Integrative management to sustain biodiversity and ecological continuity in Central European temperate oak (Quercus robur, Q. petraea) forests: An overview
Graphical abstract
Introduction
Quercus is the most important tree genus in the Northern Hemisphere, and there are about 600 extant species of oak (Mabberley, 2008). While the greatest oak diversity is to be found in the New World and in China (Nixon, 2006, Huang et al., 2013), Central Europe harbors merely four native species: the thermophilous Q. pubescens and Q. cerris and the widespread Q. robur and Q. petraea (Sayer, 2000, Leuschner and Ellenberg, 2017). Q. robur (pedunculate oak) and Q. petraea (sessile oak) are highly valued, both in economical and nature conservation terms, and temperate deciduous forests dominated by these species are the focus of this review.
Central European temperate oak woodlands were profoundly influenced and maintained by century-long human management (Kirby and Watkins, 2015). Traditional management practices, particularly pannage, wood pasture, coppicing and coppicing with standards, promoted oaks and led, at most sites, to decreasing proportions of the otherwise very competitive European beech (Fagus sylvatica) (Ningre and Doussot, 1993, Strandberg et al., 2005, Szabó, 2013, Vandekerkhove et al., 2016a). Bobiec et al. (2018) highlighted that the intensification and specialization of modern post-18th century agriculture and forestry led to the loss of transitional habitats, which had provided very suitable oak regeneration niches for centuries. Scientific forestry, however, regionally undertook great efforts to develop oak high forests in the first half of the 19th century (Ningre and Doussot, 1993, Eliasson and Nilsson, 2002, Mölder et al., 2017a). Hence, the role of oaks in the potential natural vegetation is ambiguous in some areas and has been controversial for decades (Bobiec et al., 2011, Reif et al., 2016, Leuschner and Ellenberg, 2017).
European temperate oak woodlands are highly valued for their rich, typical biodiversity, which can be mainly attributed to the life history traits and structural characteristics of Q. robur and Q. petraea. In particular, both oak species have a long natural life span of between 300 and 900 years and very durable wood, even in the decay stage (Ranius et al., 2009b, Leuschner and Ellenberg, 2017). Therefore, they offer suitable and particularly long-lasting habitats for a variety of saproxylic species with low dispersal abilities. However, even one clear-cut or deep ploughing event can destroy the temporal and spatial thread of ecological continuity in oak woodlands (Grove, 2002, Vandekerkhove et al., 2013, Fichtner et al., 2014, Nordén et al., 2014). According to Brändle and Brandl (2001), 252 phytophagous insect and mite species are oak specialists. For comparison, beech has merely 44 specialist species in these groups. In addition, the understory of oak-dominated woodlands is frequently characterized by a wide variety of woodland plant species and soil-dwelling fungi (Nitare, 2000, Leuschner and Ellenberg, 2017). However, Lindbladh and Foster (2010) critically suggested that the current populations of oak and its associated species appear to represent biological legacies in the midst of protracted decline, especially in view of the former abundance, longevity and persistence of oaks.
Current oak silviculture in the high forest system aims at producing high quality timber (e.g., sliced or peeled veneer) or construction timber. The investment in oak stands is high, due to long rotation periods, laborious regeneration techniques and frequent silvicultural interventions (Kenk, 1993, Schütz, 1993, Solymos, 1993, Lüpke, 1998). This is particularly true in the framework of close-to-nature forestry (Lüpke, 1998, Pommerening and Murphy, 2004).
In the management of mature oak stands, in particular, there are trade-offs between economic and conservation targets. Securing economic revenue and regeneration calls for harvesting of a large proportion of the mature stands, while the preservation of biodiversity requires a high level of old tree retention. Thus, the regeneration of mature oak stands results in sharp conflicts. Nature conservationists and advocates of strict continuous cover forestry have frequently criticized clearfelling and have called for complex small-scale regeneration techniques and the retention of more mature oaks (Stahl-Streit, 2004, Jedicke and Hakes, 2005, Meyer, 2013). In order to avoid these conflicts, forest planning frequently aims at establishing new oak stands on former agricultural land or at sites that previously were stocked with conifers (Saha et al., 2017). This approach, however, leads to a disruption of ecological continuity, due to the spatial decoupling of young and mature oak stands in fragmented modern forest landscapes (cf. Goßner et al., 2008, Vandekerkhove et al., 2013).
