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2.1. Importance of arthropod pests and diseases

The impact of invertebrate pests and plant pathogens is not always obvious or easy to calculate. Just because a pest is present does not mean it has either an immediate or a long-term impact. Furthermore, while the relationships between pests (in the broad sense) and their host plants are relatively well known in temperate regions, the impact of tropical pests is relatively poorly documented (Speight and Wylie 2001). In the case of the latter, the available economic assessments generally involve informed estimates rather than controlled scientific studies (see Case Study 1).

Case Study 1 : Assessing the impact of introduced conifer aphid pests in Africa

In eastern and southern Africa, pine and cypress trees are grown extensively in industrial plantation monocultures for wood-based industries such as sawn timber, paper and other products. In most countries in this region, cypress trees are also grown in village woodlots and by small-scale farmers for fuelwood, local timber, soil stabilization and wind breaks.

Three accidentally introduced aphid pests attack these trees: the pine woolly aphid, Pineus boerneri Annand (Adelgidae) (first found in 1968); the pine needle aphid, Eulachnus rileyi (Williams) (Aphididae) (first found in the 1970s); and the cypress aphid, Cinara cupressivora Watson and Voegtlin (Aphididae) (first found in 1986). The first two species attack Pinus spp. while the third attacks Cupressus spp. as well as indigenous cedars and junipers.

Since their introduction, these three aphids have spread throughout southern and eastern Africa; they were the first pests specific to conifers to invade the region and have subsequently become the most damaging pests of these trees.

In 1991, an attempt was made to estimate the losses caused by these pests, by collecting information on the area, growth and monetary value of softwood timber in the region, and on aphid distribution, feeding ecology and associated losses through tree mortality and reduced growth. It was estimated that, up to 1990, C. cupressivora had killed trees worth UK£27.5 million (approx US$44 million) and was causing an additional loss of a further UK£9.1 million (approx US$14.6 million) per year through reduction in annual growth increment (including that from dead trees). In addition, the two pine aphids caused a further loss of UK£1.5 million (approx US$2.4 million) per year due to reductions in the annual growth increment in the region's pine plantations. These figures are clearly conservative, since they do not include the impact of the aphids on indigenous junipers and cedars, or allow for any subsequent mortality due to C. cupressivora. A second study, conducted by the Kenya Forestry Institute, estimated that in Kenya the latter species might kill as many as 50 percent of all cypress trees during the 30-year harvest cycle.

These economic data were instrumental in securing the resources needed to mount a biological control programme against the three aphids, which has resulted in substantial reductions in the incidence of the cypress aphid and possibly the others, although this has not been established with certainty.

Sources: Murphy (1996). See also Watson et al. (1999).

Some pests cause obvious damage that can be readily quantified, for example damage by the mahogany shoot borer (Hypsipyla grandella) can cause loss of the leader and hence trunk distortion in attacked trees. In other cases (e.g. damage by sap suckers, chronic diseases, defoliators and weeds), effects on yield may be far less obvious. Furthermore, although the short-term effect of `outbreak' pests is usually very obvious, data on the long-term economic impact of such damage (which occurs over a relatively limited period) tend to be sparse, particularly for tropical tree crops. The particular problems associated with outbreak pests are discussed in Section 2.1.1.

The impact of potential pests often differs in different regions. Thus indigenous pests may appear in sporadic outbreaks, or may cause more frequent problems, perhaps because of sylvicultural practices such as a tendency to grow a single species in monoculture (see Section 2.5). Newly introduced exotic pests, on the other hand, will often occur as an initial massive outbreak, but may become less damaging over time as the most susceptible trees die or, in some cases, the plants develop defensive responses against them. However, most alien pest species are likely to remain damaging at some level.

In severe cases, pests may set ecological limits to the areas in which trees can be grown. For example, in Fennoscandia, severe outbreaks of the lepidopteran defoliator Epirrita autumnata are believed to be responsible for the altitudinal displacement of the boreal tree limit (Nuorteva 1963; Seppälä and Rastas 1980; Virtanen 1998). In rare cases, introduced pests may even threaten endemic tree species with extinction (see Case Study 13).

