Carbon and water vapor exchange of an open-canopied ponderosa pine ecosystem

Eddy covariance measurements of carbon dioxide and water vapor exchange wer e made above a ponderosa pine (Pinus ponderosa Dougl. ex P. and C. Laws.) f orest located in a semiarid environment in central Oregon. The stand is a m ixture of old-growth and young trees. Annual net carbon gain by the ecosyst em (NEE) was 320 +/- 170 gC m(-2) year(-1) in 1996 and 270 +/- 180 gC m(-2) year(-1) in 1997. Compared to boreal evergreen forest at higher latitudes, the pine forest has a substantial net carbon gain (150 +/- 80 gC m(-2) yea r(-1) in 1996 and 180 +/- 80 gC m(-2) year(-1) in 1997) outside the traditi onally defined growing season (from bud swell in early May (Day 125) to par tial leaf-off in late September (Day 275)). Carbon assimilation continued t o occur in the relatively mild winters, though at a slower rate (April, max imum leaf level assimilation (A(max)) of 6-9.5 mu mol m(-2) leaf s(-1)), an d ecosystem respiration was relatively low (similar to 1.6 +/- 0.1 gC m(-2) day(-1)). In the growing season, although photosynthetic capacity was larg e (July, A(max) = 16-21 mu mol m(-2) leaf s(-1)), carbon assimilation was c onstrained by partial stomatal closure to maintain a sustainable water flow through the soil-plant system, and ecosystem respiration was large (3.5 +/ - 0.1 and 4.3 +/- 0.1 gC m(-2) day(-1) in growing season of 1996 and 1997, respectively) because of high air and soil temperatures. Despite large chan ges in evaporative demand over just a few days (VPD changing from 0.5 to 3. 5 kPa), the ecosystem water use was remarkably constant in summer (similar to 1.6-1.7 mm day(-1)). Such homeostasis is most likely another result of s tomatal control. Interannual variations in climate had a large influence on the ecosystem carbon balance. in summer 1997, an El Nino year, precipitati on was more frequent (17 days with 33 mm of rain) than in summer 1996 (5 da ys with 5 mm of rain), and the net ecosystem exchange was substantially low er in July to September 1997 (10 +/- 60 gC m(-2)) than during the equivalen t period in 1996 (100 +/- 60 gC m(-2)). Although temperatures between years were similar, the carbon assimilation in 1997 was offset by increased resp iration, probably because soils were more frequently wet, encouraging micro bial respiration. (C) 1999 Elsevier Science B.V. All rights reserved.