Quantifying regional trends in large live tree and snag availability in support of forest management

Abstract In the Pacific Northwest of the United States, 20th century timber harvesting resulted in major declines in area of forests supporting large live and dead trees (i.e., snags), that are not only key habitat elements for many wildlife species but also critical components of ecosystem function. Regional forest management guidance, such as the Northwest Forest Plan (1994) and Eastside Screens (1995), may aim to conserve and foster the development of late-successional old-growth forests, characterized by large live and dead trees. Satellite remote sensing supports regional monitoring efforts of these habitat characteristics, but managers may require additional guidance in order to leverage these data for large landscape or regional assessments. In this study, our objectives were to assess long-term (historical vs. contemporary) and short-term (1993–2017) changes in lands supporting large live trees and snags across 10 wildlife habitat types (WHTs) – disparate vegetation conditions that support significantly different wildlife communities – in Oregon and Washington, USA. We generated 30-m, annual maps of large live trees (>50 cm, >75 cm, and >100 cm diameter) and snags (>25 cm and > 50 cm diameter) based on the gradient nearest neighbor (GNN) imputation method. GNN integrates Landsat satellite imagery, geospatial climatic and topographic data, and USDA Forest Service Forest Inventory and Analysis data to predict forest attributes for all forested lands in the study area. GNN classification accuracy was poor to good for large live trees (Cohen’s kappa = 0.2 – 0.6) and poor to fair for snags (Cohen’s kappa = 0.1 – 0.3) in most WHTs, though performance was substantially lower in drier WHTs where large live trees and snags were rare. Our results highlighted long-term reductions in forest supporting large live trees and snags from historical to contemporary times, especially in wetter, more productive WHTs. In contrast, we observed short-term (1993–2017) increases in areas supporting large live trees and snags. Federal forests were both more similar to reference conditions and exhibited greater recent increases in areas supporting large live trees compared to nonfederal lands. Thus, Oregon and Washington have lost a substantial proportion of forests containing large live trees and snags and recent recruitment of these trees at regional scales is a slow process primarily occurring on federal lands. However, detecting such changes through current Landsat satellite mapping technologies remains challenging, highlighting the need for new mapping methods to aid in future management.