In some cases, the impact of a particular pest or disease may be limited by using alternative, less susceptible, tree species. In contrast to agricultural systems, there are often several different tree species that can be used to produce very similar products. Hence, in the medium to long term, tree species or genotypes that are susceptible to the pests and diseases in a particular area can be phased out and replaced by other, less susceptible, alternatives. In such cases, the net effect of the pest over time is indirect (by restricting the choice of tree species), rather than direct.

However, replacement tree crops may also be damaged by new pests. For example, in eastern Africa, damage due to canker (Rhyncosphaeria cupressi) led to the replacement of Cupressus macrocarpa by Cupressus lusitanica, which subsequently came under threat from the introduced cypress aphid, Cinara cupressivora. This type of situation is particularly common in agroforestry, where new trees (both exotic and native) are constantly being adopted and abandoned because of new pest problems. This is a good indirect indication of the importance of forest pests in the tropics, despite the absence of precise data on economic losses.

In some cases, too, limitation of damage by substitution is not always practical. Some tree species will always be in demand and hence will be grown despite substantial losses due to pests or diseases. This is the case for mahogany, for example, which can suffer severe damage from the widely distributed mahogany shoot borer (H. grandella). The continuing demand for mahogany also explains why research on the shoot borer has continued for many years (Newton et al. 1993; Floyd and Hauxwell 2001), whereas little work has been done on equally intractable pests of trees that can be substituted. Similarly, the cultural and economic value of American chestnut (Castanea dentata) is such that efforts continue to be made to develop varieties resistant to chestnut blight (Cryphonectria parasitica), a disease that devastated this species during the first part of the twentieth century (see Case Study 2).

Case Study 2 : Chestnut blight changes a forest ecosystem

Chestnut blight entered the USA on nursery stock imported from China to New York around 1900. It is caused by a fungus that prior to 1978 was known as Endothia parasitica, but has subsequently been renamed Cryphonectria parasitica. The fungus is spread by wind, rain, birds and other animals; it enters through cracks or wounds in the bark and multiplies rapidly, causing sunken cankers which expand and girdle the stem, killing everything above the canker (usually in a single growing season).

Since it had never previously been exposed to this fungus, the American chestnut (Castanea dentata) was highly susceptible and was devastated throughout its natural range, which extended over the Appalachian hills and highlands from Maine to Georgia. By 1940, it has been estimated that three and a half billion American chestnuts had perished.

Prior to this, the American chestnut had been one of the most abundant hardwoods in the deciduous forests of the eastern USA, in some areas accounting for as much as 25 percent of all tree cover. In many eastern hardwood forests, it was not only the most numerous species, but also often the largest (30 m specimens were common). It was one of the most economically important trees in the eastern USA, with very durable wood that was highly valued for furniture and construction, and nuts that were both a cash crop and a staple food for wildlife. Now, American chestnuts survive mostly in the form of small understorey trees that sprout from the original root systems but which are continuously attacked by the blight. There have been attempts to produce hybrid chestnuts through cross-breeding, using Chinese species resistant to the disease as the parent line.

The demise of the American chestnut illustrates how an entire ecosystem can be fundamentally altered. Although as a species it still survives, it is ecologically extinct - no longer a functional part of the ecosystem. Its loss has permanently changed the ecology of the eastern deciduous forests.

Source: Stein and Flack (1996)

Rubber (Hevea brasiliensis), a species of South American origin, grown extensively in South-East Asia, is another example of a tree for which substitution is not an option, since it is still the only species from which rubber can be produced (in spite of the extensive evaluation of thousands of other plant species as potential sources of rubber).

For the moment, the extremely damaging leaf blight (Dothiudella ulei) that attacks this species is restricted to South America, where it prevents the establishment of commercial rubber plantations (Case Study 3). Africa and South-East Asia are currently free of the disease, which would be devastating if introduced. The disease-free status of countries in these areas is maintained almost entirely by preventing entry of the blight and, although early detection systems and emergency eradication plans may be in place, it would be optimistic to think that the disease could be contained if it was accidentally introduced (Case Study 4).

