Research Article |
Corresponding author: Michaela Vítková ( michaela.vitkova@ibot.cas.cz ) Academic editor: Gerhard Karrer
© 2017 Jiří Sádlo, Michaela Vítková, Jan Pergl, Petr Pyšek.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Sádlo J, Vítková M, Pergl J, Pyšek P (2017) Towards site-specific management of invasive alien trees based on the assessment of their impacts: the case of Robinia pseudoacacia. NeoBiota 35: 1-34. https://doi.org/10.3897/neobiota.35.11909
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Robinia pseudoacacia L. (black locust) is a North American tree, considered controversial because of the conflict between multiple uses by humans and negative environmental impacts, which have resulted in it being listed among the most invasive species in Europe. The current management of Robinia stands in Central Europe varies locally according to national legislation, preferring either socio-economic benefits or biodiversity impacts.
We collected field data from our target region of Czechia, reviewed research articles including local grey literature mostly from Central and Southern Europe, unpublished results of local projects and inquired relevant specialists. Because Robinia grows in habitats ranging from urban to forest to natural grassland, neither unrestricted cultivation nor large-scale eradication is applicable as a universal practice. In this paper we suggest a complex management strategy for Robinia stands that takes into account habitat, this species’ local ability to spread, as well as economic, cultural and biodiversity aspects.
We categorized Robinia stands growing in Europe into eight groups and proposed stratified approach to the management based on decisions that reflect local context. Depending on that, the management includes (i) establishment of new plantations, (ii) maintenance or utilization of existing stands, (iii) tolerance and (iv) conversion to original vegetation.
Our complex management strategy will provide a comprehensive guideline for the management of alien trees in Europe.
Alien trees, Robinia pseudoacacia , plant invasion, nature conservation, management strategies, socio-economic benefit
Tree species provide economic, cultural and ecological benefits to humans, often outside their native range. On the other hand, many alien trees have naturalized, subsequently become invasive and have negative environmental impacts in their introduced range. This conflict between positive and negative effects on ecosystem services poses a problem worldwide (e.g.
Robinia is listed among 40 most invasive woody angiosperms in the world (
Whereas its favourable qualities were appreciated early, the local invasions by Robinia started to be widely recognized only after ~1950 (
In the last decade, the environmental and economic impacts of Robinia provoked stormy public debates in Europe, which involved politicians, researchers, nature conservationists, land managers, foresters, beekeepers and horticulturalists, and were recently fueled by proposal for inclusion Robinia on the list of invasive alien species (IAS) of Union concern (Commission Implementing Regulation 2016/1141 of 13 July 2016 pursuant to Regulation No 1143/2014 of the European Parliament and of the Council;
Currently, most management tools have been created for specific invaders/regions and are thus often not sufficient to address the complex range of invasion scenarios (
Robinia pseudoacacia L. (black locust) is a tree, but as a heliophilous and short-lived species, it is a weak competitor. This limitation is balanced by its easy and fast propagation (mainly through root suckers), tolerance of disturbance, rapid growth and tolerance of a wide range of habitats including extreme conditions. On the other hand, Robinia is robust and persistent, therefore it is able to persist in a site once colonized for several decades largely independent of the environment, which the tree itself modifies by changing the availability of nutrients in the soil and light conditions (
Current landscape is characterized by habitat fragmentation which causes large areas of ecotones and boundary line stands, i.e. optimal conditions for Robinia. Serious large-scale disturbances (e.g. mining) provide a lot of open, well aerated and nutrient-rich substrata. Rotation of such disturbance events resulting in decades of successional development at abandoned sites enables Robinia to spread, establish and play a key role in succession. Moreover, transport of large volumes of soil containing Robinia propagules effectively compensates for the low ability of its large seeds to disperse over great distances.
