This discussion paper presents a global review of the status of the invasiveness of forest tree species outside their natural habitat. The review covers trees and woody shrubs commonly used in commercial forestry and agroforestry. Information for the review has been collated from published databases and papers, unpublished reports and personal communications.
A number of definitions of “invasive species” have been suggested in the literature, some specifically for invasive plants or woody plants. It is recommended here that, in the context of forestry, a common definition be developed that focuses on parameters of population expansion only, because in some common definitions of invasive species impacts are preconceived to be negative.
On a global basis, available information on the status of forestry trees that have become invasive is patchy. The terminology used by authors is also very variable and there is frequent overlap in the terms “invasive” and “naturalized”. The evaluation of the extent of invasions by forest trees is most often very qualitative and subjective which makes overall assessments of the magnitude of the problem difficult.
With these caveats in mind, the following summarizes the global situation:
The number of species of trees or woody shrubs that were classed as invasive, including some listed by Binggeli (1996) as possible or potential invaders (based on literature where the extent of invasion was limited or unknown), was 443. A further 74 were reported as naturalized. 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 in the review were used for more than one purpose (i.e. the same species may be used in forestry, agroforestry and/or for amenity plantings). Hence, among those species identified as invasive, 203 species were listed as being used in agroforestry and 292 in amenity plantings.
Fifty-eight families included at least one invasive forest tree species, and 34 families contained more than one invasive species. The latter represents half the total number of families with members used for forestry that are known to be grown outside their native range. Thirteen families contained no invasive forestry tree species. There were examples of both angiosperm and gymnosperm invasive species. In decreasing order, the majority of invasive forestry trees occurred in the families Leguminosae, Pinaceae, Myrtaceae, Rosaceae and Salicaceae.
Invasive species were reported in all seven geographic regions (Europe, Africa, Australasia, North America, South America, Pacific, Asia). Most invasive trees were reported from Africa (87 species) and fewest from Europe (12) and Asia (14). However, the majority of species were invasive in only one of the seven regions, and even among the most “invasive” species the number of countries where they were reported as invasive was generally much smaller than the number of countries where they had been introduced. Most invasive species had a native range that included Asia, and the fewest a range that included the Pacific.
During the examination of these data it was notable that most accounts gave little information on the use of the trees, details of their management (forestry, agroforestry, amenity, etc.), the history of their introduction or on the scale of planting and of invasion (land area). This lack of information limited discussion to the number of invasive species encountered in different situations and restricted interpretation. Geographically, it was notable that most invasive species were reported from countries and regions where investment in cataloguing or researching the number and impact of species had been high, e.g. South Africa, Puerto Rico and North America. There were distinct gaps in knowledge of the occurrence of invasive species in some other areas, namely Africa (outside South Africa, Zimbabwe, Botswana and the oceanic islands), Asia and parts of South America. It is therefore highly likely that the list of species known to behave invasively would expand, given further work in such locations.
Few studies have been conducted on the positive and negative aspects of the impacts of invasive forest trees. Positive impacts include providing fuel and other resources for resource-poor communities. The trees may also contribute to soil stabilization in over-exploited natural forest areas such as in India. Although hybridization between introduced species can potentially produce new invasive species, only a few instances have been reported in forestry: that of Leucaena and Prosopis hybrids and these are only locally important. Invasive forest trees have been reported as major problems in grassland pastures throughout the world, but there are fewer instances where trees invade other agricultural systems or forest plantations. Most reports of invasiveness relate to natural or semi-natural habitats, e.g. riparian and wetland systems. One of the most quantitative studies conducted on the impact of invasive trees in a natural habitat was undertaken in the fynbos biome in South Africa. Here trees cause substantial losses in local biodiversity and prevent natural run-off of water from catchment zones. This is reported to affect South Africa’s water supply, especially in the dry seasons.
