DIURNAL-VARIATION AND INTERRELATIONS OF ECOPHYSIOLOGICAL PARAMETERS IN 3 PEATLAND WOODY SPECIES UNDER DIFFERENT WEATHER AND SOIL-MOISTURE CONDITIONS

The diurnal patterns of twig xylem water potential, net photosynthesis rate, water use efficiency of photosynthesis, and stomatal and mesophyll conductance to CO2 in tamarack, black spruce and swamp birch growing in a natural peatland in central Alberta, Canada, were examined. The relationships of photosynthesis to other ecophysiological parameters were investigated. Data were collected on three days with different weather and soil moisture conditions in the 1988 growing season. Day 1 was clear and warm and the ground water table was 7 cm above the average peat surface. Day 2 was clear and hot. Day 3 was cloudy but warm. On day 2 and day 3, the water tables were in the normal range for that season. Major findings were: 1) Soil flooding depressed photosynthesis in tamarack and black spruce. 2) Swamp birch was better adapted to flooding than tamarack or black spruce. 3) The trees experienced water stress in the afternoons of the two days with lower water table. 4) Changes in photosynthesis of the three species were primarily affected by changes in mesophyll conductance (g(m)) and the response of photosynthesis to changes in g(m) was similar for all three species.