Increasing spruce budworm defoliation increases catchment discharge in conifer forests.

Area of Science:

• Forestry
• Hydrology
• Ecology

Background:

• Insect outbreaks, like those caused by the spruce budworm (Choristoneura fumiferana), significantly impact forest canopies through defoliation and tree mortality.
• These changes in forest structure alter water storage and flow dynamics, affecting catchment runoff and stream discharge patterns.
• While impacts on broadleaf forests are understood, knowledge gaps exist regarding these effects in northern conifer-dominated forest ecosystems.

Purpose of the Study:

• To quantify the effects of cumulative spruce budworm defoliation on stream discharge and runoff in a conifer-dominated catchment.
• To compare hydrological responses between insect-outbreak-suppressed and untreated catchments over a three-year period.
• To investigate the relationship between defoliation intensity and hydrological alterations in northern forests.

Main Methods:

• Utilized 12 experimental catchments (6.33-9.85 km²) in eastern Québec, Canada, from 2019 to 2021.
• Implemented aerial application of Bacillus thuringiensis kurstaki (BtK) insecticide in six catchments to mitigate spruce budworm outbreaks, with six serving as untreated controls.
• Established stage-discharge relationships to measure stream volumetric discharge, runoff, and runoff ratios across a defoliation gradient.

Main Results:

• Strong positive correlations were observed between cumulative defoliation intensity and stream volumetric discharge (r=0.71, p<0.01), runoff (r=0.55, p<0.01), and runoff ratios (r=0.67, p<0.01).
• These hydrological changes are attributed to reduced available water storage within the catchment, leading to enhanced runoff generation.
• Hydrological responses to defoliation were more pronounced during summer months compared to autumn, likely due to lower available water storage post-freshet.

Conclusions:

• Forest insect defoliation significantly impacts catchment hydrology in conifer-dominated forests, mirroring effects seen in other landscape disturbances.
• Reduced forest canopy cover due to insect outbreaks enhances runoff and stream discharge by decreasing water storage capacity.
• Findings underscore the importance of considering insect outbreak impacts on hydrology within forest management and pest control strategies.