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ŠUMARSKI LIST 9-10/2019 str. 17     <-- 17 -->        PDF

According to the results presented in Table 2, the difference between two analysed factors and the interaction of factors stand type x seasons was statistically significant for all microclimate variables, except dry season water deficit. Differences were established between dry season water deficit and forest stand type and seasons (p < 0.0000), while no differences were found in the interaction of those two factors (p=0.9952).
The differences in values of dry season water deficit between Holm oak forests and maquis were not statistically significant. Between garrigue and maquis, there was a statistically significant difference in the dry season water deficit in spring (p=0.0329), summer (p=0.0046) and autumn (p=0.0256). In the comparison between Aleppo pine forest stand and maquis, there was a higher value of dry season water deficit during summer (p=0.0138), while no differences were detected for spring (p=0.0577) or autumn (p=0.0731). There were also no statistically significant differences for dry season water deficit by season between the garrigue and Aleppo pine forest stand (p=0.6369).
The largest amplitudes for most of the analyzed climatic elements were found in the garrigue degradation stage. This is explained by the structure of this degradation stage, in which plant cover is from 40 to 50% and the crown cover is significantly interrupted. The development of forest from the garrigue stage to Holm oak forest contributes to significant reductions in air and soil temperature ranges. Lower ranges of air and soil temperatures were found in Holm oak forests due the full tree canopy, subcanopy tree layer, and well-developed shrub layer.
Minimal air temperatures were lowest in maquis, meaning that the maquis degradation stage could be a less favorable breeding form for the germination of Holm oak seeds in comparison to the high silvicultural form of Holm oak forests with myrtle. According to Larcher (1969) Holm oak is most sensitive in the germination phase. A temperature drop of -2 to -3 °C is sufficient to cause damage to young plants. According to Potter et al. (2001), the buds and leaves of Holm oak are sensitive at a minimum air temperature of -5 °C. The minimum air temperature on the island of Mljet can drop to -5.2 °C (Seletković et al. 2011).
When maquis is considered as less favourable breeding form for Holm oak seedling growth and development, light as an ecological factor should be considered. High density and small amount of relative light intensity in maquis form (Oršanić et al. 2011) also has s greater impact on young plants.
According to Aussenac (2000) and Ugarković et al. (2017), soil temperature is affected by stand structure and plant abundance. In general, forest cover buffers the daily and seasonal temperature differences compared to open ground and notably clear-felled areas. Soil temperature is affected by the nature and density of cover. Soils under forest cover are warmer in the winter and colder in the summer than clear-felled areas (Aussenac 2000).
The highest maximum and average air and soil temperatures were measured in garrigue due to forest gaps indicating higher light intensity. Larger openings appear in the canopy and the stand is unable to modify the climatic elements. The lowest average air and soil temperatures were recorded in the maquis stage and probably resulted from the climate-moderating influence of dense vegetation. Large numbers of plants per unit area in this degradation stage and high plant cover moderate the influence of solar radiation. Differences in air and soil temperature, relative air humidity, volumetric soil moisture, precipitation and potential evapotranspiration between Holm oak forest stand and maquis were significant, but not for dry season water deficit. Maquis is the first degradation stage and certain ecological conditions are less favorable for Holm oak than in a forest with a high silvicultural form. According to the climatic element values, stands of Aleppo pine are more arid than stands of Holm oak and the maquis degradation stage. Reforestation with Aleppo pine as the stage before Holm oak forests, i.e., as a progression toward the return of climatogenic vegetation, improves the microclimate conditions needed for the return of Holm oak.
Air humidity is higher within forests than outside them due to poor internal mixing of internal and external air, and because higher quantities of water move from the soil depths to the air due to plant metabolism (Penzar and Penzar 2000). For that reason, the mean value of relative air humidity was highest in the Holm oak forest with myrtle in which there are tree, subcanopy tree, and shrub layers.
Previous studies have indicated that forest cover has little (Aussenac 2000, Morecroft et al. 1998, Gehlhausen et al. 2000, Meyer et al. 2001) or no (Valigura and Messina 1994) effect on the relative air humidity. In the present study, a significantly lower average air humidity was found in stages with interrupted forest cover (garrigue) and in the Aleppo pine forest stand. Generally, forest cover and tree species composition influence air humidity in Mediterranean forest ecosystems.
The maximum value of volumetric soil moisture was highest in the garrigue stage. Considering the low plant cover in this degradation stage, there is less interception (Nakamura et al., 2017) and much more precipitation falls on the ground in a shorter period of time. Due to the low plant cover and low interception, we hypothesize that there is greater