Invasive alien species are the second largest reason after habitat destruction for the decrease in biodiversity all over the world (European Commission, 2013). The European Union has taken several measures to preserve biodiversity by limiting the spread of invasive alien species. Compared to herbaceous plants the problem of invasive slow-growing and long-living woody plants has not received much attention. Invasion of non-native species is one of the many threats to biodiversity and it is considered to be a major component of global change (Mooney & Hobbs, 2000). The problem of invasive species increases constantly due to the increase in people travelling and goods being transported. Although for a long time, exotic species have been valuable and desirable souvenirs brought home from travels to distant countries but they have also become a problem which is being dealt with at a national level. The shorter the lifespan of a species, the faster it reproduces, hence the greater risk of invasion by alien species. Therefore, the invasion of herbaceous plants is a rapidly growing issue. There seems to be no serious danger when talking about species with a long lifespan, such as woody plants. Alien woody plants have not been considered to be dangerous invasive species (Holm
Invasive herbaceous plants have been studied more than woody plants (Ööpik
Estonia with a total area of 45,339 km2 has about 1,100 manors (Rosenberg, 1994) and about 800 parks which have been preserved. The oldest manor parks were established already in the 17th century (Maiste, 1996) and a majority of the parks were founded in the 18th–19th century (Nutt
The use of alien plant species in park design began in the middle of the 19th century (Sinijärv, 2013). It is known that alien species can be bought from plant nurseries but also brought from trips abroad (Nutt, 2008). Although historical events have had a devastating effect on parks that once were magnificent and many exotic species susceptible to local climate have been destroyed, there are still resistant species which can reproduce without special care. The issue of invasive woody plants in Estonian parks was recognised in regard to the restoration of old manor parks (Purik & Ööpik, 2013).
The distribution, viability and invasiveness of species are largely dependent on natural conditions (Pyšek
The characteristic features of soil in Estonia are as follows (Raukas, 1995): 1) abundance of wetland soils; 2) presence of limestone in the soils; 3) abundance of carbonate soils; 4) soils are rich in stones, especially gravel; 5) the soils are colourful due to their granulometric, mineral and chemical composition, and variable water regime. About half of the territory (45.6%) of Estonia is covered with forests (Adermann, 2009) and there are 81 indigenous woody plant species (Kull, 2009).
The materials used for the analysis included lists of tree and shrub species growing in parks (compiled between 1970 and the 1990s) and data on dendrological inventories (from 2003 to 2009), which enabled to analyse which species form so-called new natural communities. The occurrence of all invasive woody plants was analysed by regions. The analysis included coniferous and deciduous trees and shrubs classified as high-risk species by Purik & Ööpik (2013): three coniferous species (
Several databases were used to assess the distribution of invasive tree species in parks as follows:
The analysis of the distribution of invasive species is based on Piret Palm's database which was compiled in 2009 (Palm, 2009). Data was collected from inventories which were prepared during the period of 1977 to 1997. The database contains 370 tree and shrub species in 304 parks all over Estonia (Figure 1). Data analysis was performed using the statistical programming language R. A distribution map was compiled about each species which helped to view the dangers of invasion region (county) by region. Comparing the distribution maps to each other enabled to assess the peculiarities and extent of the dangers of invasion. In order to characterise the frequency of distribution in each region, a percentage of the species’ occurrence was calculated in all of these parks, county by county. The viability of invasive species was analysed from 2003 to 2009 on the basis of the dendrological inventory data of 17 parks (Table 1). The data used in the analysis included 13,994 trees. Their species and breast height diameter were determined and their age was calculated based on the breast height diameter methodology (Nutt
Overview of inventories used in the research. The inventories are compiled by Artes Terrae Ltd.
