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2019561big.jpgThere is a wide range of different biological, chemical and physical factors which have an impact on the growth and development of trees in the UK.

This can range from grazing by large mammals such as deer and domestic livestock, to insect pests, and bacterial, viral and fungal pathogens.

The effects of these factors can range from changed growth rates and tree shape (or form) to reduced timber quality or in some cases death of the whole tree. Impacts from any given factor are likely to vary depending on the local site conditions, tree species and age.

Biological damage

There are a range of different biological organisms that can cause damage to woodland. This damage can be economic in nature (damaging the value of timber), or ecological (damaging the biodiversity). In either case it may be appropriate to manage the habitat and species in a woodland to provide the greatest benefit.

If you find that trees in your woodland are dying, and you suspect that they may be diseased, then the Forest Research Tree Health Diagnostic and Advisory Service is able to test samples. Further details of their service and rates are available here. There is also a Twitter feed for new information on pest and diseases here.

Inspections of trees and woodland by Forestry Commission plant health inspectors play an important role in efforts to manage outbreaks of pests and diseases. Guidance to plant health inspectors can help you understand what to expect if an inspector needs to visit your premises.

‘Biosecurity’ (short for biological security) refers to measures taken to protect, or keep secure, one group of biological organisms – in this case trees, woods and forests – from other, harmful biological organisms, such as disease-causing pathogens such as bacteria, viruses and fungi, certain insects, and invasive plants and animals. There is a guide to biosecurity best practice available from the Forestry Commission here.

The Forest Research Tree Alert Tree pest and disease reporter is available here.

Insect pests

A significant number of pests can affect trees in the UK. Many of them have limited areas of spread and affect specific species, but due to the effects of climate change and increases in the international trade in woody plant material, it seems likely that the number of different insect pests affecting trees in the UK will increase. The Forestry Commission Plant Health inspectors have published a list of the most significant new threats in the UK here, but the main insect threats are:

The Forest Research Tree Health Diagnostic and Advisory Service is available here.

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Bacterial, viral and fungal pathogens

There are a broad spectrum of different diseases that can affect trees in the UK. Many have been established in the British Isles for hundreds of years and have become part of the natural ecosystem. However, there have been an increasingly large number of new pathogens detected in recent years. The Forestry Commission Plant Health inspectors have published a list of the most significant threats in the UK here. The main pathogens are:

  • Acute oak decline
  • Bleeding canker of horse chestnut (Pseudomonas syringae)
  • Phytophthora spp.
  • Red band needle blight (Dothistroma septosporum)

If you are concerned, or need help identifying a disease in trees, then contact Forest Research Tree Health Diagnostic and Advisory Service here. The Forest Research Tree Alert Tree pest and disease reporter is available here.

Biomass can be a suitable end use for diseased material, but there might be precautions and restrictions that apply when transporting this sort of material. In particular, there are limitations on handling of Phytophthora-infected material, and only authorised companies may deal with it.

If you have any questions about plant health, contact either the Forestry Commission’s Cross-Border Plant Health Service or Forest Research.

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Invasive species and weeds

There are many invasive species in the UK. While very few of them will harm well established trees, many of them will successfully out-compete newly planted trees and can cause a significant negative impact on woodland biodiversity. This may be a particular issue in woodlands that have been managed for game shooting as two invasive species; rhododendron and snowberry were often planted as game-bird cover. There is a useful overview of invasive species at Naturenet.

NB Ivy does not harm living trees, and shouldn’t be considered to be a risk to healthy woodland. The only time trees are in danger from ivy growth is if the tree is already rotten and cannot take the additional weight, or if for some reason the ivy overshadows the tree’s crown.

Injurious Weeds Act 1959

A number of species are covered in the injurious weeds act of 1959. Under this act it is an offence to allow particular species to spread from your land onto a neighbouring property.

  • Common ragwort
  • Spear thistle
  • Creeping or field thistle
  • Broad-leaved dock
  • Curled dock

This is covered in detail on the GOV.UK website.

