Effects of harvesting and drought on CO₂and H₂O fluxes in an aspen-dominated western boreal plain forest: early chronosequence recovery

Abstract

This study examines the hydrological recovery of two regenerating boreal trembling aspen (Populus tremuloides Michx.) dominated stands and the sensitivity of that regeneration to drought within the first 5 years of establishment. The results indicate that evapotranspiration fluxes and water-use efficiency rebounded quickly as a result of new vegetation foliage growth and wet conditions found within the first 2 years following the harvest. However, a period of dry years had a significant influence on rates of postharvest growth, carbon dioxide (CO 2 ), and water fluxes at these sites. The northern study area (NSA) and southern study area (SSA) were harvested in the winters of 2007 and 2008, respectively. The first and second years of regeneration at the SSA and NSA, respectively, were marked by an early spring thaw and higher-than-normal precipitation, while air temperatures remained slightly above the 30-year normal. During this period, mean measured height of vegetation tripled at both sites, and cumulative evapotranspiration was approximately 60% of that prior to harvest by the end of the second year of growth. By the third year (2009), the NSA became a sink for atmospheric CO 2 during the snow free season (days of the year 128-238) despite low precipitation during the latter half of the summer. Volumetric soil moisture content in 2009 was the highest (on average) of the 5 years examined due to heavy snowfall and a late start to the growing season (where air temperatures consistently exceeded 0°C), resulting in sustained productivity. However, cumulative annual precipitation also declined to 79% and 57% in 2009 and 2010, respectively, of the 30-year normal for that region, leading to significant (lagged) declines in forest productivity at the NSA in 2010 and 2011. This resulted in the site becoming a source of CO 2 to the atmosphere during the 2010 and 2011 growing seasons (annual balance was not measured). Throughout the drought period (2009, 2010, and 2011), mean stand height increased by only 15%, 11%, and 14%, respectively, compared with the mean stand height in 2008. Water-use efficiency also declined in 2010 and 2011, whereas differences in light-use efficiency did not vary significantly because foliage was maintained (i.e., leaves did not abscise as a result of drought). The results of this study indicate that regenerating aspen stands are sensitive to drought and respond relatively quickly to changes in the soil moisture regime. This is important because regional drying as a result of predicted climatic changes combined with increased industrial activity may result in significant decline in productivity within these stands over broad regions. Résumé :Cette étude s'intéresse à la récupération hydrologique de deux peuplements boréaux en régénération dominés par le peuplier faux-tremble (Populus tremuloides Michx.) ainsi qu'à la sensibilité de la régénération à la sécheresse au cours des cinq années suivant son établissement. Les résultats de cette étude indiquent que le flux d'évapotranspiration (ET) et l...

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

“…Following aspen harvesting, estimated annual rainfall interception was reduced to about 14% and 21% in the respective first and second years following harvest. By the third post‐harvest year, negligible differences in interception were observed between cut and uncut areas; rapid aspen regeneration occurred with dense stands reaching heights of 1‐ to 2‐m height (Petrone et al, ).…”

“…Harvesting is predicted to result in increases of hillslope water levels of up to 3 m (Figures a,b and a,b); however, in general, peak increases do not persist beyond the initial post‐harvest years. Rapid recovery of aspen evapotranspiration (Petrone et al, ), although spatially variable (Figure a), is predicted to prevent water‐logging of near‐surface soils with low potential for run‐off generation (Buttle et al, ; Price, Branfireun, Waddington, & Devito, ). Where harvesting does result in increased hillslope groundwater levels, impacts may be difficult to identify (e.g., Figure a) as observed and predicted post‐harvest responses are similar in magnitude to observed fluctuations due to variability in atmospheric conditions (Figures a,c, a,b, and a,b) with comparable seasonal trends (i.e., peak following snowmelt and general decline during growing season) for both cut and uncut locations.…”

“…The harvested and reference catchments (Figure and Table ) are located central to the URSA where the landscape is characterized by gently rolling topography with low topographic relief of 10 to 20 m. Catchment hillslopes are covered predominately by aspen approaching maturity following regeneration from wildfire in 1962 (Petrone et al, ). Flat‐lying clay‐rich areas contain shallow ~1 m deep ponds (i.e., Pond 40 and Pond 43; Figure ) underlain by 2 to 5 m of organic‐rich lake sediments referred to as gyttja.…”

Utikuma Region Study Area (URSA) – Part 2: Aspen Harvest and Recovery Study

“…• Aspen forest lands showed greatly reduced evapotranspiration (ET) and interception (I), with some increase in soil moisture storage after harvesting (Chasmer et al 2012, Hopkinson et al 2013, Petrone et al 2015.…”

“…• Rapid regeneration creates conditions of higher water use efficiency (WUE) and returns stand ET to near (70 %) the pre-harvest rate within three years (Brown et al 2013, Petrone et al 2015.…”