DIGITALNA ARHIVA ŠUMARSKOG LISTA
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of Bukovské vrchy Mts., which are located in the east part of Slovakia – 4.18 m3ha-1y-1 (Bugala and Parobeková, 2016) and also with Sudetes Mts. in the Czech Republic, where increment ranges between 3.53 - 5.59 m3ha-1y-1 (Vacek et al., 2016). Shift of production optimum in Bukovské vrchy Mts was due to higher continentalism of the climate. There, the highest volume increment was 5.25 m3ha-1y-1 at an altitude of 400 m, which was more than 200 m of altitude lower. An altitude in which grey alder achieves its productive optimum therefore strongly depends on locality, and it should be considered also in the management plans and in greening projects. On the other side, in terms of potential economic use of alder stems, the most important indicator is their overall quality. Quality features are usually influenced by age and origin of individuals, their genetic conditionality, but also sociological position in the stand. Increasing interest in grey alder gives more attention to saving the gene pool of its valuable populations and searching for appropriate ways of its renewal and cultivation (Krstinić et al., 2002, Šmelková and Sarvašová, 2007). With proper treatment, alder stems of high quality are achievable even on sites, where other species are unable to thrive (Bugala and Migas, 2011). Since grey alder belongs to the noble woods, its value production may outweigh the volume production of other commercial tree species. In the context of greening projects, in recent years, the issue of ecological suitability and stability has also come to the fore (APA, 2015). Grey alder growing in the zone of its ecological optimum seems to be a very good alternative in choosing suitable tree species for this purpose (Rytter and Rytter et al., 2016). In our case, based on the characteristics of raw tree-ring series (Table 2), ecological optimum of grey alder is identical with its productive optimum, what is the undisputed advantage of this tree species. Low growth sensitivity of the whole population (0.143) refers to its ability to cope with the external factors and fairly wide ecological elasticity. As the between-tree correlation and expressed population signal confirm the low influence of non-climatic factors, a substantial part of the sensitivity is caused by the climatic factors. The growth trend of grey alder in the surveyed river basin is strongly decreasing and air temperatures are gradually increasing which makes them significantly negatively correlated (Figure 3 above). Standard chronology eliminates the growth trend influence; nevertheless, it is possible that increasing temperature can potentially slightly change correlation coefficients in time with the progressive global warming (Parobeková et al., 2016). Descriptive statistics indicate that the quality of standard chronology of grey alder is comparable with the other chronologies of broad-leaf trees from the temperate zone of Europe (Rozas, 2005; Douda et al., 2009). Expressed population signal (EPS) 0.85 reflects that the standard chronology is dominated by a coherent stand-level signal and is suitable for climate reconstruction (Speer, 2010). Growth–climate analysis provides generally weak correlations between temperatures and precipitations for the growing period 1969-2015. The radial growth of grey alder was significantly positively affected only by temperature in the current April and negatively by precipitation in the previous growing season. Also, Vacek et al. (2016) recorded positive influence of temperature and negative impact of precipitation. Therefore, some insignificant correlations were recorded in other months. Generally, it can be stated, that grey alder growing near to the watercourse prefers drier and warmer spring and colder and drier autumn. Negative influence of the precipitations in the previous growing season is linked to the root system condition. Long-term soil saturation causes anaerobiosis in the rooting zone and the root system may be strongly damaged (Rodriguez-Gonzales et al., 2010). Then there is a decrease in the nitrogen fixation and nutrient intake, increase in the concentration of toxic minerals in soil and deceleration of photosynthesis (Dittert et al., 2006). All these processes are associated with lower annual ring increment in the current year and are even more pronounced in the next year. The damaged root system cannot adequately supply the tree and despite favourable conditions in the next year, the tree lagging behind in the radial growth. Pointer year analysis uncovers, that population responded to the climate significantly positive twice – in 1970 and 1993, and negative only once – in 2015. On the average, only around 30% of trees revealed positive or negative reaction to the external factors, like we stated above mainly on climate. Similar results were obtained also in other localities of Slovakia (Balanda et al., 2012; Bugala and Parobeková, 2016). Indeed, several dendroclimatic studies have shown a relatively weak relationship between annual tree growth and climate in wetlands (Linderholm, 1999; Linderholm and Leine, 2004; Douda et al., 2009; Vacek et al., 2016). Both positive pointer years enveloped under different conditions of the current years (1970 – cold and wet, 1993 – cold and dry (Harris et al. 2014; http://climexp.knmi.nl)) but they followed after dry previous growing seasons. The increment of negative pointer year enveloped under warm and drier climate conditions, which are according to our results favourable for the growth of grey alder in the riparian zone, but extremely wet previous growing season caused probably the aforementioned damage of the roots. It follows that condition of the root system is the most influential factor in relation to the radial growth and it depends on water stress in the previous year. On the other hand, very warm and dry year 1992 (previous year of the positive pointer year 1993) (Figure 3), is mentioned in dendroecological studies of other tree species (Rybníček et al., 2012), but always in the connection with growth reduction. At surveyed locality, only 18% of individuals reported a reduction of growth, what confirms that individuals of grey alder can |