Equilibrium line folds. The age of trees continue increasing with time. Direction of slow processes . When the density of spruce budworm is low, then there is little mortality of trees and the average age of trees increases. The upper branch of stable budworm density corresponds to outbreak populations.
Old trees are more susceptible to defoliation and they die first.
Thus, the mean age of defoliated stand decreases, and the slow process at the upper branch goes backwards. Population dynamics can be described as a limit cycle that includes 2 periods of slow change and 2 periods of fast change. Transition to a fast process is a catastrophe. As a result, the probability of outbreak increases with increasing forest age. If an outbreak occurs in a young forest stand, then it is possible to suppress the population and return it back to the area of stability.