Revegetation Options Following the Eradication of Large Heracleum Species

Successful eradication of stands of invasive weeds leads to increased vulnerability to reinvasion by the species that was eradicated or other invasive species. Many invasions change ecosystem functioning (Richardson et al., 2000) and resulting site characteristics may be favourable to invasive species in general. Areas from which an invasive species has been eradicated may be subject to a changed disturbance regime and usually nutrition levels will have increased. If the soil is left denuded, the areas will be easily accessible for invasive weeds from the surrounding areas (Zavaleta et al., 2001).

During the search for natural regulating factors of H. mantegazzianum in the north-western Greater Caucasus (see Cock and Seier, Chapter 16, this volume), it was found that H. mantegazzianum becomes established quickly on newly denuded river banks (see Otte et al., Chapter 2, this volume). Where the melting snow in spring causes flooding of river banks and leaves the soil surface without vegetation, H. mantegazzianum is one of the most successful early colonizers. In the regions where Heracleum is introduced it often creates single-species stands along rivers and streams (see Pysek et al., Chapter 3, this volume). This is, however, not the only habitat type where large Heracleum species may out-compete native vegetation. In general, areas with a high level of disturbance and intensive human activities are more likely to be invaded by large Heracleum species (see Thiele et al., Chapter 8, this volume).

It has been suggested that successful establishment of an invasive species does not always imply its competitive superiority over all native plant species in the area. On a local scale successful establishment of native plants in unoccupied patches following sowing of seeds indicates that there is a potential for increasing diversity even where invasive plant species have established (Turnbull et al., 2000). Therefore, the outcome of invasion may depend on propagule pressure, i.e. the amount of available seed of species native to the invaded site. Use of cover crops following the control of invasive species can therefore be a practical tool for post-control management of affected sites.

Cover crops may influence environmental conditions and alter the outcome of competition in different ways. An agricultural study in no-till maize (Zea mays L.) from southern New England, USA, shows that a cover crop of Vicia grandiflora Scop. with rye (Secale cereale L.) suppresses weeds better than rye alone. Although V. grandiflora produced less biomass and nitrogen than V. villosa Roth., it survived better under suboptimal seedbed conditions and produced crops for several seasons from a single sowing (DeGregorio et al., 1995). Cover crops could act by decreasing the level of resources available to re-invading species and alter the outcome of the competition between native and invasive plants. This may be especially efficient if resources are reduced, for which native species are better competitors. During restoration of sedge meadow wetlands in mid-west and northwest USA, Phalaris arundinacea L., a perennial circumboreal grass, is recognized as an aggressive invasive plant species (Perry and Galatowitsch, 2003). Echinochloa crus-galli (L.) P. Beauv. is able to suppress P. arundinacea in these areas. However, E. crus-galli also suppresses desired, native wetland species such as Carex hystericina Muhl. Perry and Galatowitsch (2003) conclude that although some cover crops prevent native species becoming established, others may affect the outcome of competition in a more desireable way. Ecologists seeking cover crops for invasive species control should first determine the environmental conditions under which native species are more successful (Perry and Galatowitsch, 2003).

Soil cultivation negatively affects the survival and performance of large Heracleum species. Seedlings and young plants are killed by ploughing and if seeds are covered by more than 25 cm of soil, germination is hindered. Not all infested areas give opportunities for post-control management by tillage; at riversides and in open forests other methods must be applied. In central European beech (Fagus sylvatica L.) forests in Bern, Switzerland, and Denmark it was observed that H. mantegazzianum occasionally invades temporary gaps in the understorey following the fall of mature trees. When branches of adjacent trees narrow the gap, H. mantegazzianum disappears as it cannot compete for light with young beech trees. Eventually, they disappear and the gap closes again (H.P. Ravn, unpublished observations; Fig. 17.1). The capability to out-shade H. mantegazzianum will depend on the tree species. Thus, it has been observed in Denmark that where a stand of oak, Quercus robur L., meets a stand of beech and hornbeam, Carpinus betulus

Fig. 17.1. Forest gap in Fagus sylvestris forest closing on a population of H. mantegazzianum, Bern, Switzerland. Photo: H.P. Ravn.

Fig. 17.2. Forest trees may out-shade the large Heracleum species. In this picture from Denmark the borderline between the open Quercus roburforest (centre and right) and the more shading Fagus sylvatica and Carpinus betulus forest (to the far left) is also the borderline for out-shading of H. mantegazzianum. Photo: H. Lerdorf.

Fig. 17.2. Forest trees may out-shade the large Heracleum species. In this picture from Denmark the borderline between the open Quercus roburforest (centre and right) and the more shading Fagus sylvatica and Carpinus betulus forest (to the far left) is also the borderline for out-shading of H. mantegazzianum. Photo: H. Lerdorf.

L., H. mantegazzianum will survive in the oak forest only (Fig. 17.2) (H. Lerdorf, Denmark, 2005, personal communication). In a similar vein, Pieret et al. (2005) demonstrated that under an open Alnus/Salix forest the seed rain from H. mantegazzianum and the density of its seedlings were significantly higher under open canopy than under a closing canopy due to succession towards forest. Shade tolerance may vary between the Heracleum species; it has been observed that H. sosnowskyi is more shade tolerant than H. mantegazzianum (Z. Gudzinkas, Vilnius, 2005, personal communication).

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