20 July (3rd Sunday): Teach yourself trees at Umwinsidale
A successful “teach yourself trees” meeting took place at Christon Bank in May, kindly organized by Bernard Beekes. Bernard has now arranged a second such meeting which will take place in the Umwinsidale area of Harare, and this will occupy our main 3rd Sunday outing.
Bernard will organize people into 3 to 6 groups depending on attendance. He will print and provide a Mashonaland tree list for each group. Everybody should bring whatever books and aids (especially a lens, and your own lists for your own records) they require.
This is an excellent opportunity to work with others and through your own efforts to get to know your trees. We will meet at the home of Eleanor and Norman Dickens at 49 Umwinsidale Road at 9.30 am. Please bring a chair and lunch.
PLEASE NOTE: There will be no 4th Saturday walk on the 26th of this month.
Please contact Jean Wiley or Gill Short for details of the next Matabeleland function.
Deadly and Desirable
The coral tree is highly popular because of its flame coloured flowers, healing properties and ‘lucky bean’ seeds — but the seeds are also deadly poisonous. The botanical and common names are: Erythrina caffra (coast coral tree) and Erythrina lysistemon (sacred coral tree)
Let me tell you about something curious that once happened on a farm near Cradock in the Eastern Cape. Here lived twin daughters who spent most of their childhood in the large garden surrounding the farmhouse. Their favourite tree in the garden was a sturdy coral tree, eight metres tall.
Many of us are familiar with coral trees because they are popular garden trees and also line the streets of a number of South African towns — as does the African flame tree.
Both these trees have flame-coloured flowers and the twin girls would make garlands from the ones on their coral tree each spring. It was their special, magical tree because the nectar-filled flowers attracted the most beautiful sunbirds.
After the flowers came the fruits: bright, shiny red seeds with a hallmark black spot on each, encased in long, constricted, pea-like pods. The seeds are commonly known as ‘lucky beans’, but the term is loosely applied because several similar-looking seeds are also called lucky beans.
Coral tree seeds have an extremely hard coat which protects and preserves them. Seeds found in the Keurbooms region of the Southern Cape estimated to be 200 years old were still 99% capable of germinating.
The twins amassed vast collections of seeds from their coral tree over the years. One year they decided to make necklaces from them, though it took them quite some time to pierce each lucky bean and then string them together Finally the necklaces were completed and they proudly paraded them. So pleased were they with their handiwork that they wore them to bed that night.
The following morning one of the twins would not wake up when her sister came over to her bed. She was lying there, unable to move, with her lucky bean necklace slung across her mouth.
The twin narrowly escaped death that night so the story ends well, but no one could identify what it was that had threatened her life. It was only when a local botanist versed in traditional medicine saw the twins wearing their lucky bean necklaces that all became clear
“You must be very careful of those,” she said. “They can be deadly poisonous.”
“But we’ve swallowed them before and nothing happened. Like the time we played ‘Doctor Doctor’ and used the lucky beans as pills,” the twins explained.
“Ah yes, but that was because those beans were not pierced. You shouldn’t swallow lucky beans in any event, but if a seed is pierced or even cracked, it exposes its poison. This poison is so potent that it can cause paralysis or even death.”
What had happened is the afflicted twin had been sucking on her necklace as she lay asleep and the poison had entered her system.
“You are extremely lucky,” the botanist said, and went on to explain how important it is to know and understand the plants in your environment. “Some are wonderful for healing, others are both deadly and desirable.”
She described how the crushed leaves of the coral tree are applied to suppurating sores, how open wounds are treated with a powder made of the burnt bark, and how a decoction of the roots is applied to sprains.
“The red or ‘inflamed’ colour of the flowers signals the tree’s medicinal use,” she continued. “Sores, wounds and swellings all become inflamed and the flame must be extinguished before healing can begin. Fighting fire with fire, the coral tree is a wonderful natural healer in this regard.
“At the same time, we need to remember that all healing takes time. In traditional medicine, pain is respected as the body’s natural protection against further injury and as part of the healing process.”
As they grew older the twins never lost their sense of wonder for their special, magical tree, and started reading about the coral tree and many other local trees in Palgraves’s Trees of Southern Africa.
“What is a tree?” asks Palgraves. “A happy definition is that if you can sit in its shade, then it must be a tree.”
The coral tree is a magnificent member of the ‘order’ of trees and the twins have never forgotten it. They are adults now, but whenever they visit the farm, they always head for their coral tree and automatically start chanting, “Wonderful for healing. Deadly and desirable! Wonderful for healing. Deadly and desirable!”
