HS2: How rail construction has impacted ancient woodland

THE biggest railway construction project in Britain for around 150 years is now underway and completely unyielding and uncompromising to anything in its way, including ancient woodland and champion trees. That said, HS2 would have been dwarfed by what took place in and around the 1840s. Original maps of 19th-century railway construction across Britain and how it impacted on the country’s woodland cover presumably do exist, although the landscape still reveals places where railway lines were driven straight through large expanses of ancient woodland. A prime example is the Great Northern Railway (GNR), authorised in 1846 to connect London Kings Cross and York, cutting through the rural bliss of Middlesex and Hertfordshire.

The railway construction went slap bang through iconic woodland at (Monken) Hadley Common, then in Middlesex but now part of the London Borough of Barnet. According to Oliver Rackham, Hadley Common was the last remaining unenclosed 2 per cent of the 3,380 ha that was once the royal hunting ground of Enfield Chase. Rackham quotes a survey of the woodland conducted in 1702 which reported beech on the higher gravels through hornbeam, oak and ash to a little elm in the clayey bottom. Amazingly, the composition has hardly changed in over 300 years, except for the disappearance of the elm and significant ingress of sycamore into parts of the woodland.

As today’s inter-city trains from London to York and Edinburgh pick up speed through New Barnet Station, some 14 km out of Kings Cross, you begin to see the ancient woodland of Hadley common towering above you on both sides of the railway track. You can imagine the amount of environmental and ecological damage caused when the railway was constructed 170 years ago, but according to records, the GNR Company purchased only seven of the 77 ha of Hadley Common to construct the railway – a seemingly small price to pay.

Deep inside Spoil Woods is an almost pure stand of very old hawthorn trees.

RAILWAYS AND WOODLAND REGENERATION

Ironically, 19th-century railway construction created woodland as well as destroying it, and this appears to have happened along this stretch of the GNR through Hadley and beyond almost two centuries ago. This was precisely because construction of the railway required deep cuttings and a series of tunnels, one of which is the longest in the land. Railway spoil from cuttings and tunnels, typically comprising thick London clay, was apparently spread out onto the fields along the new railway.

Three miles further north along the same railway line between Potters Bar Station and Brookman’s Park station is a narrow (50 m) wood running alongside the railway track. This woodland cover tracks a deep railway cutting of some 100 m in length. I had been aware of this woodland for many years, but only recently began to appreciate how its peculiar tree composition and character give strong clues about its origins.

The main thing which caught my eye was a significant area of almost pure old hawthorn inside the wood comprising a peculiar mix of standards, low pollards and apparently self-coppiced trees. All were clearly old, with circumference at breast height (cbh) 70–90 cm for standards and 85–120 cm for low pollards as measured just below the original cut or break. Only later, when I noticed the name ‘Spoil Woods’ on a local map, did I put two and two together and realise the significance of this woodland, its origin and the reason for an old hawthorn tree stand deep inside.

READ MORE: Former ‘Tree of the Year’ felled to make way for HS2

Spoil Woods I concluded was yet another of Hertfordshire’s small ‘accidental woodlands’, described as such by H.L. Edlin in 1956, with this particular accidental enclosure serving as collateral to the original railway construction in the 19th century. Hiley had previously described accidental enclosures as ‘dribs and drabs’ of land in circumstances that lead to neglect, thus allowing the land to ‘tumble down’ to forest through intermediate stages of scrub.

Small woods make up a sizeable area of Hertfordshire and have a collective presence out of all proportion to their individual sizes. This wood, I concluded, had developed on the London clay spoil excavated from the deep railway cutting, which was spread out over the fields where the wood now stands. With this idea in mind, I set about assessing trees inside the wood to see whether species, size and age profiles were compatible with these theories on the woodland’s origin and development.

One of the large old field maples with a cbh (circumference at breast height) of 150cm, a common feature of small accidentally enclosed woodlands.

INSIDE SPOIL WOODS

There is a good smattering of oak trees (all standards) with a 19-tree sample showing a mean cbh of 135 cm (range 96–210 cm). There are several large ash trees with cbh of 120–130 cm and there is a lot of blackthorn around the perimeter of the wood. Another unusual feature is the presence of several very old field maples and crab apples with cbh up to 150 cm which is large for these normally small woodland tree species.

Under-storey is mainly elder with a ground cover comprising common ivy with dog’s mercury and dog violet flowering in early spring. Late spring brings forth a beautiful carpet of English bluebells with wild (non-variegated) yellow archangel, so dense in places it could be considered invasive. Hedge woundwort flowers profusely in summer. There is no bracken, but some other fine fern specimens including Dryopteris.

One of the old crab apple trees in Spoil Woods which according to H.L. Edlin are very characteristic of small, accidentally enclosed woodlands.

This stretch of the Great Northern Railway was built from 1846 to 1849 and it would appear the thick blue-grey London clay excavated from the deep railway cutting was spread over the field above. Hawthorn together with blackthorn would have been the first tree responders to colonise such an extreme and inhospitable soil environment. Modern-day observations on similarly deep excavations (e.g. motorway construction) in this area of North London and South Hertfordshire show hawthorn and blackthorn are invariably the pioneers and first tree colonisers of raw London clay.

Given the raw clay state of this new soil environment, the very first plants to colonise would have been robust and hardy herbaceous plants like ragwort, creeping thistle and docks, along with a wide range of grasses. Only when the clay was covered with vegetation, a good time after, would the hawthorn and blackthorn have come in, to be followed after another even more substantial period of time by the first potential high-forest trees like English oak.

