Yrjö Haila
Abstract
`Forest fragmentation' refers to a process of formerly continuous forests becoming subdivided into patches isolated from each other by tracts of another habitat type, often hostile to forest organisms. Human-induced fragmentation is ubiquitous, but it ought be distinguished from natural `fragmentedness' of forests which is a reflection of the inherent heterogeneity of nature, and maintained by discontinuities in the environmental space as well as disturbances such as wildfires and windthrows. Ecological criteria for identifying different types of fragmentation effects can be found by separating from each other ecosystem structure and function on the one hand, and fragmentation effects due to isolation and external influence on the other. All particular effects are, however, scale dependent: mere geometry is not sufficient for identifying potential harmful effects of forest fragmentation, but the geometry of a given fragment system must be related to characteristic scales of the processes of interest. Fragmentation induces a multiplicity of processes in vegetation and the specific consequences in a given situation are difficult to predict, but it is feasible to enumerate types of effects that are commonly observed such as the following: (1) a new habitat is created around the edge of forest fragments; (2) new, possibly alien species may invade forest fragments from the outside; (3) the disturbance regime in forest fragments is different compared with a continuous forest; (4) animals and soil organisms necessary for pollination, seed dispersal or nutrient acquisition of particular plants may vanish from forest fragments; and (5) decreasing area of forest fragments brings forth increasing risk of extinction of local populations within them. A major need is to avoid forest fragmentation in forest management; monitoring of forest stand type and age distribution on the regional scale using, for instance, GIS is recommended.
OE Nov 21, 1996