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White and black alder are our indigenous noble broad-leaved species. In Croatian forestry, black alder has an important place, while the white alder does not have economic importance and it is becoming increasingly neglected. White alder appears on shingle, active material in mainstream, on river banks and terraces which are periodically or at least sporadically flooded. It very rarely occurs on its own on drifts and it does not appear in swamp habitats which are characteristics of black alder. The farther the area is from the mainstream watercourse, the higher is the probability of black alder supervention. Finally, in swampy, peaty areas, black alder forms its stands.
Stands of white and black alder are relatively clearly distinguished in the researched area, although somewhere is common their joint supervention on the same stands. In the areas where white and black alder habitats are overlapped, their spontaneous hybridisation is possible. White and black alder crosses have been so far noticed only in NW Croatia.
Variability of a species is one of the most important preconditions for its adaptive potential in variable environment conditions, and in a long term, for survival of the species. That is, variability insures adaptability of populations towards environment changes through generations. The optimal variability of white and black alder in the area of riparian forest ecosystems is disturbed by negative anthropogenic activities.
This paper researches the intra-population and inter-population variability of white and black alder in five natural populations along rivers Drava and Mura on the basis of morphological leaf traits.
Material for the morphometric analysis was collected in five natural populations in the areas of Podravina and Međimurje, along rivers Drava and Mura (Figure 1). Three populations of white alder and two populations of black alder were included in the research. Each population was represented by 20 trees and each tree by 20 healthy and undamaged leaves, collected from short fertile shoots of the outer, light-exposed part of tree top. The leaves were scanned and measured by the WinFolia programme. Ten foliar traits were defined and measured altogether (Figure 2). From the measured traits, the following ratios were derived: MPW/BL, PW1/BL, PMPW/BL, PL/BL.
The measured morphological traits were shown through descriptive statistical parameters. For determining the intra-population and inter-population variability, the univariate analysis of variance was used. For determining similarities or differences of analyzed populations on the basis of measured morphological leaf traits, multivariate statistical methods were used – cluster and discriminant analysis. These statistical analyses were conducted using the statistical programme STATISTICA 8.0.
The results of the descriptive statistical analysis are presented in Table 1, by population. For both species, the trees within populations differ significantly on all analysed traits (Tables 2 and 3). Populations of white alder do not differ significantly between themselves on any trait, while the differentiation of populations of black alder is at significant level 0,01 inherent for variables BL, PMPW, LA1 and LA2. For each species individually, smaller variability among populations has been determined, while the remaining component regarding variability of the leaves within the tree takes up the largest proportion of the total variance (Tables 4 and 5).
It can be seen from the dendrogram that the inter-population variability of black alder is greater than the inter-population variability of white alder (Figure 3).
In order to determine which traits best discriminate researched populations and to additionally clarify the trend of their differentiation, discriminant analysis was conducted (Tables 6 and 7, Figure 4). For the populations of white alder, it was determined that canonical projections of trees within populations are weakly connected, in the sense that the populations are almost completely overlapping. The weak differentiation between populations and great variability within populations can be explained by similar ecological conditions of their supervention and by intensive flow of genes between populations. Although the canonical value projections for individual trees are partly overlapping, the trend of differentiation between populations of black alder is clearly perceived. Considering that the researched species belong to different river and forest systems, we can assume that the obtained differences in variability are caused by hydrological and ecological influences.
Although the trees of white and black alder clearly differ between themselves, it can be seen that individual trees from white alder population Varaždinsko jezero cross to the right side of the diagram (Figure 4). Considering that the mentioned trees are intermediary in relation to researched species, we can conclude that it is the case of hybrid individuals. Through subsequent analysis, intermediary traits on most researched variables were identified for the crosses (Table 8).
Genetic diversity is one of the basic preconditions for adaptability of a certain species to particular habitat conditions. Through this research, the knowledge of the intra-population and inter-population variability of white and black alder on Podravina – Međimurje area was gained. This knowledge represents the basis for further research that needs to be conducted in order to obtain guidelines for improvement and preservation of genetic resources of the alder species in Croatia.
Key words: white alder, black alder, variability of leaves, hybrids, Mura, Drava