Data were compiled for 1121 tree species known to occur outside of, or to be restricted to, their native range. Among species introduced outside their native range, the dataset incorporated both species reported to be naturalized (or naturalized and invasive) in at least one country and those not known to be naturalized or invasive. Figure 1 summarizes the contribution of the different groups in the dataset.
Definitions: Native range only: species occurring only within their native range or not known to have been introduced into other countries. Exotic, not naturalized/invasive: species known to occur outside their native range but not reported to be naturalized or invasive. Exotic, naturalized only: species known to occur outside their native range, known to have naturalized in at least one location but not reported to be invasive. Exotic, naturalized and invasive: species known to occur outside their native range and to be invasive in at least one location.
The 1121 species in the dataset were drawn from 114 families and 451 genera. There were examples of both angiosperms (1026 species) and gymnosperms (95 species) and, among the angiosperms, both monocotyledons (31 species) and dicotyledons (995 species). Data sources ascribed several different growth forms (shrub/small tree/large tree) to the species in the dataset, with many species, e.g. Acacia karroo (CAB International 2000) exhibiting varied growth forms across their range, or according to their provenance.
Of the 1096 species that could be ascribed to a use category, 882 species (80 percent of the dataset) were used in forestry, 445 (41 percent) in agroforestry and 512 (47 percent) were amenity species. Almost half the species (522) were allocated to more than one category of use and 221 general-purpose species (20 percent) were allocated to all three classes. Figure 2 illustrates the distribution of species among the different usage categories.
Numbers in the diagram refer to the number of species ascribed to each category. The uses of 25 species could not be determined.
Sixty-one percent of species (680) occurred outside their native geographical range, while the remaining 441 species were either not believed to occur outside their native area, or insufficient information was available to allow them to be categorized.
Overall, 517 species (76 percent of those trees known to occur outside their native range) were classed as naturalized or invasive in at least one country or region. The number of species reported to be invasive was 443 (65 percent of those known to occur outside their native range). Table 1 summarizes data on the invasiveness of species according to the various economic use categories.
Data are expressed both as a species count and as a percentage of the total number of species in each category known to occur outside their native range.
Forestry |
Agroforestry |
Amenity |
All purpose |
Unclassified | |
Not reported invasive or naturalized |
136 (30%) |
104 (29%) |
98 (22%) |
57 (29%) |
3 (13%) |
Naturalized and invasive |
282 (61%) |
203 (58%) |
292 (67%) |
114 (58%) |
15 (65%) |
Naturalized only |
40 (9%) |
46 (13%) |
49 (11%) |
25 (13%) |
5 (22%) |
Total |
458 |
353 |
439 |
196 |
23 |
Further analyses excluded agroforestry or amenity trees not known to be used in forestry, and all species not known to be planted outside their native range. The remaining data (forestry species known to occur outside their native range) comprised 458 species and 215 genera drawn from 71 families. The distribution of species among families was highly skewed, with the top 10 families making up 64 percent of the dataset (295 species). In descending order of species number these families were the Leguminosae, Pinaceae, Myrtaceae, Rosaceae, Cupressaceae, Meliaceae, Salicaceae, Palmae, Fagaceae and Moraceae. The family with most species, the Leguminosae, comprised four tribes and 133 species – 29 percent of the forestry tree list. Twenty-seven families were represented by a single species each.
Fifty-eight families (82 percent of forestry families found to occur outside their native range) included at least one invasive species of forest tree. Thirty-four families contained more than one invasive species. More species in the genus Acacia (Leguminosae-Mimosoidae) were reported as invasive or weedy than in any other group (a total of 26 invasive species). In this family, however, there was also a similar number of species that were not reported as being invasive. For the majority of the 10 largest families represented in the dataset, the number of species termed invasive was proportionally higher than the number that were not reported invasive (see Table 2). Thirteen families (18 percent of the forest families introduced outside their native range) were not found to contain invasive forestry species.
There was no difference in the ratio of angiosperms to gymnosperms in the invasive/noninvasive classes. Approximately 87 percent of each category were angiosperm species and 13 percent were gymnosperm species.
