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Introduction of non-native tree species (NNTS) to Europe dates back to the 17th and 18th century with the on-going industrialization of Europe, when new ideas for settling enormous demands for wood products had to be resolved. The management of tree species non-native to Republic of Croatia has a certain tradition within Croatian forest management practice; nowadays, their potential should be regarded and re-evaluated in the new frame of climate changes and growing society demands for forest products and services. Today, issues of adaptation to and mitigation of climate changes as well as the question whether these tree species could increase the adaptive capacity of forests to long-term climate change patterns have initiated a new and growing interest in NNTS. Thus, the variety of examples and ecological conditions opened the need for gaining insight into possibilities and complex management issues of NNTS in Croatia. Croatian Forest Research Institute (CFRI) initiated research activities on optimal NNTS for afforestation and plantation establishment during 1960’s. Numerous trials set in different bioclimatic regions of the country served as permanent trials for the comparison of growth success of native vs. NNTS (Perić et al. 2006a). Nevertheless, no comprehensive overview on possibilities of the use of NNTS has been compiled so far.
Even though the definition of term NNTS is still not harmonised globally (e.g. Convention on Biological Diversity, International Union for Conservation of Nature, United Nations Environment Programme World Conservation Monitoring Centre), all existing definitions consider NNTS as present species, sub-species or lower taxon, introduced (i.e. by human action) outside its natural past or present distribution range. Despite the fact that each country (including Croatia) has different lists of NNTS, the members of COST Action FP1403 Non-native tree species for European forests - experiences, risks and opportunities (NNEXT) decided to focus on tree species with natural past or present distribution range outside of the European continent as a whole in a geographical sense. Discussions on issues concerning NNTS introduction and use in Croatia were initiated during the Training school (23rd–27th August 2016) organized in the frame of NNEXT. The school aimed at exploring chances, risks and challenges, which managers should overcome if they target the active use of those tree species (NNEXT 2016).
This paper is a contribution to NNTS introduction and use in Croatia, aiming at providing an overview of studies and examples of possible management opportunities. It tackles the question of why to introduce and use NNTS to Europe in general and specifically in the Republic of Croatia? Investigating their possible role in forest landscape restoration (FLR), which is the most prominent activity when it comes to mitigation of human influences on natural ecosystems (Stanturf et al. 2015) is especially promising, highlighting their role in preparing the site for native tree species. Despite all opportunities, possible threats, management risks and challenges derived from the use of some NNTS should be considered as a first step in decision making. Nevertheless, there are only a limited number of available scientific publications dealing with these issues, which strongly underlies the need for further research (Celesti-Grapow et al. 2009). Because of the complexity of the matter and growing need to further investigate risks of NNTS in Croatia, authors have continued their research and will publish the results in a separate paper.
Strong, nature-oriented forestry, left only little room for the use of NNTS in Croatia. To date, growing NNTS is mostly associated with scientific research, while only a small area of NNTS is used for practical purposes. Table 1 shows an overview of the most important NNTS in Croatia based on data from National Forest Management Plan (NFMP - ”Croatian Forests”, Ltd 2016 - 2025) and National Forest Inventory (NFI - Ministry of Agriculture 2010). Date of introduction of NNTS were compiled from forest chronicles. NFI does not provide CAI or MAI, but only growing stock at the time of measurement (2005-2009), and only for NNTS, which were included into the Inventory (some of them are possibly present, but in too small amount to be detected with this method). In NFI it is clearly stated that a small share of NNTS (in growing stock) is present in Croatia (< 4% in total, < 1% in private forests). Data from NFMP (2016 - 2025) shows that all listed NNTS are present in protected forest areas as well, but only area occupied by Robinia pseudoacacia (1.74%) is detected with this methodology. This is the reason why we present data on area occupied by NNTS by individual Forest management plans. The analysis reveals that the NNT species most used in forestry practice are Robinia pseudoacacia L. (12003.88 ha), followed by Pinus strobus L. (3218.69 ha), Juglans nigra L. (2376.66 ha), Fraxinus pennsylvanica Marshall. and Fraxinus americana L. (1099.77 ha), Pseudotsuga menziesii (Mirb.) Franco. (393,05 ha) and Quercus rubra L. (51,99 ha). A difference in data between NFI and NFMP comes from different methodologies of data sampling. Two non-native Fraxinus species are present in Croatia (Borzan et al. 2006). On the area of Pokuplje, Posavina and Podunavlje Kremer and Borzan (2009) found Fraxinus pennsylvanica Marshall and F. pennsylvanica var. subintegerrima (Vahl) Fernald, stating that several