Rhododendron macrophyllum , Pacific rhododendron

This review summarizes information that was available in the scientific literature as of 2021 on the biology, ecology, and effects of fire on Pacific rhododendron in North America. Pacific rhododendron is an evergreen shrub or small tree common in coastal and montane conifer forests in northwestern United States and southwestern British Columbia. It typically occurs on well-drained, often nutrient-poor soils and is an understory dominant in several ecosystems from coastal dune forests to subalpine forests. Pacific rhododendron can form thickets of compact shrubs in forest openings, edges, and ridgetops, or take a more open, tree-like form in shaded forests. Pacific rhododendron mainly regenerates by seed. Seeds are small and are likely dispersed by gravity or wind. They lack dormancy and germinate readily in greenhouse settings. Pacific rhododendron seeds do not form a persistent seed bank. Seeds are not heat-tolerant and are likely killed by fire of any severity. Pacific rhododendron is easily top-killed by fire of all severities, and its cover is greatly reduced immediately after fire. However, it typically sprouts from the root crown within the first growing season after fire. Postfire sprouting has been described as “vigorous”. Sprouting plants can grow quickly and form dense thickets, especially in open postfire environments or after clearcuts. Pacific rhododendron is shade tolerant and is an important component in forests throughout succession. Its cover generally increases with time since disturbance, and it may take 100 years or more for cover to return to pre-disturbance levels. While some studies describe Pacific rhododendron’s response to wildfire, most postfire literature about Pacific rhododendron describes its response after clearcut logging and slash burning and subsequent conifer planting. Because sprouting Pacific rhododendron may hinder conifer seedling establishment and growth, various treatments have been studied as control methods for Pacific rhododendron. Historical fire regimes within the distribution of Pacific rhododendron range from frequent, low-severity fires to very infrequent mixed- or stand-replacing fires. However, over a century of logging and fire exclusion practices have created novel conditions that may result in larger, more severe fires in some areas. Climate change models for the Pacific Northwest project that fires may become larger and more frequent in dry forests, and some dry conifer forests may become hardwood-dominated, and possibly less likely to support Pacific rhododendron. Wet coastal forests may be less likely to transition to drier forest types and may continue to support Pacific rhododendron, especially in shaded forest interiors.