-Heather Dugmore [Reprinted in the interests of science from the series ‘Bush Medicine’, Country Life, July 2008. And, alas, without the excellent photographs of Ben-Erik Van Wyk.]
Alien Invasive Species:Their Impacts on Forests and Forestry.[Over the past four months, we have explored Lesley Henderson’s series on invasive alien plants in Southern Africa. For the next two months, we shall examine a more generic treatment focusing specifically on forests and forestry. The original of this review was published by FAO in November 2005. In the interest of brevity, I have removed all citations except for website addresses and edited out some peripheral material. The complete original version of the paper can be found at: http://www.fao.org/docrep/008/j6854e/J6854E00.HTM. Further information on forest invasive species can be found at the FAO alien invasive species website (www.fao.org/forestry/aliens).
Alien invasive species affecting forests and forestry
Alien invasive species occur in all major taxonomic groups from micro-organisms to mammals. The Invasive Species Specialist Group (ISSG) of the Species Survival Commission (SSC) of the World Conservation Union (IUCN) has gathered information on 326 alien invasive species, including 157 that negatively affect forests and the forestry sector (www.issg.org/database). An annex of this database provides a list of 204 species known to affect forests and the forest sector, including the 157 species identified by ISSG.
ISSG has also compiled a list of the ‘One Hundred World’s Worst Invasive Alien Species’ (www.issg.org/database/species/search.asp?st=100ss&fr=1&sts=#SpeciesList) that aims to illustrate collectively the range of impacts caused by biological invasion. Although incomplete, this list is a first attempt to rank the impact of alien invasive species. Included are 62 alien invasive species—among them two grasses, six plants and 19 trees and shrubs—that impact forests and forestry.
Factors contributing to the spread of alien invasive species
Biological invasions are human-assisted—humans intentionally and unintentionally introduce species into new areas or alter ecosystems in ways that promote invasions. Global factors that support the introduction and spread of alien invasive species include:
• land use changes, including forest-sector activities;
• economics and trade;
• climate and atmospheric change;
• conflict and reconstruction;
• regulatory regimes;
• biological control of pests; and
• public health and environmental concerns.
It is important to note that not all introduced species become invasive. It is widely accepted that only a small proportion of introduced species establish, and only a small proportion of these species spread and become pests. This is often termed the “tens rule” whereby approximately 1 in 10 introduced species escapes to the wild, 1 in 10 of these escapees become established in the wild, and 1 in 10 of these established species becomes a pest. In addition, species that are known to be invasive elsewhere may not necessarily become invasive in a new environment.
Economics and trade
The openness of a country’s economy and the composition of its trade routes enhance the vulnerability of nations to biological invasions. Invasions are also enhanced by the national importance of the agriculture, forest and tourism sectors. A high importance generally leads to increases in the resources allocated to quarantine and protection, however it also increases the opportunities for introduction and spread.
Research on established alien species in 26 countries in Africa, Australia, Europe, and North and South America from the early 1960s to the early 1990s finds a clear correlation between economic variables and disturbances associated with human activities and a country’s vulnerability to biological invasions. The influence of such activities becomes more apparent when considering the problem of invasions in island ecosystems. With particularly vulnerable native biodiversity and a higher percentage of imports in comparison to continental areas, islands exhibit both economic and ecological reasons for the incidence of alien invasive species. In South Africa, its long colonial history, well-developed infrastructure, and prosperous agriculture and forest sectors have contributed significantly to the introduction, establishment and spread of invasive alien plants. About 8 750 tree and plant species have been introduced into South Africa and of these, 161 species are considered highly invasive and many more are likely to become weeds in the future.
Globalization has led to more and faster trade, new travel and trading routes, and increased trade in livestock, pets, nursery stock, agricultural produce and forest products; all of which can facilitate the introduction and spread of alien invasive species. Weed seeds, plant pathogens, larval or adult arthropods and other invertebrates, and even some vertebrate species can be transported on such commodities. Sand, gravel, coal and metal ores, and other inorganic commodities can also be contaminated with seeds, arthropods and pathogens. Unprocessed wood, wood products and nursery stock are also a major source of forest pests and diseases and have introduced a number of devastating species into the United States, such as chestnut blight (Cryphonectria parasitica), Dutch elm disease (Ophiostoma ulmi sensu lato), and white pine blister rust (Cronartium ribicola). It is believed that Pineus boerneri, a pine woolly adelgid, entered Kenya and Zimbabwe on scion material.
