Unravelling the history and development of unmanaged woodland

46 per cent of English woodland is currently unmanaged or under-managed according to Forestry Commission figures, but this figure hides the fact that much English woodland is planted with no apparent intention of future management.

I see it all of the time in woodland along railway lines and on motorway embankments and out of reach of all but the most foolhardy. I have often pondered the eventual fate of such plantings as they whiz by the train window but recently was presented with the opportunity to find out when I chanced across one such site on local authority land, on the northern fringes of London and apparently planted towards the end of the last century.

A LIVING GREEN SCREEN

This woodland appeared to have been planted as a screen for the garden of an imposing property adjoining a council-owned and publicly accessed open space. The house owner had presumably decided their boundary was uncomfortably close to a heavily used public right of way, almost certainly there long before the house. If this was the aim, it was achieved because the property is now invisible from the footpath, even when the mostly deciduous broadleaves are in winter condition.

Hawthorn, blackthorn and dog rose are traditionally popular species for such thicket-type plantings in order to generate prickly and impenetrable vegetation at minimum cost. Any thought or intention of future woodland management is a red herring simply because landowners can’t get inside to do anything worthwhile even if they want to.

A twin-leader wild cherry (gean).

Fortunately the planted species in this case comprised a selection of true trees and excluded thorny shrubs or for that matter any shrubby species like hazel, which meant I could walk into and around the woodland albeit with some difficulty. Species selection turned out to be an interesting range of native and exotic broadleaves – common ash, English oak, wild cherry (gean), silver birch, field maple, Norway maple and mountain ash (rowan) with pines as the evergreen conifer component.

A sprinkling of much larger and older trees including silver birch, hawthorn and Turkey oak and clearly pre-dating planting were located deep inside the woodland. Inside the woodland and close to the garden were two veteran if not ancient English oaks. These massive pollards with CBH (circumference at breast height) approaching 400 cm are the only oaks of such provenance in the area, although it has a long and rich history as common land documented in the Domsday Book.

 PLANTING PATTERN AND GROWTH RATES

Planting pattern was reasonably well structured at around 4 m and 3 m between and within rows, respectively. Many trees had failed and recently too, given the size of stems left in situ and clearly indicating a minimal level of ongoing management. Overall, the canopy was 12–15 m high, having already attained the stem exclusion stage with early signs of understory re-initiation.

I pondered about the reaction of foresters and arborists. Thinning would be the instinctive reaction of foresters to bank any standing timber worth saving, including some perfectly acceptable common ash, wild cherry and Norway maple. The arborist would presumably go in, clean up, prune off the dead lower branches and remove all the debris on the ground with safety and tidiness as top priorities.

Deep inside the woodland were several veteran if not ancient English oaks – the only English oaks of this age in the vicinity, which is perhaps surprising given the ancient history of the area as common land.

Of particular interest were distinct differences in how the various species had fared over some three decades of growth and development. A number of pines had failed, and some relatively recently, given the size of the fallen trunks, although the vast majority had grown reasonably well with a maximum CBH of 88 cm. The most successful species among the deciduous broadleaves as measured by lowest failings and highest radial growth rates were common ash (CBH 84 cm), wild cherry (CBH 85 cm) and Norway maple (CBH 75 cm).

Confronted with a range of much faster growing species, the shade-intolerant English oak trees inside the stand had generally not fared well. Those planted at the edge of the stand had done much better, though they were heavily branched from ground level upwards and not the sort of structure aspired to or desired for English oak within the forestry fraternity. Field maple had generally done well, both inside and on the periphery of the woodland, although virtually all trees displayed multiple leaders originating less than 0.5 m from ground level.

It was common ash which had done best in the context of potential timber trees.

When viewing this woodland in mid April from the adjacent open field you are confronted by a wall of wild cherry blossom, as though Prunus avium had been purposely planted around the edge for maximum effect at flowering time. No such thing had actually occurred, but wild cherry planted in the outermost 5 m of the stand had suckered profusely in response to the high illumination, to monopolise the periphery of the planting.

In one instance, a wild cherry tree of CBH 36 cm and seemingly part of the original planting was subsequently found to be a sucker arising from a 1.5 m length of surface-growing root extending from a planted tree now 72 cm CBH. Indeed the woodland appears to have advanced some 1.0 to 2.0 m from its original perimeter due solely to wild cherry suckering. One blackthorn was found in the outermost line of trees, but with no evidence of this species having been planted across the wider site it was assumed it self-seeded.

