Eddy covariance measurements of carbon dioxide and water vapor exchange were made above a young and an old-growth ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws) ecosystem located in a semiarid environment in central Oregon. The old-growth stand (O site) is a mixture of 250- and 50-year-old ponderosa pine trees with no significant understory (summer maximum leaf area index (LAI) (m2 half-surface area foliage per m2 ground) is 2.1). The young stand (Y site; 15 years old in 2000), about 10 km southeast of the old stand, is naturally regenerating following the clear-cut of an old stand in 1978 and has at present about 40% of its LAI in understory shrubs (summer maximum LAI of 1.0).
Even though climatic conditions at both sites were very similar, ecosystem carbon exchange differed substantially between the two ecosystems. The old-growth forest with about two times the LAI of the young site, had higher carbon assimilation rates per unit ground area than the young forest, with trends similar between the two forests in spring and fall. Deviations from the trend occurred during summer when water stress in trees at the young site led to a significant reduction in transpiration, and consequently carbon assimilation due to stomatal limitations. Throughout the year, ecosystem respiration (Re) and gross ecosystem production (GEP) were generally greater at the O site than Y site, and the net of these two processes resulted in a lower net carbon uptake at the Y site.