Examining the Ecosystem Evolution of Nikanotee Fen Watershed: An Ecohydrological Perspective

Abstract

Recognition of the environmental ramifications of long-term natural resource development in the Athabasca Oil Sands Region (AOSR) has prompted the implementation of sustainable land use practices. This includes obligatory regulations that require impacted landscapes to be returned to their pre-disturbance functionality. Ecosystem function (e.g., carbon sequestration) is driven by soil-plant-atmosphere exchanges of energy, carbon and water resources. Given the magnitude of disturbance during surface mining (i.e., removal of vegetation and the subsurface) the reestablishment of ecosystem function requires extensive reclamation. This involves the complete reconstruction of surface and subsurface ecosystem components including the establishment of a hydrological regime. Thus, reclamation ultimately creates ‘new’ landscapes, often beginning with a bare ground phase followed by planting campaigns, and the eventual development of widespread plant communities. An understanding of ecohydrological processes throughout the different post-construction evolutionary phases is necessary to evaluate i) ecosystem function and trajectory and ii) reclamation design and success. As the natural, undisturbed landscape of the AOSR consists of hydrologically connected upland forests and fen peatlands, two pilot-scale watersheds incorporating these landscapes have been constructed to examine the viability and design of multi-landscape reclamation endeavours in the region.

This thesis captures the evolving ecohydrological regime during the first seven years (2013 - 2019) post-construction at one of the novel, constructed watersheds in the region, Nikanotee Fen Watershed (NFW). Ecosystem function and evolution of both the fen and upland were quantified based on key ecohydrological indicators (net ecosystem exchange (NEE), evapotranspiration (ET) and water-use efficiency (WUE)) using a multi-scale (ecosystem and plant community), multi-method (remote sensing, eddy covariance, instantaneous chamber measurements) approach. Over the course of the study period, both landscapes exhibited significant biophysical evolution, from bare ground to fully vegetated ecosystems, resulting in physical (e.g., altered albedo, surface roughness and producing plant-mediated shading) and functional (e.g., transpiration and carbon sequestration) transformations. Initially, during bare ground conditions, surface-atmosphere exchanges were driven by abiotic factors (atmospheric and edaphic conditions) and controlled by soil water availability. In the fen, due to near-surface water table, high soil moisture content, surface ponding and the low albedo of wet, bare peat, most available energy was partitioned to latent heat and a high degree of decoupling between the surface-atmosphere was observed. During this time, surface evaporation rates were consistently high and comparable to open-water values in the region. Due to the lack of plant community, the fen was a source of CO2 and WUE was low and driven by high evaporative losses. In the upland, the exposed dry sand-loam cover soil resulted in higher albedo and equal partitioning between latent and sensible heat. The drier edaphic conditions resulted in limited evaporation, with only small increases in response to precipitation events. Moreover, these conditions resulted in net carbon losses and similar to the fen, limited WUE.

Once plant community became established, edaphic controls decreased, and surface-atmosphere exchanges were driven by plant-mediated responses to atmospheric conditions. In the fen, as water availability remained high, latent heat continued to be the dominant energy flux, but a larger proportion of available energy was partitioned to sensible heat, particularly during drier periods. Widespread plant coverage and the establishment of a thick litter layer supressed surface evaporative losses and increased transpiration, resulting in lower ET rates compared to bare ground conditions and greater surface-atmosphere coupling. Coinciding with plant growth, the fen quickly evolved from a CO2 source to a sink by year three post-construction. Moreover, once fully vegetated, WUE remained relatively stable despite seasonal hydrometeorological variability. Here, stable WUE trends reflect well-developed rooting architecture of the plant community and a well-connected groundwater network between the two landscape units resulting in hydrological self-regulation sufficient to maintain adequate plant function during periods of water stress. In the upland, the growth and development of treed species resulted in a marked increase in latent heat flux, ET rates, CO2 uptake and WUE, with seasonal trends mirroring plant phenology. However, at the conclusion of this study the upland was still functioning as a minor CO2 source, but is expected to become a sink in the near future as trees continue to mature.

Overall, an examination of ecohydrological processes and feedbacks during early development suggests the constructed system is evolving towards a functional, self-sustaining ecosystem that’s able to withstand periodic environmental stress. Furthermore, rates and seasonal trends of key ecohydrological indicators (ET, NEE, WUE) at the constructed watershed were comparable to those observed at surrounding natural and post-disturbance boreal landscapes providing further support of ecosystem function. Results from this study provide insight to early ecosystem function and trajectories and can be applied to future designs and planting prescriptions to improve long-term reclamation success.