Regenerating common ash seedlings free from Chalara ash dieback disease, for the moment at least.

Noticeable was the number of twin- and triple-leader trees, including wild cherry, field maple, Norway maple and to a lesser extent, common ash. I asked colleagues for possible reasons behind the high level of disruption to terminal growth.  

David Gwillam at Prees Heath Forest Nurseries said browsing by rabbits was the most likely reason. The twin and triple leader structures of the trees indicates damage occurring at the seedling or whip stage, says Shane Lanigan, a consulting arborist based in West Hertfordshire. Browsing of terminal buds would have initiated generation of recovery shoots, hence the multiple leaders, said Shane. Lignin is a low-digested fibre in rabbits (Gidenne and Lebas, 2002) with their corresponding preference for immature plant material (Gwillam, 2016; Lanigan, 2016).

Native field maple has clearly established and grown well throughout the site, although virtually all trees have multiple leaders and are heavily branched from a low position on the stem.

Solid-wall polypropylene shelters had only been used in commercial forestry for a relatively short time by the mid 1980s (Mabbett, 2009) and were unlikely to have been deployed by the local authority owner of this woodland. The only evidence of tree protection was a remnant of brown plastic from a wraparound tree guard at the base of a single rowan tree. Such guards would have protected trees from rabbit-caused bark damage, but not from browsing and resulting disruption to terminal shoot growth, especially during periods of deep snow cover, which gives these small mammals extra height to access the terminal buds. 

The rabbit population of North London and Hertfordshire was hit hard by myxomatosis in the 1950s and 1960s (Harris et al., 1995). Shane Lanigan, who started his career as an arborist in the 1970s in the North London and Hertfordshire area, recalls how the rabbit population had bounced back by the 1980s when these trees appear to have been planted.

The next question concerned the exact nature of the pines, but with the foliage and reproductive structures already confined to the crown some 15 m up high in the canopy it was difficult to pin down exactly which subspecies/variety combinations of Pinus were present.

Planted tree species were already carrying heavy fruit and seed loads, like the wild cherry (gean) shown here.

David Gwillam said photographic evidence strongly suggested Corsican pine. Shane Lanigan concurred with Corsican pine (Pinus nigra subsp. laricio), as opposed to Austrian pine (Pinus nigra), on the grounds of tree form, and said from his experience in this area Corsican pine was the overwhelming first choice at that time. Virtually no Corsican pine is now planted due to the endemic and destructive presence of the Dothistroma needle blight fungal pathogen, but David Gwillam says at that time Prees Heath Forest Nurseries was selling 150,000 Corsican pine every year.

HOW OLD?

So exactly how old is this woodland, and in which year of the 20th century were these trees planted? Enquiries made to the local council failed to yield a definitive answer, so I resorted to measurements and recommendations of foresters and arborists old and new.

Father of English Forestry John Evelyn (1620–1706) was one of the first foresters to measure trees in his Sylva, or A discourse of Forest-trees (1664), but the expert recommendations I required were formulated and published much more recently. The late Alan Mitchell in his Field Guide to Trees in Britain and Northern Europe (1974) presented a wealth of information on and references to tree size, including many rule of thumb calculations which have proved invaluable. The particular Alan Mitchell rule of thumb I required was mean growth in girth (radial growth): in most trees with a full crown this is one inch (2.5 cm) per year (Mitchell, 1974). By these calculations, my 80 cm trees including pine, ash and gean were 32 years old and close to my guess-estimate. The site, it would appear, was planted in the mid 1980s.

Corsican was the most likely pine to have been planted back in the 1980s.

REGENERATION JUNCTION

The most interesting feature of this woodland is an already high level of tree regeneration and in particular the solution to a conundrum around regeneration of Norway maple. This continental European native is widely planted in North American forestry, where a propensity to regenerate from seed has made it an invasive species in some US states including New York and New Hampshire (Mabbett, 2014). I had only ever found Norway maple regenerating via root suckers until I ventured into this poorly lit North London woodland, which, to my amazement, I found to be covered with vigorously growing seedlings of Norway maple under and around mature, seed-bearing trees.

