Non-native trees

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What is a non-native tree?

False acacia Robinia pseudoacacia © Ita McCobb

Trees that have become invasive can change ecosystems and habitats. They can reduce or impede water flow leading to flooding. They might also change the pH or chemical composition of the soil.

Non-native trees (also sometimes referred to as “aliens”, “introduced” or “exotic”) are those that are grown outside of their native range or ecotype. This categorizes trees as native only if they are growing within an area where they have traditionally occurred, before the introduction either intentionally or unintentionally of non-native trees.

The distinction between non-native and native is important as many non-native trees are invasive and alter the ecosystem of an area.

A tree is considered invasive if it can become established in a new ecosystem and spread unaided, creating a negative impact on native trees, plants, animals, insects or fungi.

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What do non-native trees cause?

Horse chestnut Aesculus hippocastanum © Ita McCobb

Trees that have become invasive can change ecosystems and habitats. They can reduce or impede water flow leading to flooding. They might also change the pH or chemical composition of the soil.

Water tables can be lowered if non-native trees have deep rooting systems while fast growing trees that create a wide canopy can out shade native trees and other vegetation.

Non-native trees support far fewer invertebrates and are often unpalatable to native species that are not used to feeding on them.

Non-native trees can deprive native trees of much needed nutrition and cause some flora and fauna to become endangered.

As coniferous woodlands in the UK are usually predominantly composed of non-native species they tend to have lower biodiversity than broadleaf woodland.

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The threat of monoculture plantations

Monoculture plantations of non-native trees threaten ecosystem service, particularly watershed protection and biodiversity conservation. These plantations also generate less leaf litter and other organic inputs than native forests.

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Trees in urban environments

London plane Platanus acerifolia © Ita McCobb

Many non-native trees may be recognizable as native trees because collectors and plant hunters introduced them many centuries ago. Examples in the UK include the Horse chestnut Aesculus hippocastanum, Sycamore Acer pseudoplatanus, False acacia Robinia pseudoacacia L. and London plane Platanus x hispanica.

Currently many cities around the world are planting more trees and the use of non-native trees has been criticized, as some people believe a native tree is more adaptable to local growing conditions and is more resistant to disease.

This is not always the case, however, as non-native species and tree cultivars are frequently more adaptable to urban conditions than native species. The London plane tree Platanus x hispanica when first discovered in the 17th century was found to thrive and provide wonderful shade. Its ability to withstand the sooty air, drought and other adversities assures its popularity as an urban tree. More recently the Maidenhair tree Ginkgo biloba, has become popular as it has a slim profile and can therefore fit into narrow spaces where a native tree would fail to thrive.

Planning a broader range of trees means that there is less chance of a disease wiping out a large proportion of a particular species.

Non-native tree species can also help restore native ecosystems on degraded land or reclaimed land.

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Climate change and urban tree environments

Climate change may result in greater stress on native trees, making them more susceptible to disease and insect attack.

Native trees may not be able to adapt fast enough to meet the changes, whereas non-native trees may be able to thrive and be more suitable to changing urban environments if used discriminatingly to meet the needs of changing city climates.

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Trees and urban wildlife environments

Maidenhair tree Gingko biloba © Ita McCobb

It is often assumed that native trees are better for native wildlife. Although native wildlife does have preferences for some native trees many also take advantage of non-native trees.

Many non-native trees provide alternative sources of food such as fruit and are used as habitat if they have branch structure and other characteristics similar to native trees. It is believed that canopy age and tree species diversity are important for supporting diverse wildlife.

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Tree-planting in future urban environments

Research has shown that a lot more work has to be done and more specific knowledge gained before any suitable selection of trees (native or non-native) for urban areas can deliver a wide enough range of environmental, economic and social benefits.

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Some rampant non-native trees

Wild tamarind Leucaena leucocephala © United States Department of Agriculture

Shoebutton ardisia Ardisia elliptica

A shade-tolerant evergreen tree whose fast growth and attractive fruit made it a popular ornamental plant in the past. It has escaped from private and public gardens to invade natural areas. Due to high reproductive output and high shade-tolerance, carpets of seedlings can form underneath adult trees. High seed viability (99%) and seed consumption by both avian and mammalian frugivores can lead to its rapid spread across a landscape.

Pop-a-gun, snakewood tree Cecropia peltata

This is a fast-growing, short-lived tree that grows in neotropical regions. It is light-demanding and rapidly invades disturbed areas, such as forest canopy gaps, roadsides, lava flows, agricultural sites, urban locations, and other disturbed areas. It naturally occurs in tropical Central and South America, as well as some Caribbean islands and has been introduced to Malaysia, Africa and the Pacific Islands. It may be replacing, or competing with, other native pioneer species in some locations.

Red cinchona, quinine Cinchona pubescens

This is a widely cultivated tropical forest tree which invades a variety of forest and non-forest habitats. It spreads by wind-dispersed seeds and vegetatively via multiple suckers up to several metres away from the original tree once it is established. C. pubescens replaces and out-shades native vegetation.

Horse/wild tamarind Leucaena leucocephala

This fast-growing, nitrogen-fixing tree is cultivated as a fodder plant, for green manure, as a windbreak, for reforestation, as a biofuel crop etc. Leucaena has become an aggressive invader in disturbed areas in many tropical and sub-tropical locations and is listed as one of the 100 of the World’s Worst Invasive Alien Species. This thornless tree is difficult to eradicate once established, rendering extensive areas unusable.

Broad-leaved paperbark tree Melaleuca quinquenervia

This tree can reach heights of 25 meters and hold up to 9 million viable seeds in a massive canopy-held seed bank. This fire-resistant wetland-invader aggressively displaces native sawgrass and pine communities in south Florida, alters soil chemistry and modifies Everglades ecosystem processes. Melaleuca is notoriously difficult to control.

Bush currant, velvet tree Miconia calvescens

This is a small tree native to rainforests of tropical America where it primarily invades treefall gaps. Miconia is now considered one of the most destructive invaders in insular tropical rainforest habitats. It has earned itself the nicknames “green cancer of Tahiti” and “purple plague of Hawaii”. It has invaded more than half of Tahiti. Miconia has a superficial root system that may make landslides more likely. It shades out the native forest understorey and threatens endemic species with extinction.

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References: Forestry Commission UK; Oregon Government, Department of Forestry; Yale School of Forestry and Environmental Studies; www.countrysideinfo.co.uk; http://www.issg.org/database/.