Research

Trees in a Changing Climate

Key implications
Coping with the effects: advice for gardeners
Tips for tree planting and establishment
Frequently asked questions
Conference

Frequently Asked Questions

How will climate change negatively affect Britain’s trees?

The most immediate likely effects on trees are:

• Changes in seasonal patterns.
• More frequent and severe water shortages, droughts, high winds and extreme weather which could damage trees.
• In the south and east, heat and drought may increase losses, particularly of new trees and mature trees in hedgerows and towns.
• Increased risk of non-native pests and exotic diseases.
• Existing pests and diseases are likely to become more active, expand their ranges and cause more damage to trees suffering from drought.
• The geographical distribution of individual species will change.
• More frequent waterlogging will affect roots and tree stability on certain soils.
• Increased winter rainfall may raise water tables enough to kill roots, making trees more vulnerable to summer droughts.
• A change in semi-natural woodland ecosystems.

Any good news?

Species that are currently ruled out by frost and winter cold will be available for planting, with certain limits as cold episodes in winter can’t be ruled out entirely, although there will be fewer.

There could be increased growth rates as a result of rising concentrations of carbon dioxide boosting photosynthesis, where not limited by other climatic factors, and longer growing seasons.

Opportunities to restore floodplain woodland are likely to increase, bringing better flood control.

The value of woodland, including its limited role in mitigating climate change, will be promoted; positive attitudes to woodland may increase tree planting, contributing to the multiple benefits of greater tree coverage.

Garden trees will be even more valued for shade, and city trees valued for their cooling effect.

Climate change may reduce the activity of some pathogens that depend on moist conditions. (e.g. Brunchorstia which currently limits the planting of Corsican pine to the English lowlands, and a range of foliar diseases including Marssonina leaf spot of poplar).

Will the Kyoto Protocol reduce the effects of climate change?

The climate change we expect in the next 30 to40 years will be due to our past greenhouse gas emissions. Any steps to mitigate these effects, such as reducing greenhouse gas emissions (including tree planting), will take decades to work through the climate system. Some level of climate change is unavoidable. Furthermore, every opportunity to reduce emissions that is not taken now will worsen the effects of climate change later this century.

Will planting trees help reduce carbon dioxide?

Tree planting can make a small contribution through sequestering carbon, but it is not a viable solution to the threat of climate change - we can’t plant our way out of trouble.

• It has been estimated that, globally, vegetation including trees is currently removing between 30 and 40% of total carbon dioxide emissions. It is therefore essential that the planet does not lose its forests.
• However, in the UK, because our population and greenhouse gas emissions are high, trees make little contribution to lowering the nation’s carbon dioxide emissions (less than 2%).
• The carbon stored in all our woodlands is only equivalent to emissions in a single year and making car useage for the current generation of drivers carbon neutral over their lifetime would require that two-thirds of the country was afforested.

When will we start to notice the effect of climate change on trees?

We are already noticing the effects:

• Beech trees in parts of East Anglia and southern England have already experienced limited dieback during recent droughts. Such problems are likely to arise more frequently, particularly on south-facing slopes with thin, freely-draining, soils on which the species has traditionally been planted.
• Sycamores are responding fastest to climate change.
• Hawthorn, hornbeam, ash, beech and maple are coming into leaf earlier.
• This may alter the competitive advantage of some species, resulting in changes in composition of woods over the next 50 to 100 years.
• A small deterioration in the condition of oak and Norway spruce over the last 20 years may be linked to climate change.
• For a range of species, warmer winters may  lead to poor regeneration because seeds do not get cold enough to sprout.
• Lack of winter cold may lead to reduced, erratic or untimely flowering.

Which species will thrive in the changing climate?

• Species such the as small-leaved lime which is believed to have flourished in one of our warmer climatic spells, may make a comeback as the climate warms up and it can once again produce fertile seeds.
• In the early stages of climate change ash may replace beech as the most suitable commercial broadleaf species across much of southern England; in turn, ash may be replaced by pedunculate oak.
• The range of sessile oak is predicted to contract significantly to the north and west.
• Of the conifers, Corsican pine is predicted to benefit most from climate change, and expand its range to the north and west.
• The range of Sitka spruce is predicted to contract to the north and west, being replaced by Douglas fir and Corsican pine.
• In streets and gardens, we can begin to embrace trees from Mediterranean climates, such as Corsican pine. Olives and eucalyptus are already more common in Britain and more tender species such as loquats and persimmons may become more common. Figs, peaches and apricots may fare better. Ornamental trees are likely to include a wider range of palms, more tender flowering trees such as Persian lilac and the silk tree in the south. Hot, dry summers ripen wood making it cold resistant, so although we can expect occasional cold episodes in winters, they should be less damaging to ripened wood than to the soft growth resulting from cool, wet summers.

