Analysis: Grand old oak trees in conflict with UK's concrete

PLANTING the ‘right tree in the right place’ is now a crusade, but we should spare a thought for all those vintage trees which have found themselves in the wrong place at the wrong time without having moved an inch, sometimes for centuries. Yes, I am talking about thousands of veteran and ancient trees, mostly English oak, which sooner or later become caught up in conflict with concrete. Unlike animals, trees don’t move from place to place, but grow aerially and underground with root growth their main Achilles heel – especially for English oak.

These oak trees – once hedgerow, meadow or even perhaps woodland trees – have been left in situ on housing developments, particularly during the building boom in the years after World War II. But they inevitably come into contact and conflict with the very people whose lives they were meant to improve.

IN CONFLICT WITH CONCRETE

Such stand-offs between trees and concrete occur most frequently through an interfacing of tree roots and building foundations causing subsidence, but for other legitimate reasons including a risk of branch and tree failure, shading and leaf litter. And, believe it or not, a number of frivolous complaints, including that “pesky blackbird waking me up at dawn with its constant singing” or “the trees offer cover for would-be flashers and other ne’er-do-wells”.

Delayed conflict between trees and concrete is something I have been thinking about for some time, and recently crystallised when the future of two aged oak trees was in the news, though the backgrounds and likely results of the two cases could have not been more different.

First was the famous Bowthorpe Oak, a 1,000-year-old ancient oak tucked away in a safe location on a farm near Bourne in the county of Lincolnshire. The government even stumped up cash for the Woodland Trust to give tender loving care to the tree in perpetuity. 

The second tree was a much younger English oak but still with six centuries on the clock.

The last of a line now located in a suburb of Peterborough in Huntingdonshire, it was originally saved by covenant when the area was developed in the 1970s. But the ancient oak is now under threat following a further development apparently contrary to the covenant. The net result is that a recently extended house is allegedly at risk from subsidence. Insurance companies and money are involved, with the council offering temporary reprieve for the tree, though its long-term prospects do not sound good.

This made me recall a visit in late summer/early autumn 2020 to a housing development not far from where I live. Arborist Shane Lanigan was instructed by the tree manager to carry out a survey on two large, mature oak trees growing in an open green space within a cul-de-sac of houses in a south Hertfordshire town. 

The location was a post-World War II housing development on farmland, looking all the better for the presence of two mature hedgerow oaks left in situ when the houses were built during the 1950s, and which clearly pre-dated the houses by a century at least.

There were originally three oak trees, designated T1, T2 and T3. T2 had been removed in 2018 following rise of the dreaded ‘s’ word (subsidence). Further reports of subsidence to at least one property continued to put the future of these two remaining oak trees (T1 and T3) in doubt, prompting the on-site tree risk management appraisal conducted by Shane Lanigan. Shane would prepare an arboricultural report analysing the different options for tree management and provide a recommended course of action. 

Shane’s appraisal and report was to be the latest in a series of site investigations from other parties, including a milestone one carried out in September 2016. The following arboricultural report said the concrete foundations (trench mass fill) extended to a depth of 600 mm. Soil beneath the foundations comprised stiff, brown clay with a plasticity index of 49 per cent which, as Shane pointed out in his report, meant the volume change potential (shrinkage/swelling) would be correspondingly high.

The 2016 investigation observed roots within Borehole 1, at the left-hand corner of the affected property, which were identified (by anatomical root identification) as oak (Quercus spp). Readings obtained from level monitoring carried out between December 2016 and February 2018 showed there had been movement accompanied by substantial cracking in the brickwork of the affected house. However, level monitoring was stopped in February 2018, which meant no assessment could be made of any ground recovery that could have taken place after tree T2 was removed in June 2018.

ON SITE IN SEPTEMBER 2020

We arrived on site to find two fine tree specimens of a mature age class. Shane used leaf form to tentatively identify the trees as hybrid specimens from crossing of the two native English oaks, Quercus petraea [Matt.] Liebl, and Quercus robur L., thus designated Quercus x rosaceae Bechst, according to contemporary convention in tree taxonomy. 

The two trees had clearly weathered well over the preceding centuries, despite a drastic change in land use during the 1950s. But this is hardly surprising since English oak is known to do exceptionally well on the London clay substrate found in this corner of southern England. 

My grandfather used to say the only things that did well in this corner of Hertfordshire were roses and rheumatism, testimony to the inherent dampness of the ground. London clay may be ideal for growing English oak trees, but is not so good for securing and sustaining the concrete foundations of brick-built buildings. This area immediately north of London is a natural hotspot for building subsidence and even more so when you introduce the roaming roots of English oak trees into the soil mix.

