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Journal for Nature Conservation 16 (2008) 61—71

Invasion patterns of Heracleum mantegazzianum inGermany on the regional and landscape scales

Jan Thiele�, Annette Otte

Division of Landscape Ecology and Landscape Planning, Justus-Liebig-University Giessen,Heinrich-Buff-Ring 26-32, 35392 Giessen, Germany

Received 2 March 2006; accepted 2 August 2007

KEYWORDSAbandonedgrassland;Dominance;Habitat saturation;Impact assessment;Invasion success;Invasive alienspecies

SummaryHeracleum mantegazzianum Somm. et Lev. is an invasive tall forb in Europe withimplications for human health (photo-dermatitis), recreational and economicinterests, and local biodiversity. This paper presents invasion patterns of the speciesin Germany on the regional and landscape scales and assesses the species’ impactson native habitats. We conducted a survey addressing the nature conservationauthorities of all 440 German districts and carried out our own field studies in themost heavily invaded landscapes in Germany. The survey indicated thatH. mantegazzianum is present and perceived as a potentially dangerous invader inabout two-thirds of German districts, while actual or short-term hazards can beassumed for only about 15% of districts. The latter were concentrated in the naturalgeographic region ‘western low mountain ranges’. In the field studies, dominantstands of H. mantegazzianum, which bear the highest potential for adverse effectson native biodiversity, accounted for 36% of all large stands of the species. Invasionsuccess was highest in abandoned grasslands, grassland and field margins, andcorresponding tall-forb stands. The saturation (% area covered) of these preferredhabitats with H. mantegazzianum was 8.7%. The invasion percentage (% areainvaded) was 18.5%. In conclusion, our results suggest that today H. mantegazzianumhas only moderate impacts on the regional and landscape scales even in most heavilyinvaded regions of Germany.& 2008 Elsevier GmbH. All rights reserved.

Introduction

The umbelliferous tall forb Heracleum mante-gazzianum Somm. et Lev. (Giant Hogweed) is one ofthe most prominent invasive species in CentralEurope today. It was introduced from its native

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range in the Western Greater Caucasus to botanicand private gardens in several European countriesin the 19th century (e.g. Kowarik, 2003; Ochsmann,1996; Wyse Jackson, 1989). During the 20th centurythe species became a popular ornamental plant(Kobylka, 1977; Lundstrom, 1984; Pysek, 1991;Sheldon, 1982) and was also propagated as a beeplant (Adolphi, 1995; Zander, 1930). Hence, thespecies has been widely dispersed by humans(gardeners and bee keepers) which substantiallyenhanced its spread (Pysek, 1991). H. mantegaz-zianum has repeatedly escaped cultivation since itsintroduction (Ochsmann, 1996) but a massivespread has been observed in several Europeancountries only from the 1950s onwards (e.g. CzechRepublic: Pysek, 1991; Germany: Ochsmann, 1996;Great Britain: Clegg & Grace, 1974; Tiley et al.,1996; Wade et al., 1997). Today, H. mantegazzia-num is widespread in Germany and occupies atleast 57% of grid cells in the national floristic map(German National Floristic Database, ‘DatenbankGefaßpflanzen’; www.floraweb.de).

H. mantegazzianum has serious health implica-tions for humans due to phyto-photo-dermatitiscaused by furocoumarins (syn. furanocoumarins)contained in the sap of the plant (Drever & Hunter,1970; Jaspersen-Schib et al., 1996; Lagey et al.,1995). Further, it conflicts with recreational andeconomic interests, e.g. by obstruction of trails andriverbanks (Tiley & Philp, 1994), and may leadto serious erosion of riverbanks (Caffrey, 1994).Moreover, it can reduce local biodiversity (alphadiversity) by outcompeting native plant species(Lundstrom, 1984; Manchester & Bullock, 2000;Pysek & Pysek, 1995; Thiele & Otte, 2007). There-fore, H. mantegazzianum is commonly regarded asa problem plant that provokes costly and tediouscontrol actions. The total annual costs due tohealth impacts and management of the species inGermany were estimated as ca. h12 million(Reinhardt et al., 2003).

As monetary resources are frequently limited, itis necessary for managers to decide which invasivespecies and populations to control in the first placeand which ones to control later or leave alone(Hiebert, 1997). In order to take sound and sensibledecisions, managers are in need of information onthe impacts of invasive species. However, rigorousassessments of imminent impacts from non-indi-genous species have rarely been conducted (Byerset al., 2002; Parker et al., 1999).