Although most data comes from Central and Southern Europe, we considered for our assessment the whole of Europe (Table
Selected references from different European countries used for categorization and complex management strategy of Robinia stands. See Table
Phytosociological data | Robinia forests | Human-made habitats | Vulnerable habitats | Intensive short rotation plantations |
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(Categories 1, 2, 3) | (Categories 4, 8) | (Categories 5, 6) | (Category 7) | |
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Šindelářová (1986) |
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LIFE99 NAT/IT/006252 |
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LIFE04 NAT/CZ/000015 |
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LIFE08 NAT/E/000072 |
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LIFE05 NAT/H/000117 |
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LIFE08 NAT/RO/000502SFC |
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LIFE06 NAT/SK/000115 |
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LIFE07 NAT/B/000043 |
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LIFE11 ENV/FR/000746 | LIFE07 NAT/D/000213 |
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LIFE08 NAT/PL/000513 |
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LIFE09 NAT/IT/000118 | ||
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LIFE09 NAT/CZ/000363 | ||
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Information for our paper, illustrating the approach for a major IAS in our study area, was obtained from (i) more than 100 research articles and local papers referring or applicable mostly to European countries (Table
According to
Decision framework for selecting suitable Robinia management. Width of arrows indicate importance of the management. Shading indicates the number of potential sites covered (white – relatively few occurrences, black – most of the sites). Data come from the reviewed literature and project reports.
Main features used in categorization of types of Robinia stands, their description and management.
Robinia type | Physiognomy | Distribution and habitats | Source of occurence | Vegetation structure and dynamics | Status | Management |
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1. Regularly managed Robinia forests | Closed forests in natural habitats | Common, temperate and warm areas across Europe | Cultural (open habitats on initially infertile soils threatened by soil erosion, mainly sands and rocky pastures) | Monospecific tree layer | Sustainable - profitable - risky | Forestry maintenance |
Wide range of habitats (from wind-blown nutrient poor sands to the most fertile soils) | Dense and species-rich undergrowth dominated by nitrophilous herbs or grasses | Conversion is troublesome and risky | ||||
Spontaneous (open habitats in the vicinity of plantations, e.g. abandoned vineyards, orchards and fields) | Regular regeneration (forestry) | |||||
2. Regularly managed mixed Robinia forests | The most common forest type with Robinia | Cultural (open woodlands, gappy forests, clearings, deforested sites) | Mixed - Robinia combines with alien and native trees | Sustainable - profitable - low risk | Forestry maintenance | |
Wide range of habitats across Europe | Spontaneous (drier parts of floodplain forests, forest margins, disturbed sites) | Biodiversity of undergrowth depends on the share of Robinia and other aliens | Conversion is troublesome and risky, succession to natural forests is easy | |||
Regular regeneration (forestry) | ||||||
3. Unmanaged old Robinia forests | Czechia, Switzerland | Abandoned old cultures (over 50 years) | Mixed - Robinia gradually replaced by competitive trees (Fraxinus excelsior, Acer platanoides) | Instable - not profitable - risky | Conversion is troublesome and risky, succession to natural forests is easy | |
Less accessible sites, e.g. steep slopes | Spontaneous old and never managed stands, e.g. in rocky ravines | Spontaneous succession without management for several decades | ||||
4. Stands in human-made habitats | Small-scale or semi-open non-forest stands | Common, widespread across Europe | Cultural (ornamental purposes, apiculture or biological recultivation) | Monospecific or mixed stands with native pioneers, nitrophilous trees and aliens | Sustainable - context dependent risk and profit | Context dependent planting or conversion |
Urban, agrarian and industrial areas | Spontaneous (escape and succession in wasteland, public greenery, post-mining landscape and landfills) | High share of aliens in canopy or understory; many ornamental woody species | ||||
