Secrets of the forest giants

At a time when the architectural mantra for city landscapes seems to be “build ’em high, then build ’em higher”, it’s easy to become blasé about skyscrapers. For every Jin Mao Building, there’s a Taipei 101, and for every Shard there’s a Burj. The skyscrapers of the natural world, our trees, pale into insignificance when it comes to a straight fight on height. Dubai’s Burj Khalifa is 829m high. The tallest known tree in the world – a coast redwood named Hyperion – comes in at just over 115m, roughly the same height as Mukesh Ambani’s 27-storey “Billion Dollar Home” in Mumbai, and Centrepoint, one of London’s first skyscrapers.

As giddying as skyscrapers can be, to stand in the presence of the living giants that comprise the Californian redwood forest is to experience a different kind of wonderment. Perhaps the most accessible place to see large numbers of mature coast redwood (Sequoia sempervirens) is Muir Woods. Just 12 miles from San Francisco, across the Golden Gate Bridge, this 226-hectare designated national monument represents a tiny but important fragment of the old-growth redwood forests that once covered an estimated 2m hectares of the western seaboard of North America.

Leaving the freeway, the approach to Muir Woods is by narrow and winding rural roads. The woods range over hills that form ravines and canyons, and which serve to emphasise the soaring scale of the redwoods. The trees at Muir are nowhere near record breakers – the tallest is a little under 80m – but they are still massively impressive. Their arrow-straight trunks seem to form architectural columns, giving rise to names such as “Cathedral Grove”. Footpaths follow the contours of the ravines, in some places formed from decking timbers raised up on stilts to ensure that the visitor experience is a gentle one, not hardcore woodland trekking.

As well as sequoia, there are tanoaks (Notholithocarpus densiflorus), California bay laurel (Umbellularia californica) and bigleaf maple (Acer macrophyllum), all of which are equipped to cope with life in the shade of the big redwoods. The bigleaf maple has, unsurprisingly, a very big leaf to absorb every possible ray of sunlight. The California bay laurel has an incredibly strong and wide-spreading root system that enables the plant to lean towards sunlit glades without toppling over, and the tanoak’s leaf structure is adapted to absorb and photosynthesise lower light levels.

The entrance of Muir Woods

The conditions at Muir Woods are ideal for the coast redwood to thrive. The soil is humus-rich brown loam, slightly acidic and well drained, partly due to the sloping topography. The coastal location means the thick Pacific fog for which San Francisco is notorious rolls through the trees – as the comedian Eddie Izzard noted: “They don’t tell the tourists about the weather in July and August.” The fog is vital to the flora of the Californian coast as it provides moisture during the driest times of the year. In July and August, the average rainfall is little more than 3mm, and a mature redwood can consume well over 100 gallons of water a day from the ground. During these arid months fog accumulates on the combined canopy of the trees, some of which is absorbed through the leaf while the bulk falls to the ground as fog drip. Year-round streams run through many of the ravines, providing further moisture and helping to keep the shaded glades cool.

Sequoia National Park’s trees reach even taller heights than at Muir Woods

Coast redwoods and their close cousins the giant sequoia (Sequoiadendron giganteum) are extraordinary trees. Redwoods are the tallest living organisms on earth, as tall as 10 London buses parked end to end. The giant sequoia are the largest organisms on earth by volume and weight (mycologists have grounds to dispute this but it’s hard to look up to a fungus) and the largest, named General Sherman, would need at least 35 fully grown blue whales to balance the scales. Mature trees typically are 1,800-2,200 years old, some even older. During these centuries the trees will have lived through countless forest fires, and this is where the genius of the redwood shines. The bark is very thick, fibrous and rich in tannins, while the heart wood lacks resin. These factors, combined with foliage growth that usually starts a long way off the ground, makes the redwood a hard tree to burn down. After the great fire that swept through San Francisco in the wake of the 1906 earthquake, it was recorded that redwood lumber-framed buildings, with redwood exterior finishes, played a significant role in limiting the spread of the fire. In the wild, the aftermath of a forest fire means the redwood can score again. The newly cleared ground, enriched with ash, makes the perfect seedbed for redwood seedlings to grow rapidly – they can make 20m in 20 years – free from competition from slower growing plants.

