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Diurnal and Seasonal Patterns of Net Assimilation in Douglas‐Fir, Pseudotsuga Menziesii (Mirb). Franco, as Influenced by Environment

Helms, John A.
Ecology 1965 v.46 no.5 pp. 698-708
Pseudotsuga menziesii, air temperature, autumn, branches, carbon dioxide, carbon dioxide production, diurnal variation, environmental factors, leaves, light intensity, photosynthesis, relative humidity, seasonal variation, selenium, summer, thermocouples, trees
A 2—year study on net assimilation was carried out in a 38—year—old natural stand of Douglas—fir. Five trees in each of the dominant, co—dominant, and suppressed crown classes were studied using the cuvette method on intact branches and measuring the CO₂ exchange with an infrared gas analyzer. Light intensity, air temperature, and relative humidity were monitored using selenium photocells, thermocouples, and a 2—mv, 24—line recorder. The net gain in photosynthesis in 1962 was found to be two to three times that in the drier year of 1961. Expressed per unit weight of dry foliage per hour, suppressed foliage possessed higher photosynthetic efficiency than co—dominants, which in turn were slightly more efficient than dominants. Douglas—fir could photosynthesize at low light intensities. The CO₂ compensation point was commonly as low as 10 ft—c, and maximum rates of net assimilation were attained at 800 ft—c. Net assimilation could not be predicted from specific levels of air temperature, light intensity, or relative humidity, but not found to be directly related to light intensities below 750 ft—c and to air temperatures above 30°C. Apart from these extreme situations, net assimilation under natural conditions is apparently limited by the interaction of many internal and external factors. Diurnal patterns of net assimilation differed in trees of different crown class. The rates of photosynthesis and nocturnal respiration commonly fluctuated within each diurnal pattern despite apparently stable environmental conditions. The causes of midday depressions are complex as depressed patterns occurred under hot conditions in summer and under cool, foggy conditions in autumn. Bursts of CO₂ evolution immediately after sundown, lasting up to 2 hours, were commonly observed in summer and autumn.