OVERVIEW OF VEGETATION RESEARCH IN HOLM OAK FORESTS (Quercus ilex L.) ON EXPERIMENTAL PLOTS IN CROATIA

In the vegetative sense, holm oak forests are an integral part of the Mediterranean region, with holm oak appearing as the dominant tree species in the climatogenic community of the coniferous belt. The course of vegetation development, i.e. progression and regression, is seen in the succession of holm oak forests. Succession is present on the Croatian coast of the Adriatic Sea, and throughout the whole Mediterranean area. This paper provides an overview of the vegetation analyses and dynamics of development of holm oak forests. Research was conducted only on MAB plots and plots on Rab Island (Kalifront Peninsula, belonging to the Faculty of Forestry, University of Zagreb for the research of Raus, spanjol and Barcic). Vegetation inventory generated a list of all plant species in the tree, shrub and undergrowth layers that were observed and recorded on the selected plots, with values for abundance (number) and cover. The combustibility and flammability of holm oak were analysed, since forest fires are the most significant threat to forest vegetation and vegetation in general in the Mediterranean zone. Multifactor analysis of variance (Sokal & Rohlf, 1995) was applied to determine the flammability and combustibility for holm oak. Linear correlation analysis was applied to examine the relationship between individual variables. The course of vegetation development, i.e. progression and regression, shows the succession of holm oak forests (Figure 5). The figure indicates the long time period necessary for the succession of forest vegetation, particularly in the case of progressive succession and complete species conversion. This includes at least several rotations of pine stands, and is in direct correlation with the habitat conditions. The research of holm oak forests on the permanent experimental plots is particularly important. These plots are part of the international Man and Biosphere project (MAB). Vegetation research in holm oak forests can be compared between the permanent experimental plots (PEPs) in the MAB project from the islands of Rab, Brijuni, Mljet and Lokrum (PEPs no. 36, 56 and 57, 37, 77, respectively). The succession processes on the PEPs are expressed through comparisons of time series monitoring data. They indicate the formation of stand structures and the emergence of the dominant tree layer (Figure 6). Understanding the properties of forest fuels in the sense of their combustibility (ability of fuel to combust) and flammability (ability of fuel to continue to burn) is very important for understanding the start and spread of forest fires. Different methods are used to determine combustibility and flammability. The most commonly used method is that described by Valette (1990), based on the research of combustibility and flammability of leaf litter. The regression analysis established that the moisture content of fuel, mean monthly air temperature, and mean monthly precipitation levels significantly affected the combustibility of holm oak (Table 10). However, the results of the regression analysis of the flammability of holm oak on Rab Island indicated that none of these variables had a statistically significant influence on flammability (Table 12). The PEPs were established for the purpose of long-term and comparative ecosystem research, and they will continue to be used for further research to explain the succession processes in the climatogenic Eu-mediterranean community.