Later they are replace by Ponderosa pines. In the Cascades it's Alder followed by Douglas Fir which is replaced in a few hundred years by Hemlocks. In the deserts of Oregon and Washington fire suppression allows the growth of sage brush and juniper trees which crowd out native grasses. The cycle of fire varies greatly from area to area. In the wet areas of Oregon and Washington it is over hundreds of years, in the Pine country it is far shorter and in the high deserts it is even shorter.
Large animals deer, elk, moose, antelope, require areas cleared by fire for forage.
Not enough grass grows in dense timber to sustain them. The forest provides protection but clearings provide food. When a clearing returns to forest, valuable forage is lost and animal populations are reduced. In some areas clear cutting is beneficial in offsetting the loss of fire, but it has it's own side effects. There are may other benefits of fire, but I won't go into them here.