Gardening in a changing climate
Trees in a changing climate
Does climate change signal the end for the trees making up the UK's traditional landscape? This was the question raised at the landmark Trees in a Changing Climate conference. Experts in horticulture, arboriculture, forest science and ecology debated the effects and implications of climate change for tree survival, species choice and cultivation in our woodlands, parks and gardens, and how the forestry, conservation, gardening and heritage sectors should respond.
Full information on the conference
Further information on how this could affect you
Trees for an uncertain future
Selecting and planting a garden tree is a long-term investment. So which should we plant today to thrive in tomorrow’s conditions? Owen Johnson makes some predictions.
Trees are long-lived and to some extent they expect to encounter fluctuations in climate. A beech shedding its leaves in summer may be stressed, but as a species beech have also evolved a means of sitting out unfavourable conditions - even for a human generation, if needs be.
Among British native trees, only bird cherry (Prunus padus) and bay willow (Salix pentandra) - both good garden subjects - are in England at the southern edge of their natural range. Both are common in northern climes such as Scandinavia, while bird cherry is cultivated as an ornamental in southern Alaska. All the rest of our natives are at home under a fiercer sun, and generally in hotter, more dehydrating summers, although holly, beech and ash dislike prolonged drought. Hazel, alder, common hawthorn, yew, ‘English’ oak and others are just as much African species - from the Atlas Mountains - as they are British ones. Indeed the trees that grow in the UK are largely derived from southern-European populations that slowly spread northwards after the last ice age.
Most of the well-known exotics that we plant in gardens also hail from hotter, sunnier places. So-called ‘champions’ (the biggest or tallest on the Tree Register’s definitive database) tend to congregate in England’s southernmost counties, even though Scotland and Ireland have the lion’s share of the best tree-growing soils.
So most familiar trees, at least, should certainly be able to cope with the early stages of global warming. But their natural pathogens may also thrive, which should alert us to the threat of new tree-disease epidemics and remind us that high biodiversity is the only safeguard. As a case in point, the recent depredations of horse chestnut leaf-miner that have been so distressing in southeast England are due not so much to the virulence of the attack, but because of the ubiquity of the victim. If a thousand gardeners in a small town plant a thousand different trees, each pathogen can only kill one here or there. Should the thousand gardeners all choose Prunus ‘Royal Burgundy’, for example, this would be a recipe for ecological instability - not to mention potential aesthetic disaster.
Trees in trouble
A few trees do prefer it cold and wet and seem likely to be more successful in the north and west. Most are conifers (including silver firs, spruces, larches and hemlocks) from Boreal forests or high mountains. Few are common in small gardens; cultivars of Lawson’s, Sawara and Hinoki cypress are exceptions and tend to be the kind of trees that die suddenly after hot, dry summers.
Among choice broadleaved trees, a small minority have their ‘champions’ in Scotland and may be the first to fail further south, though they are either too robust or too few and far between for signs of stress yet to be visible. These include Prunus serrula (Tibetan cherry), Tetradium daniellii, Sorbus alnifolia, Kalopanax septemlobus and various rowans.
Recent mild winters in Britain have encouraged gardeners to experiment with trees that used not to be considered hardy - think how ubiquitous many Eucalyptus now are in southern UK. During 2006, my fieldwork for the Tree Register assessed the performance of nearly 1,000 such borderline-hardy species. Are many of these poised to make the choice of appropriate garden trees even wider?
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| Acacia dealbata is increasingly seen in gardens, flowering profusely in winter. A speedy grower, it will soon outgrow smaller situations unless pruned. | Several Eucalyptus are now common in the UK; species such as Eucalyptus pauciflora (above) may be too large for most gardens. | At Abbotsbury Subtropical Gardens in Dorset, Luma apiculata (above) has made a fine tree. |
Climatic extremes
Good as they are at weathering climatic shifts, trees remain vulnerable to freaks of weather such as violent storms or a month of floods and waterlogging. In the case of half-hardy trees, this will also include frost. The climate of Tresco on the Isles of Scilly is frost-free; the weather, in January 1987, brought 7.3°C of frost to the Abbey Garden, and killed many historic trees. Unfortunately, scientists believe that global warming is unlikely to make such unpredictable events a thing of the past.
Even if winters become less damaging, summers are predicted to get drier. Many somewhat tender but ornamental trees come from places with a rainy or humid growing season - the Andes, the Australian temperate rainforest, New Zealand, Florida, Madeira, southern Japan and southern China. These trees will make natural choices for mild, moist gardens by the western seaboard.
While a stressed Sitka spruce is no ornament at all, a few tender, moisture-loving trees respond to adverse conditions by growing slowly and densely, making them unexpectedly appropriate for small gardens further east.