Case Study 3 : Fordlandia - a classic story of indigenous plantation failure

In 1927, Henry Ford was making half the automobiles in the world. Then, as now, the car industry depended upon the rubber supply from South-East Asia to make tyres, and by the 1920s rubber accounted for an eighth of the value of all US imports. Seeking an alternative source of rubber, Ford set out to establish a Brazilian plantation to challenge the British and Dutch monopolies based in Malaya and Indonesia.

He bought 25,000 square kilometres of land on the banks of the lower Rio Tapajós, 100 km from Santarem. However, the area had very limited potential for plantations of any kind, since it depended on seasonal rather than regular rains, was hilly (and therefore awkward to mechanize), the soil was sandy and over-leached, and it was beyond the reach of ocean-going vessels for several months of the year. Nevertheless, Ford built Fordlandia, a town complete with miles of roads and railroads, a modern port, a factory, schools, churches, hundreds of brick and stucco bungalows and a fully equipped hospital that overlooked swimming pools, tennis courts and a golf course. Thousands of acres were cleared and planted with more than five million seeds of the finest, highest-yielding clones of rubber (Hevea brasiliensis), secured by botanists sent to Malaya and the Dutch East Indies. By 1934, 1.5 million rubber saplings were growing at Fordlandia. Initially, all went well. However, as the growing foliage began to form a continuous canopy over the fields, it was attacked by South American leaf blight (Dothiudella ulei). Within a year, the disease had ravaged the plantation.

Ford ordered his agronomists to try again, on an even larger scale. A second area of land was secured at Belterra, 20 km from Santarem; another town was built, more land was cleared and more rubber planted. The outcome was the same.

Although the plantations have persisted, they have always suffered from South American leaf blight, a loss of topsoil and poor labour relations (although the relative importance of these factors varies according to different sources). They have never made a significant contribution to the world's rubber supply. Ford never looked likely to recover his money, and by the late 1930s he had lost interest, finally selling out to the Brazilian Government in 1945 for US$500 000 - having already invested well in excess of US$10 million.

Sources: TAMU (2000); Evans (2002);

Case Study 4 : The threat of South American leaf blight to rubber in Malaysia

The rubber tree (Hevea brasiliensis) is grown extensively in South-East Asia, especially in Malaysia, for the production of natural rubber and, increasingly, timber. Malaysia is one of the main South-East Asian rubber producers: in 1997, a total area of 1.564 million ha was planted with rubber, predominantly by smallholders. In 1999, the total rubber production for Malaysia was 0.9 million tonnes and its export value was 3115 million Malaysian Ringgit (approx. US$0.8 million; 2 percent of total Malaysian exports).

In Malaysia, rubber is relatively free from attack by indigenous pests or diseases, and no such species accompanied the plant from its area of origin in the Amazon. In South America, the most damaging disease of rubber is South American leaf blight (SALB, Dothidella ulei), which is so virulent that the commercial planting of rubber in the continent is not viable (see Case Study 3). Since resistant varieties are not available (see Case Study 5), Malaysia has a substantial programme aimed at preventing the entry of leaf blight, which R.E. Schultes, Director Emeritus of the Harvard Botanical Museum, has suggested would overrun the Asian plantations in five years, reducing yields, killing trees and compromising the entire industry.

The first and principal line of defence against the disease is prevention. Quarantine regulations in Malaysia, Thailand and some other rubber producing countries have been strengthened to prevent an accidental introduction of leaf blight. The importation of rubber tree planting materials directly from the American tropics is prohibited except for research purposes, and relevant national and industry research institutions and universities are well aware of the potential risks. Airport posters are used to alert airline passengers to the hazard and travellers from tropical South America are requested to stopover in another country en route for at least two days; those arriving on direct flights are required to complete plant quarantine declaration cards and upon arrival are subjected to quarantine treatments, e.g. showering and changing their clothing and exposing their baggage to ultraviolet irradiation.

In comparison, investment in early warning systems is quite limited. Staff from the Rubber Research Institute of Malaysia and the Department of Agriculture carry out regular surveys of rubber diseases every two to three years. These are primarily intended to identify indigenous disease problems, so that recommendations for suitable clones may be made for different areas. However, a watch is also kept for symptoms of other diseases, especially South American leaf blight. A contingency plan has been prepared in the event of leaf blight being found in Malaysia, but by that stage it may well be too late for the South-East Asian rubber industry.

