Hydrologic impact of aspen harvesting within the subhumid Boreal Plains of Alberta

Abstract

This study examined the hydrological impact of harvesting aspen (Populus tremuloides) within the subhumid Boreal Plains of Alberta, Canada, following clear‐cutting of two stands in successive winters within a small catchment situated in glacial moraine. Impacts were evaluated using a combination of observational data collected from within the harvested catchment and an adjacent reference catchment and numerical simulations conducted with HydroGeoSphere. Sensitivity simulations evaluated the influence of post‐harvest evapotranspiration rates and variability in atmospheric conditions on the range of hydrologic response. Study results indicate that aspen harvesting had limited impact on groundwater levels and streamflows within these hydrologic systems because of the subhumid climate with low frequency of large storms, large soil‐moisture storage capacity of heterogeneous glacial materials, and high evapotranspiration rates of regenerating aspen. Despite an estimated increase in hillslope groundwater levels of up to 3 m, pond and peatland water levels increased by less than 0.3 m and were accompanied by increased streamflows of less than 10 mm. However, predicted increases in groundwater levels and streamflows were sensitive to regenerating aspen evapotranspiration rates, which can be enhanced by appropriate harvesting techniques but may be reduced by climate change. These results are consistent with previous results from within the Boreal Plains but differ from aspen harvesting studies conducted in other settings where appreciable increases in streamflows have been reported. This disparity highlights the need to consider the integrated response of the hydrologic system when evaluating impacts from disturbance and making comparisons between settings.

Changes in hillslope hydrology in a perched, shallow soil system due to clearcutting and residual biomass removal

“…Yet, where soils were deeper, more available soil storage limited runoff (Grip, 2015). Similarly, Thompson et al (2018) described a comparable process occurring after harvesting of aspen stands in the Western Boreal Plains, Canada. In the Western Boreal Plains, Thompson et al (2018) predicted that water tables would increase up to 3 m post‐harvest due to decreased transpiration demand but there was limited impact on streamflow due to the large hydrological buffering capacity of downgradient peatland systems.…”

“…Many hillslope forest harvesting studies have focused on systems where the hillslope water table was present throughout the year and generally observed that water table elevations increased post‐harvest (Choi, Dewey, Hatten, Ezell, & Fan, 2012; Dung et al, 2012; Johnson, Edwards, & Erhardt, 2007; Thompson et al, 2018; Whitson, Chanasyk, & Prepas, 2005). The perched and more ephemeral water table that was present at these hillslopes led to a different hydrological response than has been observed in the broader literature.…”

“…In the Western Boreal Plains, Thompson et al (2018) predicted that water tables would increase up to 3 m post‐harvest due to decreased transpiration demand but there was limited impact on streamflow due to the large hydrological buffering capacity of downgradient peatland systems. The lack of streamflow response was attributed to deep soils above the impermeable layer, resulting in a large water storage capacity, as well as the general atmospheric condition wherein potential evapotranspiration was greater than precipitation (Thompson et al, 2018). The interplay between permeable soil depth and atmospheric condition can offset the water table response on harvested hillslope by years (Carrera‐Hernández et al, 2011), resulting in potential long‐term effects that are not traditionally captured by shorter‐term studies.…”

“…Following forest harvesting, typical hydrological responses include increases in water yield/runoff and peak flow duration (Buttle, Webster, Hazlett, & Jeffries, 2019; Dung et al, 2012; Dung, Hiraoka, Gomi, Onda, & Kato, 2015; Monteith et al, 2006; Moore & Wondzell, 2005; Schelker, Kuglerová, Eklöf, Bishop, & Laudon, 2013). These effects are largely driven by soil compaction, decreased transpiration, and increased effective precipitation, which enhance runoff from uplands to downgradient aquatic and wetland systems (Buttle, Creed, & Moore, 2009; Moore & Wondzell, 2005; Thompson, Devito, & Mendoza, 2018). The increased runoff typically results in higher suspended sediment loads and changes in downstream solute yields (Ampoorter, de Schrijver, van Nevel, Hermy, & Verheyen, 2012; Eklöf, Lidskog, & Bishop, 2016; Oda et al, 2018; Vance et al, 2018).…”

“…Soil disturbance and compaction can be mitigated if harvesting occurs during the winter on frozen soils (Kolka, Steber, Brooks, Perry, & Powers, 2012). Large reductions in transpiration and interception due to the removal of the overstory canopy (Dung et al, 2012;Ide, Finér, Laurén, Piirainen, & Launiainen, 2013;Thompson et al, 2018) leads to a surplus of soil moisture, resulting in higher water tables and increased runoff generation (Buttle et al, 2019;Dung et al, 2012;Ide et al, 2013;Thompson et al, 2018). These hydrological effects can last for several years to tens of years, depending on the local hydroclimatic conditions and the regeneration rate of the forest (Buttle et al, 2018;Buttle, Oswald, & Woods, 2005;Ebeling et al, 2016;Moore & Wondzell, 2005;Oda et al, 2018;Thompson et al, 2018).…”