In 1994, a large-tree harvest standard known as the “21-inch rule” was applied to land and resource management plans of national forests in eastern Oregon and Washington (hereafter, the “east side”) to halt the loss of large, old, live, and dead trees and old forest patches. These trees and forest patches have distinct ecological, economic, and social values, as reflected in widespread fish and wildlife use, public support for protecting them, and commercial interest in harvesting them, thus they have been the topic of much discussion and debate. At the request of regional Forest Service managers, we review the scientific knowledge accrued since implementation of the 21-inch rule and discuss the rule’s role and relevance to forest planning today.

Critical to our review are new findings from the social sciences and their integration with new biophysical and ecological science to form a more holistic understanding of forest ecosystems and the values they provide. We examine how human values associated with old trees and old forests are nuanced and evolving and discuss important social and economic changes relevant to large, old trees and old forests that have occurred across the Pacific Northwest in the past three decades.

Major advances also have been realized in landscape and fire ecology, climate and carbon science, and wildlife, fishery, and silviculture sciences related to the role and importance of large and old trees in east-side forests. Key findings show that trees of early-seral species that are older than 150 years contribute important ecological values not present in younger large trees. Other findings come from climate change research, landscape assessments, and fire history studies, which have contributed knowledge about the historical and likely future variability in fire frequency and severity in various forest types, landscape dynamics, and how landscape resilience works.

Many forests are now homogenized, with conditions no longer resembling those that existed prior to Euro-American settlement. Disturbance regimes have become more severe in many places, causing widespread ripple effects. The area burned by wildfire will continue to increase under climate change, and disturbance regimes will change further, leading to even broader changes in forest structure and species composition. Moderate or severe fires or fuel treatments, coupled with maintenance burning, may be needed to remove local seed sources and competition from undesirable shade-tolerant trees and help some patches of forest better adapt to fire and climate change. Proactive management can help facilitate some transitions, leading to better outcomes for people, forests, and native species.