Stand-level gas-exchange responses to seasonal drought in very young versus old Douglas-fir forests of the Pacific Northwest, USA

Abstract

This study examines how stand age affects ecosystem mass and energy exchange response to seasonal drought in three adjacent Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests. The sites include two early seral (ES) stands (0 to 15 years old) and an old-growth (OG) (~450 to 500 years old) forest in the Wind River Experimental Forest, Washington, USA. We use eddy covariance flux measurements of carbon dioxide, latent energy, and sensible heat to derive evapotranspiration rate, Bowen ratio, water use efficiency, canopy conductance, the Priestley-Taylor coefficient, and a canopy decoupling factor. Despite different rainfall patterns in 2006 and 2007, we observed site-specific diurnal patterns of evapotranspiration rate, Priestley-Taylor coefficient, canopy conductance, vapour pressure deficit, and relative soil water content during both years. Our results suggest that ES stands are more likely than mature forests to experience constraints on gas exchange if the dry season becomes longer or intensifies because water-conserving ecophysiological responses were observed in the youngest stands only at the very end of the seasonal drought.

Citation

Wharton, S.; Schroeder, M.; Bible, K.; Falk, M.; Paw U, K.T. 2009. Stand-level gas-exchange responses to seasonal drought in very young versus old Douglas-fir forests of the Pacific Northwest, USA. Tree Physiology. 29: 959-974.