A 'common myrtle', planted by my parents in Hastings in a hot, dry border against the house, turned out to be the exquisite Andean tree, Luma apiculata. After 20 years it remains a 1m (3ft) bush, but thrills each August as white flowers polka-dot the dark foliage. Luma is proving tough enough to reach tree size and show off its tissue-paper orange-and-white bark in gardens as dry or chilly as the Lord Leycester Hospital in Warwick and Feeringbury Manor in rural Essex; it is easily grown from seed, and is a tree I would love to see planted far more widely.
Trees for hot spots
There are, however, plenty of trees from drier environments that, given mild winters, should suit many British gardens.
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| Acacia baileyana combines golden flowers with delicate foliage. | Araucaria heterophylla is tender in mainland Britain, but may be a favourite if frost becomes rare. | Palm Phoenix canariensis has been widely planted in Britain and is surviving our milder winters. |
- Olive, a shining example, has become popular; Albizia julibrissin (pink siris), with its bullfinch-breast flower plumes and foliage as intricate and lustrous as a glazed Islamic tile, should grow more reliably.
- Two species, both street trees in hotter climates, now have one fine young ambassador each in England: Melia azedarach (bead tree), with light, elaborate foliage and eye-catching, mauve blossom makes a spreading tree on St Leonards Terrace near the Royal Hospital Chelsea, London, while Firmiana simplex (Chinese parasol tree), now 7m (23ft) tall and wide in Ventnor Botanic Garden, Isle of Wight, has soft, elephant’s-ear foliage reminiscent of Paulownia, and airy yellow flowerheads.
- Australian flora is full of showy, drought-tolerant small trees, many new to British gardens, including Melaleuca, Banksia and Polyscias.
- Acacia baileyana (Cootamundra wattle), which grows fast in a run of mild winters and combines a laburnum’s floral bonanza with exquisitely filigreed grey foliage (smoky mauve in A. baileyana ‘Purpurea’).
- Grevillea robusta (silk oak), a giant, flowering tree in the subtropics, survives as an exciting 12m (40ft) spire in Swansea’s Brynmill Park - albeit on the site of a demolished hothouse.
- You have to go to Tresco to see South African trees thriving in the UK, but London gardens may soon suit Leucadendron argenteum (silver tree), its leaves as white as lambs’ ears, and Virgilia oroboides (keurboom), with soft-pink pea-blossom amid glaucous foliage.
- Tantalising prospects from the South Pacific include the fairy-tale pagoda tiers of Araucaria heterophylla (Norfolk Island pine) and Metrosideros excelsa (New Zealand Christmas tree), the billowing crowns of which turn crimson with bloom each summer on Tresco.
- Several palms are proving fairly hardy. They can carry starburst fireworks of foliage on stout trunks like pineapple skin (Phoenix canariensis) or an elephant’s leg (Jubaea chilensis), or fan-like leaves on writhing stems clad in giant spiky Velcro (Trithrinax). The leaves of Butia species are sea-grey and arch back as gracefully as magnolia petals, even on younger plants.
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| Albizia julibrissin will bloom after a hot summer. | Firmiana simplex produces bold foliage and yellow blooms. |
Palms may not be everyone’s cup of tea, but it is clear that not all trees we currently enjoy in our gardens will thrive as weather patterns shift. One thing however seems clear: we should regard the changing climate as an incitement to experiment, not an excuse to play safe.
Owen Johnson is a dendrologist, author and Assistant Registrar of the Tree Register.
He was successful in being granted a Bursary from the RHS Coke Trust Awards in February 2006 to undertake his comprehensive and extensive research into updating and extending the Tree Register's records of half-hardy trees in the UK. He visited gardens and estates in parts of southern England where such trees are widely planted. His research was recognised as a valuable documentation of the spread of tree coverage across the UK and the possible migration of species further north. His Bursary Report entitled Half-hardy trees in Britain and Ireland earned him a Bursaries Committee Prize in 2007.
Find out more about RHS Bursaries. Alternatively, e-mail bursaries@rhs.org.uk or contact the Bursaries Secretary, RHS Garden Wisley, Woking, Surrey GU23 6QB.
Further information
Champion Trees of Britain and Ireland (the Tree Register), by Owen Johnson (Editor); Whittet Books, 2003, ISBN 9781873580615.
In the September 2007 and December 2007 issues of The Plantsman, Owen Johnson summarised his recent survey of half-hardy trees in Britain and Ireland.
View the first part of Owen's article (457KB Adobe Acrobat pdf)
View the second part of Owen's article (516KB Adobe Acrobat pdf)
The Tree Register website gives access to the definitive list of national ‘champion trees’ and further details of tender trees recorded in 2006 and since.