The fire-retardant qualities of redwood lumber, along with its light weight, resistance to decay and beauty, led to high demand for the wood with trees felled at an astonishing rate. In a little over 50 years, 95 per cent of the 2m hectares of forest had gone. There was little in the way of protection or replanting programmes.

President Theodore Roosevelt and John Muir in Yosemite, 1903

It’s impossible to predict if the logging would have continued until the very last redwood fell. Places such as Muir Woods were saved in part because the topography made it harder to reach, fell and log the trees – but by the 1880s there was rising concern for America’s wilderness.

The redwood forests found their most vociferous champion in John Muir, a Scottish-American naturalist, writer and activist. Muir wandered and lived in the wilderness of Canada and California where he formed strong views about the value of pure preservation over conservation with the sustainable use of resources. Muir was hugely influential – he helped persuade President Theodore Roosevelt of the importance of a national park system by spending the day walking in Yosemite together, before camping under the stars.

Water flows through Redwood Creek in Muir Woods, 2011

Muir Woods was saved by congressman William Kent, who bought one of the last remaining redwood forests in the Bay area, Redwood Canyon, to preserve it from logging. When the local water company sought to dam Redwood Creek and destroy the canyon, Kent sidestepped the action by giving the forest to the federal government. When Roosevelt suggested naming the newly designated national monument Kent Monument, the congressman insisted it be named after Muir in recognition of his work in establishing the national park system.

Today the future is brighter for the coast redwood and giant sequoia. New plantations are being made and managed. Botanists and conservationists are finding out more about the oldest trees and discovering new giants in the woods; Hyperion was only discovered in 2006 in a remote forest in northern California, and its location is still secret. The largest coast redwood by volume, the Lost Monarch, was only found in 1998.

Muir Woods, with its comparatively small size and proximity to San Francisco, is not the wild wilderness that John Muir sought to preserve. But if you can’t make it to Sequoia National Park or one of the other redwood parks, it is a wonderfully easy way to marvel at the giants of the forest.

Rae Lakes on the John Muir Trail in Kings Canyon National Park, California

How to grow redwoods

● As in the wild, redwoods enjoy slightly acidic, nutrient-rich, loamy soil that is moist but well drained.

● Young trees have the classic coniferous shape of a Christmas tree. It’s only as they get older that the lower branches fall and the beauty of the clear stem and fissured bark becomes apparent.

● These are emphatically not trees for small or medium-sized gardens. Partly because of their size, but also their scale. As impressive as a 10 or 15 storey-high tree might be, in anything less than a couple of acres it will look ridiculous.

● The growth habit of redwoods is usually a single straight trunk with no secondary leaders. This means they really can’t be pollarded – not without totally destroying the look.

● Redwoods start producing seed from around 15 years old, bearing male and female cones. The female cones contain very small, winged seeds, but the viability rates are as low as 20 per cent. Sowing in volume is advised to raise just a few plants.

● Vegetative propagation from cuttings isn’t impossible but they don’t always transplant well. Take tip cuttings from young plants for the best chance of success, and to avoid the need for a very long set of ladders to gather the cutting material.

Record breakers: how redwoods shape up around the world

● Redwood seed began to make its way into Europe from the early 1800s, with a surge of interest from the mid-19th century.

● The oldest tree outside the US is in Italy, at Villa Sammezzano, planted in 1816.

● Stourhead has the oldest tree in the UK (1845) and the thickest (7.22m) and third-tallest trees outside the US (40.50m). There are notable redwoods at Kew Gardens, Wakehurst Place and at Benmore in Scotland, where there is also an entire avenue of giant sequoia.

● The Palace Hotel of Bussaco in Portugal has a 52m specimen, with Germany’s oldest, thickest and tallest at Exotenwald Weinheim arboretum. There are notable trees in most of the European botanical gardens, and a good collection at Ballarat Botanical Gardens in Australia.

Matthew Wilson is managing director of Clifton Nurseries in London

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