This paper reviews both the positive and negative effects of invasive exotic trees and woody shrubs from a global perspective. However, it should be noted that definitive reports on impacts are rare and on a global scale information is very patchy. This paper does not cover the basic benefits of forestry and agroforestry per se as these have already been reviewed by Mather (1993) and Evans (1982) for forestry trees and by Hughes and Styles (1989), Nair (1990) and Hughes (1994) for agroforestry.
In several of the situations discussed in Sections 4.1 and 4.2, a conflict of interest has arisen (when a tree becomes invasive) between the value that results from the introduction (including values that might arise out of invasiveness) and the perceived negative impacts; this is discussed further at the end of this chapter.
In agroforestry, there are several reports of positive effects of trees and woody shrubs that have become invasive. As noted previously, most species introduced for agroforestry have been selected for a number of traits, including their suitability for fodder and fuelwood. In many areas of the developing world, the very high demand for forest products has meant that local communities have exhausted their natural supplies. In some cases, though, the aggressive, spreading nature of introduced trees has resulted in reforestation, albeit with an introduced species. This in turn has provided rural communities with a “free” and continuous supply of forest products.
For example, Prosopis juliflora was introduced into India from Central America. It is now invasive in most of India where it grows on waste ground, along roadsides and in pastures. The rural poor in India now exploit these trees for vital supplies of fuel and pods (Muthana and Arora 1983; Pasiecznik et al. 2001). Some of the American Prosopis species also exhibit similar behaviour within their native range. Ranchers dislike these natural successional invasions of native Prosopis species in grasslands because they occur in dense stands and reduce the amount of available fodder. Comparable situations are likely to exist for other introduced agroforestry trees in other parts of the world, but have not been widely reported.
In a similar vein, many invasive alien trees are reported by some Forest Departments as providing important soil stabilization in areas where natural forests have been over-exploited (e.g. Karnataka Forest Department, India, personal communication, 2002). Both of these impacts need further study.
Reports of effects that can be classed as “negative” can be divided into the following categories:
• hybridization of tree species,
• economic and social impacts, and
• environmental impacts.
Hybridization can occur between different species introduced into the same area or between an introduced and a native species. There are few recorded instances of negative impacts of hybridization in trees. However, some examples do exist, as documented by Hughes (1998) in the case of Leucaena, a genus known to hybridize readily and of which several hybrids already exist. One hybrid, L. leucocephala × L. diversifolia, which occurs naturally in central America, Papua New Guinea and the Philippines, is self-fertile and has become weedy in at least one location (Veracruz, Mexico). Similar problems have arisen from hybrids of Prosopis species in South Africa (Pasiecznik et al. 2001). Given that an increasing number of species of Leucaena and other trees are being introduced, largely unchecked, in other parts of the world, the main concern is that other hybrids with weedy tendencies may also arise (see Section 4.3).
Hybridization coupled with backcrossing (introgression) can lead to contamination of natural gene pools. Again, few problems have been reported in the literature, but this situation is likely to occur in the case of the Leucaena complex in Mexico referred to above (see Hughes (1998)). Quercus, Eucalyptus, Erythrina and Acacia all have the ability to cross readily, but since it requires considerable effort to understand the genetic diversity of even a single species, studies of genetic diversity and the ability to form hybrids have been conducted for only a small group of taxa, including some species of Salix, Populus and Erythrina (C. Hughes, personal communication, 2002). It is therefore possible for invasive hybrids to be overlooked, particularly where invasions are discussed only at the generic level (as for Tamarix and Eucalyptus).
Economic, social and environmental impacts are considered together since some of the issues are interlinked. In many cases invasive forestry species invade natural or semi-natural ecosystems in developing countries and have an impact on the services that those ecosystems provide to local communities. However, despite the large number of invasive species that have been recorded, little economic data exists on the impact of these species and few studies have quantified environmental impacts.
Invasive trees and woody legumes can be conveniently classified according to the following types (Holzner, 1982; Hughes and Styles, 1989):
• ruderal – weeds of waste ground and disturbed habitats;
• agrestal – weeds of crops, orchards and gardens;
• pastoral – weeds of grasslands;
• silvicultural – weeds of forestry plantations;
• floral/environmental – weeds that compete with native flora (or are harmful to specific ecosystems);
• aquatic – weeds of wetlands and water bodies;
• health related – weeds that harbour pests and diseases or act as alternative hosts for such species.
As seen in Section 3, there are examples of invasive trees and woody shrubs in each of the first six categories; whilst the final category is an important one it is not considered in this report. Following the approach of Hughes and Styles (1989) the impacts of invasive trees and woody shrubs are considered under each of these categories.
Ruderal weeds. Globally, the highest proportion of invasive tree and woody shrub species is found in wasteland and disturbed habitats. As discussed above, it is likely that in these situations the species concerned are providing benefits such as soil stabilization or fuelwood for local communities.