Family |
Not inv. /nat. |
Not inv./ nat. % |
Nat. only |
Nat. only % |
Nat. & inv. |
Nat. & inv. % |
Total species |
Leguminosae |
56 |
42% |
19 |
14% |
58 |
44% |
133 |
Pinaceae |
9 |
22% |
0 |
0% |
32 |
78% |
41 |
Myrtaceae |
4 |
15% |
1 |
4% |
21 |
81% |
26 |
Rosaceae |
0 |
0% |
1 |
5% |
19 |
95% |
20 |
Cupressaceae |
7 |
44% |
0 |
0% |
9 |
56% |
16 |
Meliaceae |
4 |
31% |
2 |
15% |
7 |
54% |
13 |
Salicaceae |
2 |
17% |
0 |
0% |
10 |
83% |
12 |
Palmae |
4 |
33% |
1 |
8% |
7 |
58% |
12 |
Fagaceae |
3 |
27% |
0 |
0% |
8 |
73% |
11 |
Moraceae |
1 |
9% |
1 |
9% |
9 |
82% |
11 |
Not inv./nat: Not invasive or naturalized
Nat. only: Naturalized only
Nat. & inv.: Naturalized and invasive
The location of introduction was identified, in full or in part, for 388 of the 458 forestry tree species known to occur outside their native range (85 percent). Figure 3 shows the number of forestry species that were recorded as introduced, intentionally or by accident, into each of seven geographic regions (Europe, Africa, Australasia, North America, South America, Pacific and Asia). Introductions of forestry species were recorded for all regions, but the largest number of reports was for introductions to Africa (219 species). The dataset contained fewest records of forestry species introduced into Europe (95 species) and the Pacific (97 species). A large proportion of the species encountered had been introduced to many countries, across several geographic regions. For example, among the genus Acacia, 18 of the 46 species in the database for which the destinations of introductions were known, had been introduced into 20 or more countries. Acacia holosericea, A. auriculformis and A. farnesiana were reported to be exotic in 69, 58 and 57 countries, respectively. In contrast, approximately one-third of the Acacia species were listed as occurring in less than five countries outside their native range. On average, Acacia species were introduced into 19 countries. Seven tree species used in forestry (Artocarpus altilis, Albizia lebbeck, Acacia decurrens, Acacia melanoxylon, Salix babylonica, Acacia farnesiana and Acacia holosericea) had been introduced into countries outside their native range in all seven geographic regions.
In the majority of cases, there was a lack of information on the mode of introduction (deliberate or accidental) and the country of origin of the introduced material. There were few reports of tree species (from any of the economic use categories) having been introduced accidentally, and several reports expressed uncertainty concerning the nature of the introduction event. Among those introductions thought to be accidental, Ghate (1991) described the introduction to India of Senna occidentalis, an agroforestry/amenity species, “along with foreign goods in a very early period” before the eighteenth century, and the more recent introduction of a forestry species Senna uniflora in 1980. In the absence of conflicting information, and taking account of the economic value of many of the species concerned, it was presumed that the vast majority of introductions of forestry species were deliberate events.
Of the 322 forestry tree species in the dataset reported to be exotic and naturalized or invasive in at least one country, the locations of naturalization/invasion were identified, at least in part, for 246 species (76 percent). Figure 4 summarizes the number of species recorded as invasive (and naturalized), or naturalized (but not invasive) in each of the seven geographic regions. Appendix 3 lists in full the species reported as naturalized and/or invasive in each geographic region.
The region with the highest reported number of invasive forestry tree species was Africa, with 87 invasive species and a further 11 naturalized species. The lowest number of invasions was reported in Europe (12 forestry trees reported as invasive and a further 12 reported as naturalized). The large number of forestry species reported as naturalized or invasive in South America (114) was dominated by reports of naturalized or invasive species in Puerto Rico, where 23 forestry trees were known to be invasive and a further 59 were naturalized. A single source (Francis and Liogier 1991) contributed the majority of Puerto Rican data. The 14 species reported invasive in Asia (with a further 15 species reported naturalized) could be divided into those occurring in temperate or tropical regions. Two species (Casuarina equisetifolia and Acacia longifolia) were reported to be naturalized and invasive in temperate Asia. Twelve invasive and thirteen naturalized species were reported in tropical Asia. None of the species was reported to be invasive in both temperate and tropical parts of Asia, although Ziziphus mauritiana was reported to be naturalized in both climatic zones.
Region codes: Eu, Europe; Pc, Pacific; Aus, Australasia; N Am, North America; As, Asia; S Am, South America; Af, Africa.
