Ecological consequence of oil accidents in ecosystems of lowland forest

The paper presents the results of preliminary research into a changed condition of the forest ecosystem in lowland forests of pedunculate oak. Research was carried out in the area of Zagreb Forest Administration, Management Unit Žutica. MU Žutica is an intensively exploited oil and gas area with a very complex exploitation system consisting of as many as 289 oil and gas wells (1 well per 17.67 ha). Based on the largest proportion of the well area in relation to forest vegetation, the following compartments were selected for analysis: compartments 44 and 115 with 15 wells and compartment 127 with 9 wells. Compartments 70 and 101 (no wells) were selected as check compartments. Long increment cores were taken with a Pressler borer from pedunculate oak trees during 1998 and 2004. A quantity of 250 increment cores were taken in 1998 and 40 increment cores were taken in 2004. No samples could be taken in compartment 101 since all the trees had been cut down as a result of an accident in the year 2000 (a burst underground oil pipe). Four and five ten-year time series respectively were determined on the samples. The study of radial increments was combined with pedological research, conducted in six localities chosen in the spring of 2003. Oil and/or oil well fluid contamination in the forest soil was investigated. The analysed radial increments of all the cores taken in 1998 and 2004 show distinct variability in radial increments of all oak trees, which indicates a disturbed condition of these stands. Notably, in both measurements pedunculate oak trees in all the stands, regardless of the stands- age or tree breast diameters, show similar reactions to changes in the stands ecological condition. The analysis of mineral oil content in the soil revealed severe contamination with oil in the compartments, which renders the soil unsuitable for plant production. With regard to evident tree dieback across the management unit, the conditions and data of snag removal by felling (in the principal and intermediate yield) were taken into account. A GIS map was constructed showing the spatial distribution of snag cutting intensity, as well as the plots in which increment was determined and the soil analysed. According to the cartographic presentation, cutting intensity was concentrated in the centre of the management unit, precisely where there is the largest number of oil wells and oil transport communications. The results point to the need of introducing permanent monitoring of forest condition in this area through an interdisciplinary scientific approach in order to arrive at more exact explanations of the changes.