Importance of disturbance in forests

Disturbances exert strong control over the species composition and structure of forests. As a general rule, landscapes with frequent severe disturbance are dominated by young even-aged stands of shade-intolerant species such as aspen. Conversely, old stands of shade-tolerant species such as hemlock dominate where severe disturbances are rare. Every conceivable mixture between these two extremes can be created by the various combinations of disturbance. To understand how disturbances exert these influences over the forest it is necessary to know the basic concepts and mechanics of disturbance – the function of this chapter. Fire, wind and herbivory have been chosen for detailed discussion because they are very important influences on temperate forests and we know a lot about them and their interactions.

A definition and key concepts

The disturbance regime is simply a description of the characteristic types of disturbance on a given forest landscape; the frequency, severity, and size distribution of these characteristic disturbance types; and the interactions among disturbance types. If the forest experiences a series of unique disturbances over time, so that type, frequency, severity and size cannot be characterized, then there is no stable regime. Apparent stability of the regime, however, is a function of the length of time and size of area observed (Lertzman and Fall 1998).