Bark beetle impacts on forest evapotranspiration and its partitioning

Area of Science:

• Forest Ecology
• Hydrology
• Remote Sensing

Background:

• Insect outbreaks, like bark beetle infestations, are major global forest disturbances impacting ecosystem function.
• Understanding the effects on evapotranspiration (ET) and its partitioning into evaporation and transpiration is crucial but not well-constrained.
• Forests in the Southern Rocky Mountain Ecoregion (SRME) are significantly affected by bark beetle outbreaks.

Purpose of the Study:

• To determine the effects of bark beetle outbreaks on evapotranspiration (ET) and its partitioning at multiple scales.
• To quantify changes in the balance between evaporation and transpiration following widespread tree mortality.
• To analyze the long-term impacts and recovery patterns of forest hydrology post-outbreak.

Main Methods:

• Combined remote sensing, eddy covariance measurements, and hydrological modeling (Variable Infiltration Capacity).
• Analyzed data at both local (eddy covariance site) and ecoregion scales.
• Utilized long-term datasets (16-18 years) of ET and vegetation mortality.

Main Results:

• At the measurement scale, ET as a fraction of precipitation decreased by 30%, with transpiration reductions 31% greater than total ET reductions.
• At the ecoregion scale, ET/P reductions of 9-15% were observed 6-8 years post-disturbance, primarily during the growing season.
• Hydrological modeling indicated a 9-18% increase in the ecoregion runoff ratio, with transpiration recovery lagging behind total ET recovery.

Conclusions:

• Bark beetle outbreaks have a net negative impact on forest evapotranspiration and a proportionally greater negative impact on transpiration.
• Forest recovery is characterized by lagged transpiration recovery and potential increases in late-summer vegetation moisture stress.
• Integrated multi-scale approaches confirm significant hydrological alterations following bark beetle disturbances in the SRME.