On a geographical scale, impacts can be quanti-fied by: (i) the range size of an invader; (ii) itsabundance per unit area across that range; and(iii) the size of the effect per individual or perbiomass unit (Parker et al., 1999). It is reasonable,

for impact assessment, to try to narrow down thetotal non-indigenous range of a species to regionswhere the species actually has spread and in-creased in abundance after introduction, i.e.where it is an ‘invasive species’ (sensu e.g. Kolar& Lodge, 2001). Concerning the abundance ofinvasive plant species, dominant stands, whichH. mantegazzianum is able to build up, will haveespecially severe effects on recipient habitats.Therefore, the proportion of stands that attaindominance appears to be a useful additionalmeasure. Furthermore, better precision in asses-sing impacts will be achieved if the available areaof potentially suitable habitats is taken intoaccount. Finally, different types of invaded habi-tats should be distinguished in any assessment ofimpacts because abundances and effects mightvary with habitat type.

The aim of our study was to assess the impactsof H. mantegazzianum on native habitats at theregional and landscape scales. Our objectiveswere to:

(1) assess the large-scale pattern of H. mantegaz-zianum invasion throughout Germany and toidentify regions where the species is ‘invasive’;

(2) record the distribution and abundance of thespecies in the most heavily invaded landscapeswith regard to different habitat types;

(3) calculate the area-corrected relative invasion –

invasion percentages (i.e. % habitat areainvaded) and habitat saturation (i.e. % habitatarea covered by H. mantegazzianum) – for eachinvaded habitat type and, finally;

(4) assess present impacts and to make a prognosisfor the species’ potential to threaten regionalbiodiversity.

Methods

Germany-wide survey

In 2001, a survey on H. mantegazzianum wasconducted by addressing questionnaires to thenature conservation authorities of all 440 Germandistricts (‘Landkreise’) and cities independent froma district administration (‘kreisfreie Stadte’). Thequestionnaire asked for information on habitattypes invaded by the species (default list provided),occurrences in nature reserves (yes, no), protectedhabitat types (which ones?), and whether inven-tories of the species had been carried out. For eachhabitat type addressees were asked to estimate thefrequency class of H. mantegazzianum (absent,

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J. Thiele, A. Otte62


rare, occasional, common) and maximum spatialextent of single stands of the species (up to 100m2,4100–1000m2, 41000m2).

The received data were used to create a rankingof districts by invasion intensity which was assessedby summing up weights allocated to estimatedfrequency classes and maximum stand sizes,with higher frequencies and larger stand sizesreceiving higher weights. Extra points were awar-ded for each protected habitat type reported to beinvaded and inventories of H. mantegazzianumcarried out. Index values derived from this summa-tion were categorised into four classes of invasionintensity: species absent, low, medium, and highinvasion intensity. The first class contained zerovalues only, while the latter were derived bydividing the range of non-zero values into threeequal intervals.

Locating and mapping of study areas

For our own field research, study areas weredefined as landscape sections of 1� 1 km2 whichhad to meet the criterion of containing at leastthree stands of the species. This criterion was set inorder to: (i) avoid marginally infested landscapecontaining only isolated and maybe ‘accidental’stands; (ii) add objectivity to the sampling proce-dure (all encountered areas meeting the require-

ments were surveyed); and (iii) enable efficientdata recording.

The 35 most heavily invaded districts (andindependent cities), based on the Germany-widesurvey, were chosen as potential study regions andtheir nature conservation authorities were asked tosend copies of topographic maps (1:10,000–

1:25,000) depicting known H. mantegazzianumstands. Maps were received from 33 districts ofwhich 22 seemed to have suitable study areas.Altogether, 30 potential study areas were scruti-nised on field excursions and, finally, 20 proved tomeet the requirements defined above. These studyareas, which were distributed over 14 districts inseven German states, were surveyed in the summerseasons of 2002 or 2003. State, district, gridcoordinates, and altitude of study areas are givenin Table 1.

Within each study area all stands of H. mante-gazzianum were mapped by means of a GPS system(sub-meter accuracy). Stands smaller than 25m2 ornarrower than 1m were mapped as points or lines,respectively. Larger and wider stands, here re-ferred to as ‘large stands’, were mapped aspolygons categorised into dominant stands (densestands) and open stands. The criterion for dom-inance was H. mantegazzianum cover exceeding50% of the total surface area of the stand.Abundances of H. mantegazzianum and the per-centage of reproductive individuals were estimated

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Table 1. State, district, grid coordinates, and altitudes of study areas

No. State District (‘Landkreis’) Grid east Grid north Altitude (m a.s.l.)

