5 - The study of disturbance and landscape structure

Summary

The forest landscape comprises a collection of many contiguous stands. We may delineate and label these stands according to species composition, developmental stage, stand age, or vegetation growth stage (VGS, a concept which integrates composition, development and age defined below). If the landscape is a complex one, with more than one ecosystem type, there may be several different stand delineation/labeling systems in place, each of which is adjusted to take into account the unique disturbance–physiographic–tree species interactions in each ecosystem. The main effect of a disturbance regime on the landscape is to determine what proportion of stands are in each stage or stand age. If the disturbance regime is stable over sufficient time – two to three rotation periods for the predominant disturbance type – then a characteristic distribution of stands among growth stages or stand age will result. This chapter will start with the relatively simple concept of stand age distributions for simple landscapes and gradually work through to the more complex situations of multi-ecosystem landscapes with multiple successional webs and the presence of old-growth forest on the landscape.

Stand age distribution across the landscape

We can immediately break stand age distributions into two types: stable ones that are capable of perpetuation over time without change in shape, and unstable, or irregular ones. Only flat or monotonically decreasing stand age distributions can be stable (Figure 5.1). Stand age distributions that are unimodal or have two or more large peaks with gaps between are unstable (Figure 5.2).