The transport of forestry materials and products is a major pathway for the introduction of alien species, particularly diseases and insects. The Asian longhorned beetle, pine shoot beetle, chestnut blight (Cryphonectria parasitica) and Dutch elm disease (Ceratocystis ulmi) are just a few examples of species that have been imported into new areas on raw wood or nursery stock. The impact of such introductions can be profound: as shown in Case Studies 2 and 8, for example, while the latter two species completely altered the composition of forest communities in the eastern USA.
Within the forestry industry, several distinct pathways for the introduction of alien species have been identified. Some of these have been alluded to earlier, but are examined here in greater depth.
The importance of alien pests in forestry and the potential for introduced forestry trees to become invasive have already been discussed. However, other species may be introduced in association with forestry practices, either deliberately or inadvertently. Specific examples include cover crops and mycorrhizae.
Fast-growing cover crops are useful for reducing weeds in the early stages of plantation establishment. Typical cover crops include various vines (e.g. nitrogen fixing species) or grasses that can subsequently be grazed or harvested. In mixed agroforestry systems, vegetable and fruit crops may be under-planted and the associated weed control will also benefit the trees.
In the early part of the twentieth century, various exotic cover crops were tried and used in some countries. However, the characteristics that make these species effective as cover crops are also likely to make them invasive. Mikania micrantha (Case Study 7), for example, was deliberately introduced as a trial cover crop in plantation systems and subsequently became a major weed in both agricultural and forestry systems (Parker 1972). In general, the use of exotic plants as cover crops should be approached with caution and the introduction of such crops into areas where they do not already occur should be stopped.
Similar care should be exercised over the introduction of exotic mycorrhizae to promote the growth of forestry trees. In general, these introductions may be expected to be beneficial, but it is extremely difficult to anticipate or assess the impact of such species on an ecosystem (Case Study 19). Introductions of these species should therefore be approached with caution (particularly in the case of oceanic islands) and efforts made to monitor their subsequent establishment, spread and ecological impact.
|
Case Study 19: Possible impacts of mycorrhizal introductions associated with forestry trees There is relatively little information on the impact of introduced mycorrhizal fungi, except on their direct benefit to associated trees. Their possible effect on the invasiveness of their associated trees (or other ecological effects) has not been widely investigated. Introductions of symbiotic mycorrhizae may increase the invasiveness of exotic plants, but at present there is little evidence of this. However, there is clear evidence from Hawaii that fungal symbionts of alien tree species can significantly alter the natural ecosystem. In Hawaii, Myrica faya, an invasive alien tree, forms a symbiotic relationship with Frankia spp. (an actinomycetal genus that forms nitrogen-fixing root nodule symbioses). It is not known whether Frankia was naturally present in the soil in Hawaii, but the interaction between M. faya and Frankia significantly increases soil nitrogen levels. These high nitrogen levels, in turn, provide a nutrient-rich substrate for other fast-growing alien species, which can outcompete slower-growing native species. The high-nitrogen soil also attracts worms (another alien to Hawaii) that in turn attract pigs (also alien). The pigs disturb the soil looking for worms, creating germination sites for invasive alien weed species. There is thus a whole series of negative impacts associated with this particular tree-mycorrhizal relationship. In New Zealand, the introduced ectomycorrhizal fungus Amanita muscaria has become naturalized in some Nothofagus forests. This fungus was introduced to New Zealand relatively recently, but before 1997 it had only been reported in three or four native forests within the Nelson Lakes National Park. Recent records, however, suggest that it is now widespread in indigenous forests in the northern half of the South Island. In most localities where it is found, Amanita muscaria is present at numerous separate sites. This distribution may be due to some local characteristic of the environment, the fungus, or the host, or it may be the result of local spread following a single invasion. So far, it has been recorded only in association with Nothofagus spp., with no reports of it occurring with the only other indigenous ectomycorrhizal trees in New Zealand, Leptospermum scoparium or Kunzea ericoides. This system provides an ideal opportunity to research various questions relating to the effect of mycorrhizal species on the forest ecosystem, including:
Sources: http://www.landcareresearch.co.nz/research/biosecurity/fungal/; Candace J. Felling, personal communication, 2002; Tim Low, personal communication, 2002; Peter Johnston, personal communication, 2002. See also Lutzow-Felling et al. (1995); Vitousek (1986, 1992). |
The accidental or deliberate introduction of pollinating species can also have unpredictable effects. Most trees are pollinated by wind or by generalist pollinators - usually insects (particularly bees), but also sometimes by vertebrates. However, some trees have highly specialized pollinators, which may be restricted to the tree's indigenous range. For example, many fig species are dependent on highly specific fig wasps (Agaonidae) for pollination, and without them the fig tree will bear no seeds. Three exotic fig tree species grown in Florida gardens for over a century only started spreading approximately 20 years ago, when their fig wasp pollinators arrived. Thus Ficus microcarpa, widely planted in Florida as an ornamental, only became a pest about 45 years after its introduction, when its associated fig wasp (Parapristina verticillata) was introduced. There are similar examples from California and Bermuda.