In addition to the possibility of the commodities themselves carrying alien invasive species, the containers and vehicles in which they are transported can also facilitate invasions. Wood packaging material made of unprocessed raw wood, including pallets, crates, drums, skids, and cases can be a pathway for the introduction and spread of pests, in particular forest pests. The Asian longhorned beetle (Anoplophora glabripennis), emerald ash borer (Agrilus planipennis) and brown spruce longhorn beetle (Tetropium fuscum) are alleged to have arrived in North America among packing materials from Asia. The Sirex woodwasp (Sirex noctilio) probably entered Argentina, Australia, New Zealand and South Africa in wooden packaging from Europe or North Africa and the red turpentine beetle (Dendroctonus valens) may have been introduced into China’s pine forests through packaging made from infested North American wood. The pinewood nematode (Bursaphelenchus xylophilus) the causative agent of pine wilt disease, has spread from its native North America to Asia and Europe in wooden packing materials.
Containerized cargo can shelter alien species from microorganisms to reptiles and mammals and, since inspecting such freight is very difficult and costly, many alien invasive species may enter a country undetected. No longer are the effects of alien species invasions initially confined to areas around ports of entry since containers are typically not unloaded until they reach their final inland destinations.
Vehicles (cars, trucks, trains, planes and ships) may also be contaminated with all types of pests and, since commercial and recreational movement of vehicles across international boundaries has increased, the threats are considerable. Railway sleepers have also been blamed for spreading many insect pests; it is suspected that Phoracantha semipunctata and P. recurva, both serious pests of eucalypts, entered South Africa in freshly-cut railway sleepers imported from Australia.
New trade activities, particularly in forest products, between China and the former Soviet Union and North America has led to a dramatic increase in pest and disease problems on both sides through accidental introduction of alien invasive species.
Climate and atmospheric change
Human activities are releasing greenhouse gases such as carbon dioxide, methane, nitrous oxide, halocarbons and ozone into the atmosphere, and the rising concentrations of such gases is changing climate. Global climate change has many environmental consequences including changes in species distributions and in their abundance within existing distributions as a result of direct physiological impacts on individual species and changes in abiotic factors, reproduction and recruitment opportunities, and interspecific interactions.
Climate change may produce more favourable conditions for alien invasive species. Once dominant species in native areas are no longer adapted to the environmental conditions of their habitat, it is likely that introduced species will displace them, thus drastically changing successional patterns, ecosystem function and resource distribution.
Climate, geography and other variables define the distribution limits of a species; however with changes in climate these limits are expanding, spreading species into higher latitudes and altitudes due to increased temperatures, humidity, precipitation and other climatic factors. It has been noted that the current distribution of stands of invasive trees in South Africa was largely influenced by climatic factors. Warming trends may also allow for longer breeding seasons for invasive species, as observed in populations of the collared dove (Streptopelia decaocto) in Europe. Since the development of insects is temperature-dependent, it has been predicted that increasing temperatures will enhance the winter survival of insects and facilitate population increases and expansions in geographic range. Changes in climate and environmental factors may also allow existing introduced species to become invasive. Climatically induced stress on plants and other species also reduces their ability to resist invaders and thus makes them more vulnerable to insect or pathogen damage.
Climate change may alter production patterns and trade in agricultural and forestry commodities by species being grown more competitively in higher latitudes and altitudes. Since alien invasive species establish more easily in habitats disturbed by human and other factors, such changes can provide more opportunities for them to invade.
Climate change also affects the frequency and intensity of extreme climatic events, which may have the greatest influence on alien invasive species by disturbing ecosystems and thus providing increased opportunities for dispersal and growth of invasive species.
In addition to the effects of climate change, increasing concentrations of greenhouse gases can have significant effects on the success of alien invasive species as well. Higher concentrations of carbon dioxide increase photosynthetic rates and water use efficiency of plants and ecosystems. The resulting increased soil moisture has potential to provide habitat for late-season annuals which may be invasive. Distributions of spruce budworm in Oregon have been observed to change in relation to increased atmospheric CO2 concentrations. However, increased levels of CO2 affect plant species differently which is likely to result in substantial changes in the species composition and dynamics of terrestrial ecosystems.
A large component of the nitrogen cycle is atmospheric nitrogen, which must be fixed before it can be used by most organisms. As a result, it is this fixed nitrogen that controls the productivity, carbon storage and species composition of many ecosystems. Changes in the deposition of nitrates from the atmosphere through the burning of fossil fuels has resulted in large changes in vegetation, as observed in Western Europe, which may favour the growth of some alien invasive species.
