Genetic resources of Norway spruce (Picea abies Karst.) in Slovenia

Gregor Božič¹, Marjan Kotar², Marjanca Pavle¹, Igor Smolej¹, Sašo Žitnik¹ & Hojka Kraigher¹

1Slovenian Forestry Institute, Večna pot 2, Ljubljana, Slovenia
²Forestry Department, Biotechnical Faculty, University of Ljubljana, Večna pot 83, Ljubljana, Slovenia

Due to the highly diverse ecological conditions also the forest sites and their constituent tree species are characterised by a high diversity. The traditionally co-naturally oriented forestry management for sustainable and multifunctional use has resulted in that the species composition in 87% of the Slovenian forests is equal to or very similar to the potential one. Only in 9 % of all forests the species composition has been significantly changed, and in 4 % the species composition is completely different from the natural one (Grecs / Kraigher 1997). This may be mainly attributed to the planting of Norway spruce in the past.

Norway spruce has occurred at the territory of Slovenia relatively early as an euro-siberian eastern floral element. Palinological studies have shown that in quartar it was spread more or less over the whole territory, except for the glacial climatic periods (šercelj 1981). A well grown spruce tree from the periglacial sediments from Anhovo in the Soča valley can be dated back to 18.790 ± 300 years (šercelj 1996). Norway spruce had reached its holocen culmination in the boreal and the atlantic especially in the high montane regions. Today, it grows naturally only on ecologically suitable sites, i.e. on locally colder and edaphically more humid sites (zupančič 1980).

Its anthropogenically based distribution in Slovenia shows impacts of the middle European school, supporting high spruce timber production. Therefore its current distribution and percentage in the growing stock in Slovenia is much higher, 35 %, as to what it would be naturally according to spruce sites areas, i.e. 8 % (Table 2).

Norway spruce occurs indigenously in most parts of Slovenia, except in submediterranean and subpannonic region. It usually accompanies beech and silver fir, but can grow as a dominant species in high montane and subalpine regions from the Julian Alps to the Pohorje mountain range, while in the Dinarics it occurrs in groups on extreme cold sites, otherways its distribution is as individual trees.

Forest management through the past centuries has influenced spruce occurrence in Slovenia. In places where coal-mines, iron-works and glass-makers were important, natural beech or silver fir and beech forests have been managed into predominantly or pure spruce stands (i.e. on Jelovica, Pokljuka, Pohorje). This led to the impoverishment of biodiversity, especially in the biological component of forest soils. Furthermore, the biological instability was supported also by application of planting material using seeds and seedlings of uncontrolled provenances, unsuitable to the site conditions (PERKO / POGAČNIK 1996).

From the 1990 Forest inventory it can be concluded, that in Slovenia, from forest sites with Norway spruce as individual or dominant species, 67% are well preserved regarding their indigenous species composition, 24% have a changed species composition, 6% significantly changed and 3% completely changed species composition. Spruce grows in 77% of its area on limestone ground rock material, 9% on mixed and 14% on non-carbonate ground rock materials. In the total growing stock the proportion of spruce increases with higher altitude and is highest at 1201 to 1600 m/above sea level. This is also the zone of most of the natural spruce sites. The percentage of spruce in the total growing stock in this zone is 70% (Bončina / Mikulič 1998).

Site productivity and growth characteristics of Norway spruce in Slovenia have been studied on sites where the natural vegetation is classified as spruce and fir-beech plant communities, overgrown by Norway spruce stands. Site productivity has been determined on the basis of the total volume production of stands which have a natural basal area and by means of a site index (SI) in combination with yield tables. Site index is defined as a top height at an age of 100 years.

The mean age of the analysed stands ranges from 103 to 186 years. The analysed stands were pure, fully stocked and mostly unmanaged. The effect of thinning and other human activities were generally absent, with the exception of a small number of stands. The investigations were designed as a factorial experiment with 5 replications where the factor site unit consisted of 9 levels. The sample plots (size: 30 x 30 m) were randomly located. In every plot all trees were measured (height, DBH) and cores were taken with an increment borer for the period of the previous 30 years. The top height trees were felled and cut into 6 to 9 sections, from which a disc was taken for stem growth analysis. On every plot the growth of felled or dead trees was reconstructed with the help of a stump inventory. The volume and increment of the removed trees was determined for the last 30 years. The volume of the trees which had been removed more than 30 years prior to the research was estimated by means of yield tables. The sum of growing stock (Vol.₀) and the volume of removed trees in the last 30 years (Vol._0-30) and the volume of trees which had been removed more then 30 year prior to the start of the research (Vol._30-a) represents the total volume production (TVP). The mean annual increment (MAI) was obtained by dividing TVP by the mean age of the stand. The value of MAI at the time of its highest value is the estimate of the site productivity because the analysed stands were fully stocked and had a natural basal area. The general data about the analysed sites and stands are given in Table 3.

Table 3: General characteristics of sample plots and analysed stands