Dieback of Silver Fir (Abies alba Mill.) on Northern Velebit (Croatia)

Background and Purpose: Silver fir is the most widely distributed and the most important commercial conifer species in Croatia; however, it is also the most endangered. The first records of its dieback in Croatia go back to the beginning of the 19th century. Dieback has since caused growing concern and has become a major problem in forestry. The phenomenon of silver fir dieback is attributed to a number of causes. The objective of this study is to present the dynamics and intensity of silver fir dieback in the last decade on northern Velebit and determine the connection between tree dieback, relief and climatic factors. Materials and Methods: Data on dead silver fir trees were collected in northern Velebit (management units Begova~a, Jelovac and Lom, Krasno Forest Office) at the level of the management units for the 1998-2006 period. The dynamics of tree dieback was analyzed on the basis of dead tree volume. Regression models were used to obtain data on temperature and precipitation quantity for each compartment/subcompartment. Dependence of dieback intensity on relief and climatic factors was established with correlation coefficients. Results: The average annual dieback of silver fir in the 1998-2006 period ranged from 0.4 to 0.1 m³/ha. The highest dieback index, amounting to 2.5% of the total growing stock, was recorded in the management unit Jelovac, while the lowest, reaching 1.4% of the total growing stock, was recorded in the management unit Lom. The lowest dieback of only 0.6 m³/ha on average was detected in NW expositions, while the highest of 2.7 m³/ha was found in eastern expositions. An increase in mean vegetational air temperatures of 1.1°C and 0.7°C was recorded in the areas of meteorological stations Gospi} and Zavi`an respectively, as well as a decrease in vegetational precipitation quantities of 4.2% (Gospi}) and 0.4% (Zavi`an). Conclusions: A rising trend in the quantity of dead wood mass of silver fir (Abies alba Mill.) has been recorded in the past ten-year period. Fir dieback in the amount equaling annual increment was established. The highest dieback intensity occurred at lower altitudes and milder slopes. A statistically significant increase in air temperature values was recorded in the study area. Although precipitation quantities are reduced, they are not statistically important. Very slight to slight, but statistically significant positive correlation was found between a rising trend in temperatures and precipitation. Correlation was also found between an increase in the silver fir growing stock and dieback intensity