continuity, stand maturity and continuity in natural stand dynamics. Forest continuity refers to the temporal extend of how long a given site is wooded (i.e. the
maintenance of the forest cover over time including soil development). For example,
forest sites that have been continuously wooded for at least more than two centuries
have been described as ‘ancient forests’ (Peterken 1977; Rackham 1980) in contrast
to ‘recent forests’ that are afforested during the last two centuries on former
agricultural land. The reference date for ‘ancientness’, however, is still on debate
(see Bergès and Dupouey 2020). Stand maturity is associated with tree and stand
aging, and thereby with processes regulating the availability, continuity and diversity of habitat structures (Janssen et al. 2019). The continuity in natural stand
dynamics refers to the duration of stand development without anthropogenic disturbances, which depends on the length of forest management cessation or the frequency and intensity of silvicultural interventions (e.g. thinning or commercial
harvesting). Note that a long-term forest continuity does not necessarily imply a
high stand maturity or long-term natural stand dynamics, although each aspect
determines the conservation value (Watson et al. 2018; Janssen et al. 2019; Bergès
and Dupouey 2020) and ecological integrity of a forest.
There is mounting evidence that ancient forests harbour higher abundance and
richness of forest species on average than recent forests due to dispersal and
recruitment limitations (Brunet and von Oheimb 1998; Flinn and Vellend 2005;
Fritz et al. 2008a; Seibold et al. 2015; Flensted et al. 2016). Similarly, a large
proportion of forest species—across taxa—depends on structures associated with
late forest development phases and ‘old-growth’ forests (i.e. primeval or long-term
unmanaged forests). For example, this includes high growing stocks and a high
quantity and quality of dead wood, a wide range of tree sizes, a high spatial
heterogeneity, a high variety of host species in various microclimates and a high
abundance of senescent and large-diameter trees (Christensen et al. 2005; Bauhus
et al. 2009; Brunet et al. 2010; Krah et al. 2018). Overall, old and large-diameter
trees exhibit higher diversity of microhabitats than young trees (Fig. 2) and are
important for forest structural heterogeneity and functional complexity (Lutz et al.
2013). At the forest stand scale, forest continuity and stand maturity are key factors
regulating the availability of substrates required for many species of conservation
concern (Fritz et al. 2008a; Janssen et al. 2017)—specifically, the abundance and
continuity of habitat structures as well as the variability in microclimates associated
with late forest development phases (terminal and decay phase). Consequently,
biome-specific biodiversity increases in a wide range of taxa with habitat continuity
(Ohlson et al. 1997; Nordén et al. 2014), tree/stand age (Heilmann and Christensen
2004; Fritz et al. 2008b; Moning and Muller 2009; Moning et al. 2009) and length of
forest management abandonment (Paillet et al. 2010; Alroy 2017; Kaufmann et al.
2018). Ancient forests are therefore priority sites for species conservation (Flensted
et al. 2016; McMullin and Wiersma 2019). Consequently, land-use changes or
intensive forest management of ancient forest sites would contradict biodiversity
conservation. Moreover, forest management should focus on promoting the continuity of habitat structures, species interactions and species composition, which in
turn would benefit synergies among multiple forest ecosystem services (Felipe-Lucia
394
A. Fichtner and W. Härdtle
maintenance of the forest cover over time including soil development). For example,
forest sites that have been continuously wooded for at least more than two centuries
have been described as ‘ancient forests’ (Peterken 1977; Rackham 1980) in contrast
to ‘recent forests’ that are afforested during the last two centuries on former
agricultural land. The reference date for ‘ancientness’, however, is still on debate
(see Bergès and Dupouey 2020). Stand maturity is associated with tree and stand
aging, and thereby with processes regulating the availability, continuity and diversity of habitat structures (Janssen et al. 2019). The continuity in natural stand
dynamics refers to the duration of stand development without anthropogenic disturbances, which depends on the length of forest management cessation or the frequency and intensity of silvicultural interventions (e.g. thinning or commercial
harvesting). Note that a long-term forest continuity does not necessarily imply a
high stand maturity or long-term natural stand dynamics, although each aspect
determines the conservation value (Watson et al. 2018; Janssen et al. 2019; Bergès
and Dupouey 2020) and ecological integrity of a forest.
There is mounting evidence that ancient forests harbour higher abundance and
richness of forest species on average than recent forests due to dispersal and
recruitment limitations (Brunet and von Oheimb 1998; Flinn and Vellend 2005;
Fritz et al. 2008a; Seibold et al. 2015; Flensted et al. 2016). Similarly, a large
proportion of forest species—across taxa—depends on structures associated with
late forest development phases and ‘old-growth’ forests (i.e. primeval or long-term
unmanaged forests). For example, this includes high growing stocks and a high
quantity and quality of dead wood, a wide range of tree sizes, a high spatial
heterogeneity, a high variety of host species in various microclimates and a high
abundance of senescent and large-diameter trees (Christensen et al. 2005; Bauhus
et al. 2009; Brunet et al. 2010; Krah et al. 2018). Overall, old and large-diameter
trees exhibit higher diversity of microhabitats than young trees (Fig. 2) and are
important for forest structural heterogeneity and functional complexity (Lutz et al.
2013). At the forest stand scale, forest continuity and stand maturity are key factors
regulating the availability of substrates required for many species of conservation
concern (Fritz et al. 2008a; Janssen et al. 2017)—specifically, the abundance and
continuity of habitat structures as well as the variability in microclimates associated
with late forest development phases (terminal and decay phase). Consequently,
biome-specific biodiversity increases in a wide range of taxa with habitat continuity
(Ohlson et al. 1997; Nordén et al. 2014), tree/stand age (Heilmann and Christensen
2004; Fritz et al. 2008