Source: Wittenberg and Cock (2001).

From a global perspective, there is a great need to develop blight resistant varieties (Case Study 5) as insurance against the ever-present possibility of leaf blight escaping from South America. Disruption of the global rubber supply, on which automobiles and airplanes depend, would have a major economic impact on the world economy.

Case Study 5 : Rubber - an industry under threat

From Fordlandia (Case Study 3) emerged a sobering lesson: the rubber trees that were by far the most sensitive to leaf blight were the high-yielding strains from the Far East. In selecting for yield, the breeders in Asia had inadvertently produced strains that were especially susceptible to the blight. However, whilst millions of trees died at Fordlandia, a handful survived, all wild and native to the Amazon. Since trees with inherent resistance to the disease clearly existed, this suggested that standard plant breeding techniques could be used to produce resistant commercial varieties.

In 1943, in response to the need to secure rubber supplies during World War II, the US Department of Agriculture sent botanists into the Amazon to search for resistant trees. Some were found growing at the agricultural experimental station at Turrialba, Costa Rica (now CATIE), and by the end of the war many of the technical problems related to establishing high-yielding, disease-resistant varieties had been solved. However, shortly after the war's end, the work ceased, reportedly in the belief that a satisfactory synthetic alternative would soon be available, although also partly to protect the British rubber industry in Malaya. In Costa Rica, all records of the project were lost and the germplasm collections were abandoned and eventually destroyed.

Satisfactory synthetic rubber has yet to be produced and natural rubber still dominates the rubber market, being particularly important for vehicles and aircraft tyres. South-East Asia is the principal source of natural rubber and if South American leaf blight were to reach the region, the world's supply of natural rubber would be drastically reduced, causing considerable economic perturbations on a global scale. The need for disease-resistant cultivars seems obvious, but nevertheless is not being addressed.

Source: TAMU (2000)

2.1.1. Outbreak pests

Some forest pests are characterized by population cycles in which prolonged periods at relatively low (non-damaging) densities are interspersed with occasional `outbreaks', during which their numbers grow rapidly to very damaging levels. These outbreaks persist for varying lengths of time. Pest outbreaks in tropical forest plantations are almost inevitable at some time during the rotation and can cause major economic losses (Wylie 2001). Since there will inevitably be some impact on tree growth during the outbreak, these pests may cause considerable concern while their populations are high. However, over the entire lifetime of the crop, the damage caused is not necessarily significant. Nevertheless, synchronized or successive outbreaks of different pests (e.g. a defoliating insect followed by a disease (Case Study 6)) can be particularly damaging, and may lead to tree mortality.

Case Study 6: Concurrent outbreaks of insects and diseases: maple defoliators and maple anthracnose

Simultaneous or successive outbreaks of insect pests and disease can be very destructive to forests. For example, in Pennsylvania in 1994, the ability of sugar maple (Acer saccharum Marsh.) and red maple (Acer rubrum Linnaeus) to refoliate following outbreaks of forest tent caterpillar (Malacosoma disstria Hübner; Lasiocampidae) and elm spanworm (Ennomos subsignarius (Hubner); Geometridae) was limited by an anthracnose leaf disease caused by a fungal plant pathogen (Discula campestris (Pass.)).

The disease prevented many trees from refoliating and consequently prevented them from generating the sugar reserves in their roots needed to support new growth the following spring. Thus in 1995, the combination of the earlier defoliation and the anthracnose caused severe branch dieback and contributed to increased tree mortality: in a typical northern hardwood stand, tree mortality normally ranges from 1 to 3 percent, whereas in 1995 it reached 11 percent or more in many affected stands.

While such cases of insect outbreaks closely followed by disease outbreaks do occur from time to time (and can be very destructive), many trees do survive, and these genetically superior individuals then form the basis of a new and more pest-resistant forest community.

