California sequoia trees growing happily in UK

Redwood Trees at Wakehurst Horsebridge Woods. Image: Visual Air/RBG Kew

Imported giant sequoia trees are well adapted to the UK, growing at rates close to their native ranges and capturing large amounts of carbon during their long lives, finds a new study.

The research from University College London (UCL) and the Royal Botanic Gardens, Kew, found that biggest species of redwood trees, Sequoiadendron giganteum, known as the giant sequoia, can potentially pull an average of 85 kilograms of carbon out of the atmosphere per year.

Though introduced to the UK 160 years ago, this is the first time the trees’ growth rate and resilience in the UK have been analysed. There are an estimated half a million redwoods in the UK, and more are being planted, partly due to their public appeal.

In the wild they are endangered with fewer than 80,000 giant sequoias remaining in their native California range. Water availability is known to have a particularly strong influence on the growth of giant sequoia with drier years correlating with lower tree ring growth.

"I find it amazing to see these giants dotted across the landscape and see how rapidly they are growing.”

Mat Disney, UCL
Horsebridge Wood in south-east England is dedicated to the North American landscape.

Lead author Ross Holland, formerly a master’s student at the UCL, said, “Giant sequoias are some of the most massive organisms on earth and in their native range make up some of the most carbon dense forests in the world due to their great age.

“We found that UK redwoods are well adapted to the UK and able to capture a large amount of carbon dioxide. We hope that these findings can help guide decisions on future tree planting and management.”

The researchers emphasise the most effective way to mitigate climate change is by reducing carbon emissions from burning fossil fuels. Trees can help by absorbing carbon emissions, but they also provide other important climate, ecosystem and wellbeing benefits.

Giant sequoias grow quickly and are also some of the longest-lived organisms in the world, keeping up their rapid growth throughout their 3,000-plus year lives. They can grow up to 90m tall, and while not quite the tallest in the world, their wide trunks grow out, giving them the greatest volumes.

In addition, they are fire resistant, able to survive blazes that would wipe out forests of other tree species.

Water availability is known to have a particularly strong influence on the growth of giant sequoias, with drier years correlating with lower tree ring growth. The trees grow best in their native range in California’s Sierra Nevada mountains, so the researchers wanted to gauge how they fare under UK climates, which are milder and with a wider range of rainfall.

They compiled the first dedicated map of giant sequoias in the UK, mapping nearly 5,000 individual known trees. The team visited three groves of trees, located at Wakehurst wild botanic garden in Sussex, Havering Country Park in Essex, and Benmore Botanical Garden in Scotland.

They set up terrestrial laser scanners to map the trees in 3D, enabling them to measure the heights and volumes accurately and to create models of 97 representative trees.

3D laser scan of a giant sequoia. Image: Mathilda Digby

Co-author Dr Phil Wilkes, formerly of UCL, now at Royal Botanic Gardens, Kew, said: “Using the latest laser scanning technology has allowed us to accurately ‘weigh’ these massive trees without having to cut them down. This means we can measure many more trees as well as revisit them in the future.”

Knowing when the trees were planted allowed the team to calculate their average growth rates under the varying climate conditions between the three UK sites. They found that the trees at Kew and Benmore grew at similar rates as their US counterparts, while at Havering the trees grew more slowly, likely due to less rainfall in the region and competition from dense local woodland.

The report says, based on evidence, when deciding where to plant giant sequoias in the UK in future, it will be important to consider whether there is sufficient rainfall and year-round soil moisture, at the moment of planting and for the coming decades.

The Sierra Nevada Mountain in Yosemite National Park, California. Image: Mick Haupt/Pexels

Senior author, Mat Disney, UCL, said: “These results give us an important baseline for estimating how well giant sequoias are doing in the UK climate.

“Currently, these trees are probably more important for their aesthetic and historical interest than they are for solving the climate crisis but as more are planted we need to know how they will grow.

“The history of these trees in Britain is fascinating – initially as symbols of wealth and power, through to now being widely planted in parks and woodlands. They are iconic but there is almost no work on how fast they grow or how well they will do in the UK's changing climate. I find it amazing to see these giants dotted across the landscape and see how rapidly they are growing.”

The report was published in Royal Society Open Science.