A general observation was that, for many of the species recorded as “invasive”, the number of countries in which they had become invasive or naturalized was relatively small compared to the number of countries in which they had been introduced (Figure 5). For example, although Prosopis occurred exotically in over 70 countries (Prosopis juliflora alone was recorded as an exotic in 71 countries), and Acacia in over 60 countries (Acacia holosericea in 69 countries), the highest number of countries in which an individual species was reported as invasive was seven (A. melanoxylon and A. nilotica). A scatter plot showing the number of countries in which Acacia and Prosopis were introduced, and the number of countries in which they were reported as invasive, is shown in Figure 6.
The majority of forestry species for which the location of invasion/naturalization events were known were invasive in only one geographic region (Figure 7). The highest number of regions in which a forestry species was recorded as naturalized or invasive was six. Eleven forestry species were naturalized or invasive in five regions. This mirrored the pattern observed in the complete dataset from all categories of tree use (Figure 8). Across the whole dataset (all categories of tree use) the species with the widest geographical pattern of naturalization and invasion were Robinia pseudoacacia (six regions), and in five regions Azadirachta indica, Casuarina equisetifolia, Pithecellobium dulce, Prosopis juliflora, Psidium guajava, Ailanthus altissima, Acacia dealbata, Acacia melanoxylon, Acacia mangium, Leucaena leucocephala, Schinus terebinthifolius, Broussonetia papyrifera and Lantana camara.
Figure 9 shows the native regions of forestry species that were reported to be naturalized or invasive in at least one country. Those forest trees with a native range that included the Pacific produced the fewest records of naturalized or invasive species (11 species), while Asia produced a total of 79 invasive species, including 55 from temperate areas and 47 from tropical areas.
Data summarize the results of 246 forestry species for which the locations of naturalization or invasion events were known. None of the species was recorded as exotic and invasive or naturalized in all seven geographic regions.
Data summarize the results of 380 forestry, agroforestry, amenity and unclassified trees. None of the species was recorded as exotic and invasive or naturalized in all seven geographic regions.
The number of naturalized or invasive species originating in each of seven regions is shown. Note that the native range of several species included more than one geographic region.
Information on date of introduction was lacking for many species. In the case of forestry species, information on date of introduction was obtained for 51 (approximately 13 percent) of the 388 tree species where location of introduction was known. Information for species in other usage categories was similarly poor: of 158 tree species used for agroforestry, amenity or unclassified purposes (for which the location of introduction was known), the date of introduction was obtained for only 16 (10 percent). Almost universally, dates of introduction were found for only a fraction of the countries into which a species had been introduced, i.e. for any one species, information on introduction date was readily available for generally no more than 17 percent of the countries in which it had been introduced. Furthermore, where dates of introduction were known, it was usually impossible to separate small-scale or individual introductions from large-scale planting programmes. Hence, because of the lack of consistent, wide-scale, reliable data sources, no attempt was made to investigate the relationship between time since introduction and the occurrence of an invasion event.
Species level information on scale of planting was scarce, as were global data on plantation statistics that could be used to evaluate any potential relationship between scale of planting and scale of invasion. Country plantation statistics in the comprehensive Global Forest Resources Assessment 2000 (FAO 2001) were summarized by total plantation area and by the plantation area of eight major categories: Acacia, Eucalyptus, Hevea, Tectona, “other broadleaved”, Pinus, “other coniferous” and “unspecified”. Even at this broad level, information on the distribution of plantations among the eight categories was not available for every country. For some countries, more detailed information on genera used in plantation forestry was given in the country profiles at the FAO website (http://www.fao.org/forestry/fo/country/nav_world.jsp); however, for many countries, this more detailed information was unavailable. The lack of data at species level prevented further analysis because no distinction could be made between plantations comprising native or exotic species. Similarly, no distinction could be made between plantations of forest trees known to behave invasively and those comprising species not known to spread beyond the plantation.
An attempt was made to evaluate data on invasiveness in relation to biological characteristics such as “pioneer” (i.e. a plant occurring early in a vegetation succession), versus “climax” (i.e. a plant occurring in the latest stage of succession). Although these terms are useful and widely applied in the description of specific succession processes, it was considered that the tendency to behave as an “r- or K-strategist” (MacArthur and Wilson 1967; Pianka 1970) falls along a gradient. Typical r-strategist plants have a high rate of increase in uncrowded or otherwise favourable environments (i.e. the early stages of succession), manifested by rapid growth and the production of large quantities of easily dispersed seeds, rapid germination, etc. (Allaby 1994). Typical K-strategist plants live in a constant or predictable environment and experience density-dependent regulation. K-strategists often exhibit a smaller and delayed reproductive effort (Begon and Mortimer 1986). For many tree species represented in the dataset there was insufficient information to categorize them as “pioneer” or “climax” species on anything other than a highly subjective basis. It was also considered likely that species that occurred naturally in a range of ecosystems might occupy different stages in the successional process in different habitats, and that this could pose further problems in allocating them to a particular category. In this report, therefore, there has been no attempt to present data on the invasive behaviour of forestry trees in relation to their successional status in their native range.
