1 INTRODUCTION

Forest cover has increased considerably in Europe over the past two centuries (Agnoletti, 2007; Bičík, Jeleček, & Štěpánek, 2001; Devi et al., 2008; Piussi, 2000). At the same time, a substantial portion of woodland biodiversity has, however, declined severely across the continent (Bergman, 2001; Kopecký, Hédl, & Szabó, 2013; Nieto & Alexander, 2010; van Swaay, Warren, & Loïs, 2006). The decline in forest biodiversity is usually attributed to intensive practices of modern forestry (Agnoletti, 2007; Benes, Cizek, Dovala, & Konvicka, 2006; Brockerhoff, Jactel, Parrotta, Quine, & Sayer, 2008). For that reason, increasingly large areas of forests are protected from intensive or any use in effort to alleviate the decline (Schultze, Gärtner, Bauhus, Meyer, & Reif, 2014; Winter, 2012).

The reason behind the recent decline in woodland biodiversity despite an increase in forest cover and conservation efforts, however, lies rather in an altered structure of forest habitats. Through most of the Holocene, temperate woodlands in Europe had a diverse spatial, species and age structure characterized by a low‐contrast‐border mosaic of patches with various levels of canopy closure from open to closed forests (Alexander, 1998; Rackham, 2003; Vera, 2000), which supported both light‐demanding and shade‐tolerant woodland organisms. The varied canopy closure was primarily maintained by natural factors such as fires, grazing of large herbivores and windthrows (Bengtsson, Nilsson, Franc, & Menozzi, 2000; Bradshaw & Hannon, 2004; Hultberg, Gaillard, Grundmann, & Lindbladh, 2015; Vera, 2000). Later, the natural disturbances were supplemented or gradually replaced by traditional forest management including, for example, pasture grazing, coppicing and burning (Jamrichová et al., 2013; Kirby & Watkins, 2015; Rackham, 2003; Szabó, Müllerová, Suchánková, & Kotačka, 2015). In the past two centuries, however, the remaining natural disturbance factors have largely been suppressed and the traditional management has been replaced by the practices of modern forestry (Bürgi, 1999; Brunet, Felton, & Lindbladh, 2012). This led to a substantial increase in canopy closure and formerly common open woodlands were replaced by closed‐canopy forests in most of the temperate Europe, which led to a decline in light‐demanding woodland species (Bugalho, Caldeira, Pereira, Aronson, & Pausas, 2011; Hartel et al., 2013; Lindbladh et al., 2007; Müllerová, Szabó, & Hédl, 2014; Szabó, 2010). The increased canopy closure affected commercial as well as protected forests across the continent (Bürgi, 1999; Miklín & Čížek, 2014; Saniga, Balanda, Kucbel, & Pittner, 2014).

Biota associated with dead wood (i.e., saproxylic organisms) was generally thought to profit from an increase in forest cover and especially increased amounts of dead wood in protected forests (Vandekerkhove, De Keersmaeker, Menke, Meyer, & Verschelde, 2009; but see the meta‐analysis of Lassauce, Paillet, Jactel, & Bouget, 2011). While some saproxylic organisms are indeed favoured by such changes (e.g., Drag et al., 2015; Horák, Chumanová, & Hilszczański, 2012), others experience a severe decline similar to that of other biota associated with open woodlands such as plants or butterflies (Bergman, 2001; Hédl, Kopecký, & Komárek, 2010; Kopecký et al., 2013; van Swaay et al., 2006). For instance, saproxylic beetles requiring sun‐exposed dead wood have higher red list status than species preferring shady forests (Seibold et al., 2015). Despite the increased forest cover and high dead wood volumes in many protected forests, the decline in saproxylics is especially true for the species associated with veteran trees and their specific microhabitats (Siitonen & Ranius, 2015).

Veteran trees are trees that have passed beyond maturity and often bear dead wood microhabitats such as cavities, bare wood and dead branches (Kraus et al., 2016). The term “veteran tree” may include even younger individuals that have developed similar characteristics as a result of adverse growing conditions or injury (Lonsdale, 2013; Siitonen & Ranius, 2015). Veteran trees are considered keystone ecological structures of woodlands (Lindenmayer, Laurance, & Franklin, 2012; Lindenmayer et al., 2014; Read, 2000). Interestingly, the populations of many veteran tree‐associated organisms are often found in the last remains of open woodlands such as game reserves and chateau parks, rather than in strictly protected forests where trees are protected from cutting but where high canopy closure prevails (Ranius et al., 2005). The above suggests that biota associated with veteran trees is negatively affected by canopy closure. This assumption is supported not only by the preference for open conditions among numerous flagship species associated with veteran trees (Albert, Platek, & Cizek, 2012; Buse, Ranius, & Assmann, 2008; Ranius & Jansson, 2000) but also by generally better thriving of veteran trees in open conditions (Lonsdale, 2013; Read, 2000). The fact that canopy closure may compromise the survival and recruitment of veteran trees is, however, insufficiently documented and thus often ignored.

Moreover, open woodlands that have been affected by humans for centuries were often considered of limited value for nature conservation. Instead, management measures allowing for increasing canopy closure are widely advocated and practised in protected areas, presuming to restore “natural” conditions by minimizing human impact (Schultze et al., 2014; Winter, 2012). In the absence of natural disturbances, such approaches, unfortunately, do not lead to the desired “natural” stage and often have detrimental consequences to veteran trees and their associated biodiversity. The reason why we often fail to see the problem may also lie in a delayed response of veteran trees to canopy closure. Deteriorating health of tree veterans under closed‐canopy conditions temporarily increases the amounts of dead wood and postpones the negative effects of canopy closure on saproxylic organisms, while it supports other species than those associated with veteran trees. Veteran tree‐associated species may thus actually undergo the extinction debt (Jackson & Sax, 2010; Tilman, May, Lehman, & Nowak, 1994).

To address this important issue and provide background information for qualified management decisions, we investigated the survival and distribution patterns of veteran trees and their specialized beetle inhabitants under the changing conditions of increasing canopy closure. Using aerial photographs, we quantified the past and recent canopy closure over 146 km2 of the lowland floodplain forests and meadows of Central Europe. We mapped trees that can be considered veteran, including oaks of large diameters, hollow trees and trees occupied by three beetle species typically associated with veteran trees, that is the great capricorn beetle (Cerambyx cerdo Linnaeus 1758), the hermit beetle (Osmoderma barnabita Motchulsky 1842) and the jewel beetle Eurythyrea quercus (Herbst 1784). This study specifically aims to (1) describe changes in woodland canopy closure using old and recent aerial photographs; (2) evaluate the effect of canopy closure on the presence and mortality of veteran trees; (3) assess the importance of present canopy closure for endangered model species of saproxylic beetles, (4) evaluate the influence of the past levels of canopy closure on the present distribution of veteran trees and model beetles, and (5) propose the principles of the conservation management of protected woodlands based on our findings.