Source: Pennsylvania Department of Conservation and Natural Resources website (

The causes of such outbreaks are not well known. Outbreaks of some species seem to occur in regular cycles, e.g. those of the larch budmoth, Zeiraphera diniana, and the birch looper, Epirrita autumnata, occur at intervals of 8-10 years in the Alps and Fennoscandia, respectively. Similarly, in Japan, the beech caterpillar, Quadricalcarifera punctatella (Motschulsky) (Notodontidae), often causes serious defoliation in beech forests at intervals of 8-11 years; furthermore, the outbreaks tend to occur synchronously in different areas (Kamata 1998).

Possible causes of outbreaks include:

Outbreaks of native pests are usually sporadic and collapse quite quickly, either due to starvation or to the action of natural enemies (predators, parasitoids or diseases). Newly introduced pests are more likely to increase until food is limited and then disperse, extending the outbreak to new areas, and possibly returning to the original outbreak area once regrowth is available. For example, the pine false webworm, Acantholyda erythrocephala (L.) (Pamphiliidae), usually develops outbreaks of short duration (mainly two to three years) in its region of origin in central Europe, whereas in North America, where it was introduced, persistent and severe outbreaks have been observed since the early 1980s (Kenis and Kloosterman 2001). If indigenous natural enemies can adapt to the new pest, they may break this pattern. Often, however, they are unable to do so, in which case biological control should be considered as a long-term solution (see Section 3.4).

2.2. Importance of weed pests

In forestry, weeds are principally a problem at the establishment stage of a plantation, creating difficulties in the preparation of sites for planting and through competition with young trees (particularly the smothering action of vines). They can also cause problems in the production of nursery stock and can affect forestry activities such as thinning and maintaining access, rights-of-way and road margins, as well as increasing fire risks.

Both indigenous and alien species can be involved, but in general they are early succession species that invade a site after disturbance and which would normally be displaced by forest anyway - but only after a much longer period of time than is typical of commercial forestry.

Most weeds of forestry also affect other ecosystems, i.e. they are not specific to forestry systems. For example, Mikania micrantha, a South American vine, is an important weed in young plantations in the Asia-Pacific area (e.g. of Pinus plantations in Fiji), smothering young trees and easily growing into a 10 m canopy. However, it also affects arable crops, fruit crops, pasture, etc. (Case Study 7).

Case Study 7 : Socio-economic impact of Mikania micrantha in Kerala, India and Malaysia

Mikania micrantha is an important invasive weed in several countries within the moist tropical zones of South-East Asia and the Pacific and is still expanding its range. In southwestern India (Western Ghats) it is a recent introduction (about 1985) but it is already a significant weed throughout Kerala and has started to invade neighbouring states.

The vine is able to invade degraded natural forest ecosystems, out-competing indigenous species. It is also an important weed of home gardens and agricultural systems, particularly of tea, bamboo, reeds (for thatching) and other plantation crops such as rubber, teak, eucalyptus and plantain. It grows up into the canopy and is capable of pulling plants over, or of smothering the crop. In tea plantations it seriously disrupts picking, since it grows amongst the tea shoots being harvested. Apart from depriving crop plants of soil nutrients and sunlight, it also has an allelopathic effect on their growth.

The Kerala Forest Research Institute recently assessed the socio-economic impact of M. micrantha in Kerala. The weed affects both subsistence and tree crops and intensive weeding is now necessary, resulting in an escalation of production costs and subsequent loss of income (KFRI, unpublished report).

In India, the most severely affected crops include banana, coconut, coffee, cocoa, cassava, pineapple, ginger and teak. Yield reductions can be substantial and infestations in some areas have resulted in the substitution of highly susceptible crops with more weed tolerant alternatives. In natural forests, the collection and extraction of non-wood forest products (e.g. reed, bamboo) have become more difficult and more time-consuming because of the creeping and twining habit of the weed.

In Malaysia, M. micrantha is considered to be a highly competitive weed in immature plantations of cocoa, coconut, rubber and oil palm; large potential yield losses have been estimated in the latter two crops, and the cost of controlling the weed has significantly increased overall production costs. In 1981, about 5 percent of the total weed control costs in rubber and oil palm plantations were directly attributable to control of M. micrantha; this amounted to about 10-12 million Malaysian Ringgit (approx. US$3 million at the 2002 exchange rate).

