Over the last century, plantation forestry using exotic trees has developed as an integral and crucial part of many national economies and environmental programmes. Planted trees and woody shrubs have also proved vital in improving the livelihoods of many of the world’s poor. As a result, countries, international organizations, programmes and industries have been exchanging forest reproductive material on an ever-increasing scale. Given the current global agendas on trade, combating desertification and climate change, this trend is likely to continue.
In contrast, a separate global agenda is now focused on invasive alien species (IAS), which are currently considered second only to habitat destruction in terms of loss of biodiversity (Sandlund et al. 1999). This concern is reflected in Article 8(h) of the Convention on Biological Diversity (CBD), which calls on its members to “prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species”. Likewise, several international and national bodies such as the IUCN Species Survival Commission are focusing more attention on the issue of invasive alien species. Although large numbers of tree species have been introduced from one region to another in the past, most of these species do not naturalize and, of those that do, not all become invasive. However, there are now several well-documented studies that show the hazards that can result from an introduced tree or woody shrub becoming invasive (see Section 4). Some of these introductions are the result of horticultural activities but some are the result of plantation forestry and agroforestry programmes. This report is concerned only with the latter.
Various studies and reviews have been published regarding the “weediness” of introduced trees and woody shrubs, but little attempt has been made to evaluate information on the global status of these species and their impacts. This report attempts to fill this gap.
The aims of this study were to:
• conduct a global review of the status of invasiveness of introduced trees and woody shrubs;
• identify the possible link between planted forest development and the occurrence/risk of invasiveness;
• review the positive and negative impacts associated with invasiveness of the tree species;
• identify the planning, monitoring and management options in a forestry or agroforestry context; and
• highlight priority areas (geographic, thematic or species specific) for future in-depth studies.
In the remainder of this section the global status of forestry is considered, as well as some definitions of invasiveness in the context of trees and woody shrubs.
Although there were introductions of trees such as teak (Tectona grandis), Eucalyptus and Acacia outside their natural range throughout the eighteenth and nineteenth centuries (Evans 1982), the planting of exotic trees for forestry and agroforestry in reforestation and afforestation programmes did not become a large-scale global activity until the second half of the twentieth century. At first, the extensive scale-up of use of these species was for industrial forestry, trees being grown for sawn timber and pulp. Plantations were first expanded in the Southern Hemisphere, for example in Chile and Australasia, but other regions (such as East Africa and northern South America) soon followed. The focus of these plantations has always been on the use of a small number of fast-growing species, mostly in the genera Acacia, Eucalyptus, Gmelina, Pinus, Populus and Tectona. Evans (1982) estimates that three genera, Eucalyptus, Pinus and Tectona account for 85 percent of all plantations in the tropics.
In the 1970s and 1980s tropical and subtropical regions experienced increasing shortages of fuelwood and other timber in rural communities, coupled with increasing environmental degradation, soil erosion and desertification. As part of national and international efforts to address these issues, exotic woody shrubs and, in particular, woody legumes were promoted for use in agricultural systems and to revegetate degraded lands (Hughes and Styles 1989). The woody legumes were particularly favoured because of their ability to grow rapidly under harsh conditions. As with industrial plantations, there has been heavy reliance on a small number of species, especially of Acacia and Leucaena.
With large areas of degraded land in tropical and subtropical regions, planting targets have always been ambitious. The availability of seed, together with basic information on how to collect, handle and grow different species in both nursery and plantation situations, have been major factors in determining the species chosen for planting programmes. As a result of the importance of seed availability, those species producing seed which can be stored for long periods have often been preferred.
There are currently no widely agreed published definitions that cover the concept of an “invasive tree” or “invasive woody shrub”. This is partly because the term “invasive” is relatively new and its current wide usage to describe pest and disease problems, particularly those that affect the environment, seems to stem from the use of the term “alien invasive species” by CBD. The CBD defines this as “an alien species which threatens ecosystems, habitats or species” (Article 2). However, IUFRO’s Silva Voc are considering some working definitions which will be made available later this year. To date, the term “woody weed” is perhaps the most commonly used term in the scientific and pest management literature that can be equated with “invasive tree/woody shrub” (CABI Publishing Division, personal communication, 2002) but as yet this term has no formal definition. Unfortunately, published information on invasive trees/woody shrubs is very subjective and thus difficult to compare with other studies (see later).
However, a few authors who have published reviews or databases on “invasive trees/woody shrubs” have proposed some definitions, including the following:
1. Invasive woody plant – “[characterized by] the establishment of self-generating, usually expanding, populations of an introduced species in a free-living state in nature” (Binggeli 1996).
2. Invasive plant – “an alien plant spreading naturally (without the direct assistance of people) in natural or semi-natural habitats, to produce a significant change in terms of composition, structure or ecosystem processes” (Cronk and Fuller 1995).
3. Invasive plants – “naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from parent plants (approximate scales being more than 100m in less than 50 years for taxa spreading by seeds and other propagules; more than 6m in 3 years for taxa spreading by roots, rhizomes, stolons or creeping stems) and thus have the potential to spread over considerable areas” (Richardson et al. 2000)
Definition 2 differs in two main ways from the others: the first is the inclusion of the word “alien” and the second is the inclusion of the expected impact of the invasive plant. These impacts could be considered detrimental or beneficial depending on who “uses” the ecosystem.
For comparison, two more general definitions of invasive species have been suggested:
4. Invasive species – “an alien species that becomes established in natural or semi-natural ecosystems or habitat, is an agent of change and threatens native biological diversity” (Shine et al. 2000).
5. Invasive alien species – “species introduced deliberately or unintentionally outside their natural habitats, where they have the ability to establish themselves, invade, out-compete natives and take over the new environment” (CBD 2001).
In these definitions, alien invasive species are considered to be a subset of alien species as a whole. The emphasis is on the threat to native biodiversity and hence the overall impact of invasive species is considered to be negative. For the purposes of those working in forestry or agroforestry, the first three definitions are of more practical use because the impact of an invasive species is not preconceived. The second definition is of particular relevance for this report because of the inclusion of the term “alien”. These definitions will be discussed further in the last section of the report.
There are a few other terms that are commonly used in the literature on invasion biology. Again, these have been open to interpretation by authors but, nonetheless, useful working definitions do exist. Some of these terms, with their definitions, are as follows:
• Introduction – “the movement, by human agency, of a species, subspecies or lower taxon (including any part, gametes or propagule that might survive and subsequently reproduce) outside its natural range (past or present). This movement can be either within a country or between countries” (Shine et al. 2000).
• Alien species (= nonnative, nonindigenous, foreign, exotic) – “a species, subspecies, or lower taxon occurring outside of its natural range (past or present) and dispersal potential (i.e. outside the range it occupies naturally or could not occupy without direct or indirect introduction or care by humans), including any part, gamete or propagule of such a species that might survive and subsequently reproduce” (Shine et al. 2000).
• Naturalized – “alien species that have become completely established ferally and are breeding and holding their own in competition with the native flora and fauna” (Fitter and Fitter 1967).
This last definition is taken from British literature: more global definitions are not available.