Evidence suggests that the Coast Salish peoples of San Juan Island took an active part in managing and modifying the ecosystem they lived in. This included controlled forest burnings, where the dense forests of the island were purposefully set on fire. Natives usually set their fires in cooler weather during spring and fall. This way, the fires could be controlled easier and not get out of hand. The once heavily-forested areas of the island thinned, and as the fires died out they left behind open areas rich with nutrients for plant growth.
In western Washington, these controlled burnings of local forests impacted tribal life in a variety of ways. It cleared out densely wooded areas, which in turn improved avenues of travel for tribal members throughout the island. It opened up prairie areas and stimulated growth of the camas root, which the Indians regularly consumed. Controlled burnings also promoted a growth of wildlife in the area in the years following the fires. The resulting sprawl of thicket that rapidly followed the devastation from the fires baited animals like deer, which consumed the small plants and grasses that abundantly grew after the fires.
The long term effects of intervention on the San Juans was recognized immediately by American explorers in the 1800s. George Gibbs of the United States Boundary Commission observed that the trees in the area would be a poor choice of timber for loggers. Christy Avery, a historian of the National Park Service in the Pacific West Region, suggests that the unusual abundance of Garry Oaks on the island is another result of the fires. The controlled burnings kept the Douglas fir, which naturally grows throughout the region, in check. To people not familiar with fire ecology, the practice may seem destructive. However, fire is a necessary and naturally-occuring part of many ecosystems. Today, forest managers sometimes attempt to replicate the Native-controlled burning that created this ecosystem.