Name of the park | Year of the inventory | Name of the project |
---|---|---|
Hummuli manor park | 2008 | Reconstruction project of Hummuli manor park |
Härgla manor park | 2007 | Dendrological inventory of Härgla manor park |
Kiidjärve manor park | 2009 | Reconstruction project of Kiidjärve manor park |
Kukruse manor park | 2009 | Reconstruction project of Kukruse manor park |
Kuremaa manor park | 2006 | Reconstruction project of Kuremaa manor park |
Lõhavere Hospital park | 2009 | Dendrological inventory of Lõhavere Hospital park |
Mäetaguse manor park | 2004 | Reconstruction project of Mäetaguse manor park |
Pagari manor park | 2007 | Reconstruction project of Pagari manor park |
Puurmani manor park | 2007 | Reconstruction project of Puurmani manor park |
Püssi manor park | 2007 | Reconstruction project of Püssi manor park |
Riidaja manor park, I phase | 2006 | Reconstruction project of Riidaja manor park, I phase |
Rõngu castle hill park | 2008 | Dendrological assessment of Rõngu castle hill park and recommendations for management |
Saka manor park | 2008 | Reconstruction project of Saka manor park |
Saku manor park | 2007 | Reconstruction project of Saku manor park |
Sürgavere manor park | 2008 | Reconstruction project of Sürgavere manor park |
Õisu manor park | 2008 | Reconstruction project of Õisu manor park |
Rogosi manor park | 2003 | Dendrological inventory and assessment of Rogosi manor park |
Three species of coniferous trees categorised as high risk are abundant in parks. The most numerous invasive coniferous species is European larch (
Although European larch (
The occurrence of high-risk invasive tree and shrub species in parks by species (data of 304 parks).
No | Species name | No of parks | % of parks |
---|---|---|---|
1 | 244 | 80 | |
2 | 194 | 64 | |
3 | 192 | 63 | |
4 | 171 | 56 | |
5 | 168 | 55 | |
6 | 167 | 55 | |
7 | 117 | 38 | |
8 | 115 | 38 | |
9 | 84 | 28 | |
10 | 75 | 25 | |
11 | 71 | 23 | |
12 | 70 | 23 | |
13 | 58 | 19 | |
14 | 56 | 18 | |
15 | 42 | 14 | |
16 | 18 | 6 | |
17 | 17 | 6 | |
18 | 15 | 5 |
Compared to coniferous trees, there were fewer invasive deciduous trees or shrubs in parks (Table 2, Figure 1). The northern region was exceptional because
The most widely distributed species are mostly shrub species.
The difference in the presence of coniferous trees in the western region compared to the rest of Estonia is noticeable – two species cannot be found in any of the parks in the western region and only one species occurs in the parks of the islands. The deciduous species
In order to analyse the severity of the problem of invasion, the parks that had the highest number of species’ individuals and invasive offspring present were analysed.
The proportion of young trees growing in parks that have reproduced naturally is very small, which means that many species do not reproduce independently and thus fail to pose an invasive threat (Figure 5). The number of invasive coniferous and deciduous trees in sixteen different parks was generally under 40. The number of invasive trees is bigger in Hummuli and Räpina manor park, and in Kukruse manor park which is an exception as there is a European larch (
The three coniferous trees categorised as invasive (
The distribution and viability of invasive species (data of 304 parks).
No | Species name | No of parks present | Remarks (viability) |
---|---|---|---|
1 | 8 | shrub | |
2 | 10 | was not present, except in Kukruse is present in large numbers | |
3 | 2 | shrub | |
4 | 1 | shrub | |
5 | 8 | present in large numbers | |
6 | 6 | shrub | |
7 | 8 | present in large numbers | |
8 | 3 | shrub | |
9 | 0 | - | |
10 | 2 | was not present | |
11 | 3 | was not present | |
12 | 0 | - | |
13 | 0 | - | |
14 | 4 | a few individuals | |
15 | 0 | - | |
16 | 2 | was not present | |
17 | 1 | was not present | |
18 | 0 | - |
This article gives an overview of the invasion problem of slow-growing and long-living woody plant species in the parks of Estonia. When using the Weber-Gut risk assessment system used in Central Europe, the number of alien woody plant species was 53 in 2013, which were divided into three risk categories (Purik & Ööpik, 2013). In order to evaluate the severity of the invasion problem, the plants’ species and age were analysed which enabled to determine their distribution and viability or reproduction ability.
