Experts at the United Nations Intergovernmental Panel on Climate Change are no longer in any doubt that the world’s climate is warming and that it is human activities, including the burning of fossil fuels and clearing of forests, that are the cause. The forest industry needs to plan to mitigate the problems and benefit from the opportunities.
Thanks to diligent record keeping at the Radcliffe Observatory in Oxford, the UK has a unique series of climate measurements dating back to 1659. Using this data, scientists at the UK Climate Impacts Programme (UKCIP) have recently shown that the average UK temperature has risen by 0.7ºC since records began. This doesn’t sound like much, until you consider that nearly half of this rise has occurred in the past 20 years. Four of the five warmest years on record have occurred since 1989. Since emissions of greenhouse gas (CO2, CH4 and N2O) are still rising, such ‘forcing’ of the climate will go on well into this century, probably at a rate of up to 0.3ºC per decade. Although our climate is naturally variable, it is the rapid rate of present-day change that is highly unusual.
Climatic variability
Predictions about changes in temperature can be made with reasonable certainty but predicting other climatic changes is less straightforward. Because of this, most predictions from climate models incorporate a range of scenarios. The two bar charts below show examples for rainfall and temperature increases. Under the high rate of change scenario, northern England and Scotland could be receiving around 15% more rain per year by 2080. Predictions of average changes conceal one of the more worrying aspects of climate change for foresters – an increase in climatic variability. There are likely to be an unusually high number of hot dry years in the south-east and unusually wet years in the north. Both north and south will experience much warmer winters.
Evidence of substantial changes in the structure and composition of UK vegetation in response to past environmental change suggest that the geographic range limits of tree species may change rapidly in future. However, predicting exactly what these changes will be is a complex task. The range of a species is not determined just by climate but also by other factors, such as interactions with other species including pests and pathogens, barriers to dispersal and soil characteristics.
Rapid climate change may result in increased mortality rates in established forests. Mature trees may become stranded in areas with an environment that is no longer appropriate, leading to an increased vulnerability to herbivores and pathogens. Both pests and pathogens should respond to climatic warming faster than long-lived trees.
Climate change will undoubtedly bring new threats to commercial forestry in Britain. The number of cold winter days (an average temperature below 0ºC) has been falling steadily since the 18th century. Warmer winters and fewer cold days will bring bud burst forward in most species. But the increasing variability of the climate system means late frosts will still occur. Most conifers in northern Britain are chosen on the basis of resistance to harsh climatic conditions. With temperatures rising there is a temptation to plant higher yielding, less hardy varieties, to take full advantage of the improved growing conditions. The risk is clear, but current predictions on the likelihood of severe late frosts are not solid enough to guess the length of the odds.
Many afforested sites in Britain’s uplands are on ‘marginal’ soils. These are commonly poorly drained and prone to waterlogging, which decreases root growth and leaves the trees with poor anchorage and vulnerable to strong winds. Rainfall is set to increase in the uplands over the next 80 years, and may worsen these age old problems. The UKCIP predicts only modest increases in mean wind speed by 2080. Nevertheless, the past decade has seen the highest frequency of severe gales since 1881. It is still not clear if this represents a long-term change.
In contrast to the north, southern England summers will become significantly drier. Severe droughts are likely on well drained sites, like Thetford Forest, where summer water shortage is already a problem. There is also likely to be an increased risk of fire.
Warming winter temperatures may mean increases in the frequency and severity of insect pest outbreaks. Britain’s spruce plantations are constantly threatened by the green spruce aphid, a well-known pest that is normally kept in check by cold winters. Aphid populations have been shown to grow faster and become more damaging in the spring following a mild winter. Furthermore, the effects of frost damage, excess water or drought tend to place stresses on trees, rendering them more vulnerable to pests.
Deer are likely to benefit from a warmer climate. Protection of young trees from deer browsing is a major expense for forest managers, as is the managing of deer numbers by hunting. Unlike the spruce aphid, deer are equally as problematic in native woodlands as they are in coniferous plantations.
