Suitability of the Weber-Gut risk assessment methodology used in Central Europe for determining invasive woody plant species in Estonian historical parks

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 et al., 1997) but the problem has become significant in many regions due to greater distribution and naturalisation of woody plants. Seven woody plants (DAISIE, 2017) are listed amongst one hundred most dangerous invasive species in Europe.

Invasive herbaceous plants have been studied more than woody plants (Ööpik et al., 2008; Sander et al., 2008; Raska, 2010; Purik, 2011; Ööpik et al., 2013), but research about woody plants (Elliku & Sander, 1996; Mölter, 2011), manor parks (Uustal, 2003; Nutt, 2008; Sinijärv, 2013; Nurme et al., 2014) and the dendroflora of manor parks (Nutt, 2013; Nutt et al., 2013) have also been carried out. However, the species invading the parks’ dendroflora have not been studied. A Master's thesis titled Risk assessment of alien woody plant species recommended for restoration of parks was defended in 2013 at the Estonian University of Life Sciences (Purik, 2013). The thesis drew attention to parks as a hotbed of invasive tree and shrub species reproduction which was considered to be a problem in the restoration of parks. Fifty-three species of alien woody plants in parks were classified into three categories (Purik, 2013) according to their invasion potential in 2013 and on the basis of the Weber-Gut risk assessment system which was previously used in Central Europe (Weber & Gut, 2004). In Estonia, 18 woody plants (Purik & Ööpik, 2013) have been listed as invasive amongst other alien species. In order to evaluate the extent of the problem of invasive species that belong to the high-risk category, the distribution of alien invasive tree species in Estonian parks classified as high risk was analysed in 2013 (Purik, 2013).

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 17^th century (Maiste, 1996) and a majority of the parks were founded in the 18^th–19^th century (Nutt et al., 2013).

The use of alien plant species in park design began in the middle of the 19^th 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 et al., 2008). Estonia is located in Northern Europe (580–600 N, 220–280 E). The territory of Estonia covers 45,339 km² and lies on the eastern coast of the Baltic Sea (the coastline is 3,794 km long). The average height is 50 m above sea level (max 317.4 m). Estonia is located in the transitional temperate climate zone and the climate is strongly influenced by the North Atlantic Current, the Baltic Sea and the geographical location of Estonia. Estonia has four seasons and the annual average temperature is about +5 °C (average temperature in February is −4…−5 °C and +18 °C in July). Temperature fluctuations from the average are frequent. Precipitation surpasses the evaporation rate. The annual precipitation rate is between 550–800 mm, on average, and the average relative humidity is 80–83%. In Estonia, the vegetation period is almost 200 days, on average (between 175–195 days), being the shortest in the north-eastern part (less than 180 days) and longest in South Estonia and islands (more than 185 days). (Estonica, 2020).

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 (Abies balsamea (L.) Mill., Abies sibirica Ledeb., Larix decidua Mill.) and 15 deciduous species (Acer negundo L., Acer pseudoplatanus L., Acer saccharium L., Amelanchier spicata (Lam.) K. Koch, Cornus alba L., Populus alba L., Populus balsamifera L., Robinia pseudoacacia L., Rosa pimpinellifolia L., Rubus odoratus L., and shrub species Caragana arborescens Lam., Sambucus racemose L., Sorbaria sorbifolia (L.) A. Braun, Symphoricarpos albus (L.) S. F. Blake, Physocarpus opulifolius (L.) Maxim.).

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.

Figure 1

Three species of coniferous trees categorised as high risk are abundant in parks. The most numerous invasive coniferous species is European larch (Larix decidua) which grows in 64% of the parks. Two fir species are a little less abundant (Abies sibirica 55% and Abies balsamea 38%).

Although European larch (Larix decidua) was the most numerous coniferous tree in parks (in 64% of the parks) (Table 2), there were remarkable differences in its regional distribution. In West Estonia and the western islands, the occurrence of the species was less frequent. In addition, there was not a single Abies sibirica in the parks of the southwestern region, including the islands (Figure 2). The same pattern occurred with other coniferous species (Abies balsamea, Larix decidua).

Figure 2

Compared to coniferous trees, there were fewer invasive deciduous trees or shrubs in parks (Table 2, Figure 1). The northern region was exceptional because Acer pseudoplatanus was found in over 70% of the parks. Populus alba was present in 80% of the parks on the islands and Amelanchier spicata in 60% of the parks in the southern region. There were only a few samples of Acer saccharinum and Robinia pseudoacacia found in all the parks (Figure 3).

Figure 3

The most widely distributed species are mostly shrub species. Caragana arborescens, which belongs to the third risk category, is widespread (found in 80% of the researched parks) (Figure 4). Sambucus racemosa (63%), Sorbaria sorbifolia (56%), Symphoricarpos albus (55%)appear in about half of the studied parks. The most abundant invasive shrub species is Caragana arborescens which occurs in 90% of the parks in different regions. Symphocarpos albus and Sambucus racemosa are also present in over 40% of the parks, and Sorbaria sorbifolia is in 20% of the parks in the western region. Rubus odoratus is not present in the western region and on islands and in other regions can be found in less than 15% of the parks. The smallest number of invasive species is in the western part of Estonia where half of the invasive species are present, and on the islands where only 11 species can be found.

Figure 4

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 Acer pseudoplatanus occurred in the northern region (in 70% of the parks), Populus alba on southwestern islands (in 80% of the parks) and in the southern region. Amelanchier spicata was present in 60% of the parks. Robinia pseudoacacia and Acer saccharinum were present only in small numbers (0–20%). Distribution of the shrub species Caragana arborescens was very high. Also, Symphocarpos albus, Sambucus racemosa, Sorbaria sorbifolia were abundant. However, the distribution of Rubus odoratus and Cornus alba was very small.

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 (Larix decidua) alley with 250 trees.

Figure 5

The three coniferous trees categorised as invasive (Larix decidua, Abies sibirica, Abies balsamea) produced offspring, hence, had new individuals in these parks which shows that the species are able to reproduce. The same conclusion cannot be made about deciduous trees categorised as invasive because only one species had new individuals in one park (Table 3).