For both broadleaf and conifer species, contracting ranges generally reflect water limitations. Although, the effects of climate change on species suitability appear serious, for foresters productivity of the most suitable broadleaf or conifer species is not affected greatly. However, these predictions are only for the 2050s. Beyond then implications for tree growth are far more significant.

Planting stock adapted to hotter, drier climates represents a possible response to climate change. However, widespread planting should be considered with care because of the risk of unseasonal frost and the tendency for trees adapted to hot dry conditions to fail in wet soils.

Which species of tree are most at risk?

• Natural ecosystems that have adapted to the past/current climate are at risk, including native woodlands of oak, beech, ash and Scots pine from Cornwall to Scotland.
• Impacts are most likely in the south from the possible severity and frequency of summer drought, with beech expected to be the first affected.
• Among commercial conifers, the current problem of top-dying of Norway spruce in England and eastern Scotland is likely to be exacerbated, while the productivity of Scots pine is likely to fall in southern England.

What will be the greatest impact of climate change on trees in the UK?

The effect of summer drought, particularly in consecutive years, is likely to lead to widespread tree mortality, particularly of young and hedgerow trees.

How many trees are at risk?

We don’t know exactly how many of the estimated four billion trees in the UK are at risk. They are of enormous importance commercially and aesthetically and an essential feature of the UK’s gardens and landscape.

Won’t species adapt by changing their distribution?

Species would have to move as much as 150 kilometres (94 miles) north or 100m (100yds) uphill with each 10C rise in temperature. It appears that following the last Ice Age most trees spread across the UK at a rate of 1 km (0.6 miles) per year. This won’t be fast enough to keep pace with expected temperature changes unless mankind lends a hand as well. Barriers such as intensive farmland, road networks and towns, as well as differences in soils and underlying geology will also prevent species from dispersing.

Genetic variation within a population will allow some adaptation, but the question is whether this process will be fast enough to cope. It is unlikely that evolving populations will prove acceptable as a commercial source of wood fuel, fibre or timber.

Which areas of Britain will be most affected?

• The south and east are likely to experience the greatest temperature increases, the most severe water shortages in the summer, and the greatest rainfall in winter.

• Drought and water stress mean that dry, well-drained soils in parts of London, East Anglia and the Midlands might become unsuitable for beech in the near future. As climate change accelerates, beech could move north much more quickly than it is at the present, with its natural distribution reaching the Lake District and warmer parts of Scotland as soon as 2020.

What are the implications for our woodlands?

Climate change ranks alongside land-use change and pressure from urban expansion as one of the greatest threats to what little remains of our ancient-woodland heritage.

Why does it matter if some native species don’t thrive in a changing climate?

• The native fauna has evolved with the native flora so any changes, however small, could have a huge impact.
• The character of gardens and woods will change and some species of flora and fauna could possibly be lost from the UK.

How is British flora and fauna affected?

• As climate change accelerates, the annual cycle of mammal, bird and insect species may no longer be synchronised with the species on which they depend, which would affect survival.
• Caterpillars, for example, are responding faster to changes in temperature than the blue tits that depend on them, which could mean birds struggling to feed their chicks (Source: UK Phenology Network).
• Many rare and threatened species associated with ancient woodland are highly immobile and might not be able to move the necessary 150 km (90 miles) north for every 1C rise in temperature.
• Populations of deer, rabbits and squirrels are likely to increase as warmer winters reduce mortality and longer growing seasons increase food for the young.
• Over-grazing may harm the regeneration of trees and other flora.
• Parakeets, which are blamed for much depredation by gardeners, are already common in London and some southern areas and are likely to multiply.

How will this affect heritage gardens and parks?

• Changing climate could ultimately threaten the traditional character and even the long-term survival of some historic gardens and parks and their plant collections. In particular, the dominant role that veteran and heritage trees play could be challenged by climate change. 
• Domestic gardeners, however, may benefit from a longer growing season, a wider range of trees to choose from and opportunities to experiment with different styles.

Can we expect new pests?

• Climate change is likely to alter the balance between insect pests, their natural enemies and their hosts, making it difficult to predict the impact of insect damage on UK trees.
• While natural pests play a valuable role in the forest ecosystem, invasive or exotic pests species, without any naturally occurring predators or pathogens, can cause irreparable damage.
• A rise in temperature will favour insect development and winter survival, although there will be some exceptions.
• Some non-native species may increase or colonise, such as the pinewood nematode, gypsy moth and Asian longhorn beetle.
• The green spruce aphid is likely to survive better in winter, affecting the productivity of Sitka spruce.
  Other bark beetles such as Ips typographus, present in some parts of Europe but not the UK, could become a serious problem.
• Increased global trade in timber and wood products may combine with climate change so that exotic pests become more prevalent; it is therefore essential to remain vigilant in reporting new pests and altered patterns of damage.

