Roads support the spread of invasive Asclepias syriaca in Austria

A. syriaca is a fast growing, perennial, dicotyledonous species native to the eastern lowlands of North America. The species was brought to Europe as an ornamental plant and also for economic utilization (e.g., latex production) in the 19th century, and by now, it has become naturalized in central and southern European countries (Bagi, 2008). In its invaded range, A. syriaca is mainly found in ruderal habitats (railway tracks, roadsides) and on abandoned agricultural land, deforested areas, or degraded areas (Kelemen et al., 2016; Rutkowski et al., 2015). So far, A. syriaca was only sporadically found in crop fields, vineyards, and orchards in Austria (Follak, unpublished data), although the species is a common weed in neighboring countries such as Hungary (Novák et al., 2009) and in its native range (Hartzler and Buhler, 2000). Propagation of A. syriaca occurs through seeds and rhizomes. Reproductive stems may have about two to three follicles, and each of it contains up to 200 seeds with long white flossy hairs that are perfectly adapted to wind dispersal. The roots and rhizomes form a large and dense underground network spreading out from the mother plant. Both traits are significantly associated with its colonization ability (Bagi, 2008). A. syriaca is found in well drained soils of all textural groups, especially on sand and sandy loess (Bagi, 2008). The biology and ecology of A. syriaca have been summarized in detail by Bagi (2008) and Bhowmik (1994) with a focus on Central Europe and North America, respectively.

Records for Austria have been collated from 1990 until 2018. A wide range of sources has been used (e.g., floristic literature, floristic databases, and unpublished record from experts and botanists). All records were cross-checked to avoid double entries of identical records stored repeatedly in different data sources. The year of the records was extracted from the original data source. A database incorporating 82 records was created for this study. All records were assigned to a grid cell (5 × 3 geographic minutes, ~33 km²) of the Floristic Mapping Project of Central Europe (Niklfeld, 1999). Maps indicating the spatial distribution of the study species for two time periods (1990–2005 and 2006–2018) were produced based on the grid cells occupied.

In parts of the Marchfeld region (Lower Austria), the distribution and abundance of A. syriaca along roadsides and the role of road type and adjoining land use in facilitating its spread were studied in detail. This area was selected, because the main occurrences of A. syriaca in Austria can be found in this region (Figure 1). The climate is temperate pannonic, with cool winters and warm summers, average annual temperature and precipitation being 9.8°C and 520 mm, respectively (reference place: Groß-Enzersdorf, 48°11'47.2"N 16°33'17.3"E, 1971–2000). The study area (87.5 km²) is dominated by agriculture, with some interspersed forests, gravel pits and settlements.

Figure 1

The total length of the road network was 320.6 km (2018, http://www.gip.gv.at/) excluding motorway S1, which was not sampled for safety reasons (heavy traffic volume). The road network has been categorized into three road types: (1) regional paved (“Landesstraße L”, “Landesstraße B”, length: 64.4 km), (2) local paved, and (3) local unpaved (“Gemeindestraße”, “other street categories”, length: 256.2 km). A map of the study area was subdivided into 100 m × 100 m (1 ha) grid cells. Each grid cell of a predefined route (n = 1451) was classified according to the presence/absence of a particular road type (i.e., regional paved, local paved, and local unpaved). Commonly, A. syriaca emerges from late April to mid-June (Follak, pers. obs.; Bagi, 2008). Thus, the route was visited in May/June 2018, that is, the time of year when new shoots have already emerged and are particularly visible, and before the mowing season started. Populations of A. syriaca were recorded along the roadsides, and each location was georeferenced. A roadside was defined as the area laterally from the pavement or the vegetation-free driving surface of unpaved roads to the adjoining land-use type (Forman et al., 2003). A map indicating the distribution of A. syriaca along roadsides was produced based on the sampled locations using ArcGIS 10.6. A high-resolution habitat distribution map (Kuttner et al., 2015) was used to determine the adjoining land use type. The midpoint of each grid cell was chosen for the presence/absence of the adjacent land use type. In this study, the different adjacent land use types have been merged to the following four categories: “built up areas”, “arable land” (including orchards), “grassland” (extensive grassland, intensively used grassland), and “forests” (coniferous forest, broad-leaved forest, dry biotypes). Moreover, grid cells with a railway track inside were listed separately in order to analyze their potential role in the distribution of A. syriaca.

A generalized linear model (binomial regression model) was used to model the probability of finding A. syriaca in a grid cell depending on the abovementioned variables, road type and adjoining land use type (Gelman and Hill, 2007). Significance of predictor variables was tested by likelihood ratio tests. The generalized linear model was implemented in R, version 3.4.4 (R Core Team, 2018).

