As for many western tree species, fire suppression practices in the last 100 years threaten the health of ponderosa pine. Historically, fire played an important role in the ponderosa pine ecosystem. The thick, corky bark of mature ponderosa pines makes them resistant to the small scale fires which remove smaller trees from the understory of these forests. Without fires, many smaller trees have become established beneath the older ponderosa pines. This dense understory not only increases competition for limited water and nutrients in the soil, but may also promote larger, catastrophic fires if they are to become started. Additionally, ponderosa pines weakened by competition are far more likely to be harmed by the more than 100 different species of insects and disease that attack the species.