We need to talk about Holly
There are people who cannot identify oak trees from their leaves, but I bet they could identify holly leaves by their prickles and evergreen appearance on Christmas cards. However, it is only the lower leaves that have the prickles; the upper leaves are flatter and lack spines, presumably because they are out of reach of grazing animals. To have two such distinct forms to the leaves is unusual in British trees, although holly shares this characteristic with its Christmas bedfellow, ivy.
Holly is native to western and southern Europe, northwest Africa and parts of southwest Asia. It is now more or less everywhere in Britain except for the central Highlands and far north of Scotland. Planting may have obscured a somewhat more limited distribution in the continental climate of eastern Britain. Yet it is in Suffolk that some of the tallest hollies in Britain grow, jostling among the oak canopies at Staverton Park. It can grow on a wide range of soils, although most associated with acid conditions, where it is a common understorey to oak or beech.
Holly is reasonably well-represented in the plant record of the latter part of the Holocene; it is as much a part of our semi-natural woodland assemblages oak, ash and elm. Peterken and Lloyd (1967) commented that “holly was formerly more abundant in Britain than it is now. In Ireland, it once formed pure stands from which valuable timber was cut”…. “It was common … as a large tree…” “a great collection of very large natural hollies by the river Dee in Scotland …was cut down. At that time holly grew in great abundance on the banks of the river Findhorn where the trees grew to a very great size. Johnston’s Botany of the Eastern Borders, 1853, records the cutting for timber of many trees of very large size from natural woods in Northumberland…. Nearly pure woods of holly have disappeared from some hillsides in the Black Mountains”.
As well as being valued for timber, holly leaves were an important winter fodder particularly in the north and west of Britain. Spray (1981) however considered that the memory of its widespread use for fodder for livestock was virtually non-existent amongst the farmers that he was in touch with. The best known site for old holly pollards now in England is probably the Stiperstones, but they can be found in other places and I was recently shown an area of old pollards in Killarney National Park.
Holly pollards in the Stiperstones; an old pollard on the Bolton Estate; a holly pollard wood in Killarney.
This past abundance of holly is pertinent to how we react to its current resurgence in woods across Britain. Its distribution in Europe seems to be linked to climate, particularly the severity of winter and frequency of frosts (Iversen, 1944). Walther et al. (2005) did find that recent spread of holly in eastern Denmark, consistent with the patterns of climate warming, so this might be part of reason for its current increase.
Holly is also often associated with acid woods that were formerly managed as wood pastures but where the grazing pressure has now been reduced. The reduction in grazing allows ‘bonsai-ed’ hollies that have survived heavy grazing for some years a chance to develop and form dense thickets. These may be more-or-less pure patches under gaps in the canopy, or as an understorey to taller trees. Holly increase presents conservationists with a challenge: very little ground flora occurs below a holly thicket; the holly shades the lower trunks of oak and beech with which it is growing eliminating most of the epiphytic lichens and bryophytes that are an important feature for many of these sites; and while holly does have its own associated species these are more associated with old trees, rather than the young growth.
A developing thicket under oak in Yarner Wood, Devon; a veteran oak struggling amongst the hollies of Sutton Park.
What are the options? One is to try to stop thickets developing through maintaining/re-introducing grazing. However, the levels of grazing we want for other reasons may not be sufficient to control holly spread, once it is well-established and is perhaps no longer so limited by climate. We can try to open up some thickets by re-instating pollarding of hollies, as was once done to provide fodder for cattle, sheep and deer. This should definitely be considered where there are old hollies that have been pollarded in the past. However where new thickets have formed with potentially hundreds of holly trees in a single wood, this is committing future managers to considerable and ongoing expense.
On many sites the default option may have to be to allow the thickets to develop: to accept that at least in some places dense holly is a part of ‘future (semi-)natural’ woodland communities. A dense evergreen understorey of holly could become the British analogue of the laurel stands found in parts of the Mediterranean region, alongside the burgeoning British vineyards.