Conflict and reconstruction
Conflict and civil unrest can contribute to the introduction and spread of alien invasive species in a variety of ways:
• Civil unrest leads to the breakdown of phytosanitary and animal health controls and management systems, the loss of supply lines for materials as well as to the displacement of substantial numbers of people.
• Areas where there is civil unrest or war are more vulnerable to the entry of pests and diseases because of the lack of inspections and border controls and also because of the increased unregulated movement of military personnel and refugees.
• Displaced people and their belongings can be a dispersal mechanism for, or the source of, alien invasive species.
• Increased smuggling can relocate alien species to new regions.
• Inflows of food aid may be contaminated with pests and diseases.
• Difficulties in obtaining access to border areas because of landmines and other hazards make these areas difficult to survey.
Military transport, equipment and supplies, often covered with dirt or mud from the field, are effective means of dispersal for many species which can be detrimental to new environments. For example, the puncture vine (Tribulus terrestris), native to the Sahara Desert, may have been introduced into North America on the tires of military aircraft and vehicles returning from Europe after World War II. The brown tree snake (Boiga irregularis), native to New Guinea and neighbouring areas, was accidentally introduced to the island of Guam in the late 1940s or early 1950s, most likely in military shipments of fruit. This snake is an arboreal, nocturnal predator of birds, their eggs and young that has nearly eliminated all native forest bird species. Black rats were introduced to the Midway Islands by navy ships during World War II where they drove many species of wildlife to extinction. A desert shrub, African rue (Peganum harmala) was apparently introduced inadvertently into New Mexico and Texas at World War II airfields. The agricultural pests witchweed (Striga asiatica) and the golden nematode (Globodera rostochiensis) are also believed to have entered North America on returning military equipment. Ceratocystis fimbriata, a fungus that causes canker stain of plane trees (Platanus spp.) or platanus disease, was introduced from the United States to several Southern European ports at the end of the Second World War and subsequently spread through Italy and France. Heterobasidion annosum, a root rot of pine trees, was inadvertently introduced into Italy by American troops during World War II where it resulted in an unprecedented mortality rate of stone pines (Pinus pinea) on the Presidential Estate of Castelporziano. It is believed that the pathogen was transferred in transport crates, pallets or other military equipment made from untreated lumber from infected trees.
Emergency relief, reconstruction efforts, and humanitarian assistance after wars and disasters may also contribute to the introduction and spread of alien invasive species. Though little information is available, particularly in regards to pests of forests and forestry, foreign food aid has been accused of introducing agricultural pests into a number of African countries. An example is the larger grain borer (Prostephanus truncatus), unintentionally introduced into the United Republic of Tanzania in a food aid shipment in 1979 [and now spreading rapidly throughout eastern and central Africa and expected to arrive in Zimbabwe at any time].
A country’s lack of regulatory regimes, including resources for prevention and enforcement measures (ie, a sustainable institutional framework) as well as attitudes and views regarding risks, make it more vulnerable to invasions. Regulatory systems for managing alien invasive species are heavily dependent on the actions of both the government and private sectors and the effectiveness of such systems is determined by the level of resources that governments can provide as well as the technical capacity that exists nationally.
Countries vary in their tolerance of the risk of alien species. Since countries are linked to others by the transboundary movement of people, goods and services, the level of control applied by one country will in turn affect the vulnerability of other countries. Regulatory systems can also break down, or fail to respond effectively when faced with new challenges in alien species control, either because of systemic deficiencies or because safety measures are evaded.
With approximately 650 million tourists crossing international borders every year, the opportunities for the introduction and spread of alien invasive species are profound and increasing. Travellers can intentionally transport living plant and animal species that can become invasive or they can carry fruits and other living or preserved plant materials that contain potentially invasive insects and diseases that can have profound effects on agriculture, forestry and other sectors. Travellers themselves can also be the vectors for parasites and diseases between countries. Parks and protected areas have experienced increased biological invasions due to human activities.
Biological control of pests
Another source of alien invasive species is the intentional importation and release of insects, snails, plant pathogens and nematodes for biological control of pests. Such species can escape into other unintended areas and become pests themselves. The United States Office of Technology Assessment (OTA) (1993) noted that of a total of 722 non-indigenous species imported for biological control, 237 species had become established in the United States, some of which have become detrimental.