Mining areas | Ruderal undergrowth; diverse dynamic | |||||
5. Dwarf Robinia stands growing in natural grasslands | Pannonian lowland, South and South-East Europe | Unsuccesfull cultivation combined with spontaneous spread | Low, twisted trees (ca 5-10m) or shrubs with native xerophilous shrubs | Sustainable - not profitable - low risk | Conversion / removal is troublesome, risky and mostly not necessary | |
Dry habitats (mostly mosaic of grassland, shrubs and open woodland) | Many species of sunny open habitats; nitrophytes are rare due to drought | |||||
Stable stands; survival of rare species preserves local biodiversity in agricultural land | ||||||
6. Young Robinia stands spreading into vulnerable habitats | Dry to mesic grasslands across Europe | Spontaneous spread by root suckers from adjacent stands | Young shoots with increasing cover | Instable - not profitable - risky | Conversion / removal is troublesome and risky but necessary | |
Rare vulnerable native habitats | Valuable herbaceous vegetation is replaced by Robinia | |||||
Instable stands | ||||||
7. Intensive short rotation plantations | Biomass cultures | Across South and Central Europe – e.g. Albania, Austria, Italy, Germany, Greece, Hungary, Poland, Slovakia and Spain | Cultural | Monospecific, low height (average 5-6m), rapid regeneration | Instable - profitable - risky | Intensive cultivation |
Both in forests and arable land | Low biodiversity value, weeds or nitrophytes prevail | Conversion of abandoned plantations to forest | ||||
8. Cultivated single trees and avenues | Separate tree individuals | Common across Europe | Cultural | Horticultural treatment, protection of old monument trees | Sustainable - context dependent risk and profit | Context dependent planting or conversion |
Based on links between ecological traits such as habitat, vegetation structure, origin, utilization, benefits and environmental risks we distinguish eight types of Robinia stands (Table
Deep, well-aerated, nutrient-rich mesic soils in warm areas are optimal for the growth of Robinia since trees reach up to 35 m, form straight trunks and provide high-quality timber (Figure
Biodiversity value of such Robinia forests is mostly low, however, certain groups of organisms prefer them (e.g. macrofungi or habitat generalists among birds;
Main successional / intentional dynamic changes among the types of Robinia stands. Numbers of vegetation units correspond to stand categorization in the text.
Establishment and maintenance: Most European production of Robinia wood comes from these plantations. In the Pannonian basin in particular they are the main type of forest and their yield varies between 80 and 280 m3/ha and have an average rotation age of 30 years (
Tolerance of natural succession: Not remarkable due to economic value of these forests.
Conversion: Restoration of native vegetation is mostly not profitable, being costly and time-consuming. Because of the high sprouting ability of Robinia, it is very risky to stop eradication before totally removing all sprouts (
Admixture of Robinia with cover of up to 10% is the most common type of its occurrence (
The environmental impact of Robinia growing in mixed stands is considerably less than in monocultures. In closed mature forests, it survives only as individual trees or groups of trees in areas that were previously disturbed. The composition of shaded undergrowth is dependent on the proportion of the canopy that consists of Robinia (
Establishment and maintenance: Reasons for the establishment of these forests were either to supplement natural sparse stands, e.g. forest-steppes with Quercus spp. or to improve soil quality, yield and species diversity after logging of native forests and in inter-cropping plantations (Figure
Tolerance of natural succession: Natural decline in Robinia abundance during succession was observed only in forests without large-scale disturbances, where Robinia finally occurs only as an admixture restricted to more open sites (
Conversion: Selective cutting that reduces light availability (
Closed forests in natural habitats (A–C) and small-scale stands in man-made habitats (D). A Robinia forest regenerating and managed by coppicing in stripes B Planted mixed forest with native Fraxinus excelsior and alien Robinia pseudoacacia C Old Robinia forest overgrown by Fraxinus excelsior and Acer platanoides as a result of spontaneous succession D A spontaneously established mixed stand with Robinia growing in a quarry.
