Temporal Trends in Twig Growth of Fagus sylvatica L. and their Relationships with Environmental Factors

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Abstract
Dendrochronology and twig growth studies provide useful historical information about tree growth and permit analyses of the effects of measured environmental parameters on such growth. Recent growth histories of currently healthy and unhealthy beech { Fagus sylvatica L.) trees were reconstructed for a number of sites in southern England, by measuring annual twig extension growth back to 1960. The relationships between twig growth and a number of climatic (and one pollution) variables were then determined, using multiple regression analyses. A substantial reduction in twig growth was found in 1976 and 1977 for all trees, indicating the dramatic effect of the severe drought that was experienced throughout much of southern England during 1976. After this time, healthy trees at all sites quickly regained their pre-1976 growth rates. In contrast, unhealthy trees at two sites failed to recover and still show greatly reduced rates of growth. At two additional sites, while the effect of the 1976 drought is clear, the growth rates of unhealthy trees have been significantly lower than their healthy neighbours since before 1960. Soil type and tree age failed to provide an adequate explanation for the lower than expected growth rates of unhealthy trees seen at almost all sites. At most sites, a significant proportion of the variance in twig growth is explained by combinations of climatic variables. In most such cases, growth rates decreased as the variable concerned increased. Ozone was included in the best fit model for unhealthy trees at two sites. However, since this pollutant accounted for only a small amount of the model variance and the ozone data were of poor spatial resolution, care must be taken over the interpretation of this result.

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