Environmental Problems, Their Causes, and Sustainability 1

Sudden oak death White pine blister rust Pine shoot beetle Beech bark disease Hemlock woolly adelgidFigure 11-14 Some of the nonnative insect species that have invaded U.S. forests and are causing billions ofdollars in damages and tree loss. The light green and orange colors show areas where green or red overlapwith yellow. (Data from U.S. Forest Service)How Do Fires Affect U.S. Forests? Surface,Crown, and Ground FiresForest fires can burn away flammable underbrushand small trees, burn large trees and leap from treetopto treetop, or burn flammable materials found underthe ground.Three types of fires can affect forest ecosystems. Some,called surface fires (Figure 11-15, left. p. 208), usuallyburn only undergrowth and leaf litter on the forestfloor. These fires can kill seedlings and small trees butspare most mature trees and allow most wild animalsto escape.Occasional surface fires have a number of ecologicalbenefits. They burn away flammable ground materialand help prevent more destructive fires. They alsorelease valuable mineral nutrients (tied up in slowlydecomposing litter and undergrowth), stimulate thegermination of certain tree seeds (such as those of thegiant sequoia and jack pine), and help controlpathogens and insects. In addition, some wildlifespecies such as deer, moose, elk, muskrat, woodcock,and quail depend on occasional surface fires to maintaintheir habitats and provide food in the form of vegetationthat sprouts after fires.Some extremely hot fires, called crown fires (Figure11-15, right), may start on the ground but eventuallyburn whole trees and leap from treetop to treetop.They usually occur in forests that have had no surfacefires for several decades. This allows dead wood,leaves, and other flammable ground litter to build up.These rapidly burning fires can destroy most vegetation,kill wildlife, increase soil erosion, and burn ordamage human structures in their paths.http://biology.brookscole.com/miller14207