Within the UK, Norway maple is invariably grown as a landscape or amenity tree in high-illumination situations which apparently do not allow seeds to germinate and establish as seedling trees. But where trees are in close-spaced woodlands with low illumination levels across the forest floor the seed load is not inhibited and Norway maple apparently regenerates profusely and with ease. There was also good regeneration of field maple and common ash inside the woodland, and number of species including ash, field maple, wild cherry and rowan were already carrying heavy seed loads.

Of equal interest were other tree species present as established seedling trees although clearly not included in the planting mixture. These included three evergreens (holly, yew and holm oak) and a host of broadleaves including beech, hazel, elder and hawthorn.

Over the years Hadley Green has been planted with a wide range of trees including exotics like Quercus ilex (holm oak), now naturalised in the area. However, the nearest acorn-bearing holm oaks are a good 100 m from the woodland, probably too far away for acorn-carrying grey squirrels but certainly not for the Eurasian Jay (Garrulus glandarius). Research carried out on a heterogeneous landscape in Spain recorded 250 m as the mean distance of holm-oak acorn dispersal by jays, with some dispersal occurring up to 1,000 m from the source oak trees (Gómez, 2003).

Late April and English oaks planted at the margin, and essentially the only ones to have thrived, are re-foliating fast.

There is a large population of grey squirrels in this area of North London and Norway maple is recorded as being very prone to bark gnawing by this rodent (Kerr and Niles, 1998). The complete absence of bark damage to Norway maple in this woodland and to the even more susceptible common beech in locations nearby is almost certainly due to the suburban grey squirrel population being too well fed, dining daily at bird feeders and on bread intended for ducks in village ponds.

The last conundrum was the presence of Turkey oak trees apparently there as young saplings prior to planting. As luck would have it, I was subsequently in correspondence with a lady who grew up in the area and as a keen biologist made notes and took photographs of trees in the locality during the 1950s. Apparently there was once a huge Turkey oak tree at the entrance to the open space.

Despite low illumination across the woodland floor, ground cover was surprisingly rich, starting off in early spring with lesser celandine, dog violet and ground ivy and then giving way to cleavers, cow parsley, garlic mustard, rose campion, wild geranium (cranesbill) and wood avens; and not to forget bramble and common ivy as the inevitable woodland floor plants for all seasons.

It would be easy to criticise the owners for their lack of woodland management, but given good regeneration of planted species and others coming in naturally to provide such a wide tree mix, perhaps on this score they ought to be congratulated, especially since tree species diversity is increasingly paraded as the major bulwark against the twin evils of alien pests/pathogens and climate warming, in that order.

I thought about our ancient woodland and to what extent it could have started in this way, with a basic framework of not-too-well-organised planting complemented over the centuries by propagules of regeneration from within and outside.

References:

Evelyn, John (1664) Sylva, or A Discourse of Forest-trees.

Gidenne, Thierry and François Lebas (2002) ‘Role of dietary fibre in rabbit nutrition and digestion’. 2nd Rabbit Congress of the Americas. Pages 1 to 13. Habana City, Cuba, 19–22 June 2002.

Gomez, Jose Maria (2003) ‘Spatial patterns in long-distance dispersal of Quercus ilex acorns by jays in a heterogenous landscape’. Ecography 26: 573–84.

Gwillam, David (2016) Prees Heath Forest Nurseries, Whitchurch, Shropshire. Personal Communication.

Harris, Stephen; Pat Morris, Stephanie Wray and Derek Yalden (1995) ‘A review of British Mammals: population estimates and conservation status of British mammals other than cetaceans’. Joint Nature Conservation Committee (JNCC). ISBN 1873701 683.

Kerr, Gary and John Niles (1998) ‘Growth and provenance of Norway spruce (Acer platanoides) in lowland Britain’. Forestry, 71(3): 219–24.

Lanigan, Shane (2016) Consulting Arborist. Personal Communication.

Mabbett, Terry (2009) ‘A sheltered past – a potted history of three decades of tree shelters’. Forestry Journal, September 2009.

Mabbett, Terry (2014) ‘Norway maple the enigmatic non-native tree.’ Forestry Journal, June 2014.

Mitchell, Alan (1974) Field Guide to Trees in Britain and Northern Europe. Harper Collins Distribution Services, 415 pages.