Exclusion of fire, along with selective logging of large trees with thick bark, has had some unintended consequences, particularly in drier forests east of the Cascades and in southwestern Oregon. Greater numbers of tree seedlings have gradually filled in the space between larger trees. In areas where larger pines were selectively logged off, more and more fir trees have taken their place. Even in Westside forests, stand densities have increased and there are more true firs. Because of their thin bark, true firs are more likely to be killed by fire than thick-barked Douglas-firs.

As the number of trees occupying a particular space increases, competition for light, water, and soil nutrients becomes more intense. Trees are weakened and killed by root diseases or attacked by beetles and other insects. These pests and diseases have become much more widespread due to the greater number of susceptible trees.

Dense stands with unhealthy trees are a recipe for a disastrous fire. In technical terms, there is too much fuel, leading to hotter and larger fires. Even large healthy trees are at risk of being killed by fire, because the shrubs and smaller trees crowding around them provide a "fuel ladder" by which fire can reach their crowns.

Click here to learn more about the effects of wildfire suppression on one pest affecting forests in central and eastern Oregon.