Upland reclamation promotes forest evaporative losses in the Boreal Plains of Canada: A comparison of carbon and water fluxes

Reclaiming upland boreal forests as part of post-mining watershed construction in the Athabasca Oil Sands Region (AOSR) requires evaluation criteria as these novel ecosystems develop. Here, we analyzed 55 site-years of eddy covariance observations of constructed forests and soils on formally pit-mined landscapes (9 sites) and contrasted them to 18 site-years of post-harvested ecosystems (3 sites) and 38 site-years of mature Boreal Plains ecosystems (3 sites). After approximately 5 years, the post-harvested sites had fluxes of gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE) within the variability of the widely studied mature Boreal Plains BERMS FLUXNET sites. However, even after 10 years the constructed forests had significantly lower WUE than the mature sites. High ET fluxes drove low WUE in the constructed upland conifer sites despite similar rates of GEP. Conversely, in the constructed broadleaf forests low GEP, despite similar ET, resulted in low WUE. A climate sensitivity analysis showed that there was little impact of abnormal hot, cold, wet, or dry growing seasons on GEP or evapotranspiration (ET) at the constructed forests. It is presumed that the high moisture retaining properties of the soils used in reclamation produced the low WUE and resilience to dry and hot conditions in constructed forests. Placing moisture retaining soils incorporates a degree of resilience to climate variability but also limits downgradient water yields to low lying wetlands in the relatively dry AOSR climate. This highlights a potential shortcoming of reclamation objectives developed for specific ecosystems when scaling from ecosystem to watershed scale construction. Finally, robust relationships between satellite observed greenness (MODIS EVI and NDVI) and ecosystem scale fluxes highlight how remote sensing-based metrics can be used by land managers to identify regions within landscape units that may be under performing.