Cumulative forest disturbances decrease runoff in two boreal forested watersheds of the northern interior of British Columbia, Canada

Abstract

In relatively large, forested watersheds, different types of forest disturbances such as timber harvesting, wildfire, and insect infestation often cumulatively and interactively affect runoff. There is a general lack of studies examining cumulative hydrological impacts caused by different types of forest disturbance, particularly in boreal forested watersheds. In this study, we evaluated the impacts of cumulative forest disturbances (timber harvesting, mountain pine beetle (MPB) infestation, and wildfire) on annual runoff in two large, forested watersheds situated in the boreal northern interior of British Columbia (BC), Canada. The study period was divided into the reference period of 1981 to 2003 and the disturbance period of 2004 to 2018. Time series analysis (cross-correlation) was used to determine the statistical relationship between cumulative forest disturbances and annual runoff, while two quantification methods including the modified double mass curve (MDMC) and a sensitivity-based technique were applied to separate the effects of cumulative forest disturbances and climate variability to annual runoff. Surprisingly, our results contradict the well-accepted conclusion that annual runoff increases with forest disturbances. We found that cumulative forest disturbances overall decreased annual runoff in the study watersheds, with varied reductions depending on severities of dominated disturbance types and post-disturbance forest dynamics. Our results also revealed that both cumulative forest disturbance and climate variability played an additive role in decreasing annual runoff during the entire disturbance period. These results provide valuable implications for supporting water management in boreal forested watersheds in the context of the warming climate.