1 Rhineland-Palatinate Altenkirchen 3410.500 5623.000 1602 Rhineland-Palatinate Ahrweiler 2588.300 5594.500 135–1753 North Rhine-Westphalia Ennepe-Ruhr-Kreis 2593.800 5696.400 854 North Rhine-Westphalia Euskirchen 2545.800 5595.000 470–4905 North Rhine-Westphalia Euskirchen 2535.500 5589.000 5906 Bavaria Freising 4465.500 5362.500 480–5007 Bavaria Garmisch-Partenkirchen 4430.200 5270.000 8658 Bavaria Garmisch-Partenkirchen 4443.500 5253.500 9309 Lower Saxony Gottingen 3552.500 5710.500 23510 North Rhine-Westphalia Hagen 3396.700 5687.000 145–19511 North Rhine-Westphalia Hagen 2600.100 5695.500 9012 North Rhine-Westphalia Hagen 3397.000 5689.800 260–29013 Hesse Kassel 3529.200 5684.000 270–30514 Hesse Lahn-Dill-Kreis 3467.000 5595.500 26015 North Rhine-Westphalia Olpe 3421.500 5664.500 255–27516 Thuringia Wartburgkreis 3569.500 5620.500 325–35017 Hesse Waldeck-Frankenberg 3488.300 5668.500 26018 Hesse Waldeck-Frankenberg 3477.800 5655.500 325–34519 Hesse Waldeck-Frankenberg 3487.500 5661.200 260–31020 Saarland St. Wendel 2589.000 5482.100 360–395

Coordinates represent the south-western corner of study areas (each 1� 1 km2) according to the German geodetic system (‘Gauß-Kruger’). If the altitudinal range of plots in a study area is less than 20m, average values are supplied, otherwise the lowest and highestvalue of investigated plots.

Heracleum mantegazzianum invasion patterns in Germany 63


(not taking into account seedlings and juvenileswith only primary leaves) and habitat types wererecorded for all point-like, linear, and large stands.GPS data were imported to ArcView GIS 3.2(&Environmental Systems Research Institute, Inc.)for quantitative analysis. The total number ofindividuals of H. mantegazzianum was calculatedfrom abundance estimates and number, length, orarea of the respective stand types.

Measurement of invasion percentages andhabitat saturation

Invasion percentage was defined here as the ratiobetween the area of H. mantegazzianum standsand the total area of the respective habitat typewithin the study areas. Habitat saturation wasdefined as the ratio of the area covered byH. mantegazzianum plants within the stands andthe total area of the habitat type (cf. Pysek &Pysek, 1995). As the cover percentages ofH. mantegazzianum are mostly lower than 100%,the habitat saturation is lower than the invasionpercentage. The spatial extent of potential habi-tats was assessed by interpreting digital aerialorthophotos of the study areas. All areas withsufficient extent to allow for adequate precision ofarea measurement from the images were mappedas polygons in ArcView GIS and for each habitattype the area sum was calculated. Patches insuffi-cient in size and fringes narrower than about 5m innature were not mapped separately but subsumedto neighbouring areas. The habitat types whichcould be discerned by interpretation of aerialimages are listed and described in Table 2.

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Table 2. Habitat types and other land-cover typeswhich could be discerned in the interpretation of aerialimages

Habitat type Key traits

Abandoned grasslands,margins of grasslands andfields, and tall-forb stands

More or less nutrient richsites which have not beensubject to regular land usein recent years and whichfeature herbaceousvegetation (mostlydominated by grasses andsometimes dominated bytall forbs)

Open riverbanks Unshaded riverbanks withherbaceous vegetation

Shaded riverbanks Riverbanks shaded by treelines, copses, or forests

Table 2. (continued )

Habitat type Key traits

Open railwaysides Unshaded railwaysides(verges, embankments) withherbaceous vegetation

Shaded railwaysides Railwaysides (verges,embankments) shaded bytree lines, copses, or forests

Open roadsides Unshaded roadsides (verges,embankments) withherbaceous vegetation

Shaded roadsides Roadsides (verges,embankments) shaded bytree lines, copses, or forests

Woodlands Copses, tree-dominatedwasteland, afforested sites,and scrubland

Ruderal areas Heavily disturbed sites, suchas sand pits, rotovatedareas, etc.

