The function of forested swamps in the Boreal Plain: climate and storage shape ecohydrological interactions

Swamps can promote hydrologic connectivity in the landscape mosaic of the sub-humid Boreal Plain (BP, west-central Canada). Yet, understanding of processes controlling and interactions arising from their ecohydrological functioning, and their temporal variability throughout expressed BP climate cycles, is still limited for this drought-prone region. Such understanding is crucial in catchments where the sub-humid climate interacts with the prevalent deep, glacial deposits and vegetation to create extensive unsaturated zones; this gives rise to a tendency of vertical over lateral water flows due to expressed storage-threshold dynamics. This thesis aimed to assess the capacity of swamps to redistribute water in such catchments, as well as characteristics necessary for and controls thereof under wet, mesic and dry climatic conditions. Further, potential interactions with adjacent systems, viz. forest growth along wetland interfaces, were investigated. The swamp was forested (deciduous canopies) and located in an aspen-dominated, mixed-wood headwater catchment within an upland-wetland-pond complex lacking an apparent drainage network, and isolated from larger-scale groundwater flows. As water redistribution is most likely limited to low-storage areas, this setting represents BP landscapes that may be particularly affected by climate warming increasing evapotranspiration ET. Results showed that low storage and specific yields defined the swamp’s ephemeral hydrologic regime, promoting frequent saturation (conducive for) subsurface and surface flows downgradient and to adjacent forests in all but the driest years. These conditions were internally-generated (i.e. required no upland water inputs), beginning in zones of lowest storage capacity at the swamp’s upper end. This process was facilitated by dynamic storage-reduction through surface-near ice, as well as ET-limiting mechanisms that maintained high antecedent moisture. In particular, sub-canopy ET, as the dominant efflux, was controlled by soil-plant-atmosphere feedbacks via soil-physical and structural changes in the deciduous over- and understory. Detailed hydrological assessments of the wetland-upland complex, combined with dendroecological and wood-anatomical analyses, further allowed concluding that the wetland provided groundwater supplements across its interface which modulated effects of hydroclimatic variability for white spruce, and potentially for trembling aspen. This thesis provides novel understanding of BP swamp ecohydrology, including the first detailed ET assessment of a deciduous system, and highlights swamps as important landscape units. It is hoped that the focus on storage dynamics will inform further BP catchment conceptualizations, and allow landmanagers to direct anthropogenic disturbances away from these key water source areas