One of the red-brick terraced houses was clearly suffering subsidence, with extensive cracking visible in the exterior wall. However, other properties appeared outwardly unaffected, with many owners and occupiers of these properties still waging war against removal of the two remaining English oak trees. In fact, contractors who had recently arrived on site with the intention of removing T1 were prevented from doing so by a rigorous protest mounted by many of the local residents.

Until the 1950s this veteran oak tree provided summer shade for dairy cows. It now provides shade for passengers at a bus stop.

However, you could see the situation from both sides. What better than to open your bedroom window on a sunny spring morning with new green oak leaves at eye level, hearing breeding blackbirds and robins singing and see grey squirrels (yes, grey squirrels) leaping from branch to branch? But not so uplifting if your property is damaged through subsidence and you are forced to move out while your home is underpinned and you watch the value of your house crash before your very eyes.

THE INSPECTION REPORT

I watched while Shane sized up and measured the trees in a truly meticulous manner, leaving nothing to chance. 

Both trees were inspected from ground level only and around the entire 360 degrees of their circumference.  Binoculars were used to inspect higher areas of the crown and the principles of Visual Tree Inspection (VTA) (Mattheck & Breloer, 1994) were used to assess the trees’ structural conditions.

Shane identified the theoretical species zone of root influence according to data provided within NHBC Standards 2020. Tree species, age class, estimated heights, measured diameters at breast height and distance from the building were detailed and are set out in table 1 and table 2 (below), along with detailed comments on the respective tree’s condition.

The discussion component of the report is detailed and frequently refers to multiple, previous on-site inspections, assessments and recommendations, often dating back many years. The client clearly wanted clarity and a definitive answer and that is what Shane gave them. 

Shane concluded that tree T3 should be removed if level monitoring showed subsidence movement is ongoing.  Tree T1, not being implicated in the arboricultural appraisal report, should remain unless it is specifically shown to be a causal factor in the current, or any future episode of building subsidence. 

Shane’s in-depth discussion is way beyond the scope of this article, but one particular point really caught my eye because it applies to a number of veteran English oak trees in the vicinity of where I live. That is a small, south Hertfordshire town very similar in history, size and composition to the neighbouring town with the oak trees featured in this article. 

Veteran oak tree (left) in South Hertfordshire which survived construction of a fire station in the 1930s (centre) and an adjacent housing development in the 1960s. The tree still produces a copious crop of acorns especially in mast years.

Suburbanites take many aspects of their environment for granted, including veteran oak trees left in situ when housing and other developments are built. So I counted up the veteran oak trees within a 100 m radius of my home and was staggered to find the number approached 30 trees. They were scattered among a variety of housing and retail developments spanning a period from the 1930s to the 2000s. Like the oak trees in Shane’s report, almost all had been crown reduced at various times, the reasoning presumably being that this would keep the trees in check.

But as Shane points out, this is not necessarily the case. With regard to the two trees in his report, Shane said: “English oaks are classified as being of high-water demand (NHBC Standards 2020). Using data from this publication, a theoretical zone of influence (TZI) of 25 m can be calculated for English oaks (at full mature height). The two remaining trees have formerly been heavily reduced and perhaps as a result of this, have higher leaf cover than maiden trees.  Although not at full mature height, their extensive leaf cover probably means their TZIs would equate to those of trees at full size.”

A supportive statement taken from the archives of The Architects’ Journal says: “Implementing a policy to prune or lop trees to prevent them from growing to full height is not a workable, long-term strategy for controlling the root network. A crown reduction of 70 per cent by volume (approximately 35 per cent height reduction) reduces the water draw by only a small amount and only during the year of pruning. In subsequent years the soil moisture levels return to normal. This is partly because pruning encourages shoot growth and hence larger leaves, which then make similar demands on the water absorption capacity of the root system.”

Arboricultural consultant Shane Lanigan still likes to keep his hand in with some hands-on tree work.

Shane believes the huge number of mature and often centuries-old trees now incorporated in housing developments has its roots in the post-World War II building boom, when vast acreages of agricultural land were used for local authority housing stock. He said: “This effect is visible in all local authority developments close to me, often revealed by the estate names e.g. Woodside Estate, The Harebreaks and Hazelwood Lane. 

“All three of these developments have extensive cohorts of mostly mature oaks but also a scattering of other species – including beech, sweet chestnut, pine and larch. In other areas built on the footprint of old aristocratic estates it is cedar and other exotic species which tend to prevail.”

The message here is if you consider a tree worthy of preservation on a housing, retail or industrial development, and you want the tree to have a trouble-free natural lifespan, then careful considerations are crucial in the planning stages of the development, rather than cutting back the crown once the development is done.

www.abbotsarboriculturaladvice.com