Death of trees is necessary 

 and beneficial in forested eco- 

 systems, although excessive 

 mortality may indicate a forest 

 out of balance. 



TREE MORTALITY 



One obvious indicator of 

 potential poor forest health is 

 conspicuous numbers of dead 

 and dying trees. 



Death of trees is normal, 

 but the causes and patterns 

 of mortality are complex. In- 

 sects, diseases, drought, wind 

 storms, and fire have killed 

 trees throughout millennia. 

 More recently, other agents as- 

 sociated with people and their 

 industry — air pollution, hu- 

 man-caused fire, and forest 

 management activities — are 

 contributing to tree mortality. 

 Factors such as competition 

 with other trees or plants or be- 

 ing eaten by grazing animals 

 like deer and elk can also cause 

 tree death. Often, death is the result of a comi- 

 blnation of causes. For example, competition 

 and drought may stress a tree to such an ex- 

 tent that insects can attack and kill it. 



Ecological Role of Mortality 



Death of forest organisms is a necessary 

 part of every forest ecosystem and part of the 

 normal cycling of materials and processes. 

 Tree death contributes woody material to the 

 forest floor where it serves as moisture res- 

 ervoirs and as habitat for a variety of plants, 

 animals, and microorganisms. The wood 

 and foliage eventually decompose, returning 

 nutrients to the soil. Standing dead trees 

 provide shelter, nesting, roosting, and hunt- 

 ing spots for birds and other animals. Trees 

 that fall in or near streams give the stream 

 the structure it needs to support fish and 

 other aquatic organisms. Tree death creates 

 an ever-changing mosaic of habitats within 

 a forest, allowing light to reach the forest 

 floor, seeds to germinate, and new vegeta- 

 tion to grow. 



Patterns of Mortality 



Patterns of mortality are complex. The 

 distribution of mortality from disturbances 



is determined by patterns 

 of vegetation, the type of 

 disturbance agent, and 

 the physical environment. 

 Where species and structure 

 are similar over a large con- 

 tiguous area, larger and 

 more severe corresponding 

 disturbances are more likely. 

 Where the forest is less uni- 

 form, mortality is patchy and 

 tends to correspond to 

 groups of susceptible spe- 

 cies, ages, and structures. 



Some disturbances cause 

 widespread mortality in 

 stands of uniform species 

 and age, such as mountain 

 pine beetle in even-aged 

 lodgepole pine. Other dis- 

 turbance agents, like dwarf 

 mistletoe and western 

 spruce budworm, are favored by multistoried 

 stands where seeds and larvae can drop from 

 tall to short trees. Fire is more deadly in 

 multistoried stands where the understory 

 acts as a fuel ladder, allowing the fire to 

 reach the tree tops. 



Wind storms, like those in Oregon in winter 

 1 995, blow trees down and snap off tops and 

 branches. Mortality is severe in relatively small 

 areas and often scattered. 



Overview — I 



