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ŠUMARSKI LIST 9-10/2021 str. 15     <-- 15 -->        PDF

Holm oak (Quercus ilex L.) is an evergreen tree species with natural distribution within the Mediterranean, from Portugal and Morocco on the west to the western coast of Turkey on the east part of its range. On the eastern coast of the Adriatic Sea, it occupies warmer localities on the coast of Istria and northern Adriatic islands while on the southern Adriatic islands as well as on the narrower coastal area south of Zadar it has a more continuous distribution. Given the size of the area of distribution, holm oak shows a big morphological and functional variability so certain populations differ according to the climate conditions in their area. This fact has been very well researched in the western and central part of its range, while the data for the eastern part, and thus for the eastern coast of the Adriatic Sea, are scarce. The great morphological and functional variability of holm oak reflects its great phenotypic plasticity. Although phenotypic plasticity is conditioned by the environment, it can also have a genetic basis through local adaptation to the established environmental conditions in different parts of the species’ area of distribution. The aim of this study was to determine whether there is a differentiation of holm oak down the climatic gradient of the eastern Adriatic coast with respect to the functional characteristics of its leaves.
The research was conducted on three provenances of holm oak from the area of ​​Pula and Zadar in the Republic of Croatia and Bar in Montenegro. During the winter of 2019, 12 leaves were sampled per 12 trees in each provenance in order to determine the specific leaf area (SLA). Additionally, acorns were collected beneath those trees with which an greenhouse experiment was established. In the experiment, a total of 64 plants were grown, of which 22 plants from the provenances of Pula and Zadar, and 20 plants from the provenance of Bar. In the second part of the vegetation period in 2020, one leaf was sampled from each plant with the aim of determining the specific leaf area (SLA). In addition, the rate of net photosynthesis (A) and the relative total chlorophyll content index (CCI) were measured in the greenhouse experiment. Measurements were performed in July and September of 2020. A was measured using a portable gas exchange measuring device LCpro + (ADC BioScentific, 2007), while the CCI was measured using the CCM-200 (Opti-Sciences, Tyngsboro, MA).
Analysis of the variance revealed a statistically significant difference among the studied provenances for most of the measured characteristics both on the leaves sampled in the natural habitat and on the young plants in the experimental plantation. Previous research on holm oak indicates that there is a functional differentiation of provenances on different climatic gradients in the central and western Mediterranean, with our results confirming this on the eastern coast of the Adriatic Sea. Young plants in the experimental plantation retain the same pattern of differentiation with respect to SLA as their mother trees in natural stands (Figure 1c). Such a result indicates that the differentiation of holm oak provenances is at least partly the result of genetic adaptation to local habitat conditions, i.e. that the holm oak on the eastern Adriatic coast as well as in other parts of its range could distinguish ecotypes. The sclerophilicity of the leaves described by SLA (Figure 1c) does not coincide with the average annual rainfall (Figure 1a), but with the amount of precipitation in the summer (Figure 1b) and the climate deficit of moisture (Figure 2a). The established pattern of provenance differentiation with respect to SLA and climatic conditions is followed by A measured on young plants in the experimental plantation. Therefore, Pula, which has the highest SLA and comes in the least arid climatic conditions, has the lowest A, while Zadar, which has the lowest SLA and comes in the most arid conditions, shows the highest values of A (Figures 1 and 2). This is in line with the literature that shows that the intensity of photosynthesis per unit leaf area in holm oak decreases with increasing SLA (Niinemets 2015, Peguero-Pina et al. 2017). Namely, the anatomical structure of holm oak leaves is adapted in such a way that those leaves that have a lower SLA also have better conductivity of CO2 through the mesophilic leaves and contain more nitrogen per unit area. Accordingly, the provenances of Zadar and Bar compared to the provenance of Pula had a lower SLA (Figure 1c) and a higher CCI (Figure 2b) which has a positive correlation with the nitrogen content in the leaves.
Holm oak could have a significant ability to functionally adapt to different climatic conditions. Accordingly, when considering the impact of climate change on natural populations of holm oak, its phenotypic plasticity should be considered, as well as the observed functional ecotypic differentiation. 
Key words: Quercus ilex L., sclerophilic leaf, specific leaf area, rate of net photosynthesis, climate