Reexamining forest disturbance thresholds for managing cumulative hydrological impacts

Abstract

Forest disturbance thresholds, defined as those at or above which significant hydrological impacts are caused, are important guides to support forest and watershed management decisions for protecting hydrological functions and minimizing negative environmental impacts. Our literature review suggests that despite their significance, the research on this topic is surprisingly limited (<20 publications), where the paired watershed experiments (PWEs) primarily designed for detecting hydrological responses to forest cover change at the small watersheds were used to derive the thresholds. However, the widely used thresholds (e.g., 20%) based on the PWEs were identified from visual interpretation rather than determined from hydrological response curves, suffering from methodological shortcomings, and thus, may lack reliability. To advance this topic, we provided a robust technique (the modified double mass curve, MDMC) for quantitatively determining forest disturbance thresholds on annual mean flow as it allows the development of a hydrological response curve between cumulative hydrological effects and forest disturbance over time at the watershed scale. We applied this robust technique in eight large watersheds in British Columbia, Canada, and found that the forest disturbance thresholds ranged from 12 to 25%. We highly recommend that the widely used forest disturbance thresholds must be reexamined, and more studies are needed with rigorous methods and in consideration of other hydrological variables in forested watersheds.

Ecological restoration and water‐related ecosystem services, one step ahead: a reply to Dib et al. (2023)

“…Classical experiments of management interventions in paired catchments showed that the minimum area required for a change from high to low-biomass vegetation (or vice versa) to cause change in the annual flow is the widely accepted threshold of 20% of the watershed (Bosch & Hewlett 1982;Brown et al 2005). However, a recent study in Canada (Wei et al 2021) found a range from 12 to 25% of the watershed, instead of a sharp threshold. Differences within the range can be due to the intervention itself (type, intensity, and duration), characteristics of the watershed (size, landscape, slope, soil, geology etc.…”

“…Differences within the range can be due to the intervention itself (type, intensity, and duration), characteristics of the watershed (size, landscape, slope, soil, geology etc. ), and climate (Zhang et al 2017; Wei et al 2021). This threshold has a crucial implication for restoration planning when water yield is among the expected outcomes of the intervention within a watershed.…”

Streamflow responses to forest and climate change in the boreal Da Hinggan Mountains, Northeastern China

“…Several studies show that differences in physiological processes (transpiration) and physical processes (evaporation) before and after the change in forest species composition account for most of the variability in streamflow that cannot be attributed to climate (Wei et al, 2021;Yu et al, 2019). Due to differences in xylem anatomy type, the transpiration rates of birch trees are higher than that of larch trees for a given stem diameter (Caldwell et al, 2016;Ford et al, 2011).…”

Roles of forest disturbance and climate variability on streamflow components in snow‐dominated paired watersheds at multiple temporal scales

“…When the changes in forest accumulate to a substantial level, streamflow can be altered significantly which would cause problems for the PWE approach (Wei et al, 2021). Therefore, the history of forest change must be precisely quantified in both control and treatment watersheds; otherwise, the application of the PWE approach is questionable for long-term studies.…”