Interannual variability of water use efficiency in an old-growth forest under drought conditions

Long-term micrometeorological measurements in the past 7 years show high interannual variability in the atmosphere-ecosystem exchange of carbon for the coniferous old-growth forest in southern Washington State. Within the AMERIFLUX/FLUXNET network this forest represents a unique ecosystem and the endpoint of several gradients: age (~500 yr), aboveground biomass (619 Mg ha-1), and structural complexity (two-sided leaf area index = 7-12, tree tops at 65 m, and 427 trees ha-1). In this paper we focus on the relationship between water availibilty and carbon squestration. While this old-growth has a high biomass it experiences regular summer drought. Our observations show that the seasonal rain-forest can assimilate carbon at relatively high rates (1.6 MgC ha-1 yr-1) under favorable climatic conditions but releases carbon (0.5 MgC ha-1 yr-1) to the atmosphere under climatic stress. The forest sequestered carbon at a similar rate to younger forests at the same latitude during a “la Niña year”, but became a source of carbon during a drier, higher temperature year. The average exchange using micrometeorological technologies over a five year period is within the measurement errors of carbon inventory estimates. Observed soil respiration ranged from 8.7 to 11.5 Mg C ha-1 yr-1 with an average of 10.9 Mg C ha-1 yr-1. Water availability is driven by the amount of winter season precipitation and the overall timing of preciptation and especially the end of the rainy season. In years with low overall respiration the stand can act as a strong carbon sink (1999), whereas increased respiratory fluxes in other years can turn the ecosystem into a weak to moderate source. Climate change and global warming could cause increased CO2 emissions to the atmosphere by such ancient forests.