Forest Invasions

Forest Invasion Synthesis Centre, Prague

Forest invasions are a global problem that requires global cooperation. That is why FISC exists — to bring together science, policy, and practice in finding effective solutions.

What are forest invasions?

Forest invasions occur when non-native organisms — such as insects, tree pathogens, or plants — spread into new regions where they did not exist before.
In their new environment, these species often lack natural enemies and can grow or reproduce unchecked.

Why do they matter?

Ecological impacts – Invasive species disrupt forest ecosystems, reduce biodiversity, and can even cause the decline or extinction of native species.
Economic impacts – They damage timber resources, increase management costs, and create billions in global economic losses each year.
Social and cultural impacts – Forest invasions can affect recreation, traditional uses of forests, and cultural values tied to native tree species.

Examples of forest invasions

Invasive insects such as the emerald ash borer destroying ash forests in North America.
Tree pathogens like ash dieback reshaping European forests.
Invasive plants such as Japanese knotweed or giant hogweed altering landscapes and ecosystems.

Forests in Danger

Top 10 Worst Invaders

1. Fallopia Japonica (Japanese Knotweed)

Commonly known as Japanese knotweed, is a perennial herbaceous invader native to East Asia that now ranks among the world’s most problematic invasive plants.

It forms dense monotypic stands that displace native riparian and understory vegetation, reducing species richness and abundance.

Its extensive rhizome network can spread several meters horizontally and vertically and regenerate from small fragments, making eradication extremely difficult.

Economically, in the UK it is estimated to cost over £165 million annually in management and control.

Beyond direct control costs, it also negatively impacts property value and infrastructure, while altering soil processes and ecosystem structure.

2. Ailanthus Altissima (Tree-of-Heaven)

It is a fast-growing ornamental introduction that has become invasive across Europe and North America.

It produces very large numbers of seeds (reports up to ~350,000 per tree), spreads by root suckers, and releases allelopathic compounds that suppress other plants, allowing dense stands to displace native vegetation.

Its aggressive roots can damage infrastructure and create management costs, and control is often difficult once established.

It is also a preferred host of the invasive spotted lanternfly (Lycorma delicatula).

3. Robinia pseudoacacia (black locust)

This tree is native to a restricted portion of North America, but now invasive in many parts of Europe and North America.

It spreads by root suckers and stump sprouts, forming dense stands that displace native plants.

Black locust alters soil chemistry and light conditions, threatening dry grasslands and rocky habitats with unique biodiversity.

Its rapid growth and nitrogen-fixing ability allow it to dominate disturbed sites. In addition, its leaves, bark, and seeds contain toxic compounds that can harm livestock and reduce habitat quality.

4. Rosa multiflora (multiflora rose / rambler rose)

It is a shrub from East Asia that invades fields, forest edges, and pastures.

It spreads by bird-dispersed seeds and rooting stems, forming dense thickets that exclude native vegetation and restrict wildlife movement.

The plant tolerates a wide range of soils and light conditions, which makes it highly adaptable.

Once established, it can reduce forage value on pastures and change habitat structure. Control is challenging, as seed banks persist for years and removal usually requires repeated treatments.

5. Leucaena leucocephala (white leadtree / subabul)

It is a leguminous tree native to Central America, now invasive across many tropical and subtropical regions.

It forms dense thickets in disturbed and semi-natural habitats, displacing native species.

It produces abundant seeds, resprouts after cutting, and tolerates a wide range of soils and climates.

Its invasiveness is enhanced by allelopathic effects, and it is often considered a “conflict species” — valued in agroforestry but harmful to natural ecosystems.

6. Agrilus planipennis (Emerald ash borer)

It is a stem-boring beetle native to eastern Asia that has become a highly destructive non-native species in North America and parts of Europe.

Larvae feed beneath the bark of ash trees (Fraxinus spp.), forming serpentine galleries that disrupt the transport of water and nutrients, ultimately causing canopy dieback and tree death within 3–4 years.

Adult emergence leaves characteristic D-shaped exit holes.

In its non-native range, EAB has killed tens of millions of ash trees, with profound ecological and economic consequences.

Control efforts include quarantines, tree removal, insecticides, and biological control via parasitoid wasps.

7. Adelges tsugae (hemlock woolly adelgid, HWA)

It is a sap-feeding insect native to Asia that infests hemlock (Tsuga) trees.

It produces white woolly masses at the base of needles and causes needle loss, branch dieback, and often tree death within a decade.

In eastern North America it has devastated native hemlocks, with major ecological impacts.

Control includes insecticides, predator beetles, and efforts to breed resistant trees.

8. Ophiostoma ulmi and novo-ulmi. (Dutch elm disease)

It is a lethal fungal pathogen of elm (Ulmus) trees, spread mainly by elm bark beetles vectoring spores.

The fungus blocks water flow in the xylem, causing leaf wilting, dieback, and often tree death within a season.

It can also move through root grafts, making containment difficult.

Management includes sanitation, insecticide treatment (of vectors), and planting resistant elm cultivars.

9. Anoplophora glabripennis (Asian longhorned beetle)

It is a wood-boring insect native to Asia with incursions in Europe and North America.

It attacks maples and many other hardwoods by tunneling through trunks and branches, disrupting vascular tissues and eventually killing the tree.

Females lay eggs under the bark, and larvae bore into the wood, weakening structure and reducing quality.

Control depends on tree removal, quarantines, and planting of non-host species.

10. Pine wilt disease (caused by Bursaphelenchus xylophilus)

It is a rapidly progressing disease in pines, caused by the pinewood nematode and transmitted by sawyer beetles (Monochamus spp.).

Once inside the tree, the nematodes multiply and block water transport, causing needles to yellow, wilt, redden and then tree death — often in just a few weeks to months.

It affects many pine species, especially Scots pine (Pinus sylvestris), Austrian pine (P. nigra), and Japanese black pine (P. thunbergii).

Management requires early detection, removal and destruction of infected wood, and limiting the movement of infested material.

Why are forest invasions increasing?

Global trade and travel move species faster and further than ever before.
Climate change is creating new opportunities for non-native species to survive and expand.
Expanding areas of planted forests often include non-native species, which may facilitate new invasions.

How can they be managed?

Prevention through stronger biosecurity measures.
Early detection and rapid response to new outbreaks.
Long-term management combining ecological knowledge, practical forestry, and public awareness.

Project HIVE 101187384. Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.