The impact of water stress on carbon exchange in two contrasting stand-age coniferous forests of the Pacific Northwest

The effects of water stress on the net ecosystem exchange of carbon (NEE) in a young and ancient forest were studied in the Gifford Pinchot National Forest of southern Washington. Here we present biometeorological measurements of carbon and water exchange at a 10 year old Douglas-fir stand and a 450-500 year old Douglas-fir/Western hemlock stand using eddy covariance techniques. The early-successional forest originated from a clearcut disturbance in 1994 and is 1.25 km from the Wind River Canopy Crane Research Facility, an AmeriFlux site that has been continuously measuring carbon, water, and energy fluxes at the old-growth forest since 1998. This second tower in the early-successional forest was added in summer 2005 with the goal of quantifying how sensitive NEE, ecosystem respiration (Reco), and gross primary production (GPP) are to summer drought stress at age-endpoint stands in the Westside forests of the Pacific Northwest.

The young stand is a short distance from the old-growth forest, enabling a close examination of microclimatic differences between stands. The response in NEE at the young stand during the seasonal, summer drought is predicted to be more intense than that at the old-growth forest due to canopy-driven differences in soil moisture and soil temperature. How this affects respiration rates and photosynthetic rates at both stands is one of the questions that will be addressed by this study.

Western Cascade forest ecosystems in the snow transient zone are extremely sensitive to summer moisture stress which limits seedling establishment in the young stands and summer photosynthesis in all aged stands. Data collected during the upcoming spring months will determine differences in the hydrological budgets of both stands coming into the summer drought period and continued data will show whether the young stand is more sensitive to summer drought as predicted. Flux measurements showed differences in carbon sink/source activity between the two aged stands during the autumn months of initial data collection in 2005. The old-growth stand was a net carbon sink (-0.33 gC m-2 day-1) while the young stand was a carbon source (+2.4 gC m-2 day-1) to the atmosphere.

Specific hypotheses to be addressed during the next couple of months of data collection and that will be presented include: (1) net precipitation is greater in the old-growth stand than the early-successional forest because interception of fog by the old-growth stand exceeds any gains in snowpack depth from clearcutting; and (2) early-successional stands are more sensitive to summer drought than old-growth stands, largely because younger forests are not able to reach deep soil water reserves, soil evaporation rates are higher, and soil temperatures are greater in the open canopies of these forests.