Managed grasslands More or less nutrient richmeadows and pastureswhich are usedagriculturally on a regularbasis

Forest margins and fringes Ecotonal zone betweenforest and adjacentvegetation and theoutermost 10m of the forestitself

Housing areas Areas of coherent plots usedfor housing

Garden plots Gardens outside settlements

Nutrient-poor grasslands Low-intensity meadows orpastures at rather nutrientpoor sites

Industrial and businessareas

Areas of coherent plots ofindustry or business use

Amenity grassland Lawns in parks, sportscomplexes, etc.

Straw meadows Wet meadows on poorsubstrates which are mownonce per year in latesummer or autumn

Lakes Water body of lakes andponds

Streets Tarmacked area of streets

Railway tracks Rails and their gravel bed

Rivers Water body of rivers



Abandoned grasslands, margins of grasslands andfields, and tall-forb stands had to be combined intoone category due to methodological constraintsrelated to the interpretation of aerial images. Thearea of forest margins was determined by creating10m buffer zones inside the forest polygons. Point-like and linear stands at fringes of woodlands andscrublands were included into the category ‘wood-lands’ while stands at forest fringes were includedinto ‘forest margins and fringes’.

The area of large stands of H. mantegazzianumwithin a particular habitat type was measured byintersecting polygons mapped by GPS in the fieldwith the interpretation of aerial images. The areacovered by point-like and linear stands was calcu-lated from abundance estimates, length (in thecase of linear stands), and percentage of repro-ductive individuals under the assumption that thearea covered by each individual alone is on average1m2 for reproductive individuals and 0.1m2 forvegetative individuals.

Results

Germany-wide survey

In total, 309 (70.2%) of the 440 questionnaireswere returned. Of these, 300 stated that H. mante-gazzianum was present in the district area (68.2%of the total, 97% of returns). Occurrences in naturereserves were mentioned by 50% of the districtsthat had replied and denied by 26%, while theremaining made no statement. About 40% reportedprotected habitat types to be invaded. Amongthese were, most frequently, natural riversides andwet grasslands and, occasionally or rarely, alluvialforests, alder swamp forests, calcareous and acidicfens, lakeshores, terrestrial reed stands, andnutrient-poor (chalk) grasslands. Inventories ofH. mantegazzianum stands had been carried outin at least 21% of the districts (48% ‘no inventories’,31% ‘no statement’) and 3.7% indicated (withoutbeing asked) that management action had beenundertaken.

There were significant differences of H. mante-gazzianum frequency estimates between habitattypes (Kruskal–Wallis ANOVA: po0.001). H. mante-gazzianum occurred most frequently on ‘riverbanksand ditches’ and ‘road verges and paths’ (tested byMann–Whitney U-tests with Bonferroni adjust-ment). Intermediate frequencies were exhibitedby ‘ruderal areas’, ‘forest margins and fringes’, and‘gardens and parks’, while ‘fallow fields andabandoned grasslands’, ‘railway tracks and sta-tions’, and ‘low-intensity grasslands’ were men-

tioned noticeably less frequently. The specieswas least frequently reported from high-intensitygrasslands.

Also concerning the maximum extent of singlestands of H. mantegazzianum there were signifi-cant differences between habitat types (Kruskal–Wallis ANOVA: po0.001). Stands of the speciesmost frequently reached very large extent ofcoverage (41000m2) in ‘riverbanks and ditches’,‘ruderal areas’, and ‘fallow fields and abandonedgrasslands’, whereas the maximum extent of standswas significantly smaller in ‘road verges and path’,‘gardens and parks’, ‘railway tracks and stations’,and ‘high-intensity grasslands’ (Mann–WhitneyU-tests with Bonferroni adjustment). ‘Forest mar-gins and fringes’ and ‘low-intensity grasslands’ didnot differ significantly in stand size from all theother habitat types.

On the basis of index values of invasion intensitynine districts (3% of returns) were classified as‘high’, 57 (18%) as ‘medium’, and 234 (76%) as‘low’ while in another nine districts H. mantegaz-zianum was absent. Figure 1 shows that theparticular classes were not evenly distributed overGermany. There was a significant accumulation of‘medium’ and ‘high’ levels of invasion intensity inthe mid-western parts of Germany (Mann–WhitneyU-test: po0.001) which mostly coincided with thenatural geographic region ‘western low mountainranges’. In the regions ‘Alps’ and the ‘foothills ofthe Alps’ (‘Alpenvorland’), there was a slightaccumulation of ‘medium’ invasion intensity, sug-gesting a secondary focus. In contrast, in the‘north-eastern lowlands’, districts without H. man-tegazzianum occurrences or ‘low’ invasion inten-sity prevailed, except for Berlin and two districts of‘medium’ and ‘high’ level where giant Heracleumsp. were tested as a fodder crops in the 1960s andsubsequently spread into the wild. However, thesetest plants, at least in the district ‘Oder-Spree’,were reported to be Heracleum sosnowskyi (Zim-mermann, 1966). Throughout the remaining partsof Germany, the ‘north-western lowlands’ and‘south-western low mountain ranges’, invasionintensity was predominantly ‘low’, interspersedwith few instances of ‘medium’ level.

