Long‐Term Trends in Forest Net Primary Productivity: Cascade Mountains, Washington

Estimates of annual net primary productivity since 1880 for four high—elevation forest stands in western Washington indicated that productivity has increased 60% during the 20th century. Because these stands were separated by up to 200 km and differed in species composition, elevation, and time since establishment, the observed trends in productivity imply a response to regionwide changes in environmental factors rather than to site—specific stand dynamics. Annual production is significantly correlated with long—term variation in summer temperature and short—term variation in annual precipitation since 1983, the beginning of continuous local meteorological records. Production is uncorrelated with atmospheric CO 2 concentrations, suggesting that direct CO 2 fertilization is currently unimportant in these forests.