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ŠUMARSKI LIST 7-8/2018 str. 52     <-- 52 -->        PDF

provenances were introduced in Croatia. Fraxinus americana L. has a lower share than F. pennsylvanica Marshall and was introduced in the 19th century (Kremer 2006).
Croatian forests Ltd. and private forest owners should use only those NNTS, provenances and varieties, whose impact on the ecosystem, genetic integrity of native tree species and local provenances is professionally evaluated and whose negative impact can be avoided or diminished (Ministry of Agriculture, 2005). NNTS could have a positive impact on rural areas, which is one of the basic targets of Croatian Development Strategy (Governement of the Republic of Croatia 2017).
3. 1. Increased wood production and non-wood forest products and possibilities to increase the income from forestry sector – Povećanje proizvodnje drveta i ostalih šumskih proizvoda te mogućnosti povećanja dobiti iz šumarskog sektora
Demands on forestry products increase continuously and existing access to natural wood and timber resources may switch to exploitation and overstraining of forest capacities underlying permanent pressures by man-induced disturbances (Bengtsson et al. 2000). If recovery of those habitats is disordered profoundly, heavy damages such as degradation of land can follow. To stop degradation process, fast establishment of efficient forest ecosystems is a key management aim. To prepare forests for future purposes man-made impairments have to be compensated by artificial restoration of tree populations, which means reforestation with native or non-native tree species. Increase of productivity can provide higher income from forests, but this aspect should carefully be taken into consideration for each NNTS since some use can require additional costs.
3.1.1. Enhanced wood production and quality/diversity of products – Povećanje proizvodnje drveta i kvalitete i raznovrstnosti drvnih proizvoda
In many cases NNTS have higher growth rates and are more easily managed than native tree species. Based on growth and yield (CFRI permanent research plots) and on the general vitality of NNTS explored in European beech-dominated sites introduction of eastern white pine, Douglas-fir and experimentally, grand fir and Pacific redcedar, have initially been recommended in Croatia (Dokuš et al. 1975, Dokuš and Gračan 1977, Dokuš 1981). In lowland area (Slatki potok locality) the most productive species are eastern white pine (478.72 m3/ha) and Douglas-fir (163.83 m3/ha) (Orlić et al. 1997, Orlić et al. 1985). Three decades after the establishment of experimental trials in different bioclimatic regions of Croatia (lowland and hilly areas), introduced Douglas-fir and eastern white pine displayed more intensive growth and better vitality compared with other NNTS (Orlić 1994, Orlić and Ocvirek 1993, 1995, Komlenović et al. 1995). Out of those NNTS, better results and vitality were demonstrated by Abies grandis Lindl. and Thuja plicata L. European larch and eastern white pine were the best height and diameter growers while Douglas-fir was the slowest grower. In year 23 after the trial establishment the most productive species are eastern white pine (average growing stock 478.72 m3/ha), European larch (208.89 m3/ha) and Douglas-fir (163.83 m3/ha). Based of research on afforestation of fernery and heath areas, species to use under these conditions should be primarily Norway spruce and eastern white pine (Orlić and Ocvirek 1993, 1995, 1996). Douglas-fir is recommended on potentially most productive sites, and possibly mixed with Norway spruce or eastern white pine. The follow-up of early research had proved these species to be good silvicultural options both in the lowlands, hilly and coastal parts of the country (Perić et al. 2006b, 2009). Provenances of these species can greatly influence their growth and development in explored site conditions so strict recommendations are provided in order to select appropriate provenances and harmonise nursery production with requirements of practical afforestation (Perić et al. 2010).
According to NFI data, Robinia pseudoacacia L. also proved to be an appropriate choice for enhancement of productivity compared with native tree species. For example, the species has an annual increment of 8.13 m3/ha in the lowland area (growing stock 3,079,000 m3), 9.29 m3/ha in hilly area (9,660,000 m3), and 1.65 m3/ha in Submediterranen-Epimediterranen area (7,540,000 m3) (NFI 2010). From its first introduction to date this NNT species proved to be a significant component of Croatian forests, especially in terms of high productivity.
3.1.2. Increased production of high quality timber in short–time periods – Povećanje proizvodnje visoko kvalitetnog drveta u kraćim ophodnjama
Sitka spruce has been a fast and productive grower under quite short rotations in some European countries. Nevertheless, Croatian research results in several trial plots, established in different bioclimatic regions show the low productivity of this NNTS (Dokuš 1972b, 1981, Dokuš et al. 1972, 1975, Dokuš and Gračan 1977, Orlić and Perić 2001, Orlić 1998, 1994, Orlić and Ocvirek 1995, Perić et al. 2006a). Consequently, Sitka spruce should be avoided under Croatian conditions. A good choice for producing high quality timber is black walnut, whose wood reaches 106–800 EUR/m3 on Croatian market. The rotation age of black walnut is