Attempts were made to collate data on other biological characteristics (e.g. deciduous or evergreen, and breeding system (monoecious, dioecious, polygamodioecious, etc.)). Key reference sources such as Mabberley (1993) and the Forestry Compendium Global Module (CAB International 2000), did not, however, provide data for every species. The breeding strategies of some species varied in different regions, perhaps according to provenance or subspecies. The available data were considered to be too inconsistent to evaluate invasive or naturalization events in relation to biological characteristics, although analyses of this type conducted by other authors on smaller datasets were taken into account.
The aim of incorporating material from both standard lists of forest species (i.e. Ecocrop (FAO 1999) and Agroforestree (World Agroforestry Centre, 2002)), and sources of information on invasives (e.g. Binggeli 1999), was to present a more balanced summary of the status of invasiveness of forest trees, by ensuring that both invasive and noninvasive species were considered. The data should be interpreted cautiously, however, because both types of sources have various gaps and biases. For example, the number of species regarded as noninvasive may have been underestimated because for many of the species on the Ecocrop forest or wood list, no further data were obtained on whether introduction outside their native range had occurred. In this particular case, a negative response to the question of whether a species occurs outside its native range should be understood to mean either that the species definitely does not occur outside its native range, or is not known to occur outside that range. Similarly, because species listed as “alien and invasive” may automatically be classed as occurring outside their native range, a bias could have been introduced whereby the number of species occurring outside their native range was skewed by information from species regarded as invasive. It should be emphasized that all references to percentages and proportions of species refer to the proportion of species encountered in the review and not to a finite and comprehensive world list.
In many cases, the process of collating information on both forestry and invasiveness characteristics was made more difficult by the fact that information derived from forestry sources (on introduction, silvicultural characteristics, etc.) did not generally provide much information on invasive tendencies. On the other hand, references to a species behaving invasively tended to be set within a very local context, frequently without information on the mode, source or history of introduction, or on the distance from a site of introduction or the extent of forestry planting.
The broad approach taken to incorporating information from different types of sources (published papers, electronic databases and personal communications) allowed new information to be included, particularly from regions where there were few written accounts. However, such an approach is vulnerable to the incorporation of errors, such as misidentification of a species. Several correspondents sent incorrect species identifications, which were later corrected. A review of this type relies heavily on the accuracy of original sources, because it is difficult to verify observer or even published records.
The “species based” approach was chosen to allow evaluation of the extent to which individual species were invasive in relation to their exotic range. Sometimes authors or correspondents reported invasiveness at genus level, e.g. Eucalyptus in Brazil (S. Ziller, personal communication, 2002). To avoid conferring invasive status on whole genera, such reports were omitted from the main summary, possibly leading to an underestimate of the occurrence of invasiveness for certain groups or localities where exact identifications have not been made or recorded.
Finally, the acceptance of authors’ own terms (naturalized, invasive, etc.) conveys author perception, but it must be emphasized that variation in the use of these terms among authors places a caveat on the extent to which “naturalized” and “invasive” species can be distinguished in this summary. Some authors, for example, routinely spoke of “naturalized and spreading species” which other authors may have termed invasive. In tables listing invasive species for a region, it was not always clear whether a species was present merely as an exotic or as an invasive, and some sources combined both invasive alien and native species. In regions where there have been few studies to evaluate the presence and impact of invasive species, there was a tendency to describe the introduction or occurrence of “invasive alien species” such as Lantana camara. In these circumstances, it was not always clear whether a source was referring to the introduction of a species known to be invasive in another country (in the absence of local information) or to a species that was actually behaving invasively at that location. However, there were relatively few records of this type, so that instances of invasiveness are unlikely to have been substantially overestimated in this review. In a global context, it is more likely that invasiveness has been underestimated, due to the unequal recording of invasive species in different countries. A further complication was the large number of approaches used by authors to rank the “invasiveness” of species. So many methods were applied to ranking that it was not felt appropriate to use ranking in this review, other than providing information on those species known to be invasive in many geographical regions.
