Figure 3. Mortality spiral for a Douglas-fir tree illustrates the series of events leading to its death.  The spiral is based upon the decline disease spiral of Manion (1981).  In this example, a healthy tree is suppressed by larger trees.  If not released from competition, the tree is predisposed to attack by defoliators.  Once partially defoliated, the weakened tree is attractive to bark beetles (Wickman 1978) and is unable to resist the beetles, which carry blue-stain fungus (Berryman 1982).  The fungus blocks the transpiration stream in the tree and causes dessication of the leaves.  As the tree progresses along this spiral, its opportunities to escape death become more limited.