soil water potential over the entire root zone. The maximum, or pre-dawn, xylem water potential values gradually decreased throughout the rainless summers, from -3 to -5 atm in the spring to values ranging from -12 to -28 atm in September depending on location. The seasonal decrease in pre-dawn xylem water potential was described mathematically by an exponential function. Relative stomatal aperture was estimated by a stomatal infiltration technique. The relationship of relative stomatal aperture values to stomatal resistance was determined, where the log of stomatal resistance was found to be directly proportional to the pressure required to infiltrate the stomatal pores. The stomata of Douglas-fir were found to be open at night in the spring, but were fully closed at night during the months of July through September. Stomatal opening during the summer months was triggered only by actual sunrise--the pre-dawn diffuse light had no effect. It was suggested that nocturnal stomatal behavior may be influenced by phenological changes. The stomata tended to open to some maximum aperture in the morning, then would remain at that aperture or would close to a greater or lesser extent throughout the day. The rate of diurnal stomatal closure appeared to be related in some manner to pre-dawn plant moisture stress and other factors. When the soil was fully hydrated, the stomata would remain at the maximum aperture throughout the day. Later in the season, as soil moisture availability decreased (reflected in higher pre-dawn plant moisture stress), the stomata would tend to close: slowly when under moderate moisture stress, faster when under severe stress. The daily maximum stomatal aperture was found to be correlated with pre-dawn plant moisture stress. Stomatal behavior of Douglas-fir was found to be unaffected by soil temperatures greater than 2°C. The observations on stomatal behavior were described mathematically. The models of vapor concentration deficit and stomatal behavior were incorporated into a digital computer simulation model of seasonal transpiration in Douglas-fir. Applications of the model in forestry and plant ecology are described