3.3.1. Monoculture Plantations Using Indigenous Species
Monoculture plantations are comparatively easy to establish and manage since all trees mature at the same time. Traditionally many such plantations have used exotic species. The timbers of these species are often of relatively low value. Some indigenous species can have much higher commercial values than fast-growing exotic species. Plantations of higher value timbers may be increasingly valuable in future once natural forests have been logged over.
Intensively managed plantations can yield a high commercial value. Plantations of indigenous species also provide some modest biodiversity benefits. The key disadvantage of using indigenous species is that little is usually known about their silvicultural requirements and most are comparatively slow growing.
Industrial plantations are often large and are established as continuous blocks. This leads to the simplification of landscapes. Breaking these extensive plantations up by using buffer strips of native vegetation or ecologically restored forests along streamsides and roads can add complexity and habitat diversity.
Buffer strips can help enhance conservation benefits by introducing more spatial complexity to a landscape and increasing connectivity allowing easier movement of plants and wildlife across landscapes. These strips or corridors can have a number of other benefits, including acting as fire breaks and streamside filters to enhance watershed protection.
Instead of using only one species in a plantation, an alternative could be to use more than one and create a mosaic of different types of plantations across the landscape. The landscape diversity could be further enhanced by surrounding each monoculture by buffer strips as described above.
The advantage of this alternative is that silvi-cultural management of each plantation remains simple; the disadvantage is that precise species-site relationships must be known if productivity in each of the different plantations is to be maximised.
Site biodiversity may be enhanced if mixed species plantations are used instead of monocultures. These might be temporary mixtures where one species is used for a short period as some form of nurse or cover crop, or they may be permanent mixtures for the life of the plantation. Most mixed-species plantations usually have only a small number of species (under four), so biodiversity gains may be modest.
Mixtures can often generate benefits in addition to any biodiversity gain. These potential benefits include improved production, improved tree nutrition, and reduced insect or pest damage. There may also be financial gains from combining fast-growing species (harvested early in a rotation) with more valuable species that need longer rotations. Disadvantages include the fact that not all species' combinations are necessarily compatible and an inappropriate mix of species may lead to commercial failure. Also, having two or more species in a plantation necessarily leads to more complex forms of silviculture and management. This means that mixtures are likely to be more attractive to smallholders and farm forestry woodlots than large industrial-scale plantations.
In many plantation forests, especially those near areas of intact forest, an understorey of native tree and shrub species will develop over time with many of the species being dispersed by animals. What began as a simple monoculture forest can acquire structural complexity and considerable biodiversity.
Such understories transform the range of services provided by the plantation. There can be benefits in terms of watershed protection and fire exclusion as well as biodiversity gains.
However, they pose a number of dilemmas for managers who may find their original objectives being compromised or, at the very least, made more difficult to achieve. Difficult tradeoffs may need to be made.
Agroforestry is a form of agriculture that mixes trees and other crops in the same area of land (see "Agroforestry as a Tool for Forest Landscape Restoration"). Some forms involve mixtures of multipurpose trees and food crops; others combine scattered trees and pastures. In most cases a variety of species are used in the farm or "home garden" that differ in canopy and root architecture, phenology, and longevity.
Agroforestry has some particular advantages in landscapes where land for food production is limited and where human populations are large or increasing. Agroforestry creates spatial and structural complexity across landscapes and offers the prospect of agricultural sustainability and some biological diversity. On the negative side, biodiversity gains may be modest since many of the species used are relatively common agricultural crop species.
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