26. Health of Beech Trees in Britain
MGR Cannell and TH Sparks

Percentage of beech trees surveyed in the UK whose crowns were more than 25% less foliated than they should have been (note that large values indicate many poorly-foliated trees), shown in relation to average rainfall in England and Wales the previous July

Specific Indicator
Since 1984, the Forestry Commission has assessed the crown density of about 9000 trees each year across Britain. Crown density is the degree of transparency or leafiness of the crown, assessed visually. There are 62 plots of beech (Fagus sylvatica) which has shown the most marked annual fluctuations in crown density. This species may also be the most likely to respond to climate change, so the specific indicator chosen is the percentage of beech trees (surveyed annually in Britain) with crown density reductions greater than 25% compared with photographs of ideal fully-foliated trees.
[Source: Forest Research, Roslin, Midlothian EH25 9SY, and the Digest of Environmental Statistics, Department of Environment, Transport and Regions, HMSO, London]

Relevance
Since the 1970s, concern has been expressed in Europe over the general state of tree health. Initially, it was suggested that there was a new kind of forest decline caused by air pollution, but climatic and other factors are clearly involved. This concern led to the establishment of annual surveys of tree health both in the UK and across Europe. The thinning of tree crowns has become accepted as the most useful single indicator of tree health.

Beech is one of the most important native broadleaved tree species in Britain, for recreation, wildlife, shelter and timber. It is also sensitive to climatic and other factors which cause crown thinning.

Sensitivity to climatic and other factors
In southern Britain, drought is a major cause of annual differences in crown density, especially on beech, where drought is often accompanied or followed by heavy seed production (mast years) dieback and small leaves. In some areas, beech may also be adversely affected in warm, sunny summers by ozone. Drought is also thought to be a factor causing dieback in hedgerow trees, which are not included in the UK crown density survey. Variation in the ring widths within broadleaved trees in southern Britain is usually more closely related with rainfall in the current or previous years than with other factors.

More generally, insect pests, such as the spruce aphid (Elatobium abietinum) on Sitka spruce, winter moths (Operophtera brumata and Erannis defoliaria) and leaf roller moth (Tortrix viridana) on oak, are major causes of tree defoliation. Fungal pathogens, such as the needle cast fungus (Lophodermium seditiosum) on Scots pine can also cause needle loss in some areas and years. Other causes of crown thinning are wind damage, frost, squirrels, and physiological and nutritional disorders, such as top dying in Norway spruce. The various defoliation agents give rise to complex variation in crown density scores between years and sites around the UK.

An examination of relationships between climatic variables and annual crown density records for five species in Britain revealed a particularly strong negative correlation between the percentage of beech trees with over 25% crown density reduction and average rainfall the previous July in England and Wales. Dry summers may impair the development of roots and buds and also increase the intensity of seed production the following year (ie the incidence of >mast= years) which cause crown thinning. There is also evidence that beech is less able to control water loss from its leaves than species such as oak.

Change over time
High percentages of beech trees were poorly foliated (ie with over 25% crown density reduction) in 1987, 1989-1992, 1995, 1997 and 2000. In all instances, these years followed previous dry summers and many of the symptoms observed indicated drought damage which may have resulted from root death the previous year. In some years, such as 1990 (but not 1991) poor foliation was clearly associated with heavy seed production.