Field investigations

The study areas were primarily situated in thefocal region ‘western low mountain ranges’ (16 outof 20 study areas). They covered a total area of2000 ha (20 km2) and contained 233 large stands ofH. mantegazzianum of which 36% were dominantstands. Altogether, the stands occupied an area of

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16.4 ha (0.8% of the total study area). Open stands(11.8 ha) generally prevailed over dominant ones(4.6 ha). With both stand types, sizes of singlestands between 100 and 1000m2 occurred mostfrequently (145 stands) while stands larger than1000m2 were in the minority (32).

Apart from large stands, occurrences of H.mantegazzianum were frequently found in theform of linear and point-like structures not suitedfor mapping of spatial extent. Linear structuresbearing H. mantegazzianum were found in 16 out of20 study areas and amounted to a length ofbetween 30 and 2121m per study area. Point-likestands were found in all study areas with absolutefrequencies of between 2 and 57 per study area.The number of stands per category declined frompoint-like (322), over linear (185), and open (148)

to dominant (85), while the number of individualsper category exhibited the opposite pattern (6921;12,690; 53,979; 126,687 individuals per category,respectively) with 63% of all individuals accumu-lated in dominant large stands.

Figure 2 shows the absolute frequencies of H.mantegazzianum incidences per habitat type foundduring the field surveys of 2002 and 2003. Inaccordance with the questionnaire survey, road-sides and embankments of rivers and ditchesshowed high frequencies regardless of being openor shaded by trees. Also margins and fringes offorests, woodlands, and scrublands were frequentlyinfested by H. mantegazzianum, while this speciesoccurred less commonly in ruderal areas andon railwaysides. In contrast to the questionnairesurvey, abandoned grasslands were among the

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Figure 1. Map of Heracleum mantegazzianum Somm. et Lev. invasion intensity in districts of Germany. Classification ofinvasion intensity was based on a Germany-wide survey addressed to 440 district conservation authorities in 2001. Theline signature delineates the natural geographic region ‘western low mountain ranges’ which represents a focal regionof H. mantegazzianum invasion.



commonest habitat types of H. mantegazzianumand the species was even found in managedgrasslands quite regularly. Furthermore, the spe-cies occurred with intermediate frequencies inwoodlands (copses, tree-dominated wasteland andafforested sites), at margins of grasslands and fieldsand in tall-forb stands at disused sites (this habitattype had not been included in the Germany-widequestionnaire survey). The percentage of dominantstands among all large stands varied with habitattype and was especially high (above 50%) for openroadsides, abandoned grasslands, and margins ofgrasslands and fields. Protected habitat types werealmost completely absent in the field recordsexcept for two sites featuring abandoned andslightly wet grasslands, and some occurrences inalluvial forests which, however, did not contain RedList species (Thiele & Otte, 2006).

The highest invasion percentage (18.5%) wasfound for abandoned grasslands, margins of grass-lands and fields, and tall-forb stands (which hadto be combined into one category) followed byopen riverbanks (13.8%), open railwaysides (9.7%),ruderal areas (5.8%), and open roadsides (3.4%).The remaining invaded habitat types showedinvasion percentages of about 2% or less. Due totheir lesser frequency and spatial extent dominantstands contributed considerably less to the in-vasion percentages than open stands (Table 3).Contributions of point-like and linear stands toinvasion percentage and habitat saturation werenegligible throughout. The highest habitat satura-tion (8.7%) was again found for abandoned grass-lands, margins of grasslands and fields, and tall-forb stands.

