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among trees and is typical for the stem exclusion (aggradation) phase of the natural forest cycle (see Oliver and Larson 1990; Emborg 1998). In the stand, light in particular had become limiting and thus prevented further establishment of new plants. Top heights indicated an above average site index. The low volume of dead trees in comparison to old-growth forests was in line with similar findings in forest reserves in Europe and indicated past management and succession history (Christensen et al. 2005). Seedling coverage was low due to the dark understory; still, fir seedling densities where high compared to similar studies (Marinšek and Diaci 2011, Diaci and Firm 2011) and indicated great potential for fir regeneration. There was no indication of future colonization of the stand by broadleaves.
Analysis of ground vegetation reflects rather favorable site conditions in the Lipje forest reserve. This confirms the major proportion of species of the Carpino-Fagetea s. lat. classis. Compared to other communities of beech and horn­beam forests, there is somewhat more moderate acidophilic plant species of the Vaccinio-Piceetea classis. This is connected with partial site acidification as a result of litterfall from fir, which completely dominates the tree layer. Higher proportions of phanerophytes and geophytes also indicate potential beech or hornbeam forest habitat. Site conditions indicated by Ellenberg phytoindication values classify the fir forest in the Lipje reserve closer to hornbeam societies, which again shows that it is probably a secondary fir forest resulting from overgrowth. Further successional development of the above is likely to proceed in the direction of deciduous forest.
Our assumptions were confirmed by the DCA analysis, where the fir forest of the Lipje reserve is classified in the middle of the ordination space among the related phytocoenoses. Based on fir’s predominance in the tree layer, which partially modifies acidity and light conditions in the stand, the results give the impression of potential hornbeam or beech habitat. The most similar associations in terms of floristic composition seem to be a secondary association Asperulo-Carpinetum betuli M. Wraber 1969 and hornbeam-fir association (Abio albae-Carpinetum betuli Ma­rin­ček 1994). The first occurs as a degraded stadium following major logging or as the pioneer stadium of the overgrowing of agricultural land in more thermophilic beech forest habitats (Hedero-Fagetum (1962) Košir 1994, Ostryo-Fagetum M. Wraber ex Trinajstić 1972, Hacquetio-Fagetum Košir 1962, Ornithogalo pyrenaici-Fagetum Marinček, Papež, Dakskobler & Zupančič 1990). Also, Dakskobler and Marinšek (2009) report good fir vitality on such sites, where the origin of fir is not known. The second mentioned association, like the Lipje fir forest, is also found in the lowland and hilly belt (160 to 300 m asl.) of the Slovenian Predinaric region, on calcareous rocks or clastic sediments, but within it, fir is represented only sporadically and perhaps only sub-spontaneously (Marinček 2001).
This study analyzed a fir stand growing in an unlikely combination of conditions: it is in secondary succession on a special site (low elevation, limestone parent material). The high vitality of fir and its dominance, with no indication of future colonization by any other tree species, suggests a long-term fir stadium in a secondary succession or even a forest with an enduring fir potential. However, due to the complex interactions between fir and its competitors (e.g. reciprocal replacement) as well as climate change, it is extremely difficult to speak of potential vegetation. At the time of stand establishment, the average growing stock in Slovenian forests was tree times lower than that of today. Additionally, many activities resulting in forest degradation were taking place and have since largely been abandoned. From this point of view, the increase of late successional fir can be understood. On the other hand, there are many reports that fir is being replaced by beech in natural forests (Šafar 1952; Marinšek and Diaci 2011). This study, in showing that fir can locally colonize some new sites during secondary succession, expands the prevailing view that fir decline throughout human history has been due to anthropozoogenic influences. There may be more similar examples since the origin of many pure fir forests has not yet been explained. The overall regression of silver fir calls for additional research that would help unravel the complex interactions between fir and its competitors, environmental factors, and human-induced disturbances. Nevertheless, we can conclude that the presence and vitality of this fir stand at the very edge of its typical distribution raises optimism for fir’s future despite the threat of climate change.
The authors wish to thank Dejan Firm and Jernej Peljhan for their valuable help during field and lab work. We are also grateful to Jan Nagel for his assistance in improving the English of this manuscript. This research was funded by the Slovenian Research Agency, Research Programme P4-0059.
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Anon., 1961: Forest management plan for management unit Poljanska dolina 1962–1971. Forest Enterprise Kocevje (in Slovene).
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