Public health and environmental concerns
Concerns about the effects of pesticides on the environment and human health can also promote the spread of alien invasive species by allowing such species to spread unchecked.
Impact of land-use change and forestry on the spread of alien invasive species
Land use change has obvious direct effects on the world’s ecosystems through habitat destruction and alteration of competitive relationships but also indirect effects through alteration of fire frequency and nutrient and water balances. Land use change is a major contributor to the introduction and spread of alien invasive species which, in turn, are the second most important threat to biodiversity, behind habitat destruction.
Activities of the forest sector can contribute to the introduction and spread of alien invasive species through forest utilization practices and the intentional introduction of species for commercial forestry, agroforestry and other purposes. Trade in forest products is also a contributor.
Forest utilization, in particular practices such as timber harvesting, extraction of non-timber products, the construction of logging and transport roads and facilities for logging camps, and the conversion of natural forest to plantations can have direct and indirect negative impacts on the ecological functions of forests and on forest biodiversity by promoting the invasion of alien species. For example, logging was found to be the prevailing factor influencing the establishment of alien plants in a national park in Madagascar; and due to the dominance and persistence of the invasive species, the logged sites never recovered native species diversity. As a result, the structure and diversity of the forest remained altered even long after logging operations ceased. Clear cutting practices have also been shown to lead to the replacement of native ant communities with invasive non indigenous species.
Forest roads provide essential access for timber extraction, management and monitoring of forest resources and are thus an important requirement for sustainable forest management and use. However, when poorly designed and maintained, forest roads are often the cause of a variety of environmental problems associated with forest harvesting operations. Under some circumstances, forest roads may also initiate or accelerate the invasion of non-native species that ultimately displace native species. In addition, the increased levels of human activities in previously inaccessible areas, as facilitated by forest roads, cause many environmental problems, including the possible introduction of alien species.
Land use changes can promote the emergence or re-emergence of infectious diseases which degrade human health and that of other species. Infectious disease agents often, and perhaps typically, are alien invasive species since they are invaders over most of their range. Activities of the forest sector contribute to the spread of infectious diseases in a variety of ways.
Forest activities, such as clear-cutting and road building, may increase exposure of workers to infectious diseases such as human immunodeficiency virus (HIV), Ebola hemorrhagic fever, Marburg hemorrhagic fever, yellow fever, leishmaniasis, malaria and Ross River virus disease. The index case for the Ebola epidemic in the Democratic Republic of the Congo in 1995 is thought to be a charcoal-maker who worked in the forest outside Kikwit. Logging can change the abundance, extent and quality of aquatic larval habitats for the Anopheles mosquito vectors of malaria by disturbing the forest floor, creating depressions that catch and hold water, thus creating new sites for the development of mosquitoes.
Deforestation can result in humans coming into closer contact with the vectors for such diseases as leishmaniasis, yellow fever, trypanosomiasis (both African sleeping sickness and Chagas disease), and Kyasanur forest disease. Similarly, some animal reservoir hosts increase in abundance between forest edges and human settlements, thus increasing the risk of human exposure to pathogens.
The destruction of forest habitat may result in the removal, replacement or eradication of dominant vector species, and sometimes the replacement species are more effective vectors of disease, as observed with loa loa (tropical eyeworm) and onchocerciasis (river blindness). Deforestation and desertification may also be accompanied by changes in the distribution of vectors such as ticks, blackflies, tsetse flies and Anopheles mosquitoes. For example, forest degradation in the United States has led to the emergence of Lyme disease as a result of the loss of some predator and competitor species of white-footed mice causing increased population densities of the mice in remnant forest fragments. Higher population densities of the mice, which carry the bacteria (Borrelia burgdorferi) that cause Lyme disease, increase opportunities for ticks to acquire such infectious agents.
Reafforestation activities can also affect the population dynamics of vectors and reservoirs which can promote the emergence of infectious diseases. Such activities in the United States and Europe have resulted in an increase in the population of deer and hence the deer tick, which is a vector of Lyme disease.
The forest sector itself is a major source of alien invasive species. Many of the tree species used in commercial forestry in many parts of the world are alien or non-indigenous to the area. Hundreds of species have also been widely and successfully planted for a variety of purposes including afforestation, desertification and erosion control, and for the supply of fuelwood and other forest products. Such intentional introductions however, can bring about many unintended and costly consequences when these species escape cultivation and invade natural ecosystems.
Alien tree plantations can have negative effects on the biodiversity and water resources of afforested areas. Introduced species that spread from plantations to natural and semi-natural areas, and also into areas set aside for conservation and water production, have considerable impacts on ecosystem properties and functions.
