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Trees in a Changing Climate
Barking up the wrong tree: are we planting the wrong species?
Trees in UK gardens are expected to have the potential to last 50 to 120 years, and shrubs 15 years or more. Climatic change could affect longevity and should therefore impact on long-term planning, unless gardeners are prepared to focus on the short-term and replace and replant trees when necessary. Beech trees in parts of East Anglia and southern England have already experienced limited dieback during recent droughts and such problems are likely to occur with increasing frequency. Ash is likely to fare better than beech, but may in turn be replaced by pedunculate oak, which is good for biodiversity, by foresters, landscapers, park keepers and estate owners.
Corsican pine is predicted to benefit most from climate change so could be the new tree of choice for foresters, whereas dwarf pines and drought-tolerant broadleaf trees such as the holm oak could be an option for gardeners that are taking a long-term view.
Britain to resemble Bordeaux?
In 50 or 100 years’ time, will South East England be like Bordeaux? Bordeaux is further south and therefore has much higher light levels and a longer growing season so it won’t be identical. However, it’s likely that many of Southern France’s species will become more common - such as walnut, poplar, sweet chestnut, plums, kiwi fruits and vines. Sunflower and maize crops, which are currently marginal in Britain, may be more widely planted.
Hardwoods such as eucalyptus, which are common in Portugal, could do well in free-draining soil if it doesn’t get saturated.
Streets ahead: a touch of the exotic for our public spaces?
Silk tree, Persian lilac and the pistachio, all common in Mediterranean streets and parks, could appear in the UK. Paulownia , a tree with flowers similar to a foxglove which can be prone to frost damage in Britain, especially when young, could soon fare better in the British climate as temperatures rise and growing seasons start earlier.
Gardeners feel the heat
The climate in which trees planted now will mature will be very different to today’s. Will they survive and what should gardeners be planting? There’s no simple answer. Gardeners shouldn’t think that because the climate will be more tropical in 50 years' time, that they should plant trees that may survive in 50 years, as they may not make it through the intervening 49 years. The advice for gardeners is to not select trees at the drier end of their range or soil requirements. Planting holm oak may be a good suggestion in the south of England, but not necessarily in the north Pennines. Gardeners should go for trees adapted to the soils and site so they will be best able to resist stress. Poplars and willows may grow now in dry situations, but in future might be hard presed in hot, dry summers. In the south and east, gardeners should stop planting beech for the long term, either as specimen trees or hedges. Tree of heaven, holm oak and eucalyptus will suit the site better now and are likely to pull through whatever climate change brings.
End of the line for Britain’s traditional woodlands?
Native woods of oak, beech, ash and Scots pine from Cornwall to Scotland are most at risk from climate change. Impacts are most likely in the south, with beech expected to be the first affected.
- Small-leaved lime, once a major component in our woodland, may make a comeback.
- Sycamore and horse chestnut are responding fastest to climate change. Hawthorn and hornbeam are also coming into leaf earlier.
- This may alter the competitive advantage of some species, resulting in changes in composition of our woods over the next 50 to 100 years.
Goodbye bluebells?
Changes in timing of leafing may alter the balance of trees in native and semi-native woodlands. The larger leaves of the sycamore may shade out species that are not adapting as quickly. The fortunes of bluebells and snowdrops, which overwinter as bulbs, will depend on when they come into leaf, relative to the trees above them. They may lose further access to light and nutrients from early perennials, such as garlic mustard and cow parsley, whose development has previously been suppressed by the cold, and from non-native invasives such as perfoliate alexanders (e.g. at RBG Kew).
Skills crisis?
More experts will be needed to establish trees properly and to manage them in the changing climate. The horticultural community may have to establish tree care protocols for climate change and undertake further research into the implications of climate change for gardeners.
Cracking up: why climate change is putting our trees under stress
Drought crack is likely to increase in England and to cause defects in timber. There is likely to be a greater risk of trees spontaneously dropping limbs, which could be bad news for gardeners who are already liable in law for damage arising from trees. Subsidence could also increase as wet winters swell soils and dry summers shrink them.
Getting to the root of the problem. Can we plant our way out of trouble?
Tree planting can make a small contribution through sequestering carbon but isn’t a viable solution to the threat of climate change. In the UK, because our population is high in comparison to our land and forest area, trees make little contribution to lowering the nation’s carbon dioxide emissions, with their growth accounting for less than 2% of emissions. The carbon stored in our woodlands is only equivalent to emissions in a single year and making car usage for the current generation of drivers carbon neutral over their lifetime would require that two-thirds of the country was afforested.
Britain braces itself for new pest threat
The predicted rise in temperature will generally favour insect development and winter survival, although there will be some exceptions.
While natural pests play a valuable role in the forest ecosystem, invasive or exotic pest species, without any naturally occurring predators or pathogens of their own, can cause irreversible damage.
- Some non-native species may increase or colonise, such as the pinewood nematode, southern pine beetle, gypsy moth and Asian longhorn beetle, three exotic pests that have recently been found in southern Britain.
- More green spruce aphids are likely to survive the winter, which may lead to a decline in the productivity of Sitka spruce.
Plant disease epidemic for the 21st century?
Warmer and wetter winters could lead to more active root pathogens, so roots will be more seriously damaged. Then, when the root systems come under stress in the hotter summers, more trees and shrubs will die.
Adapt or die? The dilemma facing our heritage gardens
Planting stock adapted to hotter, drier climates represents a possible adaptation response to climate change, however they may suffer from unseasonal frost damage and would completely alter the character of our heritage gardens and landscapes.