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ŠUMARSKI LIST 11-12/2016 str. 56     <-- 56 -->        PDF

2010). Droughts in particular would have a positive influence on phloemophages and leaf-chewers, both indirectly through nutritional changes in plants and directly through better survival and/or more generations. It is proven that the increasing frequency and severity of droughts may result in increasing frequency of outbreaks and an increasing area affected by them (Csóka 1996; 1997; Jurc 2007). It is also evident that some earlier neglected native species are becoming increasingly important through the growing incidence of biotic disturbances in European forests (Grégoire and Evans 2004; Hirka and Csóka 2010). In the last years the impact of changing environmental conditions on the latitudinal and altitudinal distribution of some native forest insects has been well documented (Jurc 2007). In the Sub-Mediterranean area of Slovenia, the damage due to leaf-chewers – defoliators (Tortrix viridana Linnaeus, 1758 and Aleimma loeflingiana (Linnaeus, 1758)) has been increasing, and in the forest region of Koper in the period from 1995 to 2005, large scale defoliations were recorded on a total area of 14,374 ha. Starting in 2003, there was an increasing trend in damaged area (Jurc 2007). Many major defoliator species also show a similar trend in Hungary (Hirka et al. 2011; Klapwijk et al. 2013) and Croatia (Matošević et al. 2009).
In line with general trends, we are witnessing the appearance of new defoliator and wood-boring insects on C. australis, which until recently had been considered as a particularly disease and pest resistant species (Kišpatić 1983 in Potočić et al. 1983; Zúbrik et al. 2013). In the last ten years in Slovenia, Croatia and Hungary, a number of new pests affecting European nettle tree have been recorded, some even to the extent of local outbreaks (Jurc 2014, Hrašovec 2009). It may be that the recent trends in albeit unknown Celtis pests are actually related to the results of recent studies of insect population ecology. They can be part of global and multi-year processes of population development of individual insect species about which we know very little (Tenow et al. 2013). Recent research on the population ecology of Operophtera brumata and other early-season geometrids shows that the population ecology of a 9- to 10-year cycle cannot be fully understood on a local scale unless population behavior is known on a larger, European scale (Tenow et al. 2013). Bearing this in mind, the occurrence of harmful defoliators, including those that we are increasingly detecting on C. australis, are projected to prosper in the future due to global warming, and this needs to be taken into account (Tenow et al. 2013).
The results of our contribution are intended to provide additional insight into the question of whether or not this tree species should be introduced on wider scale in plantations of P. nigra affected by pests and diseases. However, large P. nigra plantations in Slovenia are now increasingly threatened by pests and diseases, such as sphaeropsis blight (Diplodia pinea (Desm.) J. Kickx), Cenangium ferruginosum Fr., and Sydowia polyspora (Bref. & Tavel) E. Müll., Dothistroma spp. (Jurc and Jurc 2014; Piškur et al. 2013). Also in Croatia, drought, as a trigger, weakened pines that were subsequently attacked by several species of pathogenic fungi. The largest damages were caused by attacks of the sphaeropsis blight (D. pinea) (Pernek et al. 2012).
The conversion of old P. nigra plantations into ecologically more stable deciduous forests in Craation and Slovenian Mediterranean areas is an important goal (Matić et al. 2011, Gajšek et al. 2015). Some studies indicate that C. australis is an appropriate species for the conversion of P. nigra plantations, it improved ecological conditions, it is appropriate for warmer and dry habitats (Topić 1997, Gajšek et al. 2015). The experiments with planting of C. australis on the sites of the P. nigra plantations in Croatia and Slovenia showed that the survival rates of C. australis in Slovenia (Gajšek et al. 2015) and Croatia (Topić 1997) were almost the same (91% and 92.5%, respectively) after the first year since planting. But planting large-scale monocultures of C. australis on dry and warm sites will likely increase the risk of insect outbreaks in these stands. The high concentration of any food plant might be a major triggering factor in insect outbreaks. In that scenario we might clearly expect that both specialist (L. celtis and P. millierella) and generalist herbivores (N. polychloros, A. xylosteana, E. defoliaria, C. fidella and N. acuminatus and other) would adapt to the more abundant availability of the new host, C. australis, by increasing their population densities to a more damaging level. Some reservations are expressed in this respect, and more careful planning of future afforestation or remediation strategies is encouraged in order to avoid future failures and new problems with C. australis, a tree species believed in general to be a highly resilient one.
We thank Dušan Jurc for valuable suggestions, discussions and photographs, and the reviewers for their helpful input that improved the manuscript.
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