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et al. 2011), thus there is numerous paleoecological evidence that its share has decreased substantially throughout the course of human history (Wick and Möhl 2006, Feurdean and Willis 2008). Fir is also extremely susceptible to polluted air, especially SOX emissions, which resulted in an especially pronounced decline in the second half of the 20th century (Elling et al. 2009, Diaci et al. 2011). However, there are also local exceptions to the general trend of fir regression. For example, in the period from the mid-19th to the mid-20th century, the share of fir in the Dinaric Mountains increased due to its systematic promotion by silviculture and the extinction of large ungulates (Matić 1973, Klopcic et al. 2010). Another example of localized fir expansion, which is less documented, is its role in secondary succession on abandoned agricultural lands (Mlinšek 1968, Dolezal et al. 2004, Bartolome et al. 2008). In this study a combination of two rare characteristics for fir occurrence is presented, namely a special habitat (low elevation, south open valley, and carbonate parent material) and its relatively rare role as a pioneer in secondary succession.
The main objectives of this study were to (1) analyze the forest reserve history and verify if the stand originated from secondary succession, (2) examine the structure and development of the stand and determine the role of fir, and (3) compare the composition of ground vegetation in the reserve and indicated ecological factors with other fir associations in Slovenia.
Materials and methods
Materijali i metode
Stand and site characteristics – Karakteristike staništa i sastojine
The Lipje forest reserve is situated in Poljanska dolina in the foothills of Poljanska gora (45° 30’ 54" N, 15° 04’ 52" E). It belongs to the Poljanska dolina forest management unit, Section 103c, and covers 2.43 hectares (Figure 1). The elevation of the reserve ranges from 360 to 380 m a.s.l. The parent material is composed of limestone. Karstic phenomena, such as sinkholes and rocky outcrops, are common. The reserve lies on a flat plateau with a slight slope inclination of 5 %. Stoniness is about 10 %. The soil type is calcocambisol of variable depth. The nearest meteorological station is Kočevje (463 m a.s.l.), and the station for precipitation measurements is located in Predgrad (375 m n.m.v.). The long-term mean yearly temperature in Kočevje (1960–90) was 8.3 °C and mean July temperature was 17.8 °C. Due to the lower elevation of the research site and its openness towards the influence of the sub-Pannonian climate, the estimated mean yearly temperature was about 1 °C higher. Long-term average rainfall for the station in Predgrad was 1438 mm year–1.
The entire forest reserve was protected in 1976 in an area of 5.0 ha. After denationalization a part of the reserve was returned to the village community. In the first management plan (Anon., 1961), a pure silver fir stand with an admixture of Norway spruce and broadleaves, especially hornbeam, was reported. It was also mentioned that this forest very likely developed very likely on a former pastu­re. Growing stock was estimated at 106 m³/ha, and the ratio of coniferous and deciduous trees was 96 % versus 5 %, respectively. Selection felling was prescribed as a way of management. In subsequent forest management plans (Anon., 2010), the section was referred to as a forest reserve, but only the last management plan provided a reliable assessment of growing stock (819.8 m³/ha, Bitterlich relascope method). In addition to sheep grazing on pastures in the vicinity of the reserve, there was also local production of lime, for which substantial quantities of firewood was need­ed. For this study, the forest site has been provisionally identified as a secondary association Asperulo-Carpinetum betuli M. Wraber 1969 var. Abies alba nom. prov. (Mlinšek et al. 1980) which occurs on potential beech or hornbeam forest sites as pioneer vegetation on former pastures and mostly sunny slopes in the colline and submon­ta­ne vegetation belt. Due to its small size, the reserve is under the influence of management in adjacent stands. However, neighboring stands have only recently been excluded from the reserve. The entire forest area is also characterized by a low impact selection management regime. During the course of research we did not find signs of logging or other human intervention in the research area.
Recordings and analyses – Prikupljanje i obrada podataka
In 2010 we carried out a full inventory of all trees with dbh > 5cm in the reserve. We also measured the height of 30 fir and 30 spruce trees on a transect through the reserve. Additionally, a permanent 125 x 80 m research plot, oriented in the N-S direction, was set up in the reserve core. Species, dbh, and tree coordinates were assessed. Within the larger research plot, a N-S aligned grid (4 x 4 m) was laid out with a total of 26 permanent regeneration plots on the grid intersections. The size of the plots was 1.5 x 1.5 m. There were no distinct gaps in the forest reserve; therefore, all plots were under a dense, closed canopy. All plots were permanently marked with iron stakes in order to be located with a metal detector at the next inventory. We recorded species composition and percent cover of all vascular plants on the plots. Plant cover was estimated visually from above, to the nearest 10 % from 10 to 100 %, and to the nearest 1 % from 1 to 5 %, excluding mosses. Regeneration density was recorded per species in two height classes: small seedlings ≤ 20 cm (excluding one-year-old seedlings) and seedlings (20 cm < h ≤ 130 cm). All woody plants were scored for browsing ¸