Are we at risk from new tree diseases?

• Yes. The insects that carry some pathogens such as the fungus causing Dutch elm disease are likely to respond to warmer summers by extending their ranges. Phytophthora cinnamomi, a very widespread fungus that causes root and stem-base diseases of a wide range of broadleaved and coniferous species, is likely to become more prevalent across Europe.
• Warmer winters may increase the activity of some weak pathogens, such as Phacidium coniferarum, which are active only when the host is dormant.
• Summer drought would probably favour diseases caused by fungi whose activity depends on host stress, particularly root pathogens and latent colonisers of sapwood.
• Fewer summer rain-days may cut back 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.

Are we at risk from non-native plants?

Yes. Non-native plants such as Japanese knotweed and Himalayan balsam have tightened their grip on our ecosystem and are displacing native plants. The pressures they cause are likely to be increased by climate change. There is also the possibility that as the climate becomes warmer, other non-native plants which currently grow in the UK but are limited by our low temperatures will become more aggressive and invasive.

Advice for gardeners

Overall, is climate change an opportunity for gardeners or a problem?

It’s both an opportunity and a problem. On balance, gardeners are likely to lose a little but win a lot. Gardeners will be able to grow more plants from countries such as Chile, South Africa and Australia, all of which have very desirable plants. However, with the warmer weather will come drought, which will make it harder to grow more humid, tropical plants. Although the general trend is for the climate to warm, there will still be cold years and, maybe, even cold decades. The frost sensitivity of plants from warmer climates will therefore still be an issue.

What can you do to protect trees that have already been planted?

Outside the garden nothing can be done. Gardeners could try to mulch around trees to preserve water, and to ensure that trees are watered well while they’re being established. Planting species with a low irrigation requirement, and storing rainwater, particularly from excess winter rainfall, will be sensible. ‘Grey’ water, which is ‘used’ water e.g. from baths, could also be used in gardens. However, ‘grey’ water cannot be stored for hygiene reasons.

How can gardeners protect trees from storms and flooding?

Arboricultural tree maintenance, particularly removing dead limbs, is sensible. Crown reduction is an option for trees particularly vulnerable to wind damage. Little else can be done. Changes to wind speeds and the frequency of storms are the least certain of the climate change predictions. An option is to ensure that plants requiring good drainage or susceptible to waterlogging are located in suitable soils. Installing drainage might also be considered in gardens particularly prone to waterlogging. Maintaining ‘soft’ landscaping in the urban environment reduces run-off during rainstorms. Retaining parks and gardens in towns and cities could therefore play an important role if predictions of heavier rainfall and increased winter rain are borne out.

Which trees should gardeners buy to cope best with the changing climate?

Avoid trees at the drier end of their range or soil requirements; consider winter rainfall and soil type if a species is susceptible to waterlogging. Planting holm oak might be a good suggestion in southern England, but not necessarily so in the north Pennines.

About climate change

What is climate change?

A perturbation of the earth’s climate system that will have significant impact on both mankind and natural ecosystems; currently, this is viewed as a 2C rise in global temperature. The current 2C is 2C more than would be expected from any natural cause, and is therefore thought to be due to human activity, primarily burning carbon to produce carbon dioxide.

What is causing it?

The increase in the atmospheric concentration of greenhouse gases which inhibit the transmission of some of the sun’s energy from the earth’s surface to outer space. The increased concentrations of greenhouse gases result in part from human activity: deforestation and the burning of fossil fuels such as gasoline, oil, coal and natural gas.

In August 2004 the European Commission described climate change as ‘one of the greatest environmental and economic threats facing the planet’; in January 2004, Sir David King, the UK Government’s chief scientist, described climate change as a greater threat than global terrorism.

How is the UK’s climate changing?

Climate change in the UK can be measured in records extending back over 350 years.

• The 1990s was the warmest decade in central England since records began in the 1660s and this warming of climate over land has been accompanied by warming of UK coastal waters.
• The growing season for plants in central England has lengthened by about one month since 1900.
• Heatwaves have become more frequent in summer, while there are now fewer frosts and winter cold spells.
• Winters over the last 200 years have become much wetter relative to summers throughout the UK.
• A larger proportion of winter precipitation (rain and snow) now falls on heavy rainfall days than was the case 50 years ago.
• After adjusting for natural land movements, the average sea level around the UK is now about 10cm (4in) higher than it was in 1900.

Sources

UK Climate Impacts Programme
Forest Research
Woodland Trust
UK Phenology Network
Royal Horticultural Society