The moist forests of the Rocky Mountains typically support late seral western hemlock, moist grand fir, or western redcedar forests. In addition to these species, Douglas-fir, western white pine, western larch, ponderosa pine, and lodgepole pine can occur creating a multitude of species compositions, structures, and successional stages that can be arrayed in a variety of fine to coarse mosaics. Weather, insects and diseases are often the most frequent and persistent disturbances. Large high severity wildfires do occur but their intervals have been lengthened by successful fire suppression and fire suppression has decreased the occurrence of small mixed severity fires. White pine blister rust (an introduced disease) has decreased the presence of western white pine throughout much of northern Idaho, western Montana and eastern Washington, the heart of the moist forests of the northern Rocky Mountains. Compared to the role decomposition plays in the ponderosa pine forests, decomposition is more of a regulator of biomass than is fire in the moist forests.