Over time there has been a growing national and international awareness of the possible risks of invasiveness of forestry trees, but it is likely that some stakeholders in plantation forestry remain ignorant of the risks. Awareness has been highest in environmental sectors but some of the risks have been highlighted by those in agricultural sectors. This has led to conflicts of interest in some parts of the world that are partly fuelled by the general lack of quantitative information on the ecological and economic impacts of invasive forestry trees. A compounding factor is the lack of information and tools (methodologies, etc.) to make such assessments. In the light of the above, it is recommended that a number of case studies be conducted in collaboration with countries that have a high dependence on planted forests. Such case studies should cover a range of plantation forestry situations (commercial, development, environmental) and include the development and promotion of tools for ecological and economic assessments. Particular attention should be paid to those regions of the world where little information exists concerning the invasiveness of exotic forestry trees (e.g. tropical and temperate Asia).
Many biological attributes (e.g. life history, taxonomic status, genetic constitution) are poor indicators of invasiveness if considered alone. However, some of these characteristics (in combination with other factors such as the extent of invasiveness expressed) are being used in risk assessments (see below). For those species that have already been recorded as highly invasive in at least one area, it would be of more practical significance to examine whether or not some form of control or management by local communities contributes to noninvasiveness in other areas.
Globally, the development and implementation of prevention, control and management tools for invasive forestry trees has been cautious and patchy because of the basic economic and developmental benefits of the trees concerned. Some countries, sometimes in collaboration with international partners, have made large investments in exotic trees and are therefore reluctant, given the general lack of quantitative information on the negative impacts, to take action against those species that have become invasive. An additional constraint that prevents many countries from implementing risk assessments, control and management schemes, is the general lack of both the necessary tools and relevant information concerning their use.
Work on prevention has included the development of risk assessment and risk management models. There are a number of discussions and suggested schemes in the scientific literature, some of which cover environmental aspects. All of these schemes are aimed at exotic plant introductions in general rather than forestry trees per se. Overall, the development of risk assessments poses some problems – such as factoring in the time lags that can occur before a tree becomes invasive as well as the problem of unpredictable hybridization with other tree species.
Practical risk assessments, based on information such as whether the plant is invasive elsewhere and various biological attributes, are now in use in Australia, New Zealand and the USA. A few other countries/territories have schemes under development. The most common assessment method to date is based on assigning numerical scores to various attributes. There is an urgent need for such risk assessments to be further evaluated for use in forestry and for them to be more widely promoted if they are considered appropriate.
Some research groups are developing alternative approaches to risk management, including the production of seedless clones of pines and near sterile hybrids of Leucaena species; however, propagation methods still need to be developed for the latter.
Risk assessment and management of alien plants (i.e. introduced plants that are not yet invasive) has also been considered in some cases. New Zealand, for example, already has a qualitative system in place. Many researchers in this field have called for monitoring schemes to be set up once a plant has been introduced. For forestry trees this would entail planting trials and would have to be continued for many years. Practical guidelines on monitoring aimed at forestry programmes are not readily available and this is an area where further work is required.
In some countries large-scale eradication programmes employing mechanical and chemical methods have been undertaken against woody legumes, such as Prosopis, that are invasive in pastoral systems. However, these methods are costly and it has generally not been possible to eradicate the trees concerned. In some cases, therefore, methods are now focused on control and use rather than eradication. Nevertheless, eradication methods (with sponsorship from the government) are being used effectively in South Africa to clear water catchment zones. Eradication methods have also been successful in clearing alien forest trees in areas of conservation value in Mauritius and Florida. Biological control and integrated control (biological with mechanical and chemical methods) have been used for the control of woody legumes in Australia, Southeast Asia and South Africa. These programmes are still largely ongoing and experience suggests that a complex of complementary natural enemies will be required to effect full control.
Some efforts are now being made to resolve conflicts of interest by developing management schemes rather than control programmes for invasive trees. In South Africa, on the basis of economic models, seed-feeding bruchids have been introduced to control the seed output of several leguminous trees that have become invasive (e.g. Acacia mearnsii) and these efforts are reported as being successful. These programmes are now supported by new legislation that restricts the planting of trees that have invasive tendencies. At more of a research level, work on pasturelands in other countries has shown that, under some management regimes, invasive woody legumes can complement pasture grasses for livestock feed. All these experiences can serve as models for other countries trying to resolve problems associated with invasive forestry trees.