It is a great challenge to conserve or restore highly valued oak forest biocoenoses while, at the same time, maintaining the economic viability of oak forestry and, thus, the interest of forest managers in oak. Therefore, within the framework of close-to-nature forest management (Pommerening and Murphy, 2004), it seems worthwhile to bring current ecological and silvicultural knowledge together in an overview article with the aim of identifying suitable options for forest management planning. Very few publications have, up to now, dealt with this cross-section issue (Löf et al., 2015).
Our aim is to identify management options for forest and nature conservation management that sustain both the ecological value of oak forests and the economic viability of oak silviculture. In order to identify these management options, we conduct a literature review considering the following questions:
- 1.
Oaks and close-to-nature forestry – what are the key silvicultural challenges and options?
- 2.
What is the particular significance of ecological continuity and which structural features are of importance for biodiversity conservation in oak forests?
- 3.
What are the key elements and possible strategies of forest management that sustain the ecological values in oak forests in combination with viable forestry?
Section snippets
Area of interest
According to Leuschner and Ellenberg (2017), we define Central Europe as follows: It includes Germany, the Netherlands, Belgium and Luxembourg, Switzerland, Austria, the Czech Republic, Slovakia and Poland, as well as parts of the neighbouring countries of Denmark, southern Sweden, Hungary, Romania, Slovenia, Croatia, eastern France and the Italian Alps. We are aware that Central European oak woodlands are not only characterized by large gradients of climatic and edaphic factors, but also by
Key targets of oak silviculture
Modern oak silviculture in the high forest system is aimed at producing high quality timber with a DBH of at least 70 cm within a timeframe of 160–200 years or, respectively, construction timber with a DBH of at least 60 cm within 140–180 years. To achieve these targets, the site nutrient supply should be mesotrophic or even better (Penistan, 1984, Schütz, 1993, Lüpke, 1998, Spellmann, 2001, Attocchi, 2015). While Q. robur can be found preferentially in sandy, loamy or alluvial sites in the
The importance of ecological continuity
Many of the present distribution patterns of plant and animal species in Central European cultural landscapes are closely related to land-use history. Most notably, woodland specialist species with low dispersal abilities are dependent on a century- or even millennium-long interaction of land-use types and intensities that secured the long-term availability of suitable habitat conditions (Fichtner et al., 2014, Bradshaw et al., 2015, Buckley and Mills, 2015, Flensted et al., 2016, Cateau et
Habitat trees, habitat tree islands and retention forestry
Retention forestry has become an important topic in both forest science and practice in recent times (Bauhus et al., 2009, Gustafsson et al., 2012, Fedrowitz et al., 2014). This management approach means that “single trees and/or intact forest patches are retained at the time of harvest, with the overall aim of achieving a level of continuity in forest structure and complexity” (Fig. 3; Fedrowitz et al., 2014). Bauhus et al. (2009) have coined the term “silviculture for old-growth attributes”
Strategies for systematic oak forest conservation and management planning
With reference to the spatial distribution and arrangement of the management elements for ecological continuity and biodiversity conservation in oak forests presented here, it has to be decided where the different measures should be situated and implemented in the multifunctional forest landscape matrix (Krumm et al., 2013, Vandekerkhove et al., 2016b, Borrass et al., 2017). In the following we present possible strategies for management planning, each of which, however, should be implemented
Conclusions
Considering the initial research questions, we draw these main conclusions:
1 Oaks and close-to-nature forestry – what are the key silvicultural challenges and options?
With regard to integrative oak forest management, close-to-nature forestry must not be equated with strict continuous cover forestry. In view of the light-demanding oak regrowth, the creation and subsequent enlargement of canopy gaps is indispensable for oak forest regeneration and economically viable forestry. Therefore, the
Acknowledgments
The funding of the research projects “QuerCon – Long-term conservation of ecological continuity in oak forests” (grant number DBU 32694) and “Identification and protection of forest stands of special importance for biodiversity conservation” (grant number DBU 29677) by the German Federal Environmental Foundation (DBU) is gratefully acknowledged. The authors thank Robert Larkin for language polishing and are indebted to the reviewers for suggestions that have greatly improved the paper.
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