Source: Ellison and Murphy (2000) and Teoh et al. (1985)

In northern temperate regions, the main weeds affecting forestry tend to be indigenous, although there are some significant exceptions, e.g. Scotch broom (Cytisus scoparius (L.) Link.) and gorse (Ulex europaeus L.) in British Colombia (Petersen and Prasad 1997). Species which are weedy in their area of origin often cause worse problems when introduced elsewhere. For example, Old man's beard (Clematis vitalba L.) is a relatively minor forestry weed in its native Europe, but is extremely invasive in the natural forests of New Zealand.

An important but relatively little-studied aspect of alien weeds is their ability to displace indigenous vegetation and associated animal diversity in natural, disturbed and plantation forests. This effect may involve just the forest margins and trail edges, or clearings and the ground cover within the closed canopy; all are important in terms of the biodiversity associated with each habitat. This type of impact is generally poorly documented, particularly in developing countries. However, several examples of this effect have been reported in Hawaii. For example, the South American shrub Clidemia hirta (Melastomataceae), which was an important weed of grassland and pasture in Hawaii until biological control was implemented, is now found mainly along forest trails, where it dominates the ground cover. Similarly, the margins of many Hawaiian forests are dominated by South American wild passion fruit, Passiflora tarminiana (often incorrectly referred to as P. molissima - see Coppens d'Eeckenbrugge et al. 2001), and Afro-Asian ivy gourd, Coccinea grandis (Cucurbitaceae). These plants are also present on, or threaten, other Pacific Islands. Clidemia hirta has also been introduced to South-East Asia, East Africa (Usambara mountains), Madagascar and at least some of the Mascarene Islands where, although it is less dominant than in Hawaii, it is still a significant problem. Similarly, there are anecdotal reports that the neotropical weed Chromolaena odorata (L.) King and Robinson (Asteraceae) has come to completely dominate the ground cover in primary and secondary forest in parts of Ghana (H.C. Evans, personal communication, 2000). In the context of forest systems as reservoirs for biodiversity, this phenomenon merits further study.

The importance of forest weeds in developed countries is reflected by the fact that herbicide use generally greatly exceeds that of insecticides. This is partly a consequence of various sylvicultural practices (clear-cutting, monocultures, etc.) and may have negative environmental effects in terms of water contamination as well as the removal of indigenous flora and its associated fauna (Vanden et al. 1984). Alternative approaches may be appropriate in some circumstances, for example, the use of tree borders or buffers to protect planted and natural forests from invasive weeds (see Case Study 16). Such approaches warrant further testing and evaluation (including some assessment of the potential negative impact on the biodiversity associated with forest margins).

2.3. Alien versus indigenous pests and diseases

There is no doubt that newly introduced alien pest species can be devastating, particularly to plantation forestry (e.g. OTA 1993; Canadian Forest Service 1999; Pimentel et al. 1999; Wittenberg and Cock 2001) and to the biodiversity associated with forests (CBD 2000, 2002). Historical, recent and current examples abound (Case Studies 1, 2 and 8; Annex 1), and in future increasing numbers of accidental introductions can be expected as a result of the growing internationalization of trade, the increasing movement of people, and the consequent overwhelming of quarantine services.

Case Study 8 : The origin and spread of Dutch elm disease

Evidence from pollen analysis in European peat sediments suggests that there were major fluctuations in elm (Ulmus spp.) populations in prehistoric times, related primarily to climate change but also perhaps to Dutch elm disease, caused by the fungus Ophiostoma ulmi.

O. ulmi is thought to have originated in Asia, primarily because Asian species of elm show at least a moderate level of resistance to this species. It is thought that, having coexisted for many generations with the fungus, susceptible Asian elms were slowly eliminated from the population and replaced by seedlings with greater resistance. In this way, the elms of Asia established a natural equilibrium with the disease. Dutch elm disease only reached epidemic proportions when the fungus reached Europe from Asia in the early part of the twentieth century.

A major pandemic spread across the Northern Hemisphere from the 1920s to 1940s. It was seen first in the Netherlands (hence the name, Dutch elm disease), then spread through continental Europe and into the USA, decimating the elm populations. The disease subsequently declined in Europe (though not in the USA) but re-emerged in an even more virulent form to affect Britain (in the mid-1960s) and most of Europe. The fungus isolated from this more recent outbreak showed some cultural and molecular differences from the older strains, and because it also failed to interbreed with them, was described as a new species, Ophiostoma novo-ulmi.

