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ŠUMARSKI LIST 9-10/2019 str. 18     <-- 18 -->        PDF

surface runoff, though additional research is needed to test this hypothesis.
According to Martín et al. (2014), large amounts of precipitation, soil moisture, and relative air humidity had significant and positive influences on tree growth, while high temperatures, evapotranspiration, and solar radiation had significant negative effects. Accordingly, the microclimate conditions in the garrigue degradation stage are less favorable for the growth and development of forest tree species such as Holm oak than in the maquis stage and in high silvicultural form stands. In wetter areas where annual precipitation exceeds 700 mm, soil moisture is always higher under tree crowns than outside tree crowns (Joffre and Rambal 1993). Here, average soil moisture was also found to be significantly lower in garrigue than maquis and high silvicultural forest forms as expected, due to the complete canopies in the maquis stage and high silviculture forest form as opposed to the discontinuous canopy in the garrigue. This sparse canopy and lower interception of precipitation also gave the highest soil moisture range and maximum value in garrigue.
The relative air humidity is known to have ecological significance for the supply of surface soil layers through condensation (Oršanić et al. 2011). Limited moisture will often determine the abundance of Holm oak seedlings for successful regeneration in Mediterranean areas (Oliet and Jacobs 2007). Though many factors can influence acorn production, the summer water status of plants is likely the predominant factor affecting seed growth and acorn production (Carevic et al. 2010). According to the results presented here, the best conditions for volumetric soil moisture and dry season water deficit are found in the Holm oak forest stands and the maquis degradation stage. Despite being a degradation stage, maquis is favourable for the growth of Holm oak, given the values of the dry season water deficit with regard to the ecological water requirements of Holm oak.
Under the current conditions of changing a macroclimate, microclimate conditions are also altered. According to Nunes et al. and Sardans and Peñuelas (Nunes et al. 2017, Sardans and Peñuelas 2004), longer and more frequent periods of drought can be expected in the near future in the Mediterranean forest ecosystems. By the end of the 21st century, precipitation is expected to decline in the Mediterranean as a result of climate change (Limousin et al. 2008) which will certainly have a strong impact on the negative water balance in the soil during the summer months.
Changes in the amount of available water in the soil may be a key factor in tree survival and ecosystem productivity, particularly in semiarid regions (Moreno and Cubera 2008). Due to climate change, the Mediterranean region is becoming drier, and reduced growth can be expected, which will benefit drought-tolerant species such as Phillyrea latifolia, while wetter (more mesophilic) species such as Holm oak will be lost. Holm oak has lower growth rates and higher mortality in drought conditions in comparison to the less susceptible Phillyrea latifolia and Arbutus unedo (Ogaya et al. 2003), and has a lower ecological amplitude for soil moisture and drought than the Erica arborea and Myrtus communis species which appear in the garrigue stage.
Considering the ecological requirements of Holm oak, it is not recommended for spaces to be filled in the garrigue degradation stage. Openings in garrigues should be reforested with species that have broader ecological amplitudes for soil conditions, particularly soil moisture, such as Aleppo pine, maritime pine, Turkish pine, and manna ash. Under pine stands, the soil gradually takes on properties that allow for the appearance of species that make up the composition of the Holm oak forest. This is a long-term process that could take 60 to 80 years or more, depending on the habitat conditions (Matić et al. 2011).
The values of all analysed microclimate elements in the Aleppo pine forest stand were more favourable than in the garrigue, with the exception of dry season water deficit. For that reason, the share of Holm oak in Aleppo pine stands is very low, at just a few percent, and Holm oak is almost absent in garrigues (Table 1). There are opposing opinions on Aleppo pine as a pioneer and amelioration tree species. Some claim that it improves the habitat for the return of Holm oak (Španjol et al. 2006), while others suggest that these improvements are unsuccessful (Maestre and Cortina 2004). It is our conclusion that Aleppo pine improves the microclimate conditions for Holm oak, though this is a long-term process.
An understanding of how ecological factors function and the reaction of ecosystem parts to those factors is essential to support the natural functioning of managed forest ecosystems. By associating microclimatic factors with the numerous morphological and biological properties of Holm oak (e.g., germination, height growth, mortality, etc.), this allows us to obtain a better understanding of the functioning of these complex forest ecosystems.
We thank Jakov Nodilo, B.Sc. Forest engineer, an employee of Mljet National Park, to assist with field research.
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Arx von, G., E.G. Pannatier, A. Thimonier, M. Rebetez, 2013: Microclimate in forests with varying leaf area index and soil moisture: potential implications for seedling establishment in a changing climate. J Ecol, vol. (101): 1201.-1213.