Trees may be individually adapted to fire, but how they grow in the forest and where that forest is mean a lot when a fire visits. Fires may burn quickly up steep hills, and landforms may funnel winds or shadow parts of the forest from the sun. Mountains may collect moisture or block it, changing the mix of trees from acre to acre. There’s no one recipe for managing a forest in the face of fire.

In some forests, fire is an infrequent visitor, returning to burn once every century or more when severe drought dries out the forest. These high-intensity, low-frequency fires tend to burn in wetter forests like the mist-shrouded Douglas-fir forests of the coastal Pacific Northwest or the lodgepole pine forests on north-facing slopes in the Rockies. If conditions are right, fires in these forests burn in the crowns of the trees and little can be done to slow their spread once they blow up.

At the other end of the spectrum, high-frequency, low-intensity fires burn in arid parts of the West, typically in scrubland, pi–on-juniper landscapes, oak savannas and sparse stands of ponderosa pine. These fires burn “cool” on the ground and regularly clean up fuels that would otherwise accumulate and make for more intense blazes. These fires help to keep out invasive species and to keep down accumulations of fuel that might increase the intensity of the fire. Some invasive species, such as cheatgrass, have quickened the fire return interval in some places to every few years, forcing out other plants that require longer fire return intervals.

By far the most common forest type in the West burns in what fire experts call mixed severity. These are forests that burn in a mosaic: trees on one side of a ridge may burn hot in their crowns, but a microclimate or a more recent fire on the other side of the ridge might slow the fire down and return it to the ground. These forests are made up of a wide range of trees, each differently susceptible to fire. Douglas-firs are common inland in the Pacific Northwest, but they may also be mixed with sugar pine, western red cedar, western hemlock and even ponderosa pine, among many other species. In drier forests, ponderosa pines predominate, and it’s in these forests, at low elevations, that the effect of a century of fire suppression is most severe. Big old fire survivors may be crowded by many smaller trees—“ladder fuels” that can turn a forest-cleaning ground fire into a tree-killing crown fire. These conditions, however, also occur naturally, so that a ponderosa pine forest that fire experts would call mixed severity might be expected to crown in some places and kill larger trees. Even lodgepole pines, the Roman candles of western forests, occur in mixed-severity forests, playing a role as cool fire survivors or incendiary accelerants, depending on weather conditions and nearby fuels.