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across woodland stands of different kinds (Hewson et al. 2011). Kirin et al. (2011) also concluded that for habitat selection of forest birds on the larger spatial scale both floristic and structural composition are important.
In Croatia, studies of bird communities in lowland pedunculate oak, hillside and mountain forests have been conducted (Kralj 2000, Kirin et al. 2011). Among lowland forest associations beside dominant pedunculate oak, there are still preserved alder, poplar and ash stands near river Drava belonging to the Subpannonian and Pannonian vegetational zone (Trinajstić 1998). Hole-nesting birds play a very important role in all forest ecosystems due to their specific nesting preferences and habitat selection (see Pakkalaa et al. 2018, Dolenec et al. 2005, Dolenec 2006, Kralj et al. 2009). According to recent literature, woodpeckers (Ćiković et al. 2006, Ćiković et al. 2014) and flycatchers (Kralj et al. 2009) are well investigated hole-nesters in Croatia.
Data on habitat selection in birds can be a valuable tool in forest management. Research by Schulze et al. (2019) showed that changes in forest management can considerably affect bird biodiversity, especially forest bird specialists. Therefore, we need to significantly improve our understanding of how landscape perspective fosters a multiscale approach to landscape management and landscape/conservation planning. This knowledge is especially important for large lowland riverine forests; areas which have been impacted by humans for centuries (Prpić and Milković 2005). De Zan et al. (2016) showed that the structure of the beech forest fragments, i.e. density and abundance of large trees and the diversity of fallen large branches and standing dead trees are important variables that influence positively the presence and abundance of hole-nesting birds. Furthermore, for secondary hole-nesting birds the diversity of dead wood is an essential resource for nesting, and for some species also for foraging. Hence, bird diversity can be used as a decision support tool for the application of sustainability principles in landscape management (Kubalikova et al. 2019).
This study includes secondary hole-nesting birds in riverine forest stands near river Drava in Croatia. The aim was to determine the correlation between secondary hole-nesters community characteristics and floristic and structural characteristics of their habitat. We propose these hypotheses: the number of secondary hole-nesting bird species as well as their abundance is correlated with structural habitat characteristics and older forest stands show greater bird biodiversity and abundance.
The study area is situated in the North Croatia covering riverine forests along the Drava river. In total, five study sites (Figure 1) were covered in four years research: riverine forests north of city Varaždin (16°19’N 46°38’E – 16°33’N 46°34’E, Fig 1-locality 1), Pažut forest quite near mouth of the river Mura in the river Drava (16°82’N 46°31’E - 16°88’N 46°30’E, Fig 1- locality 2), Repaš forest north of the river Drava (17°06’N 46°18’E - 17°17’N 46°13’E, Fig 1- locality 3), Repaš forest south of the river Drava in (16°99’N 46°15’E - 17°04’N 46°13’E, Fig 1- locality 4), and riverine forest near city of Đurđevac (17°11’N 46°1’E - 17°25’N 46°00’E, Fig 1- locality 5). Study was conducted in state forests in even aged stands.
Continental Croatia has a temperate continental climate and throughout the year it is in a circulation zone of mid-latitudes, where the atmospheric conditions are very variable. According to the Thornthwaite climate classification in the largest part of lowland, Continental Croatia prevails a humid climate with mean annual temperature in the study area of 10-11°C and mean annual precipitation between 700 and 1000 mm (Zaninović et al. 2008).
From the vegetation point of view, the study area is situated in the Eurosiberian – North American region and belongs to two vegetation zones: Mid-European and Subpanonian (Trinajstić 1998). Based on the majority of the cited authors, the systematisation of floodplain forests is presented in three classes, three orders and four alliances. Eleven associations have been described. A thicket of purple willow Salicetum purpureae Wendelberger-Zelenika 1952 forms a marginal forest community towards swampy phytocoenoses. The forest of almond willow Salicetum triandrae Malcuit 1929 grows on the lowest positions of the sandbanks, islands and marshes in the rivers Drava and Danube. The forest of white willow Salicetum albae Issler 1926 covers depressions on alluvial calcareous, undeveloped soils in the interior of the marshes. The forest of white willow and black poplar Salici albae-populetum nigrae Tüxen 1931 is the most widely distributed phytocoenosis, taking up central positions of the Danube islands and banks, as well as central positions in Podravina. The forest of white and black poplar Populetum nigro-albae Slavnić 1952 occupies higher positions of the Danube islands and shores, as well as high positions of the river Drava terraces. The forest of black alder with elongated sedge Carici elongatae-Alnetum glutinosae W. Koch 1926 is a distinctly relict community in the Croatian Podravina region growing on peat and base-rich humus gleyic soils saturated with water. The forest of black alder with alder buckthorn Frangulo-Alnetum glutinosae Rauš 1968 where it inhabits suitable sites of smaller, mosaic-like areas. The forest of narrow-leaved ash with autumn snowflake Leucoio-Fraxinetum angustifoliae Glavač 1959 is distributed over clayey alluvial terrains. The forest of black alder and narrow-leaved ash with European birdcherry Pruno-Fraxinetum Oberdorfer 1953 is found in Podravina near Đurđevac and in small areas around Varaždin, and