Protective monodominant forests 12–16 m tall and over 50 years old on steep slopes pose a big problem in terms of their stability. Trees gradually die, are prone to windthrow and damage and the forest becomes more open. The shrub layer is rich in species. The herb layer consists of dominating grasses, relicts or pioneers of natural forest communities and nitrophytes with cover depending on water regime of topsoil. Such protective forests provide excellent honey (
Establishment: In some countries (e.g. Czechia and Switzerland), this species was used to stabilize deforested steep eroded hillsides along rivers that were threatened by soil erosion (former pastures) and transport corridors (
Maintenance: Maintenance or restoration with native species is mostly not profitable. Old trees are often unstable, therefore logging is difficult and risky, and profit is low. Moreover, logging may trigger soil erosion and regeneration of Robinia.
Tolerance of natural succession: During spontaneous succession, Robinia is replaced by shade-tolerant competitive trees such as Fraxinus excelsior, Acer pseudoplatanus, A. platanoides, A. campestre (Figure
Conversion: Slow conversion to natural forest by means of natural succession is recommended, if there is no risk to biodiversity (adjacent natural habitats) or human infrastructure (traffic corridors or built-up sites;
A common feature of this rather heterogeneous type is a ruderal environment in urban, agrarian, industrial or mining areas (Figure
Establishment and tolerance of natural succession: As early as in the 1970’s, Robinia was used for the biological recultivation of the post-mining landscapes and landfills (e.g.
Maintenance and conversion: Active management is needed since rapid spontaneous changes tend to occur in this habitat. Consideration of the local context (e.g. role of surroundings, ornamental or utility value, claims of owner or public) is necessary, especially in urban areas. Therefore, different parts of the same stand may be managed differently, including e.g., removal of Robinia or whole stands. However, there is no reason for eradicating or banning the planting of Robinia in urban areas (
Most of these stands originated from unsuccessful planting combined with spontaneous spread in dry habitats. Robinia survives in very dry habitats where it occurs as small and twisted trees (~5–10 m in height) or even shrubs forming sparse semi-open stands with an admixture of native xerophilous shrubs, e.g. Crataegus spp., Prunus spinosa and Rosa spp. This type is common in the Pannonian lowland (Hungary and adjacent parts of Austria, Czechia, Slovakia and Slovenia) and in Southern and South-eastern Europe.
Establishment and tolerance of natural succession: In some European countries (e.g. Slovakia, Slovenia, Italy), there is a long historical tradition in Robinia planting for vineyard poles and wine barrels (at least since the late 19th century;
Maintenance and conversion: It should be left to the nature conservationists to decide whether to tolerate or remove these stands. However, most of these stands are very old and unlike those in mesic habitats, their shrubby growth does not indicate they are young plants with a potential for future growth, but are usually full-grown with their propagation greatly constrained by stress (
Non-forest habitats (A–C) and Robinia in urban environment (D). A Agrarian landscape with small-scale and semi-open Robinia stands. The spread of this species is suppressed by regular use of farming practices B Root suckers of Robinia invading a thermophilous grassland, which is the habitat of protected plant species C Intensive short rotation plantation regenerated by coppicing D Avenue of flowering Robinia in Prague (Czech Republic).
