In addition to the threats posed to forestry by the introduction of alien pest species, forestry trees may themselves become invasive and create problems of their own (Case Study 18). Curiously, relatively few countries regulate the introduction of exotic plant species and those that do (e.g. Germany and Switzerland) make a general exception for forestry species (see De Klemm 1996).
A number of definitions of `invasive species' have been suggested in the literature; in the context of forestry, it is recommended that a common definition should be developed that focuses only on parameters of population expansion and avoids any preconceived negative perceptions of invasive species.
On a global basis, little information is available on the status of forestry trees that have become invasive. Evaluation of the extent of such invasions is often very qualitative and subjective, making it difficult to assess the overall magnitude of the problem. Furthermore, the terminology used by authors is also very variable and there is frequent overlap in the terms `invasive' and `naturalized'.
With these caveats in mind, a literature review by Murphy and Haysom (FAO 2003b) found 443 species of tree or woody shrub that were classed as invasive, including some listed by Binggeli (1996) as possible or potential invaders. Some 282 species used in forestry were among those listed as invasive, and a further 40 were reported as naturalized but not invasive. The majority of the species encountered were used for more than one economic purpose. Among those species named as invasive, 203 were used in agroforestry and 292 were amenity species.
There were examples of invasive species amongst both the angiosperms and gymnosperms. Fifty-eight plant families included at least one invasive species of forestry tree, and 34 families contained more than one invasive species. In decreasing order, most invasive forestry species were drawn from the families Leguminosae, Pinaceae, Myrtaceae, Rosaceae and Salicaceae. Twelve families contained no known invasive species of forestry trees.
Invasive species were reported in all seven geographic regions (Europe, Africa, Australasia, North America, South America, Pacific, Asia), with most invasive trees being reported from Africa (87), and fewest from Europe (12) and Asia (14). However, the majority of species were invasive in only one of the seven regions, and even the most `invasive' species did not show this trait in every country in which they had been introduced. Most invasive species had a native range that included Asia, and the fewest, one that included the Pacific.
Most sources reviewed by Murphy and Haysom (FAO 2003b) gave little information on the use of the trees, details of management (forestry, agroforestry, amenity, etc.) or of the history of their introduction, scale of planting or scale of invasion (land area). Geographically, it was notable that most invasive species were reported from regions in which extensive research on the number and impact of such species has been carried out, e.g. South Africa and North America. There were distinct gaps in knowledge of invasive species in Africa (apart from South Africa, Zimbabwe, Botswana and the oceanic islands), Asia and parts of South America.
In general, there are few studies of the positive and negative impacts of invasive forest trees. Positive impacts include the provision of fuel and other resources for resource-poor communities, and soil stabilization in over-exploited natural forest areas such as in India. Research in other countries on invasive woody legumes in pasturelands has shown that, under some management regimes, such species can complement pasture grasses for livestock feed.
Amongst the potential negative impacts are the risks of hybridization between introduced species, which can potentially produce new invasive species. However, only a few such instances have been reported in forestry, notably those of Leucaena and Prosopis hybrids, which are in any case only locally important. There are relatively few cases in which forest trees have invaded agricultural systems or forest plantations, although they have been reported as major problems in grassland pastures throughout the world; most reports of invasiveness relate to natural or semi-natural habitats, e.g. riparian and wetland systems. One of the few quantitative studies conducted on the impact of invasive trees in a natural habitat was undertaken in the fynbos biome in South Africa, where trees cause substantial losses in local biodiversity and prevent natural run-off of water from catchment zones. This affects South Africa's water supply, especially in the dry seasons (Van Wilgen et al. 1996, 1998).
While there is a growing national and international awareness of the possible risks of invasiveness of forestry trees, it is likely that some stakeholders in forestry remain ignorant of the risks, particularly since there is a general lack of quantitative information on the ecological and economic impacts of invasive forestry trees. It is therefore recommended that a number of case studies be conducted in collaboration with countries that have a high dependence on forestry. Such case studies should cover a range of forestry situations (e.g. commercial and environmental) and include the development and promotion of tools for making ecological and economic assessments. Particular attention should be paid to those regions of the world where there is little information on the invasiveness of exotic forestry trees (e.g. tropical and temperate Asia).
Globally, the development and implementation of prevention, control and management tools for invasive forestry trees has been limited because of the basic economic and developmental benefits of the trees concerned. Some countries have made extensive investments in exotic trees and are therefore reluctant to take action against those species that have become invasive. The general lack of relevant information and methodologies also prevents many countries from implementing risk assessments, control and management schemes.
Nevertheless, some attempts have been made to develop risk assessment and risk management models, although these schemes are generally aimed at exotic plant introductions in the broadest sense rather than at forestry trees per se. The development of risk assessment protocols for forest trees does pose some challenges - such as factoring in the time lags that can occur before a tree becomes invasive and the problem of unpredictable hybridization with other tree species. Despite these problems, however, practical risk assessments are now in use in Australia, New Zealand and the USA, and a few other countries/territories have schemes under development. The most common methods to date are based on assigning numerical scores to various biological attributes and to factors such as whether the plant is invasive elsewhere. There is an urgent need for such risk assessments to be further evaluated for use in forestry and promoted if found appropriate.
Risk assessment and management of alien plants (i.e. introduced plants not yet invasive) have also been considered in some cases. Many researchers in this field have called for monitoring schemes to be set up once a plant has been introduced, but for forestry trees this would entail planting trials that would have to be continued for many years. Practical guidelines on monitoring aimed at forestry programmes are not readily available and this is another area where further work is required.
Where preventative methods have failed, eradication programmes have sometimes been undertaken, e.g. against woody legumes, such as Prosopis, that have become invasive in pastoral systems. Eradication methods have also been used effectively in Mauritius and Florida to clear alien forest trees in areas of conservation value.
However, such programmes are costly and have not always been successful. Hence in some cases the focus is now on control rather than eradication. For example, an integrated approach using biological, mechanical and chemical methods is being used to control woody legumes in Australia, South-East Asia and South Africa. Biological control by seed feeding bruchids has been used in South Africa against several leguminous trees that have become invasive (e.g. Acacia mearnsii), and these programmes are now supported by new legislation restricting the planting of trees that have invasive tendencies. Other approaches being developed in South Africa include making plantation owners take responsibility for the impact of invasive tree species spreading from their plantations.
Alternative approaches to managing the risks associated with forest trees include trying to develop seedless clones of pines and near sterile hybrids of Leucaena species. All of these approaches could be used by other countries trying to resolve problems associated with invasive forestry trees.
