Trends and Controls on Water-Use Efficiency of an Old-Growth Coniferous Forest in the Pacific Northwest
At the ecosystem scale, water-use efficiency (WUE) is defined broadly as the ratio of carbon assimilated towater evaporated by an ecosystem.WUE is an important aspect of carbon andwater cycling and has been used to assess forest ecosystem responses to climate change and rising atmospheric CO2 concentrations. This study investigates the influence ofmeteorological and radiation variables on forestWUEby analyzing an 18 year (1998–2015)half-hourly time series of carbon andwater fluxes measuredwith the eddy covariance technique in an old-growth conifer forest in the PacificNorthwest, USA. Three differentmetrics ofWUE exhibit an overall increase over the period 1998–2007mainly due to an increase in gross primary productivity (GPP) and a decrease in evapotranspiration (ET). However, theWUEmetrics did not exhibit an increase across the period from2008 to 2015 due to a greater reduction inGPP relative to ET. The strength of associations among particularmeteorological variables andWUE variedwith the scale of temporal aggregation used. In general, vapor pressure deficit and air temperature appear to controlWUE at half-hourly and daily time scales, whereas atmospheric CO2 concentrationwas identified as themost important factor controllingmonthly WUE.Carbon andwaterfluxes and the consequentWUE showed aweak correlation to the Standard Precipitation Index, while carbon fluxeswere strongly dependent on the combined effect ofmultiple climate factors. The inferred patterns and controls on forestWUEhighlighted have implications for improved understanding and prediction of possible adaptive adjustments of forest physiology in response to climate change and rising atmospheric CO2 concentrations.