A range of molecular tools has been used to analyse the population structure of O. novo-ulmi in the current European pandemic. Based on comparisons of random DNA fragments and the abilities of strains to fuse with one another in culture (indicating cytoplasmic compatibility), the population at the advancing front of the pandemic seems to be genetically uniform. This is to be expected where an abundance of healthy host trees creates selection pressure for the most virulent component of the fungal population. However, behind the fronts, the population shows much higher diversity - the result of stabilizing selection where factors other than virulence are favoured.

Source: and

It is not easy to predict which alien species are likely to cause serious damage if introduced. Many species are innocuous or minor pests in their area of origin, but can be devastating elsewhere. For example, of six of the most devastating forestry pests introduced into North America (including chestnut blight, Dutch elm disease, etc.), only the European strain of the gypsy moth was known as a pest in its indigenous range (Baskin 2002, quoting OTA 1993). Many ecologists have looked for predictors of invasiveness and pest potential, but at present, the best guide appears to be at least one previous record of causing problems when introduced elsewhere, especially in areas with similar climatic, geographic and ecological conditions to the area at risk. Close relatives of known invasive species are also suspect, although this is not an entirely satisfactory predictor of invasiveness. Access to reliable information is therefore critically important for assessing the potential risk of invasiveness.

2.4. Introduction of new tree genotypes

The introduction of new tree genotypes to areas where there are indigenous genotypes of the same species (or closely related species) could cause some specific problems, for example, through hybridization (see also Section 4), or by displacing indigenous genotypes, subspecies or closely related species. There are some examples of the latter situation from other systems, e.g. weedy genotypes of itch grass, Rottboellia cochinchinensis, replacing indigenous genotypes, but as yet there are few examples from forestry systems.

Alternatively, introduced genotypes may interbreed with indigenous genotypes (or closely related species/subspecies). Natural populations would then become a mixture of the two genotypes, although this is unlikely to be noticed unless the species was being monitored for such changes, or if it was linked to some evident change in appearance or ecology, e.g. a change in flower colour or a tree suddenly becoming more successful in a new habitat. In the larger scale of things, this change in the indigenous genotype may not be that important, although public concern could be an issue.

Hybridization between an introduced species and a closely related indigenous species is certainly a possibility. Generally, such crosses are likely to result in sterile or otherwise relatively uncompetitive F1 hybrids. However, unexpected and unusual things can also happen, e.g. doubling of the chromosome number of the F1 to create a new self-viable species (e.g. Spartina spp.). When congeneric species that normally occur on different continents are grown together, they are likely to hybridize successfully and indeed are often deliberately hybridized to produce plantation and amenity trees. Populus is an example of a genus whose species tend to hybridize very readily, and hybridization with introduced American species has become one of the main threats to England's endangered black poplar (Populus nigra betulifolia (Pursh) W.Wettst.) (see Case Study 9).

2.5. Monoculture versus mixed or natural plantations

Plantations provide an increasing proportion of the world's timber supply (FAO 2000). Tropical and subtropical plantation forestry has focused on a small number of fast-growing species, most notably in the genera Acacia, Eucalyptus, Gmelina, Pinus, Populus and Tectona (Evans 1987); perhaps as much as 85 percent of industrial plantations in the tropics use species from only three genera: Eucalyptus, Pinus and Tectona (Evans 1992). Furthermore, these species are normally planted as monocultures, which are often associated with an increased probability of pest outbreaks, particularly when they are composed of genetically similar trees (Gibson and Jones 1977). The underlying reasons for this include:

Case Study 9 : Hybridization threatens the endangered black poplar population in Great Britain

The black poplar, the most endangered native timber tree in Britain, is a segregated population of the Atlantic race of Populus nigra subsp. betulifolia (Pursh) W.Wettst. which is at the northwestern limit of its range in Britain. It was once a distinctive feature of Britain's lowland river valleys and appears in many of Constable's paintings. Now, however, it is sparsely distributed and the few remaining trees are coming to the end of their lives and do not appear to be regenerating from seed. In 1982, the British population was estimated at between 2000 and 3000, and it is now particularly rare in lowland England.

