Stressed native U.K. trees on the march as climate change collapses seasons

LONDON -- The native English oak could be replaced by its Mediterranean French cousin in woodlands across the country within a few generations as climate change concertinas the seasons, bringing more droughts, storms and diseases and causing stress to the trees, according to arboriculture expert Tony Kirkham.

And it is not just the traditional English oak, the tree from which the country's world-dominating navy was built for centuries, that is under threat. Other native species like the elm and the beech could also be just a memory as the stress takes its toll, either killing them outright or at the very least moving their terrain progressively northward.

"A lot of our native trees are migrating north -- they are not actually moving, but we are losing them from the southeast, which is dry. It is slow, but in evolutionary terms it is pretty fast. In another 30 or 40 years there may not be any beech in the southeast, or they may just be in isolated areas," said Kirkham, who for the past 35 years has run the arboretum at London's famed Kew Gardens.

"The landscape is changing. It always has, but from my observations it is speeding up," he added. "Trees that are now considered native here -- the elms, beech and oaks -- will no longer be. They will be replaced by trees from far away that will become the new natives as the climate changes.

"We are losing oaks in the southeast from acute and chronic oak decline. If I was going to replant them, I would go maybe for oaks from the south of France, which may grow better here than a typical English oak from this region because of the climatic factors involved."

Acute oak decline can kill a healthy tree in as little as four years. In recent years it has been found in hundreds of trees across central and southeastern England and parts of Wales.

For Kirkham, the changing climate is to blame, not just because of the altered rainfall and temperature patterns but because of the arrival of new pests and diseases from southern Europe that are steadily expanding their ranges northward.

4 seasons no longer

"If trees are under stress from drought or unpredictable weather patterns, then they haven't got the ability to fight these diseases and they go into a mortality spiral -- an American term -- and it is very difficult to bring them back," he said.

"We are also faced with a lot of pests and diseases in the tree world. That has increased dramatically from the 1980s," he added. "The climate now in this country is good for so many of these pests. With the exception of this year, we don't get the severe cold that would probably control many of them. So we don't get the winter kill."

A native of Lancashire in northern England, Kirkham is in charge of Kew's 14,000 trees. He plants up to 200 trees a year in the 300 acres of gardens, an oasis of tranquility just 8 miles west of the center of London on the banks of the River Thames.

In all, the gardens contain about 3,000 species of tree, collected over the past 250 years from all over the temperate world stretching from the Tropic of Cancer in the south to the Arctic Circle in the north as well as the west coast of Chile, South Africa's cape, and parts of Australia and New Zealand.

But while the scientists and specialists of Kew have always traveled the world in search of new species of plant and tree that will grow in the garden's well-drained acid soils, they are now having to factor the changing climate into their calculations as well.

"One of the key things that has affected the arboretum over the last 10 years or more is seasonal variation. We seem to be moving from four seasons a year to just two -- winter and summer -- and the trees really don't know where they are. Everything we get now is in excess -- if it rains, it pours, if it is cold, it is very cold, if it snows, it really snows, if it is windy, it is very windy -- and there is no pattern," he added.

The last three years have been particularly peculiar as two short and largely mild and dry winters caused a serious drought that was followed by the wettest year on record in 2012, which in turn was followed by the coldest spring in more than half a century this year.

Even the trees don't know what season they're in

Not only did the brief and mild winters in 2010 and 2011 fail to kill off pests and diseases -- many like the leaf miner moth only recently arrived from the continent -- that attack the trees, but the transition from summer to winter in 2012 was abrupt with the autumn lasting a scant three weeks rather than the usual three months.

Then, after a long, cold winter, spring seemed to get off to a running start before suddenly screeching to a halt as the cold returned. When it finally got going again, it was all over in barely two weeks.

"It is really weird. From a tree's point of view, they are not sure where they are in terms of seasons," Kirkham said. "Last year was one of the best for trees because of the heavy rainfall. But we have gone through a droughty 10 or 15 years with low rainfall, and every year the trees have been trying to catch up on water uptake. Trees are like people, they need a good rest period. If they don't have that, they get moody and sulk and then they get stressed and become difficult to manage."

Planning what to plant is a tough call in the tree world, where life expectancy is upward of 150 years. But in view of the climatic changes over the past decade or two, which with notable exceptions have been drier than the norm, Kirkham's team has focused on the more arid areas of the temperate world where the trees enjoy the rain when they get it but are also more tolerant to drought than many of their southern English cousins.

"Over the years we have been looking for trees that were good at handling drought, so trees from the Eastern Seaboard of the U.S. were very good and they are all still doing very well now -- hickories, tulip trees, a lot of the oaks. When we are collecting in China, we are collecting in northern Sichuan, which is dry," Kirkham said.

"In the southeast of England, the giant redwood is now a common sight on the horizon. It loves it here. It is a very strong tree with a really good root system in terms of anchorage," he added. "The elms are gone, the beech are going and the oaks are going. What is left is the giant redwood. If you look at the horizon on the South Downs, you could be in California."

And while the latest Met Office prediction that the United Kingdom could face another decade of soggy summers after 2012 had the dubious distinction of being declared the wettest on record, made banner headlines and was met with general groans of gloom, for Kirkham it was manna from heaven.

"The weather that we have had in the past year has been fantastic. Look 'round the trees and you see a picture of health out there. This time two years ago the trees were looking tired, they were very stressed," he said. "Wetter summers de-stress me because they de-stress my trees. I do worry about them."