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The results of the discriminant analysis suggest that the differentiation between the studied biogeographical regions is significant. Although with some overlap, the analysis revealed a clear separation between the Alpine-Dinaric and continental populations (Figure 4D). The proportion of correctly classified individuals into two studied biogeographical regions “Alpine-Dinaric” and “continental” is shown in Figure 1B. The overall classification success was 87.1% of correctly classified individuals. Within the Alpine-Dinaric group, 88.7% of the individuals were included in the correct population group. A lower rate of correctly classified individuals was observed in the continental region (85.0%).
Principal component analysis (PCA) has clearly supported a separation of the continental and Alpine-Dinaric populations along the altitudinal, temperature and precipitation gradients (Figure 4B, Table 3). The results show that continental populations are characterised by low elevations and warmer habitats with higher temperatures and lower precipitation. By contrast, high-elevation populations from the Alpine-Dinaric region are distributed within cooler habitats with lower winter temperatures and higher precipitation. In addition, PCA analysis of all individuals has supported the divergence of two morphologically distinct groups (Figure 4A).
Discussion
Rasprava
The results clearly demonstrate a high phenotypic diversity of grey alder populations in Croatia. In general, populations from the continental region had larger and wider leaves than populations from high altitudes in the mountainous Alpine-Dinaric region. In addition, large-leaf populations showed slightly lower variation possibly due to an overall reduction in the number of individuals, and fragmentation and isolation of populations due to human impact (Vukelić 2012; Poljak et al. 2014). On the other hand, environmental heterogeneity over very short distances within the Alpine-Dinaric region could result in higher phenotypic intra-population variability. A significant variability of morphological characteristics of the grey alder leaves was also reported by Krauze-Michalska and Boratyńska (2013), and Poljak et al. (2014). Nevertheless, in our previous study of grey alder populations, along the upper course of the river Drava, somewhat higher within-population variability was recorded. Furthermore, our results indicated that a natural hybridization has occurred between the common and grey alder in those populations, but at relatively low rates. However, these events may influence the diversity and structure of populations (Barton 2001; Poljak et al. 2017), and increase within-population morphological variability. In the current study, the above-mentioned influence was strongly reduced by examining the plant material for the analysis, i.e. hybrid individuals were subsequently excluded from the study.
The results of the hierarchical analysis of variance (ANOVA) were in line with the expectations of high morphological variation within populations and low differentiation between populations, as observed in alders (Krauze-Michalska and Boratyńska 2013; Poljak et al. 2014) and other wind-pollinated floodplain tree species (Jarni et al. 2011; Zebec et al. 2010, 2014). The relatively high level of among-tree variation within the populations is probably a result of both phenotypic adaptation to specific micro-environmental conditions experienced by each tree, and genetic differentiation among individual trees (Brus et al. 2011; Poljak et al. 2015). However, the AMOVA analysis showed that a great proportion of the total variation was attributable to the differences between regions, confirming the geographical structuring of populations. Moreover, most of the population pairs from both biogeographical regions had non-significant pairwise values (data not shown). The lack of ­significant among-population variation within biogeographical regions could be explained with the population longitudinal distribution along the rivers and floodplains with no barriers to gene flow, where free pollen and seed dispersion between the populations occurs (Temunović et al. 2012). More specifically, this can confirm the findings of our previous study of grey alder populations in Croatia (Poljak et al. 2014), along the upper course of the Drava ­river, where absence of inter-population variability was also observed.
The results of the hierarchical analysis of variance have been confirmed by multivariate statistical methods, suggesting the existence of a clear divergence between the populations from two biogeographical regions. Such differences in variability were also found in other woody species that occur in different biogeographical regions with contrasting climates (Škvorc et al. 2005; Temunović et al. 2012; Poljak et al. 2015; Zebec et al. 2016).
We found that the divergence among populations tended to follow an altitudinal cline along which populations from lower altitudes had larger and wider leaves than populations from high altitudes. In general, it is well known that leaf morphological traits, such as leaf length and width, are negatively correlated with altitude (Körner et al. 1986, 1989; Hovenden and Vander Schoor 2003; Bresson et al. 2011; Paridari et al. 2013). However, many environmental features accompany altitudinal changes to which plant populations adapt: those physically tied to meters above sea level, such as atmospheric pressure, temperature and clear-sky turbidity; and those that are not generally altitude specific, such as moisture, hours of sunshine, wind, season length, geology and even human land use (Körner 2007). In fact,