Forestry (pathway cause)

International Common Names

• English: Commercial forestry; Plantation forestry; Silviculture
• Spanish: Silvicultura
• French: Silviculture

Local Common Names

• Portugal: Silvicultura

A significant proportion of invasive species are trees that were originally introduced internationally for the perceived benefit of increasing wood production. These include trees for timber, such as Eucalyptus and Pinus, especially in southern Africa, and trees for fuelwood (and non-timber and environmental benefits), such as Acacia and Prosopis in many dry areas. Such invasive trees may also be grouped with other invasive trees introduced primarily for other reasons, such as fruit production, as an ornamental, for shade, shelter or cover, etc. This pathway is likely to be of declining importance with improved access to information on potential invasiveness.

Summary of organism types or species introduced

The principal organism types introduced are terrestrial plants, specifically trees suitable for the production of wood, but also shrubs for fuelwood, and other long-lived woody perennials such as some cacti, tree ferns, palms, etc. It is not regarded as a pathway for the introduction of forest (or other) pests and diseases, unless in the rare cases where trees are introduced as whole plants or other vegetative material (with or without soil), rather than as seed which is by far the most prevalent means. As far as is known, there have been no studies on tree seeds for the transmission of seed-borne diseases. There is also concern regarding the increased interest in genetically modifying tree species, and such modified organisms are also likely to be introduced via the forestry pathway.

Principle processes

The principle process of introduction is via seed, and not as live plants (with/without soil), though in the rare cases where this does occur other organisms could be introduced. Many were introduced in the 1800s and 1900s by government departments, and later by research institutes as part of species elimination and growth trials. Private timber companies have also been responsible for the introduction of some species, especially highly productive species, varieties and cultivars, such as of Eucalyptus, Pinus and Poplar for example. Seed of very many invasive trees is still available from mail order companies, and now can be purchased via the internet.

Geographical routes and corridors

These tend to link areas with similar climates and environmental conditions. The early records of tree introductions are over 2000 years old, from around the Mediterranean basin, generally from east to west, but this mostly concerned the movement of fruiting trees such as almond, carob and olive. There was also some, if limited, movement of tree seed along Eurasian overland routes, again mostly fruit trees and mostly east to west. With transoceanic shipping, there was a second and much larger phase of tree introductions across the Atlantic, between Europe and North America, and later between Africa and Latin America, and to Asia and Australasia and across the Pacific. The exchange between some areas was considerable, such as between South Africa and Australia.

Human-mediated history

Trees for timber only began to be introduced at the onset of plantation forestry from the 1700s. Before that time, natural forests were widespread and plentiful enough in relation to humanity’s requirements to allow unsustainable exploitation without the need for artificial regeneration, or only in rare and isolated cases. Then, from the 1800s, colonialization, industrialization and globalization saw the introduction of many tree species around the world intended for wood production in plantations. However, it would appear that the number of different forestry introduction events world-wide peaked sometime in the mid 1900s, and today, many of the trees that can grow well in non-native areas have already been introduced. Even with warnings about potential invasiveness, introductions of trees still occur, if at a lower level than before, and these are likely to continue into the foreseeable future. However, this pathway is likely to be of declining importance with improved access to information on potential invasiveness.

The main means of entry are by land, sea, or air, via the postal service of packets of seed. Few countries apart from Australia, New Zealand and some Pacific islands, have ‘black lists’ prohibiting the entry of seeds of selected species. Otherwise, such trade and transport of tree seed is legal if accompanied by a phytosanitary certificate (not always a requirement). Where restrictions exist, increased intervention of post would reduce illegal introduction of seed, as is significant in New Zealand at least, and more scrutiny of seed imports would reduce the risk of introduction of potential invasive tree species.

Education is also an important aspect; of foresters, those involved in forestry-related businesses, and researchers in forestry institutes and departments (Pasiecznik and Jaenicke, 2008), as well as the public at large. The general public may only order and plant a small amount of seed, relative to the other groups, and these are more likely to be ornamental herbaceous plants anyway. Education of forestry professionals may be the most cost-effective means of preventing further introductions. This could be linked to a further expansion of a global registry of weed risk assessments on forestry species. An Australian system has been adopted by Pacific Island Ecosystems at Risk (PIER), and many trees have been fully assessed, results presented as a single page summary sheet and made freely available via the internet. This provides a guide as to which species may be invasive, and others could be analysed in the same manner.

The possibility of regulating businesses involved in international trade is diminishing in parallel with the deregulation of global ‘free’ trade. However, the introduction of voluntary codes for nurseries in Australia is providing valuable lessons, though one of these is that self-regulation of profit-driven companies is largely influenced by financial gains and losses, and is having only a limited impact to date. Thus, further regulation of introductions appears necessary, in compliance with WTO SPS measures, fully transparent and based on sound scientific methodology. Existing international standards such as ISPM No. 11 (FAO, 2004) originally focusing on impacts on agriculture, now include impacts on biodiversity, the environment in general, and genetically modified organisms, and steps are being made to aid in its enforcement (Quinlan et al., 2003). The biosecurity implications of forestry as a pathway for invasive species is also acknowledged (Cock, 2003), but whether this will impact on new and firmer regulations is yet to be seen.

Once a tree species is introduced, the issues understandably change. Monitoring systems need to be developed and employed for ‘sleeper weeds’, i.e. trees known to be invasive elsewhere but which are not, currently, invasive in the area in question. As for those which are already invasive, there is the simple standard choice of eradication, control or containment.

Mechanical, chemical, cultural or biological control methods have all proved effective, on their own or in various combinations, on most invasive trees on which they have been attempted, and there is a wealth of species and location-specific information that can be found in other datasheets. A recent approach has seen the development of ‘control by utilization’ as a new convergent concept from diverse land-use managers and researchers as a means to control a number of invasive plants from around the world. It is seen as more applicable to trees than to other organism types, as trees have a known economic value (which was why they were introduced in the first place), and the necessary technologies for harvesting and processing are relatively simple, e.g. conversion of small roundwood from weedy trees to marketable timber (Pasiecznik et al., 2006). Prosopis, for example, has seen advances in control by utilization (Pasiecznik, 2001; 2002; 2006).