The changing climate is increasing the risk to trees and woodlands from pests and pathogens, with outbreaks of new pests and diseases and changes to the frequency and severity of existing outbreaks, due to climate-related factors.

Milder winters, warmer temperatures, changes in seasonal timing and phenology, in combination with other climate impacts, such as storm damage and drought stress, will increase the susceptibility of trees to damage and mortality from pests and diseases.

Trade pathways between countries inevitably provides opportunities for pests and pathogens to arrive in the UK, and a changing climate is increasingly likely to favour establishment and spread. It is essential that appropriate measures are put in place to increase resilience.

Pathogens of forest trees are becoming increasingly problematic and are likely to feature more in future, because of the changing climate. The changes to temperature and moisture patterns will lead to increased risks from:

• Phytophthoras
• Cankers
• Foliar pathogens
• Root pathogens

Pathogens such as chestnut blight (Cryphonectria parasitica) and Pitch canker of pine (Fusarium circinatum), which have previously been viewed as southern European problems, are extending their northern range as the climate warms. Bacterial pathogens are also likely to feature more.

Drought stress is likely to predispose trees to attack by pathogens and increase mortality. Latent pathogens which infect plants but can be asymptomatic for years are predicted to become more damaging as opportunities for activation and invasion increase with more water-stressed hosts.

The abundance, distribution, and impact of forest insect pests are determined by complex interactions between climate, food availability, natural enemies, dispersal opportunities and competitors, so predicting the changes is not easy.

With a changing climate, warmer temperatures may enable some species to have more generations each year, extend the period over which they are active, or extend their distribution range. For example, the Oak processionary moth (Thaumetopoea processionea), introduced to London and the surrounding area, is likely to extend its range as temperature increases.

In addition, warmer temperatures and more frequent droughts will lead to increased water stress in host trees and may make them more vulnerable. For example, the larger eight-toothed European spruce bark beetle (Ips typographus) has been discovered in south east England and control efforts are being taken to contain the spread. It prefers stressed or weakened trees, e.g. windblown and damaged spruce trees, and under the right environmental conditions beetle numbers can increase. If left uncontrolled, the beetle has the potential to cause significant damage to Britain’s forestry and timber industries.

Other indirect effects of a changing climate upon the pests may include altered relationships with natural enemies or competitors, or with control measures such as insecticides or use of fallow periods.

Different groups of insects will be influenced in different ways. Bark boring beetles could benefit from warmer temperatures, reducing the generation time of some species, whilst drought stress on trees and more frequent storm damage is likely to increase the availability of breeding material. Species of butterflies, moths and aphids may benefit from warmer summer temperatures through more rapid generation times, or range expansion. Conversely, more intense periods of rainfall might increase the mortality of insects which are exposed on the plant surface.

Many defoliators such as larvae of butterflies and moths are adapted to feed on newly flushing leaves, so cannot increase their generation time, and climate change may cause a mismatch between their emergence and budburst, but natural selection may cause rapid adaptation.

Milder winters, fewer and shorter cold spells, and longer tree growing seasons will contribute to an increase in woodland mammal populations. This increases the risks of damage to trees and poor establishment and regeneration of woodlands caused by large herbivores including deer and wild boar, and small mammals including grey squirrels. Population densities of grey squirrels are expected to increase due to improved food availability because warmer weather stimulates seed production and an increase in survival rates as a result of milder winters. Oak damage from grey squirrels in lowland Britain is expected to increase, particularly as the area of woodland increases and new woodland matures. In oak woodland, dominant oak trees >7.5 cm have been found to be damaged most.

Increasing wild boar and feral pig populations may reduce regeneration in species with seeds which they eat (e.g. oak, beech and hazel), although the increasing ground disturbance may stimulate regeneration of some other small-seeded species (e.g. birch). Recent research using national observations of deer damage in forests and woodlands has provided a tool to assess herbivore impact in woodlands and to investigate the likelihood that a forest will be damaged by deer in different locations across Britain, according to regional, landscape and local factors. At a local level, a guide is available on how to assess the risk of herbivore impact at particular sites.