Net ecosystem exchanges of carbon, water, and energy in young and old-growth Douglas-fir forests

To be able to estimate the cumulative carbon budget at broader scales, it is essential to understand net ecosystem exchanges (NEE) of carbon and water in various ages and types of ecosystems. Using eddy-covariance (EC) in Douglas-fir dominated forests in the Wind River Valley, Washington, USA, we measured NEE of carbon, water, and energy from July through September in a 40-year-old stand (40YR) in 1998, a 20-year-old stand (20YR) in 1999, and a 450-year-old stand (450YR) during both years. All three stands were net carbon sinks during the dry, warm summers, with mean net daily accumulation of -0.30 g C m(-2) d(-1), -2.76 g C m(-2) d(-1), and -0.38 g C m(-2) d(-1), respectively, in the 20YR, 40YR, and 450YR (average of 1998, 1999) stands; but for individual years, the 450YR stand was a carbon source in 1998 (0.51 g C m(-2) d(-1)) and a sink in 1999 (-1.26 g C m(-2) d(-1)). The interannual differences for the summer months were apparent for cumulative carbon exchange at the 450YR stand, which had 46.9 g C m(-2) loss in 1998 and 115.9 g C m(-2) gain in 1999. As predicted, the 40YR stand assimilated the most carbon and lost the least amount of water to the atmosphere through evapotranspiration.