Some effects of climate change will impact on forests and woodlands directly, for example the effect of increased levels of carbon dioxide on growth. Some of the effects will be indirect, such as the impacts of pests and diseases.

Direct effects

Rising carbon dioxide levels

Before the effects of a changing climate are considered, it is important to note that the concentration of carbon dioxide in the atmosphere the main driver of human-induced climate change affects the growth of trees directly. As photosynthesis is limited by the availability of carbon dioxide, increased emissions of the gas might be expected to enhance growth rates assuming all other environmental conditions remained constant. This is indeed the case. Controlled environment experiments on young trees typically show that biomass production increases by 30–50% when the carbon dioxide concentration is doubled. Although mature trees are unlikely to respond to this extent in a forest environment, some increase in productivity is likely. Other changes resulting from increased carbon dioxide concentrations may include an increase in the surface area of leaves and a change in the functioning of the stomatal pores which regulate water loss.

Increased temperature

For many tree species, budburst is likely to occur earlier in the year as a result of rising temperatures during winter months. This is already evident, with the flushing of oak advanced by up to two weeks compared with the 1950s. Although a longer growing season would be expected to increase productivity in most years, early flushing increases the probability of frost damage, which may be serious in some years. The planting of more southerly provenances, which tend to flush earlier, in anticipation of climate change could exacerbate this problem. Climatic warming may also lead to delayed or incomplete winter hardening, potentially resulting in more serious winter cold damage. In addition, warmer winters may mean that winter chilling requirements for flowering or seed germination are not met, which could affect the success of natural regeneration. Seed germination is also dependent on seed moisture content which could be affected by climate change. However, the interactions between changing temperature and rainfall patterns, together with the timing of seed germination for individual species, makes prediction of overall impacts impossible.

Changing rainfall patterns

In the southeast of England, lower summer rainfall and an increased evaporative demand are likely to lead to longer periods of drought stress on trees. Increased winter rainfall will lead to more frequent winter waterlogging, in some cases resulting in reduced stability and fine root death extending into surface soil horizons. The latter effect will also limit the ability of trees to take up water and exacerbate the problems of summer droughts. Winter waterlogging may also impact on the trafficability of forest soils and thus limit access of harvesting machinery for forestry operations. Infection by various soil-borne pathogens, including species of Phytophthora, is promoted by fluctuating water tables and would be expected to become more prevalent.

Wind speed

Prediction of changes to the wind climate have high uncertainty associated with them. They do, however, indicate that mean wind speed may increase during autumn in the south of the country, contrasting with earlier predictions of the largest changes in northwest Scotland. This is because the storm-track is predicted to move further south, together with a slightly increased frequency of deep depressions crossing the UK in winter. An increase in the occurrence of storms may make woodland more vulnerable to wind damage. However, reduced water availability in southern England and potential nitrogen deficiencies, resulting from reduced nitrogen deposition in response to the implementation of emissions control policies, may enhance allocation to roots thereby increasing anchorage. Most damage to forests is caused by extreme events and the frequency of these is very difficult to predict. It should, however, be noted that a small change in mean wind speed can have a significant effect on the frequency of extreme winds.

Indirect effects

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. However, it is possible 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.

Forest fires

Currently, the most damaging forest fires are in spring with dry vegetation and litter from the previous growing season fuelling the fires. Rainfall in spring is not predicted to change markedly, and the earlier growth of ground vegetation as a result of climatic warming could reduce the incidence of forest fires in spring and early summer. However, extended summer droughts such as those experienced in 1976 and 1995 result in a secondary peak in available fuel in late summer as the ground vegetation dies and dries off. This is demonstrated in fire statistics, which show peaks in years such as 1976, 1984 and 1995 when severe drought was experienced in some regions.

An increase in the frequency and severity of summer droughts would be expected to lead to an increase in the number of fires and forest area affected in those years. As urban and peri-urban woodland expands, the increasingly close relationship between human habitation and woodland may require that greater consideration is given to fire risk assessment and control.

The relationship between summer drought and forest fire incidence is set against a background of a downward trend in fire frequency. This is partly a result of the changing age structure of the forest estate. Since the late 1980s the area of thicket stage conifer plantation, which is generally more susceptible to fire than broadleaf woodland and mature woodland, has fallen. Thus weather conditions alone do not determine fire susceptibility and should be considered alongside woodland structure and land cover in the context of the wider landscape.