As mentioned in the previous chapter, the forest sector (and other sectors) often depends on introduced species to provide a variety of socio-economic, environmental and human health benefits to the forest sector and to the rural communities that depend upon forests. Many introduced species are highly regarded because of their exceptional adaptability to a wide variety of sites, their rapid growth, and the multiple uses of their products.
With growing concerns about the degradation and loss of natural forests, planted forests and trees outside forests, composed most often of introduced woody species, are becoming increasingly more important sources of products such as timber, fibre and fuelwood. In developing countries, fuelwood is the prime source of energy, representing over 80 percent of the wood harvest (FAO, 1999). They also provide non-wood forest products, such as fruits, leaves, roots, honey, fibres, oils, resins, cosmetics and medicines, either from the planted trees themselves or from other elements of the ecosystem that they help to create. Such products contribute to the livelihoods of rural communities by providing food, medicine and employment as well as income from their sale. In countries with low forest cover, planted forests and trees outside forests constitute the main source of wood and non-wood forest products.
In addition to providing valuable forest products, alien tree species planted in forest plantations and other areas help provide many vital ecosystem services such as:
• combating desertification;
• protecting soil and water;
• rehabilitating lands exhausted from other land uses;
• diversifying the rural landscape;
• maintaining biodiversity;
• enhancing carbon sequestration;
• providing amenity and shade.
When planted in riparian areas, trees provide spawning beds for fish and molluscs and shade which aids in the reduction of eutrophication. Trees planted on farms help to increase soil fertility by providing organic matter through litter decomposition at the soil surface or through atmospheric nitrogen fixation (nitrogen-fixing trees), both of which contribute to improvements in food production. Along roads and highways, trees and plants not only add beauty to cities and towns but also provide shade and control outdoor noise and traffic pollution. Trees also play a major role in preserving the social and cultural values attached to forests, particularly as natural forests decrease in size through deforestation or are designated for other purposes.
The forest sector also employs introduced species in biological control programmes to help combat pest problems with considerable success (McNeely, 1999).
Practical reasons for using introduced species instead of native species in the forest sector include the following (Richardson, 1998; Richardson, 1999).
• Introduced trees often grow much faster than native species, particularly in the first years after planting.
• Native species are more difficult to manage silviculturally, in part because the biology of these species is often poorly known including information on how to collect, store and germinate seeds, how to produce seedlings in a nursery and how to manage them in a forest. Introduced species, on the other hand, have been well studied in a variety of settings.
• Seeds from introduced species are more readily available and easier to handle than seeds of native tree species.
• In order to improve the trade balance by reducing the need for imported wood products, it is often necessary to develop local forest industries. Knowledge of markets and manufacturing technology is more readily available for introduced species such as pines and eucalypts.
Positive impacts may be best achieved by careful selection of species and risk assessment prior to introduction and large-scale planting, coupled with management of introduced species in order to prevent them from becoming invasive.