Discussion

Perception and extent of H.mantegazzianum invasion in Germany

The high percentage of returns (70.2%) of thesurvey indicated that nature conservation autho-rities in Germany are well aware of the invasion ofH. mantegazzianum. Nearly all returned question-naires (97%) stated that the species was presentand half of them confirmed occurrences in naturereserves. These ratios, however, cannot be extra-polated to the whole of Germany, as missing returnsmostly coincided with regions where the species isabsent or exhibits only sparse records accordingto the national floristic map as of 2002 (GermanNational Floristic Database, ‘Datenbank Ge-faßpflanzen’; www.floraweb.de). Apparently, dis-tricts in which the species is not present or too rareto be considered relevant refused to reply (exceptfor 3% of returns), whereas returned questionnairessuggest that H. mantegazzianum is perceived as apotentially hazardous invader in the respectivedistricts. If we start from this assumption, inapproximately two-thirds of German districtsH. mantegazzianum is perceived as an invader, inabout one-third it reportedly occurs in naturereserves and in almost 30% it has allegedly invadedprotected habitat types.

While keeping in mind the pitfalls of subjectivity,the index of invasion intensity based on the surveyresults appears suitable for comparing districtswith regard to H. mantegazzianum’s invasionsuccess and to identify districts likely to face im-plications. The field surveys confirm that districts

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open roadsidesshaded roadsides

abandoned grasslandsshaded riverbanks

wood-/ scrubland and forest fringesopen riverbanks

woodlands (incl. afforestations)fringes of grasslands and fields

managed grasslandstall-forb stands

ruderal areasshaded railwaysides

open railwaysideshousing areas

0 20 40 60 80

dominant standsopen standslinear standspoint-like stands

Figure 2. Absolute frequencies of Heracleum mantegazzianum Somm. et Lev. incidences found during field surveys ofdifferent habitat types in 2002 and 2003. Signatures separate different stand types of H. mantegazzianum (point-likestands, linear stands, large open stands, large dominant stands).



classified into ‘medium’ or ‘high’ level of invasionintensity comprise ‘hot spots’ of invasion. However,experience from field studies shows that invasionintensity is rather over-estimated than under-estimated. This can be concluded from the factthat one-third of all potential study areas allegedlyrepresenting invasion ‘hot spots’ in districts classi-fied into ‘medium’ or ‘high’ level of invasionintensity turned out to be only negligibly invadedby H. mantegazzianum (i.e. single stands with fewindividuals). Districts classified into ‘low’ invasionlevel prevalently reported H. mantegazzianum tobe ‘rare’ or ‘occasional’, at the most, and seldomreported large stands. Given the tendency to over-estimate invasion severity, it can be assumed thatin these districts occurrences of H. mantegazzia-num are merely sporadic and small.

Thus, it seems reasonable to narrow down thenumber of districts with actual or imminent hazardsof H. mantegazzianum to those with ‘medium’ and‘high’ invasion intensity. If we rate all missingreturns as either ‘species absent’ or ‘low’ invasionintensity, we can project ‘medium’ and ‘high’levels to 13% and 2% of all districts, respectively.Altogether, this suggests that H. mantegazzianum,although present and perceived as an invader in themajority of districts, is an actual or short-termhazard in comparatively few districts. The map ofinvasion intensity in German districts (Figure 1)

shows that districts likely to face problems withH. mantegazzianum are prevalently found in the‘western low mountain ranges’. Projections of‘medium’ and ‘high’ invasion intensities are abouttwice as high for districts overlapping with thisregion (23% and 5%, respectively) as for the wholeof Germany.

Furthermore, the survey results suggest a sec-ondary focus around the foothills of the BavarianAlps. However, the national floristic map statesonly sparse records in this region and our owninvestigations gave the impression that only a fewisolated centres of invasion exist (two study areaswere investigated). Presumably, in these casesclassification into ‘medium’ level of invasionintensity is an over-estimation which might beattributable to the greater awareness of natureconservation authorities in this region of areas ofespecially high conservation value.

Three different factors may play a role to explainthe focus of H. mantegazzianum invasion on lowmountain ranges of mid-western Germany: (i) theclimate of this region (sub-atlantic and (sub)mon-tane) closely resembles climatic conditions of thenative range of the species as compared to otherregions of Germany (‘north-eastern lowlands’,‘south-western low mountain ranges’); (ii) habitatavailability might be higher, depending consider-ably on changes in land-use regimes, especially

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Table 3. Invasion percentages of different stand types of Heracleum mantegazzianum Somm. et Lev. and totalhabitat saturation aggregated over 20 study areas (landscape sections of 1� 1 km2) in Germany

Habitat type Habitatareaavailable(m2)

Open stands Dominant stands Point-like andlinear stands

Habitat saturation

Areainvaded(m2)