Estimation of the scale of naturalization and invasion events. The lack of readily available data at species-level, both on area of planting and on area occupied by species that have spread beyond the cultivation zone, restricts discussion of scale of invasion to a single currency, i.e. species number. However, the number of invasive species can only be a very crude summary of the phenomenon, because it makes no distinction between those species that have become only locally invasive (and make little impact on a country’s economics and biodiversity), and those that cause major changes across countries and regions. Even with this caveat, the evidence obtained for this review suggests that alien invasive trees are a major phenomenon, with at least 443 species known to be invasive, including an estimated 282 forestry trees. Although these figures include 68 tree species listed as “possibly naturalized/some degree of invasiveness” in Binggeli (1996), where the extent of spread was either limited or unknown, this is in general agreement with analyses by another author (Richardson 1996, 1998) whose conclusion was that “of all the dominant tree genera used for commercial forestry, all except Abies, Fagus, Gmelina and Swietenia are noted as alien invaders”. Binggeli (1999) subsequently classed Abies alba, A. nilotica, A. nordmanniana and A. grandis as potential invaders, A. sibirica and Fagus sylvatica as moderate invaders and Gmelina arborea as a moderate invader that was naturalized in the Pacific region. Furthermore, Swietenia mahogoni is invasive in Hawaii (University of Hawaii Botany Department 1998) and S. macrophylla has been classified as invasive in Sri Lanka (Cronk and Fuller 1995).
Richardson (1996) focused on the invasiveness of so-called “wonder trees”, multi-purpose trees that could be grown in a variety of extreme conditions. In that review, all-purpose trees (those with applications in forestry, agroforestry and amenity), made up approximately 29 percent of the species encountered that were known to occur outside their native range. The proportion of this group that were rated as invasive or naturalized (71%) was in agreement with trees in other categories such as forestry (70 percent) or agroforestry (71 percent). Yet it should be noted that these categories were not exclusive: most species belonged to at least two usage categories, and the proportion of trees with a single use was relatively small. Similarly, the inherent bias of the information collecting process towards the more widely used or widely distributed species is likely to have ensured that many of these species could, at one time or another, have been regarded by foresters as “wonder trees”.
Where species have different applications (plantation forestry, amenity, agroforestry) it is likely that their establishment, management and spatial/landscape placement conditions are likely to vary, and could potentially influence their likelihood of becoming invasive. For example, compare the case of a single street tree – separated from any neighbours and isolated in an urban environment away from potential areas to colonize – to that of a large stand of a commercial forest species situated upwind of a suitable area for seed establishment.
Rouget et al. (2001) recommended the careful siting of potentially invasive species at landscape level, particularly in relation to neighbouring areas of high wildlife value. However, in general, few papers were found in which a species invasion was considered in relation to the scale of a planting or its position in the landscape. Evidence that some species have become naturalized following introduction in relatively small trial plots (Hughes 1994) suggests that the threshold planting area required to precipitate invasion is likely to vary among species. Similarly, species that are harvested before they reach sexual maturity and set seed may not express an otherwise invasive character.
This may account for the apparent difference in the behaviour of Pinus contorta var. latifolia in the UK and Scandinavia (David Wainhouse, personal communication, 2002). In Scandinavia, where it is harvested after sexual maturity, it is regarded as potentially invasive (Andersson et al. 1999). In Britain, where it is harvested at a younger age, fewer problems are encountered, although Richardson and Higgins (1998) (cited by Andersson et al. (1999)), noted that this species spreads in Scottish forests that are disturbed by sheep grazing. The fact that this species is prone to insect defoliation in Britain may help to limit its invasive tendencies (Heritage 1997).
In general, more accurate data are required on the scale of planting, the scale of invasion events and variations in management practice, before species invasion can be understood and, potentially, managed.
Taxonomic characteristics of invasive species. In this review, the families that included the largest number of invasive species were, in decreasing order: Leguminosae, Pinaceae, Myrtaceae, Rosaceae and Salicaceae. This is broadly in agreement with those named in Binggeli (1996)(although this is not surprising as many of the trees included in this review were drawn from the latter publication). This does not lead conclusively, or automatically, to the view that some families are inherently more invasive than others. Testing this hypothesis would require the separation of scale of planting from reproductive and growth biology, since species in these families are among the most widely introduced of forest trees.
