Comparing close-to-nature silviculture with processes in pristine forests: lessons from Central Europe

The term close-to-nature silviculture already contains some ambiguity since (silvi) culture implies the act of forest use. This notion was coined a long time ago by well-known silviculturists (Gayer 1885; Engler 1905; Leibundgut 1943 and more) to characterise a new form of forest management that differs from plantation forestry and the clear-cut system. The aim was a paradigmatic alternative to create multi-storeyed and rich mixed forest stands, with forest management emulating natural processes and seamless replacement of generations. Which of these processes should be selected and the degree to which natural forest dynamics should be imitated have not yet been specifically defined because the term was conceived in a general and conceptual sense and, furthermore, the aim has never been to let nature work alone. We can here quote Leibundgut (1943, p. 152) [transl. from German] “The main task of silviculture is to maximise and maintain the value produced by the forest”. Such a formulation is still quite relevant today, when the term “value” is understood in a broader sense. Actually, it would be more appropriate to use the term “utility” (original term “utilité”) in the sense of Biolley (1901).

Meanwhile, the notion of close-to-nature silviculture has been largely accepted and has gained popularity in practice. In an increasing number of countries, close-to-nature silviculture, also known as “continuous cover forestry” (Helliwell and Wilson 2012; Otto 1992), has become the main or only management approach. Close-to-nature management should be considered more as a set of guiding principles that are concerned with the whole ecosystem and with ensuring small-scale heterogeneity and stability, and uneven-aged silviculture systems are used as a means of implementing these principles (Helliwell and Wilson 2012). These general principles were first defined as plenter principles by Leibundgut (1979) and are now well recognised and formalised by ProSilva (2012), the association of European foresters practising management which follows natural processes. See also Diaci (2006) and http://www.prosilvaeurope.org. Bauhus et al. (2013) consider the following attributes as particularly representative for characterising close-to-nature management: site-adapted species composition, avoidance of clearfelling, focus on stability, reliance on natural processes and focus on development of individual trees and mixed and uneven-aged structurally diverse forests.

Research in pristine forest is necessary to understand the characteristics of natural forest dynamics. For this reason, the founders of the close-to-nature school of thought advocated the preservation of sufficient areas of untouched forests (hereafter pristine forest, even if this term is sometimes criticised because of the inevitable influence of man) in different vegetation types in order to study their particular developmental characteristics. The conservation of adequate areas of protected forests remains valid today (Leibundgut 1959; Parviainen et al. 2000; Nagel et al. 2014; Anderson-Teixeira et al. 2015). To gain insight into developmental processes, an extensive network of permanent research plots has been established. After decades of periodic remeasurements, they have provided a solid foundation for understanding stand development characteristics (e.g. Saniga et al. 2015). Moreover, in the last decades, a number of new techniques such as dendrochronology, canopy gap analysis, remote sensing and LiDAR analysis have been applied, enabling the process of generation renewal to be understood on different scales, from single trees to small clusters.

The best-represented preserved pristine forests in Europe are pure beech and mixed fir-beech forests, particularly in the Carpathians and Dinaric Mountains (Schütz 1969; Leibundgut 1982; Korpeĺ 1995; Šebková et al. 2011; Nagel et al. 2012; Král et al. 2014; Keren et al. 2014). Pure beech forest represents a significant part of potential forest vegetation in Central Europe (Ozenda 1994; Spellmann 1999) and can therefore be used as a representative example for study. In addition, beech-dominated forests represent an interesting case study because of the simplicity of the system in terms of the tree species involved and because seed dispersal and germination do not constrain regeneration to the extent that they do in other species. The main limiting factor in this case is evidently the minimal light requirements for allowing the new generation at the thicket to pole stage to reach the upper storey.

The first pillar of close-to-nature management is stem-by-stem renewal, known as the plenter system (or selection system). It has been proven in practice as well as by scientific analysis, particularly when considering conditions for sustainability, or so-called equilibrium (e.g. Schütz 2001). The plenter model applies mainly to conifer-dominated forests and particularly to shade-tolerant species, such as fir and spruce. Extension of the stem-by-stem renewal system to broad-leaved trees appears much more difficult (Schütz 1992), which is why there are very few examples which have persisted for a long time. Even in regions with a strong tradition of plenter silviculture, such as Canton Neuchâtel (Switzerland), its long-term application within broad-leaved forests has been largely unsuccessful (Peter-Comtesse 1972). The second pillar of close-to-nature management is oriented more towards small collective renewal known as the group plenter system or irregular shelterwood system (Germ. Femelsystem), depending on the size of the regeneration cohorts (Leibundgut 1946). The latter has been conceived to achieve irregularity, particularly within broad-leaved forests. The main difference between the two approaches lies in an emphasis on individual trees in the plenter system vs. groups of trees in the irregular shelterwood system as well as in the principles for controlling sustainability—on a stand level in the former and on a much larger scale (landscape) in the latter.

The quintessence of close-to-nature silviculture is diametrically different from most other ways of treating forests because of its liberal conception (the free use of felling) and because the silviculture technique is subordinate to the goals. Worldwide, most silviculturists still consider the particular silvicultural system as determinant in deriving the silvicultural tools. The concept of a systemic free and undogmatic silviculture represents a paradigmatic shift in this field. However, there are two important questions that remain open or are misinterpreted: (1) how to apply liberal silvicultural tools based on close-to-nature principles and (2) to what extent can the dynamics in natural forests be mimicked using these tools. Thus, the main aspiration of this paper is to relate close-to-nature silviculture with processes in natural forests and show examples of how to implement site and stand-specific management tools. To achieve this, we firstly present the special case of tree species mixtures in Central Europe, then discuss the results from old-growth forest research regarding stand ontogeny and regeneration patterns for two species with contrasting life traits: beech and sessile oak, and finally, we discuss silvicultural limitations in terms of maximum basal area during stand renewal. We conclude the paper with a historical overview of the evolution of close-to-nature silvicultural tools with particular emphasis on the irregular shelterwood system and the free selection of felling regime.