The study showed that based on the Weber-Gut risk assessment system used in Central Europe all the species categorised as high risk (total of 18) had not reproduced or produced an offspring in these parks. Therefore, adding them to the list and classifying them as high risk is not justified in Estonia. The species
Although invasive species may cause big problems on a wide scale, this cannot be observed within the tree and shrub species of old manor parks in Estonia, because most exotic species growing there do not compete with native species. There are species that are called invasive but their offspring do not exceed the new individuals of native species and therefore, they do not dominate the native species.
Bioinvasion goes with other processes triggered by man, such as global changes in the environment and climate, and this is considered to be an equal danger to biota next to over-exploitation, change and fragmentation of habitats and pollution (Kangur
However, there are still some introduced species (
The methodology used to determine the viability of trees cannot be applied to shrubs and therefore, a methodology that considers the specific characteristics of shrub reproduction should be used. The distribution of shrubs in parks has to be evaluated. The database that was available did not include the necessary data. A high percentage of young trees in the parks indicates that they are able to reproduce independently. Due to the fact that there is no diameter data on shrubs, it is not possible to assess the natural renewal of shrubs according to their age and hence, assessment of their reproductive abilities should be based on the shrub population and distribution data.
The severity of invasion cannot be evaluated based on the total number of trees. Although the tree species are categorised as high risk, their viability and offspring have to be analysed. Therefore, a comparison of old and young trees was carried out, which showed that in Kukruse park, there are many invasive tree species, although the number of new individuals is small. However, there are still parks (Hummuli, Kiidjärve, Kuremaa, Räpina, Riidaja, and Saku) where the number of new individuals exceeds the number of old trees (Figure 5), which represents a species’ ability to reproduce naturally.
From the point of view of the aim of this study, it is clear that the problem of invasiveness cannot be addressed the same way everywhere. It has clear regional peculiarities (Figure 2, 3, 4) and the species categorised as high risk are not distributed similarly in all the parks of the studied regions. Also, all the species do not produce new individuals. Corrections have to be made in the invasive species list: 4 tree species (
Undoubtedly, current results are not final as the growth and distribution of woody plants is a long-term process. The first alien woody plants were brought to Estonia in the 17th century and from then on the number of introduced species has been growing. The results of this research confirmed the assumption that there are regional variations in the distribution of species. The mild climate of northern Estonia might be the reason why these species can be found in large numbers in this area compared to the inland. But without further research this is not certain because the reason may be historical, i.e. the mentioned species have never been introduced into these parks.
Another aspect is connected with the maintenance of parks. However, in the case of invasive species it may be suggested that maintenance does not play an important role because they should be strong competitors for native species. Still, we estimated that the presence of the so-called invasive species should be viewed in connection with the maintenance of parks.
Our research showed that the risk assessment for Central Europe developed by Weber & Gut is not suitable for determining the invasiveness of tree and shrub species in the historical parks of Estonia.
Only half of the 18 species (determined to fall in the high-risk category) have characteristics of invasive behaviour (producing reproductive offspring, often in very large numbers, at a considerable distance from parent plants, and thus having the potential to spread over a considerably wide area).
Even in the relatively small territory of Estonia covered in our research (45,339 km2), regional differences in the occurrence of invasive species are noticeable.
Based on the Weber-Gut risk assessment system we did not recognise mass-independent reproduction of most of the tree species determined as invasive.
Our research showed that the dangers of invasion cannot be assessed with a single uniform methodology, because each habitat and species needs to be analysed individually and according to their specifics.