The period of time was sufficiently long for the hawthorn to grow into sizeable trees, further inhibiting the establishment of oak, which requires high-light situations for regeneration. This scenario would account for the almost pure stand of old hawthorn that covers a substantial portion of the woodland today. The blackthorn would have been quickly shaded out and banished to the margins as the woodland developed. W.H. Hudson, one of the great ramblers and writers of the late 19th century, mentions pure hawthorn woods he encountered in Savernake Forest and Albury Park. One, which he described as the very best, was on the high South Downs east of the village of Findon (West Sussex), about halfway between Cissbury Hill and Chanctonbury Ring.

According to the Woodland Trust’s method of estimating the age of oak trees, the oldest specimens in Spoil Woods are nearer to 100 years old than 170 years old, but this could well fit in with the scenario thus presented. Despite the clay substrate being laid down 170 years ago, there would have been a considerable time lag before colonisation by English oak, initially struggling on the raw clay substrate. Taking these factors into account, the mean cbh of 135 cm (which at first glance seems low) is what you might expect given the peculiar and oppressive site conditions presented to and experienced by colonising and regenerating trees over the last 170 years. H.L. Edlin said ‘unusual’ species like crab apple and field maple are a common feature of ‘accidentally enclosed’ small woodlands, while seldom featuring in larger tracts of forest and plantation.

Spoil Woods is a living document that describes and informs on past management practices, way of life and history of the location. Excavation of the GNR railway in the 1840s would have blighted an essentially unspoilt rural landscape to the north of London, but a beautiful woodland environment grew out of the carnage.

The Cubbington pear in the path of HS2, unceremoniously felled last month. (Picture credit: Warwickshire Museum.)

A LESSON FOR HS2

Maybe there is a lesson to be learned in relation to the ongoing construction of HS2, increasingly surrounded by controversy for its destruction of ancient woodland and individually valuable trees including veteran and champion specimens. Many are reported to have been uprooted and destroyed even before the railway construction was given the official go-ahead. 

Woodland and tree casualties already destroyed or due for the chop include about one half of the Jones’ Hill Wood near Great Missenden in Buckinghamshire. Jones’ Hill Wood, which is authentic ancient woodland, is credited as the inspiration behind Roald Dahl’s Fantastic Mr Fox because the author was a regular visitor.

A 300-year-old English oak in Warwickshire was nominated for Tree of the Year in 2020 but too little and late to save it from destruction. This magnificent tree, called the Hunningham Oak, which had stood alongside a lane in Leamington Spa, was felled in late September (not because it was in the direct path of HS2, but to make way for an associated service road and realignment of the Hunningham Road). Local protestors from miles around said the tree could have been saved by re-locating the service road by just a few metres.

Spoil Woods as viewed from the south. The original GNR railway is on the right.

Most recently, thousands of signatures were gathered in an effort to save a 250-year-old wild pear tree voted ‘Tree of the Year’ in 2015. This irreplaceable tree growing in a hedgerow at Cubbington on the outskirts of Leamington Spa was listed in the ‘Champion Tree Register’. However, anyone hoping for a reprieve would have been left bitterly disappointed as it was unceremoniously felled in late October. Two and a half centuries of growth, development and biodiversity locked up in the finest remaining specimen of an authentic native wild pear tree disappeared under the onslaught of men and machinery in a matter of hours.

To counter these claims of carnage, HS2 says it will plant seven million native trees along the first phase of the railway from London to Birmingham. Hundreds of thousands of trees have already been planted, but observers claim that an unacceptably high proportion of the trees planted in 2017/18 died through improper planting and must now be replaced. HS2 is reported to have said that putting in replacement trees is cheaper than keeping the original ones alive – presumably by proper mulching and watering.

Later in spring, the floor of Spoil Woods is covered with English bluebells.

However, perhaps the ongoing lesson from Spoil Woods offers HS2 a golden opportunity to do something really worthwhile and lasting. Why not spread spoil from the railway construction over fields along the new railway and commission contemporary scientists and their descendants to monitor colonisation of these sites by plants over the following 100 years or more? And continue the study until climax woodland is achieved, much like Spoil Woods in South Hertfordshire spawned from construction of the Great Northern railway in the 1840s?

On learning of my theory on the origins of Spoil Woods, a landowner in Warwickshire described woodland on his estate with a very similar tree species composition and age profile. And like Spoil Woods, the woodland straddled a main-line railway which, if I remember correctly, was part of the original London and North Western Railway out of London Euston, like GNR, constructed in the 1840s.

Bluebells and yellow archangel cover the woodland floor in spring. The yellow archangel is sufficiently profuse to perhaps be regarded as invasive.

AUTHOR’S NOTE

Claims by HS2 Ltd that the increased capacity generated will free up the traditional rail network are beginning to sound a bit hollow, since COVID-19 has already done that job for them.

In September 2020, I travelled 100 miles from London Kings Cross to Kings Lynn. I wore a mask for the whole journey, but for what reason I don’t know as I was the only passenger in the carriage. The company claims HS2 will reduce the journey time from London to Birmingham from one hour and 21 minutes to 52 minutes, a saving of 29 minutes.

I travel this route to the Saltex show at the NEC, Birmingham, and it is by no means a bad journey, even during rush hour. Potters Bar to Kings Cross takes 17 minutes, followed by 20 minutes waiting for and piling into a packed tube train for the one-stop underground train journey from Kings Cross to Euston.

From my personal perspective, we are carving up the countryside to save roughly the same time as it takes to travel one stop on the Victoria Line. And what is the best part of the entire journey? A leisurely 81 minute appreciation of trees, hedgerows and woodland on the traditional inter-city service from London to Birmingham.

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