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 (0-15 years old) and an old-growth (OG) (∼ 450-500 years old) forest in the Wind River Experimental Forest, Washington, USA. We use eddy covariance flux measurements of carbon dioxide (FNEE), latent energy (λE) and sensible heat (H) to derive evapotranspiration rate (ET), Bowen ratio (β), water use efficiency (WUE), canopy conductance (Gc), the PriestleyTaylor coefficient (α) and a canopy decoupling factor (Ω). The canopy and bulk parameters are examined to find out how ecophysiological responses to water stress, including changes in relative soil water content (θr) and vapour pressure deficit (δe), differ among the two forest successional stages. Despite different rainfall patterns in 2006 and 2007, we observed site-specific diurnal patterns of ET, α, Gc, δe and θr during both years. The largest stand differences were (1) at the OG forest high morning Gc (> 10 mm s-1) coincided with high net CO2 uptake (F NEE = -9 to -6 μmol m-2 s-1), but a strong negative response in OG Gc to moderate δe was observed later in the afternoons and subsequently reduced daily ET and (2) at the ES stands total ET was higher (+72 mm) because midday Gc did not decrease until very low water availability levels (θr < 30%) were reached at the end of the summer. 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.