As the climate warms and large and severe fires increase across the western United States, identifying mechanisms of forest resilience to fire is critical for understanding which land management strategies may help forests cope in a changing climate. One region where information is especially needed is the western Cascades of Washington and northern Oregon, an area referred to as ‘northwestern Cascadia’, where insights are limited due to the role of fire in this ecoregion. Fire patterns in northwestern Cascadia are in part characterized by infrequent and severe wildfire events relative to other forest ecosystems in western North America. Forests can often experience many centuries between severe wildfire events, but when episodes of large and severe fires occur, they can exceed one million hectares with much area burning with stand-replacing severity (i.e., nearly all trees killed). Forests have persisted in northwestern Cascadia in the face of such large and severe wildfires for thousands of years, though the mechanisms supporting their resilience to fire and whether these mechanisms will persist in a warming future, are key unknowns.

An essential component of forest recovery after severe wildfire is successful tree regeneration, or tree seedling establishment after disturbance, which relies on the availability of a seed source. Within stand-replacing burn patches, it is often assumed that cones are lost to fire and that forest re-establishment relies on wind-dispersed seed from nearby living forests. In northwestern Cascadia, large fires can include burn patches that exceed maximum tree seed dispersal distances. Yet, abundant post-fire tree regeneration in northwestern Cascadia has been observed at surprising distances from a live seed source. This suggests these forests may have built-in or fire-induced mechanisms to retain a temporary local seed source following stand-replacing fire. One potential mechanism for retaining a local seed source is the phenomenon of a stress crop, which occurs when a tree is stressed or dying and sends hormonal signals to invest in cone production. This mechanism has profound implications for post-fire recovery and management, yet is poorly understood.

In this project, Madison will partner with the Washington Department of Natural Resources, the U.S. Forest Service, the National Park Service, and other collaborators to explore how stress crops may help increase forest resilience to wildfire by analyzing seed abundance and composition in the first few years following fire across a range of different fire severities. Understanding seed availability across these conditions will help land managers decide where to prioritize post-fire planting efforts and anticipate what areas are likely to regenerate on their own, which is increasingly important as wildfire potential in northwestern Cascadia is expected to increase.