In total, 82 records of A. syriaca from 52 grid cells of the Floristic Mapping Project of Austria were obtained (Figure 1). At present, A. syriaca is not widely distributed in Austria. The species occurs primarily in eastern Austria (Vienna, Lower Austria). Large and naturalized populations can be particularly found in parts of the Marchfeld region (Lower Austria) (Figure 1). Recently, a few scattered populations were observed in the Graz basin and Drau valley in Styria and Carinthia, respectively. In western Austria, records of the species were rare and almost restricted to a few isolated populations (e.g., in the cities of Salzburg, Innsbruck). However, the results indicate that the species has spread over time in terms of the number of occupied grid cells. Their number increased from 20 (in 1990–2005) to 36 (in 2006–2018) (Figure 1).

A total of 78 populations of A. syriaca were recorded, and they were almost evenly distributed throughout the study area (Figure 2). A. syriaca was found along the roadsides of all road types (Figures 2 and 3a–c) Population size varied from solitary and small populations (<25 stems/population) to large populations several meters in length along the roadside (data not shown). The probability of recording A. syriaca was significantly higher (p<0.001) along a local unpaved road than along the two other road types (Table 1). Compared to roads with adjoining “built up areas”, the model also indicated that in 2018, there were significantly higher odds of finding an A. syriaca population along a road in a grid cell with “forests” (p = 0.003) and “grassland” (p = 0.006) as an adjoining land use type. In addition, the odds of finding A. syriaca in a grid cell along a road with “arable land” as the adjoining land use type was higher compared to the reference category “built-up areas”. However, with a p-value of 0.083, this was not significant at the 5% significance level. Moreover, in grid cells where a railway track was present, the chance of finding an A. syriaca population was also significantly higher (p = 0.007).

Figure 2

Figure 3

A. syriaca is currently most abundant in a small part in the eastern lowlands of Austria. Early occurrences of the species from this particular area were described by Janchen (1977). Since then, it locally spread as indicated by the occupation of adjacent grid cells in the recent years (Figure 1). It seems that short-distance spread has become a dominant mechanism of range expansion for A. syriaca. Its rapid spread in this particular area was most likely assisted by the development of the road network (motorway and interchanges) that was carried out between 2007 and 2009 and gravel mining. Road construction (Figure 3d) landscaping, and mining caused disturbed soil and barren sites creating environmental conditions that were favorable to A. syriaca (Bagi, 2008). Recent observations of A. syriaca in other parts of Austria may be attributed to former cultivation for beekeeping or as an ornamental plant (e.g., Sengl and Rehorska, 2017) and, likewise, were probably human mediated by unintended transport of rhizome fragments by contaminated soil.

Kleinbauer et al. (2010) showed that A. syriaca currently occupies less than 1% of all the potentially suitable area (grid cells) in Austria. In particular, in the southern part where only a few records are presently available, further spread and range filling is probable. The western part of Austria (alpine foreland, valleys in the Alps) is less climatically suitable, which is also indicated by the few occupied grid cells (Figure 1). Similarly, in the Czech Republic, the species is most abundant and invasive only in the warm lowlands, in particular in southern Moravia (Kaplan et al., 2017). In Hungary and Serbia, the species is also confined to the warmer regions (Hungarian Great Plain, Vojvodina; Bagi, 2008; Popov, 2016).

In its native range, A. syriaca is a common element of the roadside vegetation (e.g., Hartzler and Buhler, 2000). In its invaded range, likewise in the Marchfeld region, the species can be found along roadsides in neighboring countries such as Hungary (Bagi, 2008), Slovakia (Pauková et al., 2014), the Czech Republic (Kaplan et al., 2017), Serbia (Popov, 2016), Poland (Rutowski et al., 2015), and the Ukraine (Dvirna, 2018). The specific ecological properties of roadsides (e.g., light and soil conditions) seem to favor A. syriaca, and, concurrently, it is also believed that its life history characteristics (i.e., rhizomes up to 1 m depth) buffer the species against harsh environmental conditions (e.g., drought). The results of the survey in the Marchfeld region showed that A. syriaca occurred along roadsides in spatially limited populations up to larger ones extending several square meters. This reflects the vegetative propagation of the species by its rhizomes that allow the production of large clonal populations (Bagi, 2008).