Agrestal weeds. In general there are few reports of trees and woody legumes in these situations, since such systems tend to be intensively managed. Nevertheless, the woody legume Mimosa pigra has been reported as an important weed in rice fields and sugar plantations in Asia, and Leucaena leucocephala as a weed of cocoa in Hawaii, where it is planted as a shade tree (see Hughes and Styles 1989). Impacts in terms of yield loss and increased labour costs are not available. Other examples probably exist but are unlikely to be significant.
Pastoral weeds. World wide, there are many records of the invasion of grassland pastures by exotic woody legumes, especially species such as Acacia farnesiana, Mimosa pigra, Prosopis glandulosa and P. velutina. These legumes reduce the amount of grass fodder available for grazing and also create dense thickets through which livestock cannot move. The principal countries affected include Australia, South Africa and Zimbabwe. Large-scale eradication and control programmes have been undertaken in most of the affected countries, but have been costly and generally ineffective (see Section 4.3).
Silvicultural weeds. Many of the trees used in commercial forestry are known to be highly invasive, especially species of Acacia and Pinus (Richardson, 1998). In Sweden, the lodgepole pine (Pinus contorta var. latifolia) has been reported as invasive, regenerating spontaneously even within plantations. This phenomenon is more common in older stands. In Scotland, this species also affects forest plantations of other species (Andersson et al. 1999). The costs involved in managing these self-sown trees within plantations are not readily available. In the more tropical parts of Australia, Acacia species commonly invade Pinus radiata plantations.
Floral/environmental and aquatic weeds. Globally, most reports of invasions of trees and woody shrubs relate to semi-natural or natural habitats. These reports include species from a wide range of genera and in general, most of the affected habitats are either semi-natural or natural with some degree of disturbance, e.g. the invasion of shrublands in Hawaii and California by Eucalyptus globulus and of semi-natural pine–oak woodlands in South and Central Europe by Robinia pseudoacacia.
In most cases, reported impacts are qualitative assessments, including, for example, the replacement of native flora and the disturbance of feeding and breeding grounds for invertebrates and vertebrates. Interestingly, the likely impact of invasive forestry trees on nutrient cycles and soil biodiversity has received very little attention, although in New Zealand, Pinus radiata has been shown to alter the hydrology and nutrient levels of plantation areas (Richardson et al. 1994). Another study in northern India has shown that Lantana camara (which is not used for forestry) can significantly reduce soil nitrogen and phosphorous levels in oak–pine woodlands (Bhatt et al. 1994).
Some studies have been made of invasions of pristine habitats. Most cases relate to invasions of open forest, grasslands and fir-dominated ecosystems, but other cases have been reported. For example, the Australian pine (Casuarina equisetifolia) has invaded the Florida everglades and shorelines, where it is reported to negatively affect crocodile and turtle nesting grounds (Randall et al. 1997); Mimosa pigra is also a major invasive species in the wetland areas of Thailand and Australia (Lonsdale et al. 1995).
Some of the best examples come from South Africa, where several pines and woody legumes (particularly Acacias) have invaded the fynbos biome in the south and the veld biome in the Transvaal (Richardson 1998). A number of economic and ecological studies (e.g. Geldenhuys 1986) have been conducted on the benefits of the trees versus their associated risks. For example, in the fynbos biome, the run-off from watersheds cloaked in invasive trees (mostly pines and Hakea sericea) is 30–70 percent lower than in pristine areas. Since fynbos catchments supply about two-thirds of the region’s water needs, and large areas of these catchments have been invaded (Higgins et al. 1997; Richardson 1998), such invasions are of major concern. Furthermore, Richardson et al. (1989) reported that the species richness of fynbos ecosystems could be reduced by almost half when invaded by alien trees and shrubs.
Over the past 10 years or so, there has been a growing national and international awareness of the possible risks of invasive forestry trees (see Hughes and Styles 1989; Richardson 1998). However, as mentioned above, apart from the basic benefits of forestry and agroforestry, some communities have derived additional benefits from invasive trees (e.g. an example from North Africa showing that some degree of invasion of grasslands by woody legumes can be beneficial for livestock grazing, particularly in time of drought (Hughes and Styles 1989)). In a number of cases, therefore, conflicts have arisen over the positive and the negative impacts of invasive tree species. Such conflicts include, for example, those between economic development and conservation, which have become increasingly common as conservation agencies have become more aware of the risks to biodiversity that can result from biological invasions.
The available literature includes few case studies with quantitative ecological and economic data that would help to resolve such conflicts. This lack of information has hindered the ability of national governments to understand the issues and to develop policies on further introductions. In many countries, an additional compounding factor is likely to be a lack of information and training in the use of environmental impact assessment. This has been identified as an important constraint in relation to the development and implementation of management schemes for existing invasive species (see Section 6).
In summary, more case studies are required in representative countries to examine the benefits and invasion risks of introduced forestry trees. This would increase awareness of the issues and help national governments to make balanced assessments of proposed tree introductions and expansions of plantations.