In the case of the oil palm (Elaeis guineensis), pollinators were deliberately introduced into South-East Asia from the tree's centre of origin in West Africa. For many years, oil palm was thought to be wind pollinated, and in South-East Asia hand-pollination was necessary to obtain satisfactory fruit set. However, research in West Africa revealed several weevil species whose adults pollinate oil palm flowers. After detailed studies on host specificity, one of these species (Elaidobius kamerunicus) was introduced into Malaysia. It soon became established and spread rapidly, providing excellent pollination and obviating the need for hand-pollination. The introduction was considered to be an outstanding success and the weevil was subsequently introduced into other oil palm growing areas with the same results. The host specificity testing, which was based on the practices for weed biological control, was considered to have accurately predicted the risks. This approach might be applicable to some forestry trees which show low levels of seed production in areas outside their native range, but it should be approached with caution, since the host range testing is a long, detailed and expensive process, and, perhaps more importantly, improved fertilization could help an exotic tree to become invasive (Section 4).
A rather more speculative example in a similar vein is worth mentioning. Eucalyptus trees are very widely planted in the tropics, but conservationists often do not like them because they show allelopathic effects and the leaf litter beneath them breaks down only slowly, tying up nutrients and maintaining the allelopathic action. The biodiversity associated with exotic eucalypts is also low. In Australia, the breakdown of Eucalyptus leaves is facilitated by a group of specialized and probably highly specific detritus feeders. It may be that the introduction of one or more of these detritus feeders into the exotic range of Eucalyptus could improve nutrient cycling under these trees, to the benefit of both forestry and conservation. However, the ecological implications of this idea would obviously need careful evaluation before it could be taken further.
The importation of plants is a major pathway for the accidental introduction of alien species. For example, of 23 alien insect species that have become established in California since 1980, 20 arrived on imported plants (two others were on fruit and one was on infested wood) (OTA 1993). The Sternorrhynchan bugs, particularly the most sedentary groups such as scale insects and mealy bugs, are particularly prone to dispersal in this way. Less obvious threats include the seeds of other plant species, which may be carried either on the above-ground parts of the plant, or in the soil. The latter may also harbour arthropod pests, soil-borne diseases and other soil organisms. Knowledge of the taxonomy and natural distribution of some of the latter is rather limited and the distribution of some so-called cosmopolitan species is probably human-mediated. The increasing availability and use of tissue culture should reduce such problems in the future.
Extensive literature is now available on the safe movement of germplasm, some directly concerned with forestry trees, e.g. Eucalyptus (Ciesla et al. 1996). National and regional trade protocols and legislation may also encompass the movement of forest reproductive material (e.g. Ackzell 2002), but this area is beyond the scope of this report.
Packaging material is frequently composed of raw wood from any available species and represents one of the most important pathways for the introduction of alien organisms inhabiting woody substrates (Case Study 20).
