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 photosynthetic active radiation (PAR). 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.