Native hornbeam overlooked as an ‘upstairs’ forest tree

Common ash (Fraxinus excelsior) has completely succumbed to chalara ash dieback in little more than five years, with the fungal pathogen (Hymenoscyphus fraxineus) now infecting and killing common ash in all parts of the country.

English oak (Quercus robur and Quercus petraea) does not face such an acute, survival-threatening problem despite the advance of acute oak decline (AOD), caused by several plant pathogenic bacteria in association with the oak jewel beetle (Agrilus biguttatus). Oak processionary moth (OPM) already breeds in every London borough and all adjoining home counties but has not yet proved fatal to oak trees in its own right. However, who is to say what will happen to oak trees weakened by AOD when OPM spreads into the predominantly oak woodlands of southern England? While grey squirrels continue to run amok, landowners will always think twice about planting common beech (Fagus sylvatica).
Overlooked in all of this is hornbeam (Carpinus betulus), a member of the birch family (Betulaceae). Not because of any intrinsic deficit but simply because the tree has traditionally been cut mostly as coppice. As such, hornbeam has grown up and evolved into a ‘downstairs’ (understorey) tree or, at best, a ‘mid-storey’ component on the less-frequent occasions when trees were pollarded. However, look hard enough and you will find hornbeam as an ‘upstairs’ woodland tree, uncut, untouched and growing for several hundred years, striking for light at the top of the woodland canopy alongside classical climax trees like oak, ash and beech.

Hertfordshire is hornbeam country. Trees were traditionally cultivated as coppice and historically harvested for the hard, dense wood with a calorific value approaching anthracite. Living in south Hertfordshire, I don’t have to look far to find ‘upstairs’ hornbeam of an exceptional age and vintage.

Monken Hadley Common was described by the late Oliver Rackham as the last authentic two per cent of Enfield Chase. This unique remnant covers 70 ha on the northern fringes of the London boroughs of Barnet and Enfield, close to where they coalesce with the county of Hertfordshire. 53 ha is classed as semi-natural, mixed deciduous woodland with open glades, while the remainder is open grassland planted with native trees.

Enfield Chase was finally enclosed in the late 18th century. Most of the land was allocated to local parish councils but the Enfield Chase Act of 1777 ordered control of Hadley Common to be passed to the trustees of Monken Hadley Common, who would hold the land in trust for the ‘commoners’.

However, it appears the commoners were not allowed into the woodland to cut and coppice hornbeam for firewood or to allow their livestock to forage on hornbeam’s succulent spring foliage. Indeed the many hundreds of Hadley’s hornbeams are standards to the last tree. Hadley Woods provides a rare opportunity to assess hornbeam as a standard tree and as an integral component of semi-natural ancient woodland.

Delineation of the survey area
The 75 m x 75 m square  selected for my survey is situated in the north-west section of Hadley Woods, delineated to the north by a dry gulley, and to the south by a wall of sycamore coppice. A stream running north to south through the wood provides a natural eastern boundary and the western boundary was sited 20 m from the woodland edge. This was done to avoid inclusion of tree species like blackthorn, sallow and willow, which thrive in the higher light conditions of the woodland margins but are not generally present inside.

Population profile 
Every standing hornbeam tree within the 75 x 75 m area was measured for circumference at breast height (CBH) to the nearest 5 cm using a cloth tape measure. CBH for all other trees within the same area and identified by species was also measured. Trees with a CBH of less than 40 cm were not recorded in this survey and were classed as understorey. Of the 56 trees that were measured, 32 (56.1 per cent) were hornbeam, followed by ash (14/24.6 per cent), sycamore (6/10.5 per cent), beech (3/5.3 per cent) and oak (2/3.5 per cent).

Mean and median CBH for the 32 hornbeams was 
142.9 cm and 150 cm respectively. Six, 16 and 27 trees had recorded girths greater than 175 cm, 150 cm and 100 cm respectively. Only two hornbeam trees came in with girths less than 90 cm. Range of girth (CBH) for ash, sycamore, beech and oak was 330–120 cm, 165–70 cm, 250–105 cm and 255–195 cm respectively.

The age of most broadleaf tree species can be estimated by dividing CBH by a constant number ranging from 2.5 to 1.5 and representing average increase in girth (cm) per year (Mitchell (1974) A Field Guide to the Trees of Britain and Northern Europe). By using 1.5 as the dividing number, ages of the biggest trees in Hadley Woods come in at between 160 and 220 years.

Wokingham (Berkshire) District Veteran Tree Association’s tree sizing guide says most broadleaf woodland trees with girths of around 250 cm will come in at just over 200 years old. For much-faster-growing common ash, a 330 cm girth tree comes within the same age range. The much smaller maximum size for sycamore (165 cm) fits in with the relatively recent invasion of Hadley Woods by this naturalised and domiciled tree species.

Hornbeam – seedling regeneration
Mature standard hornbeam casts a dense shade offering little opportunity for regeneration of seedling trees, whether hornbeam or other tree species. This was certainly the situation under the unbroken hornbeam canopy, where light levels reaching the woodland floor were very low, with correspondingly poor tree seedling regeneration. However, the area featured a number of failed hornbeam trees, all of which were windblown.

The woodland canopy had been opened at various points across the area and for varying periods of time, from just a few months to 10 years or more. Regeneration of the various tree species was well underway, with young trees at different stages of growth and development, and crucially of different heights, depending on when the light-facilitating hornbeam had failed, thus offering an opportunity to gain an insight to species succession.

Seedling tree regeneration across the site was assessed using 1 m square quadrats and counting the number of seedling and sapling trees according to species. Each seedling/sapling tree was assigned to a specific height range category, with the range of categories being appropriate to the overall stage of seedling/sapling growth within the sampled area. A summary of the areas and findings are as follows:

• Under a closed canopy with low light levels where there was some seedling germination but no evidence of sustained seedling regeneration: hornbeam and ash were in the majority with a scattering of sycamore and holly. Seedlings were small, sickly and not progressing beyond this stage.

• Around a recently windblown hornbeam tree which had opened up the canopy just months before: Early stage regeneration with all seedlings less than 10 cm in height. Majority of the 98 seedlings were hornbeam, with 23 ash and 19 sycamore seedlings.

• Within several established open spaces created by hornbeam trees windblown over the previous five years: Fewer trees but a wider range of species with some ash and sycamore tree seedlings exceeding 100 cm height. Of the 139 trees counted in two quadrats, 58 were ash followed closely by hornbeam on 55 and sycamore on 16. Other species present but in very small numbers were holly, hawthorn and elder. 

• Inside well-established open spaces created by trees windblown at least 10 years previously and where sycamore and ash had completely taken over with saplings of 2 m or more: 

This snapshot of tree species succession does indicate that when up against much faster-growing and aggressive species like common ash and sycamore, hornbeam may struggle to regenerate.

No apparent obstacles for hornbeam
Despite its long-standing reputation as an understorey tree, hornbeam should be seriously considered for planting and culture as a standard high-forest tree.

There are no identifiable potential disasters from insect pests and pathogens waiting in the wings for hornbeam.
Its credentials for combatting climate change, including warmer and drier growing conditions, appear encouraging. Together with beech, hornbeam was one of the last tree species to cross the land bridge which once connected the British Isles with the rest of Europe. As such, it should be in a better position than most to withstand any future climate warming in the UK. This calculation is supported by the natural distribution of Carpinus betulus, which extends right across Europe, through the Balkans and well into western Asia.