Quickflow response to forest harvesting and recovery in a northern hardwood forest landscape

Abstract

Forest harvesting often increases catchment quickflow (QF, water delivered rapidly to the stream channel), a metric of high‐flow events controlling a catchment's solute and sediment export. Nevertheless, our understanding of QF responses to various silvicultural strategies (e.g., clearcutting, selection harvest, and shelterwood harvest) is incomplete. We present a 31‐year examination of QF delivery from treatment (clearcut, selection harvest, and shelterwood harvest) and control catchments in a deciduous forest landscape in central Ontario, Canada. Growing season root‐zone storage capacity was estimated using a water balance approach to evaluate temporal changes in QF response to precipitation (P) for pretreatment and posttreatment periods. Threshold relationships between QF and P were assessed for control and treatment catchments for pretreatment and posttreatment periods using piecewise regression. Root‐zone storage capacity demarcated shifts in the hydrologic regime arising from forest harvesting and subsequent regeneration. This was particularly pronounced for clearcutting where postharvest decline in root‐zone storage capacity was followed by a rise to preharvest values. Similar pretreatment threshold relationships between QF and P, and near‐identical P thresholds for producing significant QF, reflected similar soil and overburden depths in the catchments. Harvesting effects were indicated by increases in QF/P ratios for relative small P and the number of P events that generated QF, thus changing treatment QF vs. P threshold relationships. Prior to harvesting there was no significant increase in QF with P below a threshold P of 35–45 mm; however, there was a significant QF vs. P relationship below this threshold for all treatments postharvest. Clearcutting increased the number of QF events for the entire postharvest period and the first 9‐year postharvest compared to the other treatments; nevertheless, evidence for intertreatment differences in total QF depth delivered from the catchments during the growing season was inconclusive. Our work suggests that changes in threshold relationships between QF and P, coupled with knowledge of the physical processes underlying them, are useful when evaluating hydrologic responses to forest harvesting.

Travel times for snowmelt‐dominated headwater catchments: Influences of wetlands and forest harvesting, and linkages to stream water quality

“…If the latter, higher runoff ratios should be associated with longer mean travel times (Hale & McDonnell, 2016). The negative relationship observed in the pre‐harvest period suggests annual runoff ratios are strongly influenced by the proportion of quickflow to total streamflow, which is consistent with the flashy hydrographs for these small catchments (Buttle et al, 2019). In addition, wetland cover likely influences the relationship between runoff ratio and mean travel time.…”

“…The harvested catchments show a post‐harvest decrease in mean travel times which is consistent with an expected increase in runoff being dominated by shallow flowpaths following harvest (Buttle et al, 2018, 2019; Hewlett & Helvey, 1970; Swank, Swift, & Douglass, 1988). There is potential for differences in climate pre‐harvest and post‐harvest to confound these results; however, most of the unharvested catchments show either negligible change or an increase in travel times during the post‐harvest period.…”

“…Alternatively, we might expect longer travel times associated with higher annual runoff ratios if annual runoff ratios are strongly influenced by augmented baseflows due to groundwater contributions (Hale & McDonnell, 2016). Shorter travel times following forest harvesting due to an increase in the quickflow proportion of total streamflow, as was previously documented for the harvested catchments used in our study (Buttle, Webster, Hazlett, & Jeffries, 2019). …”

“…Hillslope soil cover is orthic humo‐ferric podzols (spodosols) with well‐defined L and F (Oi and Oe) horizons with a combined average thickness of 0.05 m (Hazlett et al, 2001), while dispersed organic soils occupy depressions and riparian areas (Creed, Beall, Clair, Dillon, & Hesslein, 2008). In the headwater catchments, average depth of soil to basal till or bedrock is around 0.5 m (Buttle et al, 2019; Hazlett et al, 2011).…”

Non‐linear quickflow response as indicators of runoff generation mechanisms

“…Thresholds in quickflow response were assessed with respect to maximum hourly rainfall intensity and the sum of Gross p and ASI (Gross p + ASI) using a piecewise regression analysis (PRA). Following previous analyses (Buttle, Webster, Hazlett, & Jeffries, 2019), the segmented package in R (R Core Team, 2019; Muggeo, 2003) was used to quantitatively compute breakpoints (i.e., thresholds) and slopes. Due to the spatially heterogenous nature of soils, within catchment values of ASI varied as much as between catchment values.…”