This type, which complements the previous one, refers to current invasion of natural habitats by Robinia (Figure
Establishment and tolerance of natural succession: Compared to native trees, Robinia has a high sprouting ability and is extremely resistant to disturbance. It produces numerous root suckers that enable it to disperse at up to 1 m per year (Central Europe;
Maintenance and conversion: The spread should be restricted if Robinia stands occur in or adjacent to fallow land, grassland or other habitats with rare native plants, such as those on rocky slopes. The eradication should be rapid and persistent although expensive and risky due to use of herbicides and the disturbance causing vigorous regeneration of Robinia and erosion resulting in the release of nutrients and growth of weeds. For detailed list of suitable and unsuitable methods see (
Whole Robinia clones must be removed as the roots of the individual plants are connected. For quick eradication the best choice is felling followed immediately by spraying the area felled with herbicide. Removal by incomplete girdling (
Planting short-lived Robinia plantations for renewable bioenergy production (Figure
Establishment and maintenance: These plantations should be established only in areas where an abundant metapopulation of Robinia already exists. The most common methods are either planting seedlings or rooted cuttings, however, a more environmental friendly and cheaper method is to transform Robinia forests at low quality sites (
Tolerance of natural succession and conversion: Extreme caution should be taken when such plantations are abandoned. There is a great risk of an intensive growth of suckers of Robinia, especially as the spontaneous succession of native vegetation is very slow. In northeastern Greece, succession to near natural riparian forest was not recorded even 14 years after abandonment. Site preparation for establishment of plantations as well as relatively low production of litter and periodic removal of organic matter through wood cutting caused a long-term changes in availability of soil nutrients and light, thereby affected species composition in behalf of ruderal species (
This type includes individual Robinia trees occurring solitarily or in groups in parks, gardens and at sites such as chapels or crossroads (
Establishment, maintenance, tolerance of natural succession and conversion: Planting is usually easy. Trees need to be pruned and suckers removed regularly to prevent invasion into surrounding habitats. Consideration of the local context is necessary, Robinia should not be planted close to vulnerable natural habitats. Old trees are desirable because they provide shade and habitat for, e.g., rare saprophytic fungi or saprophagous beetles (
Based on the environmental conditions and human land use we reconcile the main contradictory approaches to Robinia pseudoacacia in Europe, where it is planted for multiple beneficial purposes, but also escapes from cultivation and becomes invasive, with impact on species diversity and ecosystem functioning. At the moment the management of Robinia stands varies locally, depending on the socio-economic benefits vs. biodiversity impacts, from enthusiastic embrace to planting restrictions to complete rejection. Unfortunately, the information sources related to possible management are biased by narrow focus of the parties involved (environmental vs. forestry). Furthermore, the legislation in several European countries governing the management of Robinia is often contradictory.
For these reasons, an integrated solution to harmonize the different views of various target groups is needed. We propose a stratified approach to the Robinia management, which takes into consideration both the ecological and economic aspects associated with its occurrence. Because Robinia grows in a wide range of habitats ranging from urban environment and agricultural landscape, to forest and natural grassland, neither unrestricted cultivation nor large-scale eradication is feasible. We offer several decision scenarios suitable for specific situations in particular landscapes, where Robinia is tolerated in selected areas, but eradicated in others. We distinguish eight types of Robinia stands; for each of them we describe ecological conditions, economic benefits, and environmental risks and propose sustainable management practices.
This study was supported by the long-term research development project no. RVO 67985939, the Praemium Academiae award to P. Pyšek from the Czech Academy of Sciences and Centre of Excellence PLADIAS (project no. 14–36079G, Czech Science Foundation). We would like to thank Tibor Benčať (Technical University in Zvolen, Slovakia), Robert Brus (University of Ljubljana, Slovenia), Richard Büchsenmeister (Federal Research and Training Centre for Forests, Natural Hazards and Landscape, Austria), Marco Conedera (WSL, Switzerland), Walther Gian-Reto (Federal Office for the Environment FOEN, Switzerland), Ivan Jarolímek (Institute of Botany SAS, Slovakia), Robert Kanka (Institute of Landscape Ecology SAS, Slovakia), Bata Kinga (Ministry of Agriculture, Hungary), Ingo Kowarik (Technical University of Berlin, Germany), Milan Lichý (National Forest Centre, Slovakia), Wolfgang Nentwig (Institute of Ecology and Evolution, Switzerland), Károly Rédei, (Hungarian Forest Research Institute, Hungary), Daniela Ribeiro (Slovenian Academy of Sciences and Arts, Slovenia), Hermann Schmuck (Office for Forests, Nature and Landscape, Liechtenstein), Wojciech Solarz (Polish Academy of Sciences, Poland), Esther Thuerig (WSL, Switzerland) and Czirák Zoltán (Ministry of Agriculture, Hungary) for providing unpublished data, literature and consultations. We thank also Tony Dixon for editing the English and Ivana Rajznoverová, Zuzana Sixtová and Helena Zbuzková for technical assistance.