Spatial Analysis of Pedunculate Oak Mortallity Rate for Adaptive Forest Ecosystem Management in Croatia

Pedunculate oak (Qercus robur L.) is one of the most important tree species in Croatia. As a dominant species, it covers about 10% of the forested area (about 210 000 ha), which accounts for about 12% of the total growing stock. This tree species is distributed in the lowland and floodplain area of the rivers Drava, Sava and Kupa and their tributaries. Forests of pedunculate oak are managed according to close-to-nature principles and with rotations of over 140 years. In the past 100 years, the management of pedunculate oak forests has been hindered by a serious management and ecological problem: the occurrence of tree dieback and decline. From the management point of view, the problem entails the increased quantity of dead or severely damaged trees, increased management costs and a disturbance in natural forest regeneration. From the ecological point of view, the problem centers around changed site conditions, which is unfavorable for both present and future growth of pedunculate oak trees. In the period from 1995 to 2007, an amount of 35% of the current pedunculate oak volume declined in Croatia, with mortality rising to 40% if the past several years are taken into account. Tree dieback incurs grave financial losses, which reach up to 40% of the potential market value of undamaged and healthy wood mass. The losses are further aggravated by increased costs of managing such forests and reduced non-timber forest functions. The mortality rate of pedunculate oak trees is an indicator of the instability of lowland forest ecosystems, especially when dieback occurs in optimal site conditions for the pedunculate oak (micro-elevations and micro-depressions). Individual, intensive and catastrophic tree dieback has been recorded. Salvage cuts are applied in the attempt to mitigate the effects of disturbances caused by tree dieback. These cuts are undertaken continuously, depending on tree dieback intensity, and are based on the assessment of tree condition. Tree condition is assessed on the basis of tree damage criterion, which comprises crown defoliation and leaf discoloration. These are indicators of lessened tree vitality, but have so far not proved adequate for the reduction of pedunculate oak tree dieback. In order to advance the current condition of forest ecosystems, additional indicators of decreased tree and stand vitality are being sought, and so are the indicators of their disturbed stability. Mortality rates that take into account the volume of dead tree mass are used for this purpose. The objective of this work is to establish spatial distribution of d