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excelsior L. (Alnion glutinosae Malcuit 1929, Alnion incanae Pawlowski in Pawlowski, Sokolowski & Wallisch 1928, Salicion albae Soó 1930); riparian mixed forests of Quercus robur L., Ulmus laevis Pall. and Ulmus minor Mill., F. excelsior or F. angustifolia Vahl (Ulmenion minoris Oberd. 1953); and on higher elevations Illyrian oak-hornbeam forests (Erythronio-Carpinion Horvat 1938/ Marinček et Wallnöfer et al. 1993), which are otherwise predominantly forests of colline belt (Davies et al., 2004). In all these forests, invasive alien species are present (Marinšek and Kutnar, 2017); however, the Illyrian oak-hornbeam forests are the least studied in this regard. This is why we focus on this forest type in our study.
Because of the long presence of humans in these plains, conversion of Illyrian oak-hornbeam forests into productive agricultural land has resulted in a very reduced, patchy distribution of forest remnants (Čarni et al., 1998). The remaining forest patches vary in size and are under continuous anthropogenic pressures.
What is more, previous studies in similar forest types have shown that among life forms, alien phanerophytes and therophytes are significantly over-represented compared to native plant species in lowland forests (Wagner et al., 2017). Therefore, we selected two very common alien invasive plants in studied forest patches (and Slovenian lowland forests in general): the phanerophyte Prunus serotina Ehr. and the therophyte Impatiens parviflora DC., aiming to assess the drivers behind their occurrence. The second goal of this study was to introduce a model based on forest patch characteristics to explain the presence of both species in a forest patch. We focused on environmental data and particularly on human pressure. Therefore, we included patch metrics in the assessment of occurrence drivers. We hypothesized that by combining environmental characteristics, patch metrics, and the influence of human pressure, we could assess which patch characteristics influence vulnerability to invasion and contribute to the successful spread of P. serotina and I. parviflora into the studied lowland forest remnants.
Study area – Područje istraživanja
The study was conducted in Illyrian oak-hornbeam (Erythronio-Carpinion) forest patches of various configurations on the alluvial plains of the Mura and Drava rivers in the Danube basin (according to EUNIS habitat classification; Davies et al., 2004), with an average altitude between 220 and 250 m a. s. l. The study area is situated in the sub-Pannonian biogeographic region, with a temperate continental climate. Mean annual precipitation and mean annual temperature are approximately 900 mm and 10.9°C, respectively (ARSO, 2021). The dominant trees are Quercus robur, Q. petraea Liebl., Carpinus betulus L., Prunus padus L. and F. excelsior.
The geological bedrock of the study area are silicate gravel deposits. The prevailing soil is a dystric ranker containing around 10% of water. As the rankers dry quickly drought is common phenomenon during hot summers with little precipitation. Soil reaction is acidic, soils are poorly saturated with bases and have low cation exchange capacity (Vovk, 1996; Vidic et al., 2015).
Both riverbeds are regulated, and in the past, extensive oak-hornbeam forests were converted into intensively managed arable land. Only small patches of forest have been preserved, mostly in areas less favourable for agricultural use. The main crops on both plains are corn and various cereals for livestock. On Apaško polje, oilseed rape and oilseed pumpkins are also planted frequently (Ščap, 2018). Land ownership structure is traditionally fragmented; therefore, modest small farm cultivation is predominant. Beside, the settlements are scattered, with the mostly rural inhabitants living in family houses with courtyards behind (Ščap, 2018). On the other hand, settlements on the Dravsko polje are relatively large, dense and located at roadsides (Rebernik, 2011). The majority of inhabitants is concentrated in two major urban centres: Maribor and Ptuj.
Studied species – Istraživane vrste
Prunus serotina is deciduous woody perennial native to North America. The species has been globally naturalized, including South America, South Africa, Oceania, Asia and Europe. In several European countries, P. serotina is widely naturalized and invasive (CABI, 2020a). Even more, the species is recognized as one of the most invasive trees in Europe (Klotz, 2009). Its negative impact on native ecosystems is shown by reductions in forestry production and in the biodiversity of native trees (CABI, 2020a). Prunus serotina is fast growing and tolerant of shaded habitats. In central Europe, P. serotina trees mostly exhibit shrubby growth, rarely reaching 20 m in height, while in its native range it can grow up to 38 m. Its juicy fruits are eaten by various birds and mammals, who disperse the seeds over long distances and thus contribute to the expansion of its range (CABI, 2020a).
Impatiens parviflora is an annual herb native to central Asia. Its invasive range extends in the northern hemisphere to North America, including Canada (CABI, 2020b) and Oregon in the USA (USDA, 2013) and to most of Europe, from the southern to the most northern countries (CABI, 2020b). Florianová and Münzbergová (2017) reported that I. parviflora in the Czech Republic has had a negative impact on native understorey plant richness by reducing the