Human activities on a global scale enable an increasing number of species to colonise regions outside of their native range, establish self-sustaining populations and expand further into natural habitats. This phenomenon is known as biological invasion (Elton, 1958; Nentwig
Exotic species have long been considered valuable and desirable souvenirs imported from distant places (Nutt & Kubjas, 2020). A study of the economic uses of plant species that are invasive in various parts of the world showed that most are ornamentals (Weber, 2003). These species have been introduced for horticultural use by nurseries, botanical gardens, and horticultural enthusiasts (Reichard & White, 2001). Some of the exotic species introduced to Europe, initially planted in botanical gardens and later in parks and as urban greenery, have now become invasive (Stace & Crawley, 2015).
After the introduction of non-native tree and shrub species into Europe, their acclimatization and propagation for landscape gardening began. Alien tree and shrub species were often planted in parks and urban greeneries. In the 17th – early 18th century, mainly non-native berry bushes like
Dendrological plantations in Latvia were intensively studied in the 20th century. The dendroflora of urban and rural parks and dendrological plantations (in total 4,806 sites), including trees and shrubs was investigated between 1971–1990 by dendrologists of the Botanical Garden of the Latvian SSR Academy of Sciences under the leadership of Dr Raimonds Cinovskis (Cinovskis
Over the last 200 years, the number of alien tree and shrub taxa in synanthropic habitats have significantly increased. Most were introduced as garden and park plants. Furthermore, plant species invasions begin in these greeneries. In the context of global warming trends, the number of such taxa escaping from greeneries will increase in the near future (Laiviņš
Currently, the largest dendrological plantations in Latvia are found in the National Botanic Garden (about 4,000 tree and shrub taxa) and the Kalsnava Arboretum (2,268 taxa; Evarts-Bunders
Recently the global database of invasive trees and shrubs was updated, including a total of 751 species (434 trees and 317 shrubs) from 90 families (Rejmánek & Richardson, 2013; Rejmánek, 2014). The first list of alien organism species in Latvia was compiled in 2007 and includes 637 vascular plant taxa, of which 155 are trees and shrubs (Svilāns
Successful invasion by alien species is almost irreversible, because most invasive alien plant species are successively established in large numbers before they are observed. Subsequently, they are almost impossible and very expensive to eliminate (Anonymous, 2019). Some of the most significant invasive plant species are cultivated trees and shrubs, which have been cultivated for a long time, successfully naturalized, and are now classified as invasive, ecologically aggressive ‘problematic’ plants. All seed-producing (and even some sterile) trees and shrubs have the ability to spread (Rejmánek
The aim of the study is to increase knowledge about alien tree and shrub species distribution in Latvia and to evaluate their invasiveness in relation to significant species traits. The objectives of the study are: 1) to update the list of alien tree and shrub species in Latvia; 2) to evaluate the invasiveness of alien trees and shrubs (degree of naturalization); 3) to assess the invasiveness risk in association with species origin and important traits: status and reproduction strategy. Our hypotheses are as follows: 1) species introduced to Latvia from distant regions (e.g., Asia, North America) will have a significantly higher invasiveness risk; 2) the risk of invasiveness and the status of the species are closely related to their reproduction strategy.
The list of invasive tree and shrub species was updated based on earlier research of woody flora in dendrological plantations in Latvia conducted from 1971 to 1990 under the guidance of the dendrologist Raimonds Cinovskis of the Botanic Garden of the Latvian Academy of Sciences (currently the National Botanic Garden), when a total of 4,806 dendrological plantations were inventoried. The lists of native and alien tree and shrub species were later published in the Atlas of Latvian Woody Plants (Laiviņš
In order to provide as detailed information as possible on the 178 listed tree and shrub taxa, the following information is provided in Appendix 1:
Taxonomic affiliation of the listed trees and shrubs to genera. First report in the area. This column provides a summary of when the non-native woody plant species were introduced into the territory of Latvia, based on the first publication date (Laiviņš Origin. Non-native tree and shrub species were classified as alien, casual alien, naturalized, invasive and transformers or weeds following Pyšek Vector. The mode of entry into Latvia is indicated for each taxon. The status describes the types of introduction of alien species as classified according to Stace & Crawley (2015). The evaluated trees and shrubs have two types of introduction in Latvia: F – as edible (human food), including herbs and spices, plant seeds, fruits and other plant parts; H – horticultural introductions. The type of spreading indicates whether the species reproduces in Latvia from seed and/or by vegetative propagation. The data were summarized based on the authors’ observations and literature. Species distribution in Latvia was estimated by applying the square method, which is related to the geographical coordinates, where one square corresponds approximately to 7.6 × 9.3 km or 71 km2 for Latvia. The total number of the grid cells in Latvia is 1,017, from which 822 are completely and 195 partially located in the territory of Latvia (Laiviņš Invasiveness risk. There are many challenges facing the field of risk assessment of species invasiveness. After revision of previously used methods (e.g., Pheloung, 1995; Van Wilgen Risk class. Each non-native woody plant species was assigned to one of the three risk classes, based on the total point score as follows (Weber & Gut, 2004): 3–20 low risk – species is unlikely to pose a threat to natural communities; 21–27 intermediate risk – species requires further observation; 28–38 high risk – species is likely to become a threat to natural communities if naturalized.
