An index of photosynthesis for comparing forest sites in western Oregon

An index was developed that takes into account the combined influence of air and soiltemperatures, light, and the availability of soil moisture upon photosynthesis by a unit area offully exposed foliage of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). The index isderived from a summation of daily estimates of carbon assimilation (milligrams CO2 assimila-tion per decimeter per day) for an entire year. In a comparison of forest environments inwestern Oregon the index was correlated to a measure of forest productivity (r2 = 0.99).Furthermore, it suggests that much of the annual carbon fixation occurs during the mildwinters characteristic of the region. The ability to assess the effects of frost, soil drought, andother variables separately was valuable in explaining differences between coastal, valley,and mountainous sites