Jaross W FEMC2007

Influence of Windstorms on Marbled Murrelet Habitat Development in Managed Coastal Forests of Washington State; A Scenario-based Analysis

Weikko Jaross, Washington State Department of Natural Resources, Washington St. SE, P.O. Box 47016, Olympia, WA, 98504, USA. weikko.jaross@wadnr.gov
Scott Horton, Washington State Department of Natural Resources, Olympic Region, 411 Tillicum Lane Forks, WA, 98331, USA, scott.horton@wadnr.gov
Mark Teply, Washington State Department of Natural Resources, Washington St. SE, P.O. Box 47016, Olympia, WA, 98504, USA, mark.teply@wadnr.gov

The Washington State Department of Natural Resources is developing a long-term habitat conservation strategy for the marbled murrelet (Brachyramphus marmoratus); a small seabird nesting in coastal forests of the Pacific Northwest and listed as a threatened species under the Endangered Species Act. The potential abundance of nesting habitat resulting from alternative landscape management strategies is uncertain due to stand replacing windstorm events. The Woodstock Spatial Planning System (Remsoft Inc.) is used to compare future landscape conditions with an objective of seeking effective and efficient management strategies for increasing nesting habitat and minimizing nest predation.

The current landscape is comprised mostly of simple-structured stands originating from turn-of-the-century windstorms, and decades of intensive, even-aged forest practices. Management strategies aimed at developing higher quality nesting habitat may lead to complex multi-canopy structures that will support nesting platforms in large, tall trees having deep broad crowns. Of concerns are devastating habitat losses to windstorms and the role of silviculture and landscape based management in mitigating this risk. Studies of windstorms, silviculture, and nest predation provide insights into the fate of nesting habitat, and the bases for a working hypothesis for determining where management for nesting habitat is most likely to be successful. Complex stand structures generally do not persist in locations exposed to severe windstorms because stands are repeatedly blown down. Potential pathways for sheltered locations are best illustrated by single tree losses and small gaps that drive late-successional dynamics not affected by windstorms. Nest predation is minimized by a landscape mosaic of large patches of both early and late seral stages. By identifying exposed and sheltered locations to windstorms, it may be possible to develop a complex landscape of varied opening sizes, scattered overstory, and clumps of variable retention, which increases nesting habitat and reduces predation.

A topographic model identifies locations exposed to windstorms. Susceptibility of stands to windthrow is evaluated by stand density management diagrams. Weibull distributions are applied to windstorm records to develop landscape-level return intervals and gap size distributions. Habitat quality and predation models are applied to simulate outcomes of landscape patterns over 70-years. Regeneration harvest and variable retention harvests are available choices as well as variable density thinning, except in high windthrow situations.
The majority of current forest conditions are susceptible to windthrow. Nearly half the forest studied is topographically exposed to windstorms. The expected return interval for stand replacing windstorms is 40-years. As a result, exposed locations might provide only a third of the potential nesting habitat of sheltered locations. Harvests, which mimic historical landscape patterns, minimize predicted nest predation.

The simulations demonstrate that managing for nesting habitat in locations sheltered from windstorms reduces the uncertainty of achieving long-term habitat conservations objectives. Emulating gap size distributions across the landscape using even-aged strategies in locations exposed to windstorms minimizes landscape level predictions of nest predation. The analysis is encouraging in that windstorm modeling can be useful for determining where management for marbled murrelet habitat is most likely to be successful.