Eddy-covariance and biometeorological methods show significant net uptake by an old-growth Douglas-fir forest in southern Washington state, the oldest forest ecosystem (500 years old) in the AmeriFlux network. Annual net ecosystem exchange (1.3 - 2 tC ha^{-1 yr-1}) was comparable to younger ecosystems at the same latitude, as quantified in the AmeriFlux program. Data acquisition at the Wind River Canopy Crane research Facility (WRCCRF) site is ongoing since 1997/98. A detailed analysis of the ecosystems response to changing climatic conditions will be presented. Quantities of interest are the carbon flux and water vapor flux between the forest ecosystem and the atmosphere in dependence of environmental variables such as soil and canopy temperature, soil moisture, vapor pressure deficit and photosynthetical active radiation (PAR).

Analysis will group the data set into several subcategories for inter-annual comparison: (1) Summer Drought (June - August) and loss of CO₂ to atmosphere; (2)Main Uptake of CO2 (March - May) during Spring; (3) Steady State of CO2 budget in the Winter (December - February) and (4) Fall recharge period (September - November). The actual temporal boundaries between the different categories are not static but depend on climatic factors. The main carbon dioxide uptake period for example can extend well into June and the beginning of July if rains continue to fall and temperatures stay cool. Summers can either be hot and dry (1998) or wet and relatively cool (1999). The year 1998 was an El Nino period, where conditions were warmer and drier than normal. By September 1998, soils were at their driest on record, and air temperature and atmospheric vapor pressure deficit (VPD) were both large. In contrast, temperatures and precipitation in 1999 were more typical of long-term means and soil moisture was larger than for corresponding summer months in 1998. It is important to note that this coniferous ecosystem has a very different seasonality than eastern hardwood and coniferous forests.

Extent and severity of the summer drought as well as spring temperature and precipitation regime are the main factors in determining respiration, evapotranspiration and photosynthetic uptake and hence ecosystem exchange with the atmosphere.