DIGITALNA ARHIVA ŠUMARSKOG LISTA
prilagođeno pretraživanje po punom tekstu



ŠUMARSKI LIST 1-2/2018 str. 30     <-- 30 -->        PDF

our results showed that the leaf measured characteristics that varied in the grey alder populations were significantly related to climate features. Populations of the grey alder from high altitudes in the mountainous Alpine-Dinaric region occupy habitats with lower temperatures accompanied by high irradiance and larger temperature oscillations. On the other hand, low altitude populations from the continental region are exposed to higher temperatures, lower radiation and higher precipitation variability. In addition, growing season tends to increase toward warmer low-elevation sites. Furthermore, common-garden experiments have shown that populations from high elevations have lower growth rates than populations from high altitudes (Vitasse et al. 2009, 2013; Montesinos-Navarro et al. 2011). Similar trends were observed in our study (authors’ personal observations). This pattern was likely due to differential selection pressure among the biogeographical regions, i.e. populations from low altitudes tend to be adapted to the warmer climate under which selection has favoured a high allocation to growth and competitive ability, whereas populations from high altitudes display lower growth rates and greater cold-tolerance (Körner 2003).
Surprisingly, low- vs high-elevation populations significantly increased leaf area at decreasing annual precipitation and precipitation of the warmest and coldest quarter. Similar results were also obtained by Meier and Leuschner (2008) for Fagus sylvatica stands in central Germany. Authors concluded that stand leaf area of the beech along this precipitation gradient is not a simple function of water availability, but is controlled by several abiotic factors including spring temperature and possibly also nitrogen supply, which both tend to increase toward drier sites, thus overlaying any negative effect of water shortage on leaf development. Since the grey alder is a hygro-mesophilous species which mostly occupied riversides, we assume that differences in precipitation among biogeographical regions did not influence population divergence. Similar conclusions were reported by Krauze-Michalska and Boratyńska (2013). Nevertheless, our results revealed that leaves of grey alder populations decrease in almost all measured leaf traits with increasing distance-to-water. This variation trend can be explained by the specific differences in the geomorphology of the terrain among biogeographical regions. The majority of grey alder populations in the Alpine-Dinaric region inhabit a drier riparian part, on elongated terraces and wet slopes along the course of the river Kupa and its tributaries, which are generally 2 to 6 m above the average water level. By contrast, continental populations of the grey alder, along the river Drava, grow along an active watercourse with intensive shifting of pebbly, and sometimes sandy sediments, on which aerated and humus soils are formed. The presence of different species in these stands also reflects different biogeographical characteristics (Trinajstić 1964; Franjić et al. 1999; Vukelić et al. 2012, 2017).
In general, tree species have high levels of morphological variability (Poljak et al. 2012; McKown et al. 2014; Zebec et al. 2015), and phenotypic differences among populations are often a result of the environmental distances between populations. Nevertheless, our results suggest that phenotypic divergence of the studied grey alder populations is the result of a significant level of isolation both by distance and by the environment. The isolation by distance assumes that phenotypic differentiation between populations increases with increasing geographic distance because of limited pollen and seed dispersal (Wright 1943, 1946; Orsini et al. 2013). On the other hand, gene flow among ecologically divergent habitats is reduced because of reduced success of the establishment of immigrants from different environments, as a result of local genetic adaptation (Nosil and Crespi 2004; Noisl et al. 2005, 2008, 2009; Orsini et al. 2013; DeWoody et al. 2015). However, we cannot exclude the possibility that adaptive differentiation and historical migration processes acted in combination to produce the observed pattern of morphological variation in the studied populations (Temunović et al. 2012; DeWoody et al. 2015). Although the main refugium during the last glacial period was located in Central Europe (Huntley and Birks 1983; Douda et al. 2014; Mandák et al. 2016), A. incana probably survived in situ in river canyons and protected valleys along the north-western Dinaric Alps, while north-eastern populations from the continental biogeographical region could represent newly colonised populations from the refugium in Central Europe.
Conclusions
Zaključci
The study revealed the existence of two morphologically and, to a large extent, geographically and environmentally distinct and well-defined groups of grey alder populations in Croatia. These patterns confirm that multiple evolutionary processes influence the morphological diversity and structure of the populations (DeWoody et al. 2015). In ­general, two distinct morphotypes were found: small-leaf in the mountainous Alpine-Dinaric region, and large-leaf in the continental region. These ecologically divergent ­habitats may have led to the general subdivision into two distinct ecotypes. Adaptation to local environmental conditions in the continental region probably resulted in a higher growth rate and large-leaf morphotypes which increase the species’ competitive ability and long-term success, whereas in the Alpine-Dinaric region natural selection has probably favoured greater cold-tolerance (Körner 2003; ­Vitasse et al. 2009). We also believe that our results could be valuable baseline data for the development of more efficient management plans for this boreal tree species.