Manipulating retained structures to improve wildlife habitat

Managed forests throughout the Pacific Northwest are subject to forest practices rules that require the retention of green trees and snags for a variety of reasons. These reasons include protecting water quality, protecting unstable slopes or sensitive soils, improving aesthetics, and providing wildlife habitat.

When it comes to understanding how much retention is enough and how it should be placed in and around clearcut timber harvest areas to support wildlife, foresters and biologists have little available information to guide operational decisions. To better understand how retained green trees and snags are functioning as wildlife habitat in Pacific Northwest clearcuts, NCASI, in partnership with Michigan State University, studied bird use of retention in young stands at three landscapes spanning a region from Northern California to Southwest Washington from 2008 to 2010. Bird occupancy varied with stand-level retention patterns within each of the three landscapes. However, patterns of occupancy were not consistent across the larger regional study area. Researchers documented cavity nesting bird use of high stumps and found more interior species using clearcuts where retention was available. Size of retention patches within clearcuts affected bird use, especially by forest interior species, with threshold levels occurring when patches exceeded 12-15 trees.

Building on that retrospective work, NCASI and Michigan State researchers are beginning a new forestry experiment (Phase II) with five treatments designed to investigate wildlife response to aggregation, composition, and slope position of structural retention in recently harvested stands. Snags will be created with a mechanical harvester in two of the five treatments. Study areas are located on lands owned and managed by six private industrial cooperators as well as on state forest lands in Oregon and Washington.

Unique to this study, treatments are designed to test variations in retention pattern that could occur under current state forest practices rules. The experimental framework for Phase II will allow researchers to assess causal mechanisms for bird and bat response to a variety of retention practices, while holding total retention per acre constant. Treatments will be applied to 45-50 stands over the next year, with the investigation of bird and bat response beginning in 2016 once the structures have begun to decay.


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