Fire has long been a keystone ecological process in Western forests. In ponderosa pine (Pinus ponderosa)/Douglas-fir (Pseudotsuga menziesii) forests of the Colorado Front Range, historical fires are believed to have been "mixed severity" in nature. That means that these fires are believed to have typically burned within a range of severities from low severity surface fire where few trees were killed to high severity crown fire where all trees died. As a result, these historical fires created and sustained a heterogeneous mosaic of forest conditions. These fires are also believed to have played a critical role in sustaining diverse, lush understory plant communities.