The impact of seasonal drought on different aged forest stands in the Pacific Northwest

This research looks at the environmental and climatic drivers of net ecosystem exchange of carbon (NEE) in two early seral (10-15 years since disturbance) and an old-growth (450-500 years old) forest in southern Washington State. The Pacific Northwest is one of the most productive forest regions in the world and its future role in the terrestrial carbon cycle will depend on the amount of timber harvested; in effect, how the age structure of these forests will change. Here we analyzed two years of data from three carbon flux towers to elucidate the impact of seasonal drought and clear-cut disturbance on landscape carbon and water fluxes. The region is subject to a seasonal drought, with only 10% of the annual precipitation falling during the warm summer months. Flux data from the contrasting forest ecosystems together with the process-based model Biome-BGC 4.2 are used to examine stand-level ecophysiological response differences to seasonal water limitations and to identify potential climatic and hydrological constraints on Douglas-fir tree regeneration and succession. Biome-BGC is used to gap-fill missing flux data as well as model carbon fluxes during the first fifteen years at the regenerating stands.

The early seral or regenerating forests are only 2 km from the old-growth forest, close enough to allow for a direct comparison of climatic-driving variables. Differences in abiotic variables between forest ecosystems are expected to result from the microclimate characteristics of each stand. The early seral forests originate from clear-cut disturbance in the early 1990's making them similar in age (10-15 years old), although they differ significantly in their residual carbon pools, i.e., the amount of wood left behind after harvest.

Flux and meteorological data were collected from November 2005 through October 2007. Early Seral North was a net source of carbon (NEE = + 2.4 g C m-2 day-1) during autumn 2005 while the old-growth forest was a slight sink (NEE = -0.3 g C m-2 day-1). Net carbon uptake was higher at the old-growth forest during spring 2006 as well, although the younger stand was a larger sink than expected during the summer. June and July 2006 were the strongest net sink months for the younger forest and net uptake values approached those of the old-growth stand. Surface (0 to 30 cm depth) water limitations began one month earlier at the younger forests (end of June) than at the old-growth stand (end of July).