The Asian longhorned beetle entered the USA in wooden packing material from China and is now established in New York and Illinois (Case Study 21). This is just one potentially devastating species that has arrived on wood packing material - to date, insects from 54 families have been intercepted in solid wood packaging materials (SWPM) by the USDA.
|
Case Study 21 : Asian longhorned beetle, a threat to North American forests3 Anoplophora glabripennis, the Asian longhorned beetle (ALB), probably entered the USA inside solid wood packing material from China, and has been intercepted at ports and in warehouses throughout the USA. The insect is a serious pest in China where it has few natural enemies; in North America no natural enemies have been recorded as yet and if this insect becomes established it could destroy millions of acres of America's hardwoods. The female beetles lay their eggs in the bark of trees, and on hatching, the larvae bore into the trunk, feeding on the heartwood and girdling stems and branches. When mature, the beetles burrow out of the tree leaving a ? inch exit hole. The adults then feed on the bark and leaves. Repeated attacks lead to dieback of the tree crown and, eventually, to the death of the tree. In the USA the beetle prefers maple species (Acer spp.), including Norway, red, silver, sugar and sycamore maples. Maples are not only a dominant tree species in the northeastern part of the USA, but are the basis of the US$40 million maple syrup industry. The beetle also attacks many other hardwood trees, including horse chestnut, mulberry, black locust, elm, birch, willow, poplar and green ash. Currently, the only effective means of containing ALB is to remove and destroy infested trees. The area is then quarantined to prevent the movement of infested trees and branches. Early detection of infestations and a rapid response are critical if the spread of the beetle is to be controlled. However, the chances of eradicating the beetle are low: in 1996, State and Federal Governments spent more than US$4 million on a suppression programme in New York City and Amityville, NY, but these efforts are not believed to have resulted in eradication. Sources: Wittenberg and Cock (2001); USDA (2002); USDA-APHIS (2003); Illinois Department of Agriculture (2001) |
Given the uncertainties concerning the origin of SWPM, only globally accepted measures that can be applied to wood packaging material by all countries can reduce the risk of inadvertent introductions of organisms associated with this material. The International Plant Protection Convention has recently published some guidelines for regulating wood packaging material (IPPC 2002), which specify approved measures for preventing pest infestation, including 1) heat treatment to a core temperature of about 56ºC for a minimum of 30 minutes and 2) fumigation with methyl bromide at a minimum of 10ºC and for an exposure time of more than 16 hours. However, wood packaging material is very often reused or recycled and, if it is stored in places where it can be attacked by pests it needs to be treated again before being used. An alternative approach would be to use different materials, e.g. plastic or chip wood (which would exclude larger organisms, but not necessarily pathogens).
Timber is the most important forestry commodity of international significance and is a major source of forest pests and pathogens. Timber shipped in containers is a particular threat, since the container provides additional protection. Moreover, as mentioned above, modern containers limit access for inspection and the sheer number of containers arriving at ports makes it impossible to check all shipments.
Strict importation regulations are necessary, and risk analysis with regard to specific import routes may be appropriate (e.g. Case Study 22).
This issue deserves separate consideration, despite its relation to earlier sections, because of its importance in the movement of alien species. Those promoting forestry aid programmes should consider the full implications of their activities, and prior to implementation should consult conservation authorities to prevent introductions of alien organisms. Otherwise, the benefits of the aid programme could be outweighed by larger losses due to new invasive species.
In regard to forestry, the Central American tree Cordia alliodora is a classic example of a problem arising from an introduction via an aid programme. This species was introduced to Vanuatu with the best of intentions as a potential timber plantation tree, but, for various reasons, failed to live up to expectations (probably due to climatic differences between Central America and Vanuatu). Cordia subsequently started to penetrate the native bush and became a nuisance (Tolfts 1997). Other well-known examples include the promotion of pines and eucalypts as timber trees in various places around the world.
Conifer cones, nuts, fruits of forest trees, etc. can harbour a wide variety of immature stages of insects, and although detection and treatment techniques are available they are not often used, since these products are not major commodities. For example, X-ray techniques can detect infested conifer seeds amongst healthy ones, but are rarely used even in the seed trade. Furthermore, this type of material is often moved by tourists (e.g. as souvenirs of their holidays), so that public education and support from the tourist industry should help reduce this traffic.
An example of a pest probably imported via this pathway is the Douglas-fir seed chalcid, Megastigmus spermotrophus (Forster) (Torymidae), which was introduced to Europe from North America, subsequently becoming Europe's most important pest in conifer seed orchards.
3 Editorial note: Since this document was prepared, ALB has been found in eastern Canada (September 2003).
