Pests and diseases

 

The implications of climate change for pests and diseases of woodland are difficult to predict. This is because there is a fine balance between pests and disease-causing organisms (known as pathogens) and their host trees. It is possible however to make two generalisations: stressed trees are more susceptible to insect pests and diseases, and the majority of insect pests that currently affect UK forestry are likely to benefit from climate change as a result of increased summer activity and reduced winter mortality. Some insect pests that are currently present at low levels, or that are not considered a threat at this time, may become more prevalent. In addition, the ‘effective’ range of existing pests or pathogens may change, including a northwards expansion of those with a southern distribution and the likely appearance of some from continental Europe.

For some pests and diseases, likely trends cannot be predicted even on the basis of expert judgement; in this category, and of particular concern, is Phytophthora ramorum, the agent responsible for sudden oak death. The higher level of uncertainty associated with the biology of fungi compared to insect pests is reflected in the less specific predictions of future trends in the incidence of fungal diseases and disorders.

Insect pests

• Climate change is likely to alter the balance between insect pests, their natural enemies and their hosts; predictions of the impact of climate change on insect damage to UK forests are therefore difficult to make.
  
• One of the most important effects of climate change will be to alter the synchrony between host and insect pest development, particularly in spring, but also in autumn; the predicted rise in temperature will also generally favour insect development and winter survival, although there will be some exceptions.
  
• The green spruce aphid is one example of an insect that is likely to benefit from the increase in winter survival, leading to more intense and frequent tree defoliation. A decline in the productivity of Sitka spruce might therefore be expected.
  
• Modelling work suggests that under a warmer climate, exotic pests such as the southern pine beetle could establish populations in Europe, and that climatic warming could make UK forests susceptible to damage; other bark beetles such as Ips typographus, which is present in some parts of Europe, but not the UK, could become a serious problem.
  
• Rising atmospheric CO2 concentrations may lead to a decline in food quality for plant-feeding insects, as a result of reduced foliar nitrogen levels.
  
• The planting of exotic tree species may exacerbate the beneficial effects of climate change on insect pests, as the natural predatory fauna may not be present to limit population growth.
  
• Changes have already been observed in the distribution of native European butterfly populations, with northern ranges extended and southern ranges reduced. The same is likely to be the case for forest insect pests.
  
• The combined effects of increased global trafficking of timber and wood products and climate change are likely to result in exotic pests such as Asian longhorn beetle becoming more prevalent; it is therefore essential that we remain vigilant in reporting new pests and altered patterns of damage.

Fungal diseases

• The effects of predicted climate change on fungal diseases of trees can, to some extent, be judged by analysing the existing roles of climate and of fluctuations in weather; it is, however, more difficult to predict the effects of climate change on host–pathogen relationships than on the individual organisms.
  
• The impact on those pathogens whose reproduction or dispersal is clearly affected by temperature is relatively predictable.
  
• Warmer summers may in particular favour certain thermophilic rust fungi on poplar, which are currently rare or non-native in Britain; this has important implications for poplar breeding programmes.
  
• Insect vectors of pathogens such as the fungi causing Dutch elm disease are likely to respond to warmer summers by extending their geographic ranges and hence the ranges of disease incidence.
  
• The likely effects of higher year-round temperatures have been modelled in the case of Phytophthora cinnamomi, a very widespread fungus which causes root and stem-base diseases of a wide range of broadleaved and coniferous species. The models show a probable significant increase in the activity of this fungus across the UK and Europe in general.
  
• Warmer winters may increase the activity of some weak pathogens, such as Phacidium coniferarum, which are active only when the host is dormant.
  
• An increased incidence of summer drought would probably favour diseases caused by fungi whose activity is dependent on host stress, particularly root pathogens and latent colonisers of sapwood.
  
• A reduction in the number of summer rain-days may reduce the incidence of various foliar diseases such as Marssonina leaf spot of poplar. Generally, however, it is difficult to predict the impact of climate change on pathogens whose reproduction or dispersal is strongly affected by rainfall or humidity.
  
• The protective effects of mycorrhizas against various root diseases may be altered by changes in the relative fitness of different mycorrhizal fungi under conditions of altered soil temperature or moisture regime.