Robinia (Robinia pseudoacacia) are native to the southeastern United States. Early in the 17th century, the species was introduced to various European countries, where it gained favour first as an ornamental tree, and later as a timber tree. Accordingly it was soon being cultivated in many countries.
Originally, as a naturalised tree species, robinia had no major negative impact on ecosystems in Central Europe. Only after World War II did the species start to show its strong dispersal potential by conquering the ruins of bombed-out cities. Later, it also colonised numerous marginal expanses of agricultural land that had been left uncultivated during the post-war socio-economic turnaround. The first articles containing detailed recommendations on controlling the robinia were published in the early 1950s.
Today, the specialist literature is ambivalent, simultaneously describing robinia as both an extremely useful and a highly invasive and thus undesirable tree species. The properties that make robinia so attractive from the silvicultural point of view – its broad habitat range, rapid growth and abilities to resprout and fix nitrogen – are the selfsame characteristics that make the species problematic from the viewpoint of nature conservation and trigger conflicts.
Where robinia are widely established, they can no longer be eradicated. In such areas, the aim should be to find management options that exploit the species' positive characteristics and minimises its negative impact.
Robinia's ecological impact
Robinia already fructify in their seventh year, producing up to 12,000 seeds per square metre in pure stands. Their seeds remain viable for longer than 10 years, though the germination rate is rather low. Since robinia seeds are heavy, their dispersal distance is relatively short. Their dispersal over long distances is largely down to human activity, through planting, the unintentional transport of seeds and roots or the redistribution of soil.
The greatest threat of robinia becoming invasive is posed by vegetative reproduction following mechanical damage caused to roots or stems. Combined with ecological tolerance and an ability to bind nitrogen, this gives the species great potential to colonise adjacent fallow land, ruderal habitats, dry grasslands, heathlands, rocky outcrops and river banks or lake shores. Since robinia leaf late and have comparatively skimpy, transparent foliage, their stands are sparser than naturally occurring forests, leading to the formation of very dense herb layers and the loss of rare plant species characteristic of the colonized sites. On dry sites, a dense layer of grass with a varying proportion of ruderal species often forms, whereas moderately moist sites are dominated by nitrophilous species. Robinia's greatest influence on the environmental conditions in dry grasslands is through shading and nitrogen fixation, which is detrimental to endangered light-loving plants and invertebrates.
Removing robinia from valuable habitats is arduous work. Moreover, efforts to restore the original plant community are rarely successful because of irreversible changes to the ecosystem with regard to soil properties, the microclimate and light conditions. Contrary to the popular belief that robinia lower biodiversity, recent findings show that, at least where birds are concerned, more species may be found in robinia than in oak stands. Another positive fact is that, by colonising structurally and species-poor industrial and agricultural areas, robinia form 'islands' of growing biodiversity, creating stepping stones for connecting valuable and endangered species.
Robinia's economic potential
From an economic viewpoint, robinia wood is of particular interest. Depending on the location, fast-growing energy plantations can produce between 50 and 135 t of dry matter per hectare every 15 years. Robinia between 30 and 40 years old are used for timber production. By then, they have a target diameter of 30 cm and provide up to 300 m3/ha of solid wood. Under favourable conditions, robinia put on average annual growth of 11-13 m3/ha, which is equivalent to between 8 and 9.5 t of dry matter per ha.
It is harder to produce high-quality timber on poorly watered, badly aerated soils, as robinia then tend to form curved stems with a short, knotless shaft section. Furthermore, late frosts can damage young shoots and cause crooked stems. To improve stem wood quality, breeding programmes have been set up in some European countries, including France, Germany and Hungary.
When dealing with controversial tree species like robinia, it is very important to define clear management principles that enable potential to be tapped, but at the same time minimise the risks involved. Recommendations issued by Czech scientists for managing robinia in Europe not only take account of environmental and economic aspects, but also take on board the distinct needs of different stakeholder groups (e.g. foresters, conservationists, beekeepers and the general public). The aim is to ensure that managers take site-specific decisions about whether to encourage, tolerate, reduce or eradicate robinia.
a) Robinia in forests
Traditional coppicing strongly favours the spread of robinia. This is an important finding, because where robinia occur in mixed forest and the species' further spread is undesirable, keeping the stand contained is crucial for avoiding the species' uncontrolled spread (seed dispersal and root suckers). Exchanging pure stands of robinia for native tree species is extremely arduous. Barring any priority biodiversity-related objectives, robinia should preferably be managed with rotation periods and interventions geared to forest functions (production, recovery, protection) and the desired end-products (e.g. posts, honey).
If the aim is to change the tree species, a different approach dependent on the initial situation is recommended. If the desired native tree species is already growing on the site or has good propagation prospects, no action should be taken to interfere with robinia, because this would only encourage the growth of coppice shoots. Instead, robinia should be monitored until their natural death. If a site has none of the desired native tree species or if doing nothing is not an option for safety reasons (protection forest, wooded railway embankments, etc.), individual robinia should deliberately be encouraged after coppicing so they can be used for advance planting for natural regeneration or for underplanting indigenous semi-shade-loving tree species.
b) Robinia on agricultural land and dry grasslands
In open areas, robinia are mostly found in very warm, arid sites with poor soil quality (e.g. fields, gorges, rocky hills) surrounded by agricultural land. Often they grow in abandoned plantations, now forming old, open stands, often with twisted and crooked stems up to roughly 10 m tall. Some European countries, like Italy, Slovakia and Slovenia have a long-standing tradition (dating back to at least the 19th century) of robinia cultivation for the production of vineyard poles and wine barrels.
Surrounding land that is regularly farmed (e.g. fields, vineyards and orchards) creates a buffer zone that prevents robinia's vegetative propagation. Problems arise when robinia stands border dry grasslands, which are the ecosystems under the greatest threat from robinia (Fig. 5). Root suckers enables robinia to advance swiftly into such dry grasslands.
Once robinia gain a foothold, it is hard to control and eradicate them because they form subterranean root intergrowth between individual trees, so removing individual stems does not have any effect. There is currently no generally recommended eradication procedure. A combination of mechanical and chemical processes has proven successful, though the use of chemical agents near water, in forests and in nature reserves is prohibited in Switzerland. Where the use of chemical agents is permitted, the most efficient method involves felling/stripping away a stripe of bark and subsequently applying herbicides to the cut surface.
c) Robinia in settlements
Provided that robinia do not grow in residential areas near sparse forest, nature reserves or other special biotopes, generally they do not pose a problem. However, robinia should not be planted in potential problem areas, where any existing trees should be removed as a precautionary measure.
Robinia are ideal for a case study on the development of various approaches for dealing with invasive tree species because they have long been cultivated, were subsequently allowed to grow wild in various European ecosystems and thus qualify as an invasive species that is both useful and dangerous. Since robinia can irreversibly change ecosystems, nature conservation should take priority over economic returns.
In intensively farmed agricultural areas and in urban environments, the risks of robinia spreading in an uncontrolled fashion are minimal. In such settings, they contribute to the diversity of landscape structures and mosaics and represent a favourable habitat for many organisms. There is no reason not to commercially exploit robinia in these areas. But wherever robinia spontaneously start to gain a foothold in a forest, their further spread needs to be controlled as far as possible by regulating their exposure to light.
In ruderal habitats damaged by storms or fire or in logging areas, robinia can potentially be highly invasive. Sensitive ecosystems such as dry grasslands, lowland meadows, rocky outcrops and sparse, dry forest (e.g. dry pine forests or thermophilic and acidophilous oak stands) are under the most pressure. There, large-scale eradication of robinia is the only solution.