Increased streamflow in catchments affected by a forest disease epidemic.

Area of Science:

• Forest Ecology
• Hydrology
• Forest Pathology

Background:

• Natural disturbances are crucial for ecosystem health, but changing regimes pose risks.
• The Swiss needle cast (SNC) epidemic in Oregon has rapidly expanded, affecting forest health.
• Understanding the impacts of forest diseases on hydrology is critical for ecosystem management.

Purpose of the Study:

• To investigate the effects of Swiss needle cast (SNC) on catchment-scale hydrologic regimes in western Oregon.
• To determine if changes in forest canopy due to SNC influence streamflow, runoff ratios, and flow timing/magnitude.
• To assess the relationship between the area affected by SNC and observed hydrologic changes.

Main Methods:

• Analyzed 20 years of SNC aerial survey data and 15-26 years of stream discharge, precipitation, and temperature data.
• Examined 12 catchments in western Oregon, varying in size and percentage of area affected by SNC.
• Tested for trends in hydrologic variables including annual streamflow, runoff ratio, and peak/low flow characteristics.

Main Results:

• Runoff ratios increased in five catchments with >10% area impacted by SNC.
• Increased runoff is likely linked to reduced canopy interception and transpiration due to SNC.
• Two catchments with the highest SNC impact showed no hydrologic changes, possibly due to compensatory factors.

Conclusions:

• Chronic canopy disturbance from foliage pathogens like SNC can significantly influence catchment hydrology.
• Forest diseases can alter ecosystem processes, impacting water resources and aquatic habitats.
• This research provides the first evidence of a foliage pathogen affecting watershed-scale hydrology.