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ŠUMARSKI LIST 3-4/2021 str. 42     <-- 42 -->        PDF

Narrow-leaved ash (Fraxinus angustifolia Vahl), distributed in central-southern Europe and northwest Africa, is an ecologically and economically important species in riparian and floodplain forests because of its fast growth and specific adjustments to the habitat, especially in central Europe, the Pannonian Basin and the Balkans (Temunović et al. 2012, Caudullo and Durrant 2016). It is currently suffering severe dieback in its distribution area (Gross et al. 2014), being one of the most endangered forest tree species in Croatia, mostly due to a pathogenic fungus Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz & Hosoya (Diminić 2015, Milotić et al. 2016). Yield and health status of narrow-leaved ash seeds are thus of great significance for seedling production and consecutive afforestation or regeneration of damaged forest stands. Previous studies showed that freshly collected seeds have a very high viability even after two years of storage (87%) and are of relatively good health status regarding fungal presence after two months of storage (Drvodelić and Oršanić 2016, Kranjec Orlović et al. 2019). However, in the last six years there is a concerning decrease in seed production of the narrow-leaved ash in Croatia in comparison to amounts collected 10-15 years ago, with yearly yields varying between 148 and 4.844 kg (Vincenc 2015, 2016, 2017, 2018, 2019), which emphasises the importance of good health status of these smaller amounts of collected seeds.
Another ash (Fraxinus spp.) species, Fraxinus pennsylvanica Marshall or green ash, is very common in Croatian lowland forests. Originally a North American species, it was introduced to Europe in the 19th century mostly for ornamental purposes and timber production (Schmiedel et al. 2013), and since then it has successfully spread outside designated planting areas, earning the status of an invasive species in some countries (Prots et al. 2011, Drescher and Prots 2016). In Croatia it was first planted over more than 130 years ago with the objective of improving and preparing very wet and difficult habitats for the native narrow-leaved ash. Although usually replaced with the native species after successfully fulfilling the ameliorative role, it has spontaneously spread in numerous locations and today there are individual trees, groups or even whole stands still widely present in Croatian lowland forests, taking a significant place in these ecosystems (Kremer and Čavlović 2005). Additionally, it has been observed that in both natural stands and urban areas, green ash yields more seed in comparison to the native narrow-leaved ash (unpublished data).
One of the most important biotic factors causing seed deterioration are insects, especially during the pre-dispersal phase of seed development (Janzen 1971, Turgeon et al. 1994, Hulme and Benkman 2002), as their attacks can lead to fruit abortion (Sallabanks and Courtney 1992), facilitated introduction of microbes and pathogens (Battisti et al. 1999, Luchi et al. 2011) or to reduction of the amount of seeds available for afforestation and natural regeneration of tree species (Turgeon et al. 1994, Boivin and Auger-Rozenberg 2016). Most common and known insects found in ash seeds are weevils belonging to genus Lignyodes (Coleoptera: Curculionidae) (Clark 1980). There are three known species that can be found in Europe: L. enucleator (Panzer, 1798) and L. suturatus (Fairmaire, 1860), which are native, and L. bischoffi (Blatchley, 1916), an invasive species which came from the North America with green ash seeds (Dieckmann 1970, Caldara 2013). Amongst the entomofauna in seeds, hymenopteran parasitoids can also be found, as a natural way of reducing the number of seed-eating insects. Most commonly, larvae of beetles are parasitized by several wasp species that oviposit their eggs in the seeds from the seedpod surface (Nakai et al. 2011).
Generally, most studies focusing on entomofauna in the ash seeds in Europe and their impact on seed health status were conducted on common ash (Fraxinus excelsior L.) (e.g. Gardner 1977, Tapper 1992, Hayatgheibi 2013, Gosik et al. 2017), and there is little or no information about insect presence in seeds of other ash species. Therefore, this research had two main objectives: (1) to explore entomofauna of narrow-leaved ash and green ash seeds in order to gain insight into the insect species present in the most common native and introduced ash species in Croatia for future comparisons; (2) to show the frequency in which insects appear in seeds, as this could be an important factor affecting ash seed health status.
Narrow-leaved ash seeds were collected in the period from August to November 2017 from visually healthy trees in natural or specially managed forest stands registered as seed sources or seed stands (five different locations). Additional seed sampling was conducted from August to September 2018 in natural forest stands registered as the seed source HR-FAN-SI-121/305. Green ash seeds were collected in November 2018 from two different locations, one in an urban area and another in a natural forest stand (Figure 1).
One hundred seeds from each location were separated from samaras and examined for presence of insects under the stereo microscope (SMZ 168-TLED, Motic, Hong Kong, China). The seeds were categorized as undamaged (without any sign of insect presence), damaged (with presence of different life stages of insects or their exit holes), or empty (partially developed but with no endosperm). Collected insect specimens were analysed and photographed under the stereo microscope and grouped according to their