Forest water use in the initial stages of reclamation in the Athabasca Oil Sands Region

Following large‐scale surface oil sands mining, large tracts of the boreal forest in the Athabasca Oil Sands Region of Western Canada are legally required to be reclaimed. A greater understanding of how these novel ecosystems function and develop with regard to water use is crucial to aid in the development of regulatory practices and protocols based on information from ecosystem recovery. In this paper, a 12‐year (2003–2014) eddy covariance measurement record of latent and sensible heat fluxes and gross ecosystem productivity of carbon dioxide is analysed to evaluate how a reclaimed boreal forest has developed during its initial growth period. The study site is a reclaimed oil sands saline‐sodic clay shale overburden deposit that was topped with 100 cm of glacial till and 20 cm of peat mineral mix. The site was seeded with barley (Hordeum spp.) in 2001 to reduce erosion of the soil cover whereas aspen (Populus tremuloides Michx.) and spruce (Picea glauca [Moench] Voss) boreal tree species were planted in 2004. Changes in structure and function corresponded to the transition of dominant vegetation cover from early successional species to forest. Leaf area index increased from a growing season peak of 0.9 in 2003 to 4.0 in 2014 and was associated with an increased growing season gross ecosystem productivity (4.9 to 8.9 g C m^?2 day^?1), an increased evapotranspiration (1.6 to 3.4 mm day^?1), and a decreased partitioning of energy to sensible heat (Bowen's ratio decreased from 1.1 to 0.4). Although canopy conductance increased throughout the 12 years, the shift from early successional species to trees with more conservative water use resulted in a decrease in conductance normalized by leaf area. Water use efficiency has increased slightly since 2008 with an average of 10.0 g CO₂ kg^?1 H₂O for the last 6 years. No prolonged dry periods were observed during the study period. The functioning of this novel ecosystem is evolving as expected on the basis of the trends observed for other natural and disturbed boreal forests.