Tree-ring Width of European Ash Differing by Crown Condition and its Relationship with Climatic Factors in Latvia

The spreading dieback of European ash (Fraxinus excelsior L.) that is a serious threat to the existence of the species in Europe, has been related to climatic changes. Still, not all trees in stands are damaged equally, suggesting that sensitivity to weather conditions might have affected the susceptibility to the disease. Climate-growth sensitivity of ash with visually healthy and damaged crowns growing in four stands in the central and eastern part of Latvia was assessed by dendrochronological techniques. The patterns of tree-ring width variation showed high diversity amongst trees, stands and regions; differences were observed between the damaged and healthy trees. Tree-ring patterns showed higher diversity amongst the healthy trees in the central part of Latvia, but, in the eastern part of Latvia, amongst the damaged ones. Mainly, the damaged trees were ca. 10-15 years older than the healthy ones suggesting age related differences in susceptibility, which might be related to vigour. The damaged and healthy trees differed also by growth trends, suggesting affiliation to different crown class, particularly at younger age. The sets of the significant climatic factors differed between the central and eastern part of Latvia. In the central part of Latvia, ash was mainly affected by the precipitation and daily temperature difference in the summer preceding formation of the tree-ring. Although the damaged trees were more sensitive to daily temperature difference and precipitation in the preceding August, the healthy trees were also additionally affected by maximum temperature in the preceding August. In the eastern part of Latvia, the sets of the significant factors were site specific, however, trees were mainly affected by temperature in the preceding autumn and current spring. In one site, the damaged ashes were more sensitive to temperature in July and September, while in other site the damaged trees were more affected by precipitation in July; the healthy trees were additionally affected by precipitation in September and temperature in April. Hence, the susceptibility to the disease appears partially related to the climatic sensitivity of trees.