Forest dynamics

Forests change, sometimes imperceptibly, sometimes dramatically, such as when a tree falls or a hurricane passes. The notion that forests are stable and unchanging is a perspective that relates to human life expectancy and does no justice to the fundamental importance of change in forests. Forests change in structure and composition due to internal processes (competition between species for water, light and nutrients; differences in regenerative capacity of species, increasing tree age) and external processes (climate change, large scale disturbances, etc). In a very general sense, undisturbed forests undergo a predictable succession in which hard wooded, long-lived, large seeded, slow growing species with specialised (mammal) seed dispersal replace soft-wooded, short-lived, small seeded, fast-growing species with generalist (wind or small bird) seed dispersal. During this process, forests increase in mean height, mean tree size and the number of leaf layers (i.e. the forest becomes darker).

Disturbance sets back succession or, in other words, it rejuvenates the forest. Disturbance can be characterised in three components: scale, intensity and frequency. The larger the scale, and intensity and the higher the frequency, the larger the impact on the existing forest vegetation and its inhabitants. Forest communities are more or less adapted to the disturbance regime that prevails at a certain site. For example, forests in the hurricane belt of the Caribbean or the Philippines have evolved in conditions in which large swaths of forest are destroyed more or less predictably on an infrequent basis. Such forests are characterised by fast growing, early reproducing well regenerating species, often with high vegetative sprouting capabilities. In contrast, forests like those in the Guianas, that have evolved in the absence of large-scale disturbances but with frequent small-scale single tree falls that cause limited damage, are rich in hardwoods with heavy seeds that reproduce late. A change in the disturbance away from the prevalent regime puts pressure on any forest ecosystem, as the species of plants and animals that are best adapted to the new conditions may be rare or even absent. Logging is a disturbance of high intensity (soil disturbance and compaction, gap creation), on a large scale and of moderate frequency (on a scale of decades). It will have a relatively low impact on forests that are regularly subjected to large disturbances, but a strong impact on forests, which are adjusted to low dynamics.

TBI sources

Steege, H. ter (1993) Patterns in tropical rain forest in Guyana