Stand age- and climate variability-effects on ecosystem exchange at the Wind River Experimental Forest, Washington, USA

The conifer forests of the Pacific Northwest, USA have the highest levels of carbon sequestration in North America and the greatest potential for future carbon uptake of any terrestrial ecosystem. Any major changes to carbon uptake in these ecosystems will influence the North American CO2 budget despite the relatively small size (105 km2) of the Pacific Northwest forest biome. This dissertation is organized into three scientific papers which examine how year-to-year climate variability and clear-cut logging affect the carbon budgets of these forests. The forests include the Wind River AmeriFlux site, an old-growth Douglas-fir/western hemlock forest and two nearby young Douglas-fir forest stands. Carbon exchange is measured with the eddy covariance technique.;In Paper 1, year-to-year variability in three Pacific Ocean teleconnection patterns (Pacific Decadal Oscillation, Pacific/North American Oscillation and El Nino-Southern Oscillation) are examined to determine if CO 2 and H2O fluxes at the old-growth forest are affected by climatic changes associated with these events. The largest ecosystem anomalies in net ecosystem production, gross primary production, respiration, light use efficiency and water use efficiency occurred when all three teleconnections were either in a negative or positive climate phase. Results from this paper suggest that any increases in the frequency of negative-phase Pacific teleconnection patterns will increase annual carbon sequestration in some Pacific Northwest forests. However, stronger and more frequent positive phases will likely decrease net carbon uptake.;The influences of clear-cut logging on atmospheric-forest CO2 exchange are examined in Papers 2 and 3. Age-specific and seasonal-specific feedbacks between carbon and water exchange, radiation, canopy conductance, vapor pressure deficit and soil moisture were measured in the old-growth forest and two very recent clear-cuts (early seral forest stands). Early seral stands had lower net carbon uptake rates than mature stands and highest uptake occurred during different seasons. Peak net carbon uptake was measured in March-May at the old-growth forest but not until July-August at the younger stands. The results from these papers suggest that young Douglas-fir stands will be more susceptible than older forests to any increases in water stress if the Pacific Northwest summer drought becomes longer or more intense.