We applied Pearson’s Chi-squared test for count data corresponding to the number of species (Hope, 1968) to evaluate the dependence between species invasiveness risk class and the following species characteristics: native distribution range (Europe, North America, Asia/Eastern Asia, Eurasia, Europe/Asia Minor, Eurosiberia, cultivar) for 178 species, status (transformer, invasive, casual, dual, natural) for 178 species and reproduction strategy (seeds, vegetative, seeds and vegetative) for 171 species.
To identify the most contributing associations of the Pearson’s Chi-squared test results among species invasiveness risk class and native distribution range and among invasiveness risk class and status, Pearson’s residuals were calculated from the Pearson’s Chi-squared test results and visualized using the ‘corrplot’ package (Wei & Simko, 2017). All data analyses were conducted in R (R Core Team, 2020).
The new annotated list of invasive trees and shrubs includes 178 taxa. The previous list of alien species (Svilāns
In total, 23 taxa from the previous alien plant list are not included in the current updated list. We did not find confirmation of the migration of these 23 alien taxa into the wild in the areas inspected during the study, for example,
The list of alien tree and shrub species in Latvia includes representatives from 28 families. There are five families with at least 10 species classified as alien that together comprise 62.36% of the total alien taxa of the country. The most frequent are from the Rosaceae family, containing 51 taxa (corresponding to 28.65% of all alien tree and shrub species), the Salicaceae family with 20 taxa (11.24%), the Aceraceae family with 15 taxa (8.43%), the Fabaceae family with 13 taxa, (7.30%), and the Pinaceae family with 12 taxa, (6.74%) (Appendix 1).
Of the taxa analysed, conifers are represented by 14 taxa (7.86%), belonging to two families Cupressaceae – 2 taxa (1.12%) and Pinaceae – 12 taxa (6.74%). The remaining 92.14% of species belong to deciduous trees.
Most of the 178 alien taxa introduced in Latvia are native to North America (46 taxa, 26%), and Europe (39 taxa, 22%), the rest originated from Eurasia (17 taxa, 10%), East Asia (20 taxa, 11%) or are cultivars – decorative varieties, hybrids and other taxa without a natural native area (32 taxa, 18%; Figure 2, Appendix 1). In total, 18% of the studied alien taxa are cultivars.
During this study, 178 tree and shrub taxa were evaluated according to the selected criteria. In Latvia, all these species are successful at self-reproducing, of which two species (1%) are recognized as transformers:
The results show that 89% of the identified invasive plants dispersed from horticulture, 6% as edible plants, including herbs and plant seeds, fruits and other plant parts, 3% horticulture and human food and 2% horticulture and timber.
This study provides a risk assessment of invasive plants for all 178 tree and shrub taxa that show signs of invasiveness after introduction in Latvia. According to the scores obtained through the risk assessment, the taxa are divided into three risk classes. Class III (high risk) includes 32 species (18%). The alien taxa to belong to the high-risk class are:
Class II (intermediate risk) includes 123 taxa or 69% of the alien species, while class I (low risk) includes 23 taxa or 13% of all alien taxa (Appendix 1). All three risk classes were present in each of the five alien species status categories (Figure 4). Class III included 23 species from the invasive taxa group (Appendix 1), the transformer
We did not find significant differences between the area of origin of the taxa and invasiveness risk classes (X2=16.26, p=0.28). Of the 32 taxa in Class III (high-risk class), 10 taxa or 31.25% are cultivated, 8 taxa or 25% are of North American origin, and four are from East Asia. Eurosiberian taxa were not found in Class III (Figure 5, Appendix 1).
We did not find a significant association between the studied species invasiveness risk class and species native distribution range (X2=16.26, p=0.30). A significant association was found between the species invasiveness risk class and species status (X2=55.92, p<0.01) and between the invasiveness risk class and species reproduction strategy (X2=61.25, p<0.01).