Several factors are thought to be contributing to the decline of black poplar in Britain:

  • Drainage of farmland and loss of wetland habitats. The black poplar has slowly adapted over time to human impacts on the British countryside, but it remains a tree of wetland regions and land drainage has reduced the area of its preferred habitat.
  • Most of the individuals remaining in the country have been propagated as cuttings from around 600 individuals, so genetic variation is low.
  • The individuals selected for propagation were primarily male, for aesthetic reasons, leading to a lack of female trees.
  • Male and female trees need to be in close proximity for pollination to take place.
  • There is a predominance of old trees, which are not regenerating.
  • Indiscriminate felling of 'unsafe' trees.
  • Several poplar species have been introduced into Britain since the 1700s, some of which can cross-pollinate with the native black poplar, creating hybrid stock. Two American species in particular are implicated, namely Populus deltoides Bartr. ex Marsh. and Populus angulata Ait.
  • The use of cultivated poplars all over the continental Europe has also created a potential threat to the remaining black poplar populations in the form of introgression (i.e. movement of genes from plantations to natural populations through hybridization).

Thus, although hybridization is recognized as a threat to black poplar, its relative importance has not been quantified.

Although black poplar lacks a direct commercial use, it is used as a parental pool in breeding programmes in many parts of the world. Therefore, active conservation and propagation by cuttings is needed to maintain the original stock. Genetic conservation is actively carried out through a dedicated network of the International Plant Genetic Resources Institute's (IPGRI) European Forest Genetic Resources Program (EUFORGEN), which has made considerable efforts to integrate ex situ and in situ conservation efforts for black poplar and has developed guidelines for in situ conservation. It has also generated a standardized list of descriptors for inventories of natural stands and established a database of black poplar clones.

Source: Wildlife Trust Action Plans;; and< /A>.

Monoculture also tends to transform sporadic pests into more permanent problems. In Canada, for example, valuable trees such as Sitka spruce (Picea sitchensis (Bong.) Carriere (Pinaceae)) can only be planted in small areas and numbers because of damage by Pissodes strobi (Peck) (Curculionidae). Similarly, the pine shoot moth, Rhyacionia buoliana (Denis & Schiffermüller) (Tortricidae, Olethreutinae) is still an important pest where monoculture continues (e.g. Spain and Chile), although in central Europe it stopped causing substantial problems when pine monoculture declined.

Mixed planting of native (and exotic) trees is therefore preferred if pest problems are to be minimized.

2.6. Susceptibility of indigenous versus exotic trees in plantations

There has been considerable debate over the relative risks of pest outbreaks on exotic versus indigenous tree species, especially in plantation settings (e.g. Nair 2001). A priori, one might argue that since exotic trees will be introduced without their associated specialist diseases and herbivores, they should be largely free of pests (e.g. Case Study 4). Conversely, one might argue that exotic trees will be exposed to new pests to which they have no resistance and to which they may therefore be extremely susceptible. In practice, it is not uncommon for newly introduced exotic trees to experience an initial period with few pest problems, followed by a period in which indigenous pests gradually adapt to these new hosts (Nair 2001). In a similar way, it can be argued that since indigenous trees are already resistant to, or tolerant of, their indigenous pests, they should escape severe damage. However, converting mixed natural forest to monoculture might be expected to disrupt pest and natural enemy population dynamics and create outbreak situations.

Nair (2001) reviewed the empirical evidence from nine commonly planted trees, grown in both indigenous and exotic settings, and concluded that the only clear trend was that, in all cases, monoculture increases the chance of pest outbreaks. Several distinct factors influence the likelihood of an outbreak, amongst which Nair identified:

It is worth noting that any such review is likely to be biased in favour of those tree species which have managed to avoid substantial damage and which have therefore been widely planted: those tree species which have proved to be highly susceptible to local or exotic pests will not have been widely grown and hence will have been excluded from the analysis. It is therefore appropriate to evaluate the pest risks associated with a particular tree/location combination on a case-by-case basis, and to establish pilot plots before scaling up production.

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