Invasionpercentage(%)

Areainvaded(m2)


Areainvaded(m2)


Areacovered(m2)

Habitatsaturation(%)

Abandoned grasslands,grassland margins andtall-forb stands

427,804 50,720 11.9 27,398 6.4 958 0.2 37,214 8.7

Open railwaysides 19,647 808 4.1 786 4.0 320 1.6 830 4.2Open riverbanks 65,747 7,077 10.8 1,537 2.3 428 0.7 1,855 2.8Ruderal areas 79,259 1,806 2.3 2,707 3.4 56 0.1 2,189 2.8Open roadsides 67,001 1,057 1.6 307 0.5 899 1.3 1,085 1.6Shaded riverbanks 219,569 3,809 1.7 462 0.2 299 0.1 2,108 0.7Woodlands 1,284,723 10,414 0.8 11,320 0.9 649 0.1 5,760 0.7Shaded railwaysides 172,833 364 0.2 445 0.3 161 0.1 706 0.4Shaded roadsides 212,431 1,126 0.5 48 0.0 520 0.2 339 0.2Managed grasslands 3,871,259 37,897 1.0 593 0.0 12 0.0 2,498 0.06Forest margins/fringes 1,115,017 1,777 0.2 251 0.0 168 0.0 393 0.04Housing area 1,062,694 86 0.0 124 0.0 0 0.0 54 0.01

Invasion percentage is calculated here as the ratio between the area sum of H. mantegazzianum stands and the total available habitatarea. Habitat saturation is calculated as the ratio of the area covered by individuals of H. mantegazzianum and total available habitatarea.



abandonment of grasslands (Thiele & Otte, 2006);or (iii) the number of local introductions by humans(e.g. sowing in the wild by bee keepers, cultivationin gardens and parks) per unit area might have beenhigher. It seems quite possible that all three factorshave an effect on the intensity of H. mantegazzia-num invasion. However, confirming their signifi-cance is beyond the scope of this study.

Invasion pattern in study areas

Saturation of suitable habitats with stands ofH. mantegazzianum best represents the invasionsuccess (Pysek & Pysek, 1995). According to thismeasure (defined as the ratio between habitat areacovered by H. mantegazzianum and total availablehabitat area), H. mantegazzianum is most success-ful in abandoned grasslands, grassland and fieldmargins, and tall-forb stands at disused sites. Anadditional measure of the invasion success and theinvasibility of habitats is dominance of the invader(Lundholm & Larson, 2004). The moderate percen-tage of dominant stands (36%) among large standsof H. mantegazzianum suggests that this species isnot always dominant although stands are notnecessarily in equilibrium with their environmentand possibly could further increase in density.Comparing the percentages of dominant standsfor the mentioned habitat types, H. mantegazzia-num seems to be less successful in tall-forb stands(24% of large stands dominant) than in the formertwo habitats (both above 50% dominant stands).Thus, it can be stated that H. mantegazzianum isespecially successful in abandoned grasslands andgrassland-like fringe habitats in the open landscapeand these habitats are most vulnerable to invasion.

H. mantegazzianum is similarly successful inopen riverbanks with respect to invasion percen-tage but the percentage of dominant stands (25% ofall large stands) is rather moderate in this habitattype resulting in moderate habitat saturation(Table 3). Hence, riverbanks are considerably lessvulnerable to invasion of H. mantegazzianum thanabandoned grasslands. Nevertheless, they certainlyrepresent an important habitat for the species,particularly with regard to long-distance dispersal.The same applies to open roadsides which also playan important role in the spread of the species(Thiele & Otte, 2008).

In western Bohemia (Czech Republic) Pysek andPysek (1995) found that ‘water courses’ and ‘pathmargins’ had a much higher habitat saturationand, conversely, unmanaged grasslands showed aconsiderably lower habitat saturation than in thepresent study. This opposite pattern might be

attributable to differing maintenance regimes ofwater courses and roads, and to unfavourableconditions of the unmanaged grasslands which werecharacterised by either drought or wetness in theCzech study.

