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ŠUMARSKI LIST 9-10/2019 str. 11     <-- 11 -->        PDF

of these habitats, such as Pinus spp. (Baeza et al. 1991, Vallejo and Alloza 1999, Navarro Cerrillo et al. 2001). Stands of Holm oak (Quercus ilex L.) degrade to the maquis and garrigue stages, and the final result of the degradation process is bare karst or rocky vegetation (Figure 1) (Matić et al. 2011). In the Croatian part of the Mediterranean, the high silvicultural forms and coppice of Holm oak forests cover 29,000 ha. The degradation stages cover 120,000 ha as maquis and 23,437 ha as garrigue (Čavlović 2010).
Afforestation by Aleppo pine improves habitat characteristics, especially soil conditions. The main role of pine culture is ameliorative: protecting the soil, creating favorable microclimate conditions, improving physical and chemical soil characteristics, and returning the autochthonous broadleaved vegetation and economic value (Španjol et al. 2006). Maestre et al. (2003) reported higher soil organic carbon and total nitrogen at a depth of 0–20 cm under pine plantation 30 years after planting. Caravaca et al. (2002) found higher aggregate stability at a depth of 0–15 cm under pines 6 years after planting.
While the ultimate mechanism by which Holm oak is degraded appears to be anthropogenic disturbance, the proximate mechanisms are less well-understood. One hypothesis is that, once enough canopy trees are removed or damaged, the understory microclimate becomes inhospitable to the Holm oak understory species. Cutting or thinning forests results in alterations to microclimate conditions (Aussenac 2000). According to Aussenac (2000), it is necessary to study the interactions between forest stands and the microclimate, especially in climatic areas characterized by large water deficits such as those common in the Mediterranean, to improve the understanding of the ecology of forest species, and in particular, their reactions to water deficits under natural conditions. There has been some research into the microclimates below sparse and dense forest canopies, mostly in the subalpine and temperate climate zones (Aussenac 2000, Morecroft et al. 1998, Latif and Blackburn 2010, Arx von et al. 2013, Ugarković et al. 2018). However, there is a lack of research on the subcanopy ­climate in the degradation stages of forest ecosystems. An analysis of the microclimate data of forest ecosystems could allow for better understanding of the ecological conditions in forest habitats. The forest microclimate is crucial for the growth and survival of tree seedlings and understory ­vegetation. The microclimate conditions in the degradation stages of a forest ecosystem have not been thoroughly ­studied. Moreno et al. (2007) outlined the positive effects of trees on the microclimate, physical properties of soil, and soil water dynamics. Modifications to the microclimate with evolving forest cover through different stages should be considered when studying the survival and growth of young natural regeneration or in relation to afforestation operations.  
The objectives of this study were to determine the microclimate differences in degradation stages of Holm oak forests and stands of Aleppo pine with Holm oak. Future additional work will be done on the growth of regeneration and understory plants.
The study was conducted within the area of Mljet National Park on the island of Mljet, Croatia. The park covers an area of 5480 ha and is situated on the Northwestern part of the island. The geological substrate is carbonate and silicaceous rock and their various forms. Limestone is the most common of the carbonate rocks. The most common soil types are calcocambisol and lithosol, with a lesser share of rendzine on limestone and dolomite. The mean annual air temperature is 16.4 °C, and the mean annual precipitation is 770 mm (Seletković et al. 2011). The vegetation of Mljet is composed of 5 forest communities: Junipero phoeniceae–Pinetum halepensis Trinajstić 1988, an Aleppo pine forest with Phoenician juniper; Querco ilici–Pinetum halepensis Loisel 1971, an Aleppo pine forest with Holm oak; Myrto–Quercetum ilicis (Horvatić 1963) Trinajstić 1985, a Holm oak forest with myrtle; Fraxino orni–Quercetum ilicis Horvatić (1956) 1958, a Holm oak forest with manna ash; and Fraxino orni–Quercetum cocciferae Horvatić 1957, a Kermes oak forest with manna ash.
Experimental plots were situated in the same soil type and with the same relief characteristics. Relief characteristics were measured by Suunto Tandem 360 PC/360R DG and Garmin GPS64 devices. The study was conducted in 2 forest communities: Querco ilici–Pinetum halepensis and Myrto–Quercetum ilicis. Measurements were performed in the high silvicultural forms of Holm oak and Aleppo pine forests and at the degradation stages of maquis and garrigue. Observers measured the diameter at breast height (DBH) with Haglof tree calipers (cm) for every tree from 2 cm DBH and the height (m) of dominant and co-dominant trees in experimental plots by digital Vertex IV to calculate the main structural elements, as follows: number of trees (N), basal area (G), and wood volume (V). On the basis of the measured heights and parameters of the Schumaher–Hallove function of trees, the local tariff for each tree type was calculated. For the green olive tree (Phillyrea latifolia L.), Laurustinus (Viburnum tinus L.), strawberry tree (Arbutus undeo L.), tree heath (Erica arborea L.), and Cade juniper (Juniperus oxycedrus L.), we used Holm oak parameters for the Schumacher–Hall function. For experimental plots in the garrigue degradation stage, structural elements were not measured; instead, only the heights of the stand were measured.