Wildlife and Countryside Act 1981

The release of a number of invasive species is also controlled by the Wildlife and Countryside Act 1981 this includes many animal and plant species including: rhododendron, Himalayan balsam, giant hogweed and Japanese knotweed. Further information is available on GOV.UK.

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Damage caused by mammals

There is a wide variety of different mammalian species in the UK which can cause damage to trees. These species range in scale from the edible dormouse (Glis glis) to deer and domestic livestock, leading to varying degrees and types of damage. Damage to trees from mammals usually falls into two categories: browsing (eating leaves and buds) and bark stripping.

Forest Research has a useful guide for determining which mammals might be causing damage available here. We also have a list of methods for restricting and controlling mammal damage in woodlands here (and further reading here.)

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Chemical damage

Air Pollution

Air pollution can affect woodlands in a number of different ways. There is a page from Forest Research on Air Pollution and Forestry from Forest Research, and one on Air pollution – practical considerations.

  • Acid Deposition

Rainwater is naturally acidic due to dissolved CO2, but man-made emissions (usually Sulphur and Nitrogen dioxides from combustion) can increase this to a point where it becomes harmful to growing plants. Emission control measures have reduced the prevalence of SO2 and NO2 significantly over recent years. Most of the SO2 and NO2 produced in Britain comes from power stations and large industrial units, but cars and heavy vehicles are also important sources of the oxides of nitrogen.

  • Particulates

Trees can be an effective barrier to particulate emissions from industry and busy roads. However, these particulate deposits can cause damage to woodlands in a number of ways :soil quality can be affected if the particulates contain heavy metals or other compounds poisonous to plants;
particles can clog leaf pores leading to reduced photosynthetic ability;
in extreme cases, particles can coat the leaf surface causing abrasions and reducing the leaf’s light capturing ability.

  • Low Level Ozone

Ozone (O3) at high altitude is an important component of the atmosphere, absorbing harmful UV radiation from the sun. However, it can also be formed as a result of incomplete combustion of hydrocarbon fuels. At low altitude Ozone is a serious contributory factor to smog and can cause irritation to eyes and lungs in humans. Low Level Ozone also causes “scorching” to vegetation and can lead to premature leaf loss.

Soil and Groundwater Pollution

Contamination of the soil can lead to damage to tree roots and a range of impacts on plant health. It is rare to find serious soil contamination in established woodlands, but new woodlands and regenerated brownfield sites may encounter problems. Woodlands which are adjacent to sources of pollution, such as landfill sites, may become affected due to the movement of contaminants in ground and surface water. If you have concerns about an area of woodland, we recommend contacting Forest Research for more detailed advice. There is a page on Forestry and surface water acidification. Some specific issues can include:

  • Spray Drift

Salt spray from the sea and gritted roads can cause a decrease in soil fertility and harm vegetation as can drift from herbicide application on neighbouring land.

  • Heavy Metals

Research has shown that trees can be effective in absorbing heavy metal elements from the soil, and there is considerable interest in this property for site remediation. You should bear in mind though, that these heavy metals will be present in the wood ash after burning, and make sure that it is disposed of safely (rather than contaminating other sites with the same problem.)

  • Organic Pollutants

Some trees species are also effective at removing organic contaminants from the soil (such as solvents, petrochemicals, wood preservatives, explosives and pesticides) the organic components of these contaminants will be completely destroyed by burning, but a heavy metal component may remain in the ash if present.

Physical damage

There are a wide range of different factors that can cause physical damage to trees. Effectively, most of the biological and chemical factors could also be considered to be physical damage. Most sources of physical damage are relatively easy to spot and identify; these may include:

  • Wind
  • Fire
  • Lightning
  • Damage from strimming
  • Poor fencing practice (wire stapled to trees etc.)
  • Vandalism

There are a number of other factors that can damage trees which are harder to distinguish. These include, frost, ground compaction and site waterlogging / drought. The best way to determine the cause of damage is to look at the site conditions over a period of time and to examine specific species characteristics (such as a lack of frost tolerance for example). If you are still unsure, and there is significant damage to a number of trees across the site, we recommend speaking to an expert. The Forest Research Tree Health Diagnostics and Advisory Service may be able to give advice.

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