Climate change has the capacity to dramatically change the British landscape over the next century, potentially making British-grown tea and olives quite common, as Dr Terry Mabbett reports.

TWO dimensions to the perceived effect of climate change on trees in Britain are its net negative impact on native trees and the opening up of opportunities for trees more closely aligned with warmer climates. The negative impact on native species is not a done deal. On current trends, exotic insect pests and microbial pathogens will have depleted many native and naturalised species long before climate warming really kicks in.

The second dimension is an altogether more fascinating scenario, with the capacity to change the British landscapes over the next century. But many candidate tree species suffer from insufficient frost hardiness while requiring longer hours of stronger sunshine than can be furnished by current English summers; Brighton will not become Barcelona overnight.

However, to see where we are now and how far we have to go, where better to look than south-west England – especially the south-west corner of Cornwall, mostly frost-free and recording some the highest hours of sunshine in the country? In 50 years’ time we could be taking tea and olives on the terrace in Truro, with black tea-leaf infusions and olive fruits soaked in brine grown and processed in England.

That’s not as crazy as it sounds, because tea is already grown commercially in Cornwall at the Tregothnan Estate, first planted in 2005. An olive grove planted in 2012 at Huggit’s Farm on the Isle of Oxney in Kent is scheduled to produce British olive oil.

Forestry Journal:  Olives have a fighting chance for a future in southern England – patio olives seen fruiting in Hertfordshire. Olives have a fighting chance for a future in southern England – patio olives seen fruiting in Hertfordshire.

JUST IN TIME FOR TEA – IN CORNWALL

Growing tea in England appears high risk, especially when you consider the exotic locations like Assam in north-east India, Ceylon (Sri Lanka) and Kenya with tropical postcodes currently delivering your daily cuppa. However, tea from these exotic origins is grown in the highlands, up to 1,400 metres above sea level in Assam and 2,700 metres in the Rift Valley of Kenya. As a rule, temperature suffers a 6.5°C drop for every 1,000 metres above sea level.

Tropical highlands are not the universally sunlit uplands you might imagine, but dull, overcast, wet and humid for much of the time. Such conditions are just the job for tea (Camellia sinensis), which requires very high annual rainfall – an absolute minimum of 1,000 mm/year, average 2,500 mm, but thriving in tropical uplands receiving up to 5,000 mm. Despite Britain’s worldwide reputation for rain, few areas of England can even come close to these levels, although Truro, with an annual rainfall of 1,300 mm, clearly goes a long way.

Forestry Journal: Do not underestimate the capacity of trees to adapt! The scene is Carrick Hill, Adelaide, South Australia in late summer (February) 2015. English oak in the foreground against the silvery-sheen background of mature olive trees behind. The backdrop is stone pine.Do not underestimate the capacity of trees to adapt! The scene is Carrick Hill, Adelaide, South Australia in late summer (February) 2015. English oak in the foreground against the silvery-sheen background of mature olive trees behind. The backdrop is stone pine.

OLIVE OIL TO FLOW – IN KENT

The ideal growing conditions for olive trees contrast starkly with those for tea. Olives thrive in hot, full-sun climates and can tolerate drought due to extensive root systems. Light, well-drained calcareous soil with an alkaline reaction is preferred. Kent, as the choice for the first commercial olive grove in England, goes a long way to meet these requirements. Kent is one of the warmest and driest parts of Britain, with more of the same predicted from climate warming, while much of the county’s soil fits the chalky status preferred by olives.

Both soya beans and sunflowers are already grown commercially in Kent, so why not olives? Olive groves may be few and far between in England but individual olive trees are now part and parcel of suburban patios as trendy ornamental trees.

Climate change predictions point towards a more Mediterranean-type climate, which is exactly what the olive tree is suited for. Olive is a classic xerophytic species (plant adapted to survive environments with little liquid water) carrying out its most intense physiological activity at favourable times (spring and autumn). Olive is relatively dormant in winter, which means periods of moderate frost are no problem. And, if we do get extended mild periods, as has been the case in winter 2019/2020, this evergreen tree will continue to grow.

If upper-end climate predictions prevail, then olive trees are just the job. This Mediterranean native is well adapted to avoid cellular oxidation and tissue damage by synthesizing large quantities of cellular antioxidants (polyphenols), and to reduce transpiration by partially rolling up its leaves inwards to insulate the stomata, which are exclusively on the abaxial (lower) surface of the leaf.

Forestry Journal: Blister blight becomes a widespread necrosis of the young leaves which represent the tea harvest.Blister blight becomes a widespread necrosis of the young leaves which represent the tea harvest.

THE DISEASE TEST

Climate change remains a longish-term consideration, but diseases are very much with us now and should be assessed for any candidate tree species.

Forestry Journal:  Spraying tea in Indonesia using cuprous oxide which is a standard fungicide treatment to control blister blight disease (picture courtesy of Trond Kristiansen at Nordox). Spraying tea in Indonesia using cuprous oxide which is a standard fungicide treatment to control blister blight disease (picture courtesy of Trond Kristiansen at Nordox).

TEA

Blister blight disease caused by a basidiomycete fungus called Exobasidium vexans is the most damaging disease of tea and, in symptomology, does exactly what the common name implies. The fungus grows into the leaf and generates spore-bearing structures which erupt as blisters. Blister blight has been in Asia for centuries but, having never moved into other tea-growing areas, is unlikely to turn up and survive in the UK. However, there is something nearer to home which may be a cause for concern.

Tregothnan Estate in Cornwall has a long experience in growing flowering Camellias such as Camellia japonica and Camellia reticulata, closely related to Camellia sinensis (tea) with similar requirements. Camellia japonica is a known host of Phytophthora ramorum. In the years following the first UK finding of P. ramorum (on viburnum in 2002) this fungus-like pathogen caused significant disease on Camellias in the horticultural trade.

OLIVES

Common olive (Olea europaea) along with oleander, lavender and rosemary, is highly susceptible to the bacterial pathogen Xylella fastidiosa. The threat is currently unnerving the UK plant health authorities in relation to imports from European countries such as Italy, Spain and France where it is causing havoc, with olives at the forefront of the carnage.

However, it’s not as bad as it first seems, as far as UK olives are concerned, because there are at least four different subspecies of the bacterium, each with its own specific plant host range. The subspecies currently destroying olives in the heat of southern Italy is Xylella fastidiosa subsp. pauca, truly tropical in origin, having entered Italy on ornamental coffee plants from Central America.

The Xylella sub species of most concern to the UK is the cold-tolerant Xylella fastidiosa subsp. multiplex, the only one considered to have a chance of spreading in current UK conditions. The main fear around it relates to its enormous host range, which includes native broadleaves like pedunculate oak and wych elm.

In the wider arena, Xylella fastidiosa clearly presents huge risks, but in the narrower olive context I would keep my eye on Hymenoscyphus fraxineus, causal fungus of Chalara ash dieback of Fraxinus excelsior (common ash), a member of the same plant family as Olea europaea (common olive).

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