In 2000, forest plantations covered 187 million hectares. Pinus spp. (20 percent) and Eucalyptus spp. (10 percent) were the dominant species planted worldwide, followed by Hevea spp. (5 percent), Acacia spp. (4 percent), Tectona spp. (3 percent), other broadleaved species (18 percent), and other coniferous species (11 percent); the remaining percentage was unspecified. Globally, broadleaves made up 40 percent of forest plantation area, with Eucalyptus the principal genus, while coniferous species made up 31 percent, of which Pinus was the principal genus; the remaining percentage was unspecified. Among the most widely planted trees, Acacia and Pinus species are the most prominent on weed lists and in reviews of invasive tree species. This prominence is perhaps because many species in both genera are highly capable of adapting to a wide variety of environments. In a summary of data on conifer taxa from 40 countries, it was found that 80 alien taxa were known to be naturalized (13 percent of species) and 36 taxa were known invaders (6 percent). Twenty-eight of these known alien invasive conifer taxa belong to one family (Pinaceae) and 21 of these to one genus (Pinus). The Pinaceae has a higher proportion of invasive species than any other angiosperm family.
Considerable attention has been given to the effects of alien invasive trees from plantations in the southern hemisphere, in particular in South Africa. Plantations cover over 1.5 million hectares of land in South Africa and are an important part of the economy–contributing US$300 million to GDP and employing over 100 000 people. The forest sector however has been one of the country’s major sources of alien species infestation; a large proportion (38 percent) of the area invaded by woody alien plants in South Africa is occupied by species used in commercial forestry, in particular Pinus and Acacia species. An analysis of the distribution of forestry plantations and invasive stands of Acacia mearnsii and Pinus spp. in South Africa discovered that these two taxa account for 60 percent of the area under commercial plantations and 54 percent of the area invaded by alien trees and shrubs. Black wattle (Acacia mearnsii), introduced into South Africa from Australia in the mid-19th century, has been widely planted in the country and forms the basis of a small but significant industry. A highly invasive species, A. mearnsii has spread to an area of 2.5 million hectares in South Africa where it negatively affects water resources, biodiversity, and the stability and integrity of riparian ecosystems. Due to their ability to fix atmospheric nitrogen, Australian Acacia species (A. cyclops and A. saligna), have radically altered nutrient-cycling regimes in nutrient poor fynbos ecosystems in South Africa. The invasion of these species and some Pinus species into large areas of the tree-poor fynbos have caused many ecosystem changes by altering factors such as biomass distribution, plant density and vegetation height, leaf-area index, litterfall and decomposition rates. Such changes radically alter habitats, resulting in major changes in the distributions of many animals. Invasive alien pines and acacias in South Africa can also reduce water tables.
Many countries are also reporting significant problems with introduced eucalypts from Australia that can be particularly harmful environmentally because their leaf litter contains chemical compounds that prevent other species from growing.
Other notorious examples of invasive trees that negatively affect ecosystems include melaleuca and black cherry. The melaleuca or paper bark tree (Melaleuca quinquenervia), native to Australia, was introduced from South America into southern Florida in the United States as an ornamental in the early 1900s. Decades later, the population began to expand and the species is currently spreading at a rate of 11 000ha a year throughout forests and grasslands of Florida’s Everglades National Park, where it damages native vegetation and wildlife.
The North American black cherry, Prunus serotina, was initially introduced into central Europe as an ornamental and over the years was planted to produce valuable timber and to function as windbreaks, firebreaks and shelter for game. By the mid 1980s, the species was considered an aggressive invader (particularly in Germany), which formed dense thickets threatening plant diversity and natural regrowth of indigenous forest tree species.
Of the 2000 or so species used in agroforestry, perhaps 10 percent are invasive. While only one percent are considered highly invasive, these include many frequently planted species such as Casuarina glauca, Leucaena leucocephala and Pinus radiata. The spread of introduced species from agroforestry plantings however is less a threat than those from commercial forestry plantations, since plantations are typically planted to much greater numbers over larger areas while agroforestry areas often adjoin natural vegetation, and the spread of trees will likely be held in check by agricultural practices and human use.
With the increase in afforestation and land use changes over the past few decades, the magnitude of such detrimental consequences has increased significantly. Generally, the species that cause the greatest problems are those that have been planted most widely and for the longest time, and the areas most affected tend to have the longest histories of intensive planting.
End of part 1