The disturbance of forest ecosystems: the ecological basis for conservative management

Abstract

The extensive literature on natural disturbance in forests is reviewed in terms of the hypotheses: (1) that disturbance is a major force moulding the development, structure and function of forests; and (b) that management of forests for all their benefits can be controlled so that the effects can be contained within those which result from natural disturbance. The causal factors of natural disturbance are both endogenous and exogenous; there are major difficulties in the formal characterization of disturbance and of recovery after disturbance. As to the latter, the acceptance of classical generalizations of the nature of succession has led to particular difficulties in the assessment and interpretation of recovery. Tree fall, which creates gaps, is fundamental to the development of many forests, and has been most intensively studied in tropical forests of Central America and the Amazon and in temperate forests of North America. Tree fall is part of autogenic change; mechanisms of gap-filling and subsequent growth and species composition vary widely with forest type and geography. Disturbance by wind is particularly difficult to characterize. Wind varies along a continuum; the blow-down of an individual tree may be mostly due to autogenic processes of ageing and decay, whereas catastrophic hurricanes and cyclones may be defined as wholly exogenous. Nevertheless, the resilience in terms of species diversity of tropical forests following catastrophic disturbance by hurricane is remarkable. A number of studies support the view that the tropical forest in hurricane-prone areas is not a stable steady-state ecosystem but rather that heterogeneity is maintained by catastrophe. The ability to regenerate by suckers and the coincidence of regenerative space and gregarious flowering are important components of the response of rainforest following disturbance. For much of the world, ‘fire is the dominant fact of forest history’. As examples, fire and its effects are reviewed for the northern boreal forests, oak-pine forests and north-western sub-alpine forests of North America. The effect of fire on species composition varies with intensity and frequency. That, together with the popular view of fire as unnatural and therefore unacceptable, places great demands on management of forests for all of their benefits, including national parks and reserves. These difficulties also affect management of other ecosystems, such as Mediterranean-type shrublands and heathlands where species diversity, productivity and cycles of regeneration and degradation are governed by fire as a natural disturbance. Shifting agriculture is a traditional form of agriculture used by at least 240 million people in the humid tropics. Shifting agriculture, together with wind, lightning and fire, is an exogenous disturbance which has little effect on soil fertility and on structure and composition of the rainforest which re-establishes after abandonment. As the intensity of disturbance of rainforest increases, resilience of the forest decreases and the current problems of extensive clearing for improved pasture and of uncontrolled logging are resulting in degraded ecosystems. Regeneration follows the often extensive death of trees caused by outbreaks of insects in many coniferous forests of northern America. This disturbance by herbivory halts increasing stagnation (as measured by decreasing rates of ecosystem production and nutrient cycling) and reinitiates succession. Other disturbances to forests occur through damage from ice-storms, snow avalanches, erosional and earthquake landslides, and volcanic activity; the development of Nothofagus forests in Chile and New Zealand is determined by such catastrophic mass movements. An extensive literature supports the hypothesis that natural disturbance is fundamental to the development of structure and function of forest ecosystems. It follows that our management of natural forest should be based on an ecological understanding of the processes of natural disturbance. Whether or not we want to do this, and the extent to which we want to derive all of the benefits from the forest, including timber, depends on social attitudes. Whereas humanism may treat conservation as the wise husbanding of forests in the interests of social traditions and harmony, animism may give nature unalienable rights. The conclusion from this review is that the ecological framework of natural disturbance and the knowledge of its component processes and effects provides the basis on which we can manage our forests as a renewable resource which can be utilized so that the forests ‘retain their diversity and richness for mankind's continuing benefit’. Nowhere is this management more desperately needed than for the protection of the world's tropical forests, its peoples and their cultures.