Thinning is a means of manipulating the development of woodlands or plantations, either for quality or quantity of trees. Conifers is usually quantity, broadleaves is usually quality. Traditionally the primary aim is to remove a proportion of the growing/competing trees to concentrate the growth of volume (not height) of the remainder.
Thinning may be selective, where the poorest quality trees are cut to free space for better trees, which are usually the biggest and healthiest anyway, and have the best form. The trees taken might be supressed, diseased, twisted, forked, damaged or leaning. Otherwise, thinning may be systematic, where trees are removed according to a plan devised in advance. This is easier and cheaper but has less control of quality. Thinning can also be used to favour different species, provide an income or increase light to the forest floor, increasing biodiversity. Timing of thinning is also important; leaving the crop too open can result in stress, sun scorch and increase the risk of wind damage, uprooting, snapping or breaking. Thinning too late will result in spindly stems and supressed growth, and could result in inappropriate species dominating.
Conventional forestry thinking was always to produce the most profitable crop but recent woodland management priorities may mean that benefits to nature, conservation and recreation have changed thinning policies. At Wenlock, all the managed woodland was acquired pre-planted or in mid rotation, so all the work has been playing the hand that we bought. All Wenlock thinnings are selective and look to create a balance of some veteran trees, log production, cordwood production, natural regeneration, deadwood, open areas and a healthy understorey. A mix of heights and ages is also desirable. Thinning allows for a gradual transition to this end objective, while retaining woodland cover and avoiding the large visual impact of clear felling and replanting.
Thinning policy has generally been to favour native broadleaves and there has been a general presumption against “exotics” and heavy shade bearers (sitka spruce, norway spruce, western red cedar), particularly on ancient woodland sites. Japanese larch, has not been so much of a problem, until the arrival in the UK of Phytophthora Ramorum (fatal disease). At Wenlock some plantation conifers were unsuited to the site or were poorly maintained allowing local seed bank tree species to take over, producing mixed woodland. With the increasing threats to woodland (disease, climate change) maintaining this diversity is one way of “future proofing” Wenlock woods. However, with the new demand for woodfuel, and the unknown impact of ash die back pending, the rationale behind thinning is constantly evolving.
Thinning at Wenlock is very much a balancing act where priorities often carry different weighting depending on the site, the potential and the practicality. The woods have been actively managed for hundreds of years by patrons who believed their prescriptions were the best – now in the twenty first century we know best – don’t we?
By Alistair Heath (Area Ranger at Wenlock Edge)