Invasive Neophytes in the Forest

In Switzerland more and more plant species originating from other regions, or even other continents, are making an appearance. When these species escape into the wild, and appear in large numbers, they can become a threat to indigenous species, cause economic damage or affect people’s health. In forests such species grow predominantly at lower warmer altitudes, near to settlement areas and are usually plants which have been purposely imported for horticultural use and which have then escaped into the wild.

Hardly a week goes by when the media does not report on a new animal or plant species which has appeared in Switzerland and has spread. Especially since the case of the common ragweed (Ambrosia artemisiifolia) the public has come to realise that changes in flora and fauna not only bring losers in terms of biodiversity loss, but also winners. And Ambrosia is not alone here. In the past decades various alien plant species have been able to spread rapidly in Switzerland. Have they also spread into the forests?

Figure 2 shows the altitudinal distribution of Neophytes using current data from the Swiss Biodiversity Monitoring programme (BDM). It can be seen from the chart that neophytes are mainly to be found at lower altitudes. The number of such species is especially high in the canton of Ticino. Forests at lower, warmer altitudes, such as those in Ticino, are more likely to harbour neophytes than forests in mountainous areas.

In Switzerland lower altitudes with a higher forest cover have, on average, less neophyte species than areas which have a sparser forest cover (Fig. 3). This difference is due to the fact that neophytes are usually present outside of forests. Therefore, in built up areas in Zurich the percentage of neophytes is around 50% (Landolt 2001), whilst forests in the immediate vicinity have a percentage of only 8%.

Invasive neophytes which are also considered to be forest species are listed on the "Black List" as well as on the "Watch List" (Table 1). They are referred to in the following, for purposes of simplification, as forest neophytes, although the species are not only limited to forests. What is particularly noticeable about these species, in comparison to the general neophyte flora of Switzerland, is that a large percentage are bushes (Cornus, Mahonia, Prunus, Viburnum) and vines (Lonicera, pueraria). Many of these are also, in contrast to indigenous flora, evergreen.

The most frequent species is the black locust which originates from North America and which appears as a neophyte especially in dry-warm places. Apart from the black locust there are other invasive alien tree species which appear locally or which are not (yet?) considered forest species in Switzerland. The tree-of-heaven (Ailanthus altissima), which originates from China and Korea, is a neophyte frequently to be found in settlement areas and along roadways or railway lines and sometimes in the forests of Ticino.

Table 1 – Invasive neophytes on the "Black list" and the "Watch list" which are present in forests (according to Landolt 1991, Delarze et al. 1999, Moser et al. 2002). The relative frequency refers to the 593 monitoring areas according to ZDSF, as well as, to 458 recordings from BDM samples. Non-determined species of neophytes are marked by a (?).

Diagram 4 shows a map of country- wide know occurrences of the above listed neophytes. Apart from the distribution data from the ZDSF data from the smaller recording areas of the BDM are also shown. The map shows that invasive forest neophytes are present at lower altitudes all over Switzerland. In Ticino they are especially numerous – even in the smaller BDM areas.

However, at lower altitudes in Northern Switzerland they are rarer and are often absent from the BDM areas. The ZDSF data show focal points in the towns (Zurich, Basel, Lausanne, Geneva) but this is partly due to the better information situation there. For instance data on the Armenian Blackberry (Rubus armeniacus, see below) had mainly been collected in the Zurich area, although this species is known to have a much larger distribution within Switzerland. Knowledge about the present speed of spread of alien species is often inadequate.

Is it therefore possible to find a universal law for the appearance of invasive forest neophytes? Results from standardised recorded BDM data show a clear relationship between the appearance of these species and the settlements in a monitoring area. (Fig. 5) In the BDM data the presence of wild but not cultivated plants is taken into account. Although countrywide the existence of invasive forest neophytes is facilitated by higher temperatures in the north of Switzerland the proximity to settlement areas is without doubt the most important factor. This can be understood when the paths taken by a species when spreading are taken into consideration. Apart from the black locust, which has been promoted in forests for over 200 years, all of the species named in Table 1 are used for urban greening. From this source the species are able to spread into the nearby forests; the proximity to settlement areas is therefore the deciding factor for their spontaneous appearances.

An example of an invasive forest neophyte spread pathway can be seen in Fig. 6. Henry’s honeysuckle (Lonicera henryi), which is available from garden centres and is often used for greening around buildings, has spread into nearby forests. The vine behaves in a similar way to the Japanese honeysuckle (Lonicera japonica), an invasive forest neophyte, which is widely distributed in the Ticino and has in certain areas even grown into the tree layers. (Fig. 1).

A further example is Fig. 7 – the already mentioned Himalayan blackberry (Rubus Armeniacus). This is also an invasive neophyte (Black List) which is planted as a fruit plant in gardens. This blackberry has spread from the forests onto industrial sites or along railway lines and is causing increased maintenance costs. The Armenian blackberry is sturdier than the majority of indigenous blackberry species and can be found on the edges of forests and even in rejuvenation areas within forests.

Blackberries are regarded as a problem species in silviculture causing increased costs in the care of young forests. These are is usually indigenous blackberry species, but it can be expected that the Armenian blackberry will spread out further along forest edges and into young forest areas. In Figure 7 this species can be seen together with raspberries (Rubus idaeus) in a rejuvenation area. This example shows that invasive neophytes are not always immediately noticeable, even when using the word “exotic” when talking about them.