The role of the road type is potentially important for the spread of invasive alien plants (Joly et al., 2011). The results indicated that A. syriaca has a comparatively higher preference for local unpaved roads. In this study, local unpaved roads were in part characterized by high disturbance of the upper soil layer along the road verges (specifically in the mining areas) providing beneficial growth conditions for A. syriaca. Roadsides of high-ranking roads (i.e., regional paved road type; to some extent, also the local paved road type) are generally covered by dense vegetation (grasses) and mown if necessary (i.e., more than twice a year, throughout the growing season). Less open ground, competition, and the high frequency of mowing may influence the establishment and performance of A. syriaca. However, Bhowmik (1994) pointed out that mowing (cutting) leads to the formation of larger colonies of A. syriaca by inducing lateral root buds to sprout. Indeed, Zalai et al. (2017) demonstrated that shoots of A. syriaca were able to regrow and recover within 2 weeks after repeated cutting in a nature conservation area in Hungary.

In addition, A. syriaca can be found along railway tracks in the study area (Figure 2), and the model also suggests that grid cells with railway tracks are significantly more populated by A. syriaca than without (Table 1). Similarly, railway construction (upgrading, station renewal along the railway) in the past decade may have fostered its local spread. Moreover, the turbulence of passing trains may have enhanced the wind dispersal of its seed and the colonization of other habitats including roadsides (Csontos et al., 2009). Railway tracks are also a typical habitat for A. syriaca in other countries (e.g., Rutowski et al., 2015).

The results reveal that specific adjacent land use types foster the presence and spread of A. syriaca along roadsides. More particularly, a roadside crossing a grid cell dominated by “forests” and “grassland” seemed to be more likely invaded by A. syriaca than a roadside surrounded by “arable land” and “built up areas”. In the study area, the lowland temperate forests (mainly stands of Robinia pseudoacacia, Quercus sp. and Pinus sp.) are broadly characterized by poor growth, and their relatively open canopy (in particular at the forest edge) allow light-demanding species such as A. syriaca to thrive in the undergrowth and to build up large populations (Essl et al., 2012). Likewise, extensively used grassland and abandoned fallow land (e.g., old fields) are hotspots for the establishment and expansion of A. syriaca because of former soil disturbance and nutrient enrichment, respectively (Kelemen et al., 2016). Thus, there is a high pressure from these populations of invading adjacent roadsides, and equally, roadside populations of A. syriaca can certainly establish in the forest and grassland habitats. The species has a high colonization ability because of its wind-dispersed seeds and rhizomes that can spread for a distance of >1 m/year from the point of origin (Bhowmik, 1994). “Arable land” as an adjacent land use type seems to have not an important influence on the presence of A. syriaca. Large infestations of crop fields along roadsides have not been frequently observed, yet, most likely because of intensive farming (i.e., high level of herbicide input, soil cultivation) in large parts of the study area. Populations of A. syriaca were commonly restricted to the field border of the crop field next to a roadside (Follak, pers. obs.). This, however, suggests that populations of A. syriaca intruded the crop fields by vegetative spread. Nevertheless, when infested crop fields are cultivated or harvested, machinery and vehicles can spread rhizome fragments of A. syriaca to and along roadsides.

Control of the species is legally binding, as it is categorized as an Invasive Alien Species of Union Concern (http://data.europa.eu/eli/reg_impl/2017/1263/oj) Problems arise when roadside populations spread into crop fields (Hartzler and Buhler, 2000) or natural habitats of nature conservation value (Kelemen et al., 2016). Moreover, it may also be problematic for road safety, because it forms dense stands (Figures 3b and 3c) that may prevent drivers from recognizing traffic signs (e.g., delineator posts) as well as wild animals.

How can the expansion of A. syriaca along roadsides be prevented? It is generally believed that it is difficult to get rid of established A. syriaca populations because of the large root network (Bhowmik, 1994). Mowing is the chosen option for vegetation control along roadsides in Austria. An effective long-term control of A. syriaca might best be secured by continuous mowing that repeatedly destroys the shoots of the plant and thus depletes stored carbohydrate reserves in the root system (Bhowmik, 1994). However, optimal and frequent cutting dates need to be determined and cannot always be achieved in practice because of organizational challenges for road maintenance depots and the increase in workload and costs. In this respect, A. syriaca is supposed to be more susceptible to mowing before flowering than at an early growth stage (Zalai et al., 2017). Although herbicides can be used to control the species (e.g., Bhowmik, 1994; Zalai et al., 2017), they are generally not used along roadsides in Austria. If A. syriaca densities are to be lowered in the study area and elsewhere, emphasis should be placed on a proper roadside management (e.g., continuous mowing, removal of A. syriaca populations followed by seeding fast-growing and/or competitive plant species; Schuster et al., 2018) and the control in the respective adjacent habitat (periodic maintenance of old fields, control along the crop field border by spot treatment and/or frequent soil cultivation; Bhowmik, 1994).