Comparing the results of the Germany-widequestionnaire survey with the findings of our fieldstudies, the most striking difference is in theranking of abandoned grasslands and grasslandmargins. According to the questionnaire survey,these are among the least occupied habitats ofH. mantegazzianum while they are among thecommonest and most preferred habitats of thespecies in the most heavily invaded landscapes.There are two possible explanations for thisconspicuous difference: (1) the survey estimatesmight be influenced by pre-existing studies aboutinvaded habitats which often found roadsides,riverbanks, and waste places as common habitats(e.g. Neiland et al., 1987; Ochsmann, 1996; Pysek,1994; Pysek & Pysek, 1995; Wade et al., 1997) butrarely (abandoned) grasslands (e.g. Tiley et al.,1996); and, (2) the spectrum of invaded habitattypes may differ between heavily and margi-nally invaded landscapes. This would imply thatH. mantegazzianum has managed to spread fromriversides and roadsides into the open landscape inits invasion ‘hot spots’, while it is still restricted torather rare occurrences in habitat types outsidethese foci.

According to the Germany-wide survey, H.mantegazzianum has invaded nature reserves inapproximately one-third of all districts and pro-tected habitat types in almost as many. However,within the 20 selected study areas virtually noprotected habitat types were found to be invaded.This might primarily be attributable to the fact thatthe study areas hardly contained habitats ofinterest for nature conservation. But an analysisof plant communities and preferred site conditionsindicated that H. mantegazzianum is barely cap-able of invading sites offering suitable conditions(drought, wetness, poor nutrient status, shade,management) for protected plant communities(Thiele & Otte, 2006). These findings seem tocontradict reports of occurrences in protectedhabitat types. An explanation may be found in thedetails of spatial arrangement of H. mantegazzia-num stands and habitats of conservation concern.Possibly, in the questionnaire survey, stands ofthe species in close proximity to rare or endan-gered communities were interpreted as ongoing orimpending invasion into those habitats. One exam-ple could be observed by comparing the que-stionnaire of one district with a case study of anature reserve in the same region (Schepker, 1998).

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The questionnaire stated that H. mantegazzianumoccurred within the protected habitat types of thenature reserve (calcareous marsh, acidic marsh,salt meadows), whereas the case study showed thatH. mantegazzianum was growing close to thesehabitats but not inside them. An alternativeexplanation might be that invasion of protectedhabitat types has occurred after deterioration ofhabitat quality (e.g. due to abandonment oreutrophication).

Assessment of impacts

H. mantegazzianum has managed to become acommon feature in landscapes of the ‘western lowmountain ranges’. Hence, here the species isprobably sufficiently abundant and widespreadtoday to sustain pools of metapopulations, and,from a medium- or long-term perspective, it maydisperse to landscapes of this region where it hasnot been present until now without furtherdeliberate assistance by humans (i.e. sowing ingardens or in the wild). Thus, concerning theinvasive range it can be stated that H. mantegaz-zianum fulfils the prerequisites to be a hazardousinvader and to have negative impacts at theregional and landscape scales within the focalregion ‘western low mountain ranges’. However,at present, even in the most heavily invadedlandscapes, the species occupies only moderate orlow proportions of potentially suitable habitats,and thus current impacts are moderate at thelandscape and regional scale.

Concerning the future development of the inva-sion of H. mantegazzianum, we presume that thisspecies, just as competitive native species, will notbe able to exhaust its potential growth sites in thefuture. Hence, the ability to displace native speciesand their communities seems to be limited at thelandscape scale and regional endangering or ex-tinction of natives by H. mantegazzianum appearsto be unlikely unless the invasion pertains torarities.

As H. mantegazzianum seems not a serious threatto nature conservation and regional biodiversitylarge-scale control programmes appear not to bemandatory. Nevertheless, the species bears otherimplications, e.g. for human health (Drever &Hunter, 1970; Jaspersen-Schip et al., 1996; Lageyet al., 1995), river management (Caffrey, 1994;Tiley & Philp, 1994; Williamson & Forbes, 1982),and public accessibility of sites, such as riverbanks,amenity areas, and trails (Lundstrom, 1984; Tiley &Philp, 1994). Hence, it is of concern to stakeholdersand land managers. Where problems arising from

the species are imminent or extant, suitablemeasures of management should be taken. Instruc-tions about how to manage H. mantegazzianum anda comprehensive list of references on this topic areprovided in Nielsen et al. (2005).

Acknowledgements

This study was carried out within the frameworkof the EU-Project ‘Giant Hogweed (H. mantegaz-zianum) – a pernicious invasive weed: Developing asustainable strategy for alien invasive plant man-agement in Europe’, Acronym: ‘Giant Alien’,Project no.: EVK2-2001-CT-00128. We would liketo thank the EU for funding. In addition, we wouldlike to cordially thank Dr. Lutz Eckstein and oneanonymous referee for helpful comments andrevision of the manuscript.

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