3.1 Pest and disease incidence in monocultures
3.2 Risks associated with plantation forestry practices
3.3 Storms and fire
A serious threat to plantations can arise from a massive build-up of a pest or disease. It has been disputed whether monocultures per se are more susceptible to such devastation (see Working Papers FP/3 and 10). An accepted ecological principle states that the stability of a community and its constituent species is positively related to diversity. Thus some argue that substitution of natural forest by even-aged monocultures may remove many of the natural constraints on local tree pest and pathogens and thus increase risk of attack. Some evidence supports this, see for example Gibson and Jones (1977) though these authors point out that increased susceptibility mostly arises from conditions in plantations rather than because only one tree species is present.
The relative susceptibility of monocultures to pests and disease is complex ecologically. For example, applying the idea that diversity is beneficial by cultivating mixed crops may not offer much protection since only small amounts of the right kind of diversity are needed to maintain stability (Way 1966). Further, the influence of diversity on stability of (say) insect populations depends on what population level is deemed acceptable. Often stable, equilibrium levels are too damaging and artificially low populations are sought. Pest controls to maintain low levels are very different from those required to achieve stability (Speight and Wainhouse 1989). These authors stress that artificially created diversity, i.e. mixed crops, does not necessarily improve ecological stability. It is certainly inferior to naturally occurring diversity; complexity of organisation and structure is as important (Bruenig 1986).
It is prudent, nevertheless, to spell out why plantations are perceived to be in danger.
1. Plantations of one or two species offer an enormous food source and ideal habitat to any pest and pathogen species adapted to them.There are several major examples where plantations have faced major disease or insect problems that have stopped the use of particular species or clones. For more detail (see Working Paper FP/10). While they may have prevented the planting of some species, impaired the productivity of others, overall they have not caused such widespread damage as to seriously question plantation silviculture as a practice.2. Uniformity of species and closeness of trees allow rapid colonization and spread of infection.
3. Very narrow genetic base e.g. one provenance or use of clones reduces the inherent variability in resistance to attack (see Working Paper FP/3).
4. Trees grow on one site for many years. This may allow a pest or disease to build up over time with little opportunity to destroy infection. The forest plantation cannot be changed quickly in face of a devastating outbreak.
5. Many plantations are of introduced exotic species and are without the insect pests and pathogens that occur in their native habitat. This has undoubtedly contributed to the great success of eucalypts across the tropics freed from numerous leaf-eating insects that occur in the Australian environment (Pryor 1976). Conversely, many natural agencies controlling pests and diseases are also missing and destruction can be swift and uncontrolled. Zobel et al. (1987) concluded that exotic stands are not more at risk, other than clonal plantations, and that problems arise mainly when species are ill suited to a site. Further, introducing biological control has been successful in many instances
There remain two concerns.
Many pest and disease problems in plantations arise from the nature of forest operations, and not directly from growing monocultures.
First, extensive planting of one species, whether indigenous or exotic, inevitably results in some areas where trees are ill suited to the site and suffer stress. This sometimes occurs in large-scale planting programmes where insufficient attention is paid to sites or where exotics are used extensively before sufficient experience has been gained over a whole rotation e.g. Acacia mangium in Malaysia and Indonesia and the discovery of widespread heart rot.
Large amounts of wood residue from felling debris and the presence of stumps are favourable for colonization by insect pests and as sources of infection. There are many examples but modification of silviculture or application of specific protection measures generally contains such problems (Evans 1999b; Working Paper FP/10).
Thinning operations can damage remaining trees and provide infection sites for diseases. In the case of Fomes the stumps are colonised and this can lead to death of adjacent trees. Delayed thinning, ragged pruning, and poor hygiene can also increase risk to remaining trees. However, none seriously threaten plantation sustainability but emphasise the need for good husbandry (Evans 1992).
Plantation uniformity possibly increases risk from hurricane and storm damage if only because trees may be planted in locations that increases their susceptibility. Sub-optimal productivity can result if a site’s potential not fully realised. Minimising hurricane damage in the tropics can be helped by planting wind-firm species such as Cordia alliodora or choosing Pinus caribaea var bahamensis over P. oocarpa.
Most forest fires in plantations are caused by arson; only a few by lightning or encroachment of fires from neighbouring land. While there are a few examples of frequent fires preventing plantation development, it is more often due to poor community relationships than any inherent shortcoming with forest plantations.