Pearson’s residuals showed the association between all three invasiveness risk classes and invasive and casual species status (Figure 6 A) which contributed to Pearson’s Chi-squared test results. The highest positive association was found between a high invasiveness risk class and invasive species status, but the highest negative association was found between a high invasiveness risk class and casual species status (Figure 6 A).
Pearson’s residuals showed the association between all three invasiveness risk classes and all three reproduction strategies (Figure 6 B) which contributed to the Pearson’s Chi-squared test results. The highest contribution was found between a high invasiveness risk class and species having both seed and vegetative reproduction strategies, which showed a positive association. A high contribution was also found in the positive association between vegetative reproduction and low invasiveness risk. A high contribution was also found in the negative association between a high invasiveness risk class and species reproducing by seeds (Figure 6 B).
As a result of this study, a national list of alien tree and shrub species was compiled, assessing observed invasiveness traits for the included species. The aim of a risk assessment for invasive trees and shrubs was to estimate which species should be listed on the national invasive plant list and to decide which new species infestations should be controlled or removed in order to prevent their spread and associated ecological consequences (Weber & Gut, 2004).
We did not prove the hypothesis that species from distant locations like Asia and North America show more pronounced invasiveness traits, namely, a higher risk class of invasiveness (Figure 5). This can be explained by the number of cultivars in our dataset. Accordingly, in other similar research (Andreu & Vilá, 2010; Fayvush
In the present study, of the 32 tree and shrub taxa included in the high-risk invasiveness class, a considerable number of them were cultivars or hybrids of horticultural origin – 10 (31.25%), and an addition eight (25%), originated from North America. The majority of cultivars with a high risk of invasiveness belong to Rosaceae:
The family Rosaceae is the most widely represented in the list of invasive tree and shrub taxa (51 taxa, corresponding to 28.65% of all trees and shrubs aliens). Asia is the major source of invasive Rosaceae shrubs, as well as invasive Oleaceae species. Most of the invasive Salicaceae are of Eurasian origin. Because of increasing connections with many Asian countries, even more invasive tree and shrub species will be introduced from Asia (Rejmánek, 2014). The number of tree and shrub species with Asian origin has increased in Latvian nurseries in recent years, for example,
The second hypothesis stated that the risk of invasiveness and the status of the species is closely related to the reproduction strategy. We have shown that species that reproduce both vegetatively and by seed dispersion are more aggressive and invade larger areas more quickly in comparison to other alien species (Stace & Crawley, 2015). For example,
When creating parks, squares, and backyard greeneries, we recommend avoiding taxa included in the high invasiveness risk class in order to prevent the invasion of large areas by adult trees and shrubs that are already capable of producing seeds. In old parks where they were planted, taxa such as
Our study resulted in the compilation of an updated list of 178 alien tree and shrub taxa, that were identified as migrating into natural ecosystems (self-seeding). A total of 44 tree and shrub taxa have been identified as aggressive invasive species, either potentially invasive (with transformer status) or invasive.
Three classes of invasiveness risks were distinguished. The majority of alien tree and shrub species are already invasive – 18% of evaluated species belong to the high-risk class of invasiveness, or have a great potential to become invasive in future – 69% of species belong to the intermediate invasiveness risk group, and only few alien taxa (12% of species) have a low risk of invasiveness. High-risk class species should be given special attention and monitoring in order to prevent the invasion of large areas by these species in the future. In addition, species with intermediate risk of invasiveness also require monitoring and periodic assessment as these taxa may become potentially hazardous over time.
Invasiveness risk and species status are closely related to reproduction strategy. We have proved that species that are able to reproduce both vegetatively and by seed dispersion are more aggressive and invade larger areas more quickly than species with only vegetative reproduction (have low risk of invasiveness). The planting of invasive species with both reproduction strategies should therefore be avoided. Therefore, we recommend planting varieties and hybrids that do not produce germinating seeds, thus avoiding self-seeding.
The Weber and Gut method is suitable for risk assessment of alien tree and shrub taxa in Latvia. Subsequently, the assessment of the tree and shrub taxa yielded interesting and reliable results, which can be used to assess the potential invasiveness risk of a species. The updated tree and shrub species list can be included in the list of invasive species of Latvia. The results of the present study can be used in future studies about alien tree and shrub species’ distributional changes due to the global climate change. The results also contribute to practical nature conservation and landscape gardening in regions with similar climatic conditions.