City of Seattle, Seattle Public Utilities, Restoration Thinning Trial, 2005 - 2017, Cedar River Municipal Watershed, King County, WA

The Restoration Thinning (RT) Program in the Cedar River Municipal Watershed (CRMW) was one of three forest restoration programs (the others being Ecological Thinning and Planting) defined and funded through the Cedar River Watershed Habitat Conservation Plan (HCP) that was signed and initiated in April of 2000. Restoration thinning and ecological thinning projects were combined into the 'Upland Forest Thinning' project and are ongoing today to meet objectives outlined in the Habitat Conservation Plan and Forest Managment Plan.

The primary goal of the RT program, which is analogous to pre-commercial thinning, was to actively thin dense young second-growth forest stands (generally less than 30 years old) to facilitate ecological development towards old-growth forest habitat conditions.

Objectives of RT include:
Reduce competition among trees.
Stimulate tree growth.
Increase light penetration under the top tree canopy.
Increase tree and understory plant species diversity.
Accelerate forest development beyond the competitive exclusion stage towards a more biologically diverse stage.
Extend the forest development stand initiation stage such that diverse species become established and diverse stand structures develop.
Provide multiple development pathways for variable forest stand structures.
Reduce long-term fire hazard.
Increase resilience to catastrophic windthrow, insect, or disease outbreak.
Increase habitat connectivity and structural variability of riparian areas.

This data package describes a forest restoration trial in young conifer forests of the western central Cascade Range in Washington State, USA. Young second-growth forests often regenerate as very dense, homogeneous stands following harvesting. These forests have low species diversity and trees often experience strong competition for resources. To increase tree vigor and growth and stimulate development of diverse understory, shrub species stands are thinned with the long-term goal to restore diverse functional older forests that provide habitat and are resilient to disturbance. Seattle Public Utilities (SPU) implemented the Restoration Thinning Program under the Cedar River Watershed Habitat Conservation Plan. Under this program, SPU conducted thousands of acres of thinning with varying prescriptions.

This trial was part of the restoration thinning program and was designed to test what type of prescription would lead to better tree growth and understory development. SPU tested two different thinning densities as well as uniform and clumped distribution of residual trees. The trial areas were sampled twice following treatment and tree and understory data were collected. Thinning prescriptions differed in response of overstory tree growth and understory response. Initial trial design and overstory measurement reports are included in the data package. This project was completed in 2017. Analysis from the restoration thinning trials influence future young stand management decisions and thinning prescriptions in the Cedar River Watershed today.