Sierra Nevada Watershed Ecosystem Enhancement Project

The goal of the Sierra Nevada Watershed Ecosystem Enhancement Project is to quantify the potential for thinning of Sierra mixed conifer forests to enhance the ecosystem services they produce including fire resiliency, carbon storage in trees, and water in streams. Much of the Sierra Nevada is covered with forests that are dramatically denser than before fire suppression policies led to extinguishing all wildfires over a hundred years ago. The forest is denser because smaller trees that would have been killed by fires have grown to mature size. Forest restoration involves thinning to reduce the density of trees and shift species composition to a more sparse and fire resilient historic level.

Today’s denser forests are more prone to experiencing high severity fire in which most trees are killed and forest litter is consumed. This can lead to soil erosion, reduced ability of forests to absorb precipitation, and increased risk of flooding.

Compared to historic forests, dense forests store more of their carbon in smaller trees that historically would have been killed by frequent low and moderate severity fires. There is a higher risk in the Sierra Nevada of losing much of this stored carbon to high severity fires. Climate change also means that the risk of wildfire is increasing as the snowpack melts faster and the annual fire season is lengthening.

Dense forests also use more water than less dense forests and physically interfere with the ability of snow and rain to reach the forest floor. This decreases the amount of precipitation that is eventually discharged into streams and available for use by downstream water users. Dense shrubs will also transpire much more water than grassy meadows or bare ground.

Thinning reduces the number of smaller trees and removes ladder fuels which reduces wildfire risk and severity, meaning it is more likely that trees and ground cover will survive fires that do occur. This reduces the soil erosion potential created by wildfires. Biomass stored on site is more likely to maintained into the future. If forest products are captured from thinning, these products also store biomass. Thinning also reduces the amount of vegetation using water and so results in more precipitation, particularly snowmelt, infiltrating into the soil and eventually running off as stream water. In west-side Sierra forests, the snowpack provides an important seasonal storage of water that, when melted, is transmitted downstream and used throughout California.

SWEEP will use scientific methods to learn how forest thinning in the Sierra Nevada affects the fire risk, carbon storage and water yield provided. Water research will measure a number of hydrological parameters and will leverage instrumentation and results from other sites to rigorously quantify the effect of thinning on stream flow. We will then use economics to identify the value of increases in fire resiliency, carbon storage, and water yield in collaboration with forest stakeholders. Multiple research sites will be used to allow for analysis of how tradeoffs between these ecosystem services differ in different locations.

Download Report: SWEEP_published_112911