Crown defoliation and causes of dieback of silver fir (Abies alba Mill.) trees in Croatia

Silver fir is the most damaged and threatened forest tree species in the Republic of Croatia, and the appearance of crown defoliation and tree dieback has been attributed to different causes. The objective of this study was to determine the relationship between crown defoliation and tree dieback of silver fir with regard to climatic and structural factors, to determine the causes of dieback (crown defoliation, wind) and the differences in the intensities of tree dieback of silver fir in a beech-fir forest, and in a fir forest with hard ferns. Data on crown defoliation and tree dieback of silver fir were collected from two permanent experimental plots in Gorski Kotar. Crown defoliation was assessed as percentages according to the ICP Forest Programme Guidelines, and dieback intensity was expressed as the percentage share of dieback and living trees. In both forest ecosystems, there was a significant increase in crown defoliation. The average defoliation time of the observed trees was higher in mixed beech-fir forests in comparison to fir forests with hard ferns. With regard to the intensity of tree dieback expressed as the number of trees, tree basal area and wood volume, the majority of dieback was found in fir forests with hard ferns. Greater tree dieback was recorded due to crown defoliation in comparison to wind breakages and uprooting. Of the climatic factors, crown defoliation was most affected by air temperature and the amount of potential evapo-transpiration. With the increase in crown defoliation, the life of the tree and its girth increase were significantly decreased. The correlation between crown defoliation and tree dieback was significant and positive, and this correlation was stronger in beech-fir forests, where it measured moderate to strong, while it was very weak in fir forests with hard ferns.