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

Dendrochronology and twig growth studies provide useful historicalinformation about tree growth and permit analyses of the effectsof measured environmental parameters on such growth. Recentgrowth histories of currently healthy and unhealthy beech {Fagussylvatica L.) trees were reconstructed for a number of sitesin southern England, by measuring annual twig extension growthback to 1960. The relationships between twig growth and a numberof climatic (and one pollution) variables were then determined,using multiple regression analyses. A substantial reduction in twig growth was found in 1976 and1977 for all trees, indicating the dramatic effect of the severedrought that was experienced throughout much of southern Englandduring 1976. After this time, healthy trees at all sites quicklyregained their pre-1976 growth rates. In contrast, unhealthytrees at two sites failed to recover and still show greatlyreduced rates of growth. At two additional sites, while theeffect of the 1976 drought is clear, the growth rates of unhealthytrees have been significantly lower than their healthy neighbourssince before 1960. Soil type and tree age failed to providean adequate explanation for the lower than expected growth ratesof unhealthy trees seen at almost all sites. At most sites, a significant proportion of the variance in twiggrowth is explained by combinations of climatic variables. Inmost such cases, growth rates decreased as the variable concernedincreased. Ozone was included in the best fit model for unhealthytrees at two sites. However, since this pollutant accountedfor only a small amount of the model variance and the ozonedata were of poor spatial resolution, care must be taken overthe interpretation of this result.