Evapotranspiration and water use efficiency in different-aged Pacific Northwest Douglas-fir stands

We used the eddy-covariance technique to measure evapotranspiration (E) and gross primary production (GPP) in a chronosequence of three coastal Douglas-fir (Pseudotsuga menziesii) stands (7, 19 and 58 years old in 2007, hereafter referred to as HDF00, HDF88 and DF49, respectively) since 1998. Here, we focus on the controls on canopy conductance (g c), E, GPP and water use efficiency (WUE) and the effect of interannual climate variability at the intermediate-aged stand (DF49) and then analyze the effects of stand age following clearcut harvesting on these characteristics. Daytime dry-foliage Priestley-Taylor α and g c at DF49 were 0.4-0.8 and 2-6mms⁻¹, respectively, and were linearly correlated (R ² =0.65). Low values of α and g c at DF49 as well at the other two stands suggested stomatal limitation to transpiration. Monthly E, however, showed strong positive linear correlations to monthly net radiation (R ² =0.94), air temperature (R ² =0.77), and daytime vapour pressure deficit (R ² =0.76). During July-September, monthly E (mm) was linearly correlated to monthly mean soil water content (θ, m³ m⁻³) in the 0-60cm layer (E =453θ -21, R ² =0.69), and GPP was similarly affected. Annual E and GPP of DF49 for the period 1998-2007 varied from 370 to 430mm and from 1950 to 2390gCm⁻², respectively. After clearcut harvesting, E dropped to about 70% of that for DF49 while ecosystem evapotranspiration was fully recovered when stand age was ~12 years. This contrasted to GPP, which varied hyperbolically with stand age. Monthly GPP showed a strong positive linear relationship with E irrespective of the stand age. While annual WUE of HDF00 and HDF88 varied with age from 0.5 to 4.1gCm⁻² kg⁻¹ and from 2.8 to 4.4gCm⁻² kg⁻¹, respectively, it was quite conservative at ~5.3gCm⁻² kg⁻¹ for DF49. N-fertilization had little first-year response on E and WUE. This study not only provides important results for a more detailed validation of process-based models but also helps in predicting the influences of climate change and forest management on water vapour and CO₂ fluxes in Douglas-fir forests.