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ŠUMARSKI LIST 7-8/2020 str. 22     <-- 22 -->        PDF

adapted to various conditions, it grows best in areas ­marked by moderately warm summers and high amount of precipi­ta­tion (Seletković et al., 2003a).
Climate change and other global changes are supposed to greatly influence the forest ecosystems in Europe (Askeyev et al., 2005, Kellomaki and Leinonen, 2005, de Vries et al., 2014). The forested area is expected to expand to the north and contract in the south (IPCC, 2007). Climatic conditions, especially extreme climatic events such as drought are regarded as critical in the process of forest tree decline, as they govern the water relations (Zierl, 2004). Direct effects of climate change include responses of vegetation to temperature and/or precipitation; indirect effects occur primarily as soil-mediated phenomena, such as the influence of precipitation on soil moisture regimes (Watson et al., 1998).
Defoliation is a non-specific symptom of tree vitality widely used in forest practice (Polák et al., 2006) and forest health monitoring, most notably the UNECE International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests), a pan European forest monitoring program. Dobbertin and Brang (2001) demonstrate that tree defoliation assessed in 5% steps is a useful parameter to predict year-to-year tree mortality. Typically, the defoliation of trees will change from year to year and will rise sharply under heavy stress (such as drought), often not in the same, but the following year (Potočić et al., 2018). Trees are also known to be able to recover from leaf loss, the interval for the return to pre-stress values depending as much on the environmental conditions as on the tree species (Prpić and Seletković, 1992, Potočić et al., 2008).
The cycling and uptake of nutrients have been shown to be critical processes for the health of a forest ecosystem. It is clearly evident from previous research that nutrition has a profound multifarious influence on the vitality of trees (Hallenbarter et al., 1999). Loss of nutrients from the system, disruption of nutrient cycling and uptake, or imbalances in nutrient status may be associated with declines in forest productivity and stability (Nilsson et al., 1995).
Trees respond to environmental stresses, among others, with increment decrease (Dobbertin, 2005). Growth reduction is a consequence of a reduced photosynthetic activity due to limitations in the environment and result in altered carbon allocation. In extreme situations, such as long lasting drought, growth reduction can lead to mortality (McDowell et al., 2011). Radial increment is sensitive to environmental conditions and local and/or regional climate (Fritts, 1976) and as such it can be used as an indicator of tree vitality.
Landmann et al. (1995) consider the results of various ­research dealing with tree decline lacking as they concentrate only on stands exhibiting great loss of vitality. As with any other influence on a forest ecosystem, climatic change would have a stronger impact on a previously stressed forest stand exhibiting poor vitality than on a forest growing in near-optimal conditions. On the other hand, climate change might have a bigger economic impact on optimal sites than on extreme, less fertile sites. Therefore, it is crucial for practical silviculture and forest management to be aware of the potential reaction of beech to climate change in order to plan mitigation measures to preserve the economic value of beech forests. Furthermore, in contrast with data from large-scale monitoring, case studies are able to provide more data from a limited area, therefore more precisely showing the ecosystem response to a specific set of climatic factors present in the research area (Seletković et al., 2009).
We hypothesise that climate has a significant influence on common beech vitality indicators (crown defoliation, foliar nutrition and radial growth). Higher temperatures and lower precipitation as well as extreme climate events should result in in higher defoliation values and reduced growth. Climate influences the absorption of nutrients, as well as physiological processes within the tree and therefore the effect should be observed on the nutritional status of beech trees. We also expect that it is possible to observe the interconnections between these vitality indicators. Therefore, the aims of this paper are to (i) investigate the interrelations of common beech vitality indicators as well as their (ii) dependencies on climatic conditions in a mature and healthy beech stand.
MATERIAL AND METHODS
MATERIJAL I METODE
Research area – Područje istraživanja
Intensive monitoring plot Sljeme, a part of ICP Forests Level II programme (plot number 103) is situated in a European beech - silver fir (Abies alba Mill.) community (Festuco drymeiae – Abietetum Vukelić et Baričević 2007), at 954 m altitude on a south-exposed slope of Medvednica massif, located in the northwest Croatia (Figure 1). The plot is 1 ha (100 x 100 m) in size. Soil type is dystric umbric cambisol on greenschist parent rock (Potočić et al., 2003). The plot is also a part of UNESCO’s Man and the Biosphere Programme (since 1981), when all forest management operations were stopped. The western part of the Pannonian area of Croatia is encompassed by the climate type Cfwbx’’ according to Koeppen’s classification (Šegota and Filipčić, 2017). For this climate type precipitation should occur uniformly throughout the year, with the smallest amount in winter, and precipitation maximums occuring in spring and late summer. According to modelled climate data from E-OBS gridded dataset (Haylock et al., 2008) for the period (1950 - 2007), annual mean temperature for our research plot was 10.8 °C, and annual precipitation was 929.6 mm.