Location of naturalization and invasion events. Although the data used in this review documented the large-scale introduction of forestry species across many countries and continents, there were many cases in which the source or history of an introduction was either unknown or unidentified. Without information on the location, scale and history of an introduction it is impossible to calculate the rate of spread, or to estimate any “time lag” between introduction and invasion. It is possible that certain provenances or subspecies have the capacity to become invasive more frequently that others, yet it is difficult to predict where problems may arise when knowledge of local provenances and their behaviour is so fragmented.
The overwhelming conclusion from the distribution of invasive events was that the invasion and naturalization of forestry, and other commercially used trees and woody shrubs, is a global phenomenon. Invasive species were reported in all seven regions (Europe, Africa, Australasia, North America, South America, Pacific, Asia), although the relative number of invasive species in each region is likely to reflect, in part, the availability of reliable data sources. Investment in cataloguing the invasive species “problem” has been particularly high in South Africa and North America and the availability of data sources such as Henderson (2001), the INVADERS Database System (Rice 2002) and the lists produced by the various North American Exotic Pest Plant Councils make it unsurprising that Africa and North America are among the regions with the highest numbers of reported invasive species. Figure 3 may infer as much about the scale of reporting as about the scale of invasion in the different regions.
The case of Puerto Rico demonstrates the sensitivity of this type of data to the contributions of individual research groups. Of 114 forestry trees reported as naturalized or invasive in the region, 68 were listed in Francis and Liogier (1991) as naturalized or invasive in Puerto Rico. Excluding all the species for which the only South American record was from Puerto Rico would have reduced the South American list by 61 species (54 percent). Of these 61 species, 53 (87 percent) were reported by Francis and Liogier (1991), underlining the large contribution to the South American list that was made by a single scientific paper. Similarly, it is interesting to note that the African region is dominated by reports of invasions in South Africa, Botswana and Zimbabwe, countries that have been the focus of intensive research on invasion ecology (e.g. Richardson et al. 1994; Richardson 1996; Henderson 2001) or the subject of recent reviews (e.g. Buss 2002; Nyoka 2001). A disproportionately low number of species were recorded in Africa outside these countries, particularly in central and northern Africa. The only exception to this distinct bias was a group of reports for African islands, e.g. Mauritius, Reunion, St Helena, etc., where again there has been considerable investment in the cataloguing of exotic species. It seems likely that further research in other parts of Africa will uncover previously undocumented cases of invasive species.
Binggeli (1996) reported the distribution of invasions in the temperate zone “from most regions with the exception of southern America and Asia and in the tropical zone from Pacific and Indian Ocean islands without any records from South America”. Correspondence conducted for this study resulted in the reporting of numbers of invasive species from South America which were similar to those reported from the known centres of invasive species research, i.e. Australasia and the Pacific. If naturalized species were included, South America became the region with the largest number of naturalized or invasive species. The more limited number of reports from Asia (see Figures 4 and 5) would appear to be due to the fact that, although many countries are aware of the potential problem, detailed inventories of invasive species or studies of their impact have not been conducted.
At a recent workshop on the prevention and management of invasive alien species in South and Southeast Asia, many country delegates commented that research on invasive species and their impact was required in the region (e.g. Paudel and Kaini 2002; Sinohin and Cuaterno 2002). There appears to be an additional information gap in temperate Asia: for example, a Chinese website on invasive species (Chinese Biodiversity Information Centre 2002) lists no invasive forest trees. Furthermore, Wang (2001) stated that although some species “had naturalized” there were no accounts of invasive species from China. This is despite the fact that China has the largest area of plantation forestry in Asia (nearly 40 000 ha) (FAO 2001).
The low number of species recorded as invasive in Europe is likely to reflect the comparatively low number of commercial tree species planted in the region.
Invasiveness as a species trait. Although many species have been introduced into 20 or more countries, most species were reported as invasive in only a fraction of their exotic range (see Figure 4). Several factors may account for this disparity, including a lack of reporting (as discussed above). In other cases it is possible that there has been insufficient time since introduction for invasion to be expressed. A third possibility is that invasiveness is not purely a species trait, but a complex interaction between a species’ reproductive biology, the local habitat, management, landscape characteristics and the established local flora. Further research and monitoring are needed to understand the extent to which a species may be classed as invasive across its exotic range and what factors are involved in the occurrence of local invasions (Section 5).
It is also desirable for the impact of invasive species to be monitored over longer time scales. For example, Walters and Savill (1992) reported the alternation of regeneration between invasive sycamore (Acer pseudoplatanus) and native ash (Fraxinus excelsior), and questioned the extent to which A. pseudoplatanus threatens British woodland in the face of such an interaction.
