Tree of the month: From coast to coast and home from home

One of the oldest and best groves outside the North American native range is at Leighton in North Wales. First planted with seedlings imported from California in 1857, before the American Civil War, the grove boasts the biggest volume of timber per acre in Britain at around 24,766 hoppus feet (20 m³ volume), 32 per cent of which is bark.

NATIVE RANGE AND NATURAL HABITAT

The tree’s native range is a coastal strip 450 miles long from south-west Oregon to Monterey County in California. The strip is 5–35 miles wide and occupies 2.2 million hectares. Old-growth stands occupy 200,000 acres, mostly in state and national parks, with another 600,000 acres in commercial timber production. Summer fog is considered the most important component of the watery environment demanded by this thirsty tree. Coast redwood grows in virtually pure stands as well as mixed stands with, among others, Douglas fir, Lawson’s cypress, western red cedar and Pacific yew.

It only grows in pure stands on exceptional sites characterised by moist, flat ground alongside rivers with gentle slopes below 1,000 m. By putting out new roots higher up in the soil profile, coast redwood is well adapted to overcome flooding while other tree species are killed. Exceptionally fast growth rates (2–6 feet per year for first 10 years), a tolerance of deep shade and the ability to re-sprout when cut mean coast redwood outgrows other trees, with the exception of Douglas fir on some sites.

Cone and seed collection high in the coast redwood canopy at Huntley in Gloucestershire (Picture courtesy Forestart).

MORPHOLOGY AND ANATOMY

The biggest trees are 115 m tall with trunk diameters of 9 m. Crowns are conical with horizontal to slightly drooping branches. Bark is soft, fibrous and extremely thick (up to 30 cm deep), bright red-brown when freshly exposed but weathering darker. The root system comprises widely spreading shallow laterals. Needles are 2 cm long with sharp tips, shaped like double-edged swords and borne in a flat plane to form feathery sprays. Cones are 2.5 cm long, hard, woody and comprising thick, wrinkled scales.

REGENERATION – STUMPS, SPROUTS AND SEEDS

Coast redwood regenerates by seed, stump sprouting and artificial cuttings. Trees start to bear at 5–15 years old and seed viability increases with age. Germination rates are inherently low.

Seedlings require more moisture than do associated conifers because they do not have root hairs. Coast redwood is a superb shooter, sprouting from stumps and root crowns within weeks of cutting. Over 100 sprouts may grow from one cut tree, each with its own root system, to create circles of new trees around the old stump. These new clonal trees may have interconnected root systems to aid stability.

WOOD AND TIMBER PROFILE

You can’t write about this prolific producer of biomass without commenting on its wood and timber profile. Despite being soft and lightweight, coast redwood timber has a decent strength-to-weight ratio. Wood weighs in at an average dried weight of 415 kg/m³.

The wood is rated durable to very durable with respect to decay resistance. Timber from old-growth trees is generally more durable than that from younger, second-growth trees. Wood is easy to work using hand tools or machinery, but planer tear-out can occur on figured pieces with curly, wavy or irregular grain.

Massive coast redwood log at Leighton recording between 80 and 100 growth rings in 2017.

BACK TO THE BRITISH ISLES

Coast redwood was discovered in 1795 and brought to Britain in the 1840s. The original Leighton stand was planted in 1857 and by end of World War II was almost 100 years old. H.L. Edlin, a forward-thinking forester of the time (British Woodland Trees, 1944), said coast redwood was planted mostly as an ornamental tree, having done especially well along the western seaboard, particularly in deep valleys with rich soil. 

“Coast Redwood should prove outstanding in Britain,” said Edlin in 1944, although plantations were still on an experimental scale. “Timber from experimental trees has proved satisfactory apart from faults inherent in its mode of growth, where side-branches cannot be suppressed, and how too rapid expansion leads to a soft and open-grained type of timber. The range of sites suited is limited, but includes mountainous country well-adapted to afforestation,” he said. 

In 1956’s Trees, Woods and Man he described the Leighton stand as “a plantation of giants and one of the most spectacular sights of British Woodlands”.  

Another 60 years passed before interest finally began to gel. The threat of climate warming is now claimed by the Forestry Commission (FC) as the driving force behind increased coast redwood planting, but those doing the planting appear to have a more immediate reason.  

David Gwillam at Prees Heath Forest Nurseries in Whitchurch, Shropshire, says the 10,000 trees he sells annually are mostly going to West Country estates in Devon, Dorset and Somerset to restock areas of Japanese larch wiped out by Phytophthora ramorum. Significant amounts have also been sold to estates in Shropshire and Staffordshire.

“Coast redwood and Japanese larch do well under similar conditions and to be frank landowners now have far fewer options due to pests and diseases,” said David. That said, identification of coast redwood by the FC as an alternative productive species has clearly given official backing to the positive planting trend. 

David Gwillam buys seed from Forestart at Shrewsbury in Shropshire. Forestart traditionally used provenances from the northern end of coast redwood’s natural range, but since 2014 are using UK-grown seed. 

“Demand has increased to the point where we could look at making UK seed collections from stands suitable for registration under Forest Reproduction Material Regulations in the UK,” said Rob Lee of Forestart.

“We are collecting seed from a select stand at Huntley, Gloucestershire. These 60-year-old trees have fabulous form and are up to 40 m in height, having clearly performed and adapted well to UK conditions, important traits to consider when choosing progeny for planting in the UK. 

“Our first UK collection in 2014 produced sufficient seeds to cover three years of supply to the UK industry. We made smaller collections from 2015 to 2017 to learn more about crop cycles and yields. There was no crop in 2018 and it will be interesting to see if there is a crop in 2019 after the dry summer of last year. Knowledge gained so far suggests crops in the UK over a five-year period can be fair to abundant, not too dissimilar to seed production in the native range.” 

Erratic, inherently low seed viability is the elephant in the room for this gigantic tree. Rob Lee says viability of the seed they have harvested (after processing) ranged from 18–47 per cent.

“Only 2.5–12.4 per cent viability has been recorded for seed dispersed over a five-year period in an old-growth forest in the USA, and I would imagine our collections have been similar before processing.”

Coast redwood clearly has a seed viability problem, so I asked Rob what they do to compensate and if he knew what was behind it. “Due to non-viable seeds weighing the same as viable seeds it is very difficult to separate good seed from bad. However, our lab test results can be used to calculate seed quantities required to match plant production requirement, and to ensure nurseries are able to produce consistent numbers of plants,” he said. 

“There is a theory that low viability is an evolutionary trait discouraging birds from predating seed from cones due to the effort involved in finding a good seed.”

COAST REDWOOD CURIOSITIES

The potential for coast redwood as a plantation conifer under a range of English and Welsh conditions has stared UK forestry in the face since the 1860s. Why it should take climate change to make the FC warm to coast redwood is beyond me. Leighton in Powys is Welsh border country, recording some of the lowest temperatures on record south of Hadrian’s Wall. Proof of the pudding for coast redwood’s cold hardiness in the UK stands aloof at Leighton in Wales.

Pest and disease assessment is clearly a sensible precaution. Forest Research claims coast redwood foliage is infected by Phytophthora ramorum, but says it is “a curiosity rather than of major significance” – a peculiar thing to say after P. ramorum infected Japanese larch in 2009.

That was a “curiosity” because the disease was called Sudden Oak Death, although that didn’t stop a curiosity graduating into a calamity for the hapless Japanese larch. However, perhaps the comment is more reassuring than at first seems, because Forest Research appears to be referring to the infection history of NA lineages of P. ramorum on coast redwood in North America and not the EU1 and EU2 lineages here in UK.