Changes in growth
All plants require a number of raw materials for growth including CO2, water and mineral nutrients. Experiments have demonstrated that the growth rates of some seedlings increase dramatically with more CO2. The Forestry Commission recently observed that oak seedlings grew 25% faster in CO2 enhanced growth chambers at Alice Holt Research Station. However, these results did not hold for all the species tested, and the balance of scientific opinion suggests that the ‘CO2 fertilisation effect’ will occur only when other factors (such as water or nutrients) are not limiting growth. Furthermore, individual plants have been shown to ‘acclimate’ to higher CO2, by down-regulating their photosynthetic systems to prevent other factors becoming limiting. Overall, the effects of more CO2 will prove positive for tree growth but are likely to be much less important than increasing temperatures.
There is a strong correlation between average summer temperature and growth in the UK. This can be seen from tree rings and also from the effect of altitude on timber yields. The observed temperature trends show important warming in autumn and spring, meaning that not only will trees grow faster during the growing season, but the season itself is growing. From the information available, production gains of around 40-50% by 2080 are estimated as a consequence of temperature changes alone. Again, this assumes no other factors are limiting growth.
The forecasted warming and changes in rainfall patterns will alter the ecological conditions for growth and open the possibility that a wider range of valuable timber species could be grown in the UK. Beech is currently limited to southern Britain by its temperature requirements but there is every chance it could be grown at more northerly latitudes in the future. It may be especially favoured in new native woodland management schemes, since warmer summer temperatures promote better seed production. Some species will be hindered by warmer winters. Many temperate trees require a certain amount of chilling during the winter months to stimulate correct development in the spring. Such species (eg the bird cherry) may become less common in the south.
Of the exotic timber species trialed in Britain, southern beech and false acacia are likely to be most suited to a warmer climate. Holm oak, which is grown widely on the Continent for its high quality timber, may be well suited to the drier south.
The changing climate presents new opportunities to British foresters. The Kyoto-Protocol to the United Nations Framework Convention on Climate Change (FCCC), which was adopted in December 1997, proposed that changes in forest cover would be taken into account in determining compliance with national commitments to reduce greenhouse gas emissions. This idea has been hotly debated both within and between governments, and was one of the reasons for the failure to reach agreement at the FCCC summit in The Hague in January. Many believe that large-scale afforestation can help to remove CO2 from the atmosphere, because growing trees lock up carbon from the air. This idea is strongly supported by the US, Canada and Japan. They favour investing in forestry plantings and using newly created forest to offset their industrial CO2 emissions. If this policy is adopted (something that the EU opposed in The Hague) we will see substantial afforestation motivated by indirect financial incentives.
Many businesses would be keen for forest carbon sinks to be counted, as forestry investments are usually more cost-effective than direct emission reductions. However, the UK government aims to meet its Kyoto target through direct reductions of emissions, and not to rely on such mechanisms as forest sinks or trading.
Growth in planting schemes
Even if no policy agreement is reached, we are likely to see the proliferation of smaller planting schemes by private companies wishing to demonstrate their commitment to carbon sequestration. The consequences for the UK timber market of ‘carbon subsidised’ forestry should be considered carefully.
Attempts to find sustainable alternatives to fossil fuels have led to a growing interest in the use of tree crops for biomass energy. Techniques have been developed to generate electricity from fast grown, short rotation crops of willow and poplar. Technological developments have steadily improved the cost and efficiency of energy conversion from biomass to the extent that the government is promoting the conversion of agricultural land for the growing of fuelwood crops in some regions. As winter temperatures rise, fast-growing plantation species such as mimosa and eucalyptus may be the optimum species for biomass forestry. The high yields and dense wood contains much more energy per unit of dry mass than the lower density willow, which may become vulnerable to summer droughts on drier sites in the south.
Related Files
Temperature increase by 2080 according to the four UKCIP climate change predictions
Rainfall increase by 2080 according to the four UKCIP cliamte change prediction
