In order to evaluate the extent to which the phenomenon of “invasiveness” is shown by introduced forest tree species it was necessary to gather data on the global use of introduced species within the forestry industry. In an attempt to reduce potential biases (i.e. over- or underestimation of the proportion of forestry tree species classed as “invasive” as opposed to “noninvasive”), two approaches to data collation were adopted.
In the first approach, two major online databases, Ecocrop (FAO, 1999) and the ICRAF Agroforestree database (World Agroforestry Centre, 2002) were used to compile a comprehensive list of tree species used in world forestry. This list included species known to occur only within their native range as well as those known to have been introduced to other countries. The trees with an exotic distribution included species not known to behave invasively, as well as those reported to be naturalized or invasive in at least one location.
In the second approach, online invasive species databases were used to identify trees or shrub-trees that were categorized as “invasive” by one or more authors. Key electronic data sources included the web-based database on invasive woody plants prepared by Pierre Binggeli (Binggeli 1999) and web pages prepared by various state agencies, e.g. the Government of Western Australia Department of Agriculture (2002), California Exotic Pest Plant Council (CalEPPC) (2002), Florida Exotic Pest Plant Council (2001), Southeast Exotic Pest Plant Council (2002), the Institute of Pacific Islands Forestry (2002), USGS (2002) and the Invaders Database System (University of Montana 1997).
To reduce biases towards electronic material, or to particular regions, these data were supplemented by a traditional literature review, further details of which are given in Appendix 1. Appendix 2 details all those who contributed unpublished information and ideas.
For those species analysed in more detail and used in case studies (species from the genera Acacia and Prosopis), the dataset was expanded to include information on biological characteristics, environmental factors across species ranges, silvicultural characteristics, the social, economic and biological impacts of invasion, habitats invaded and factors reputed to drive invasiveness.
This review considers only naturalized or invasive alien species, i.e. it does not catalogue species that behave invasively within their native range. The wide variety of definitions used by different sources in connection with terms such as “invasive”, “naturalized”, “exotic”, etc. has already been acknowledged (Section 1.3). This variety of terms presents a problem for any study seeking to provide a broad summary of the global status of invasive species. The policy of this review was, wherever possible, to accept the terms used by authors and correspondents, because in most cases it would not have been possible to verify or redefine the terms they used without further information. Where standard terminology such as “naturalized”, “invasive”, etc. was not used in the original source, the term naturalized was applied to any species that was reported to occur in the wild, but was not reported as spreading. The term invasive was applied to any species that was mentioned in the context of unassisted spread or described as an alien species that was thought to be in need of a control strategy. Ranked terms of invasiveness used by various authors were ignored, although species listed as “possibly/potentially invasive” by Binggeli (1999) were included on the grounds that they were classified according to a local event (Binggeli 1996).
Countries included in the native range were taken directly from author and database sources (e.g. (International Legume Database & Information Service (ILDIS) 2002; World Agroforestry Centre 2002). For many species, native and exotic ranges were extracted from the Forestry Compendium Global Module (CAB International 2000). Range information from this source was interpreted cautiously because the term used (“natural forest”), while often referring to a species within its native range, was defined as late successional forest or “old-growth” forest. Such forests could also arise from introduction and naturalization processes and could potentially have been misclassified in the data summary. The contrasting term “planted forest” was also used in the context of species planted within their native range. The term “exotic range” comprised countries in which a species was planted but which were outside the “natural forest” region. Special care was taken with species that occurred in large land masses, for example North America, Australia, etc., where it was possible for a country to be listed both in a species’ native and exotic range. Wherever possible, more detailed information on occurrence was collected (e.g. from a state or governmental unit) to clarify a species’ native or exotic status (particularly in large countries).
Countries that were part of native, exotic, naturalized or invasive ranges were allocated to one of seven geographical regions (Europe, Africa, Australasia, North America, South America, Pacific or Asia) following the widely used International Working Group on Taxonomic Databases World Geographical Scheme for Recording Plant Distributions (TDWG (International Working Group on Taxonomic Databases) 1992). Brummitt (2001) gives full details of the countries included in these regions.
In brief, Europe was delimited by Portugal in the west and by Kamchatka in the east, by Svalbard in the north and by Crete in the south. Africa included mainland Africa and the Macronesian, Middle Atlantic Ocean and Western Indian Ocean islands. Asia extended from Turkey and the East Aegean Islands in the west to Papua New Guinea and the Solomon Islands in the east and from Mongolia in the north to Malaysia and the Cocos islands in the south. Australasia comprised Australia, New Zealand and their islands. The Pacific group extended from the Caroline Islands in the west to Easter Island in the east and from the Hawaiian islands in the north to Tonga and Niue in the south. The USA, Canada and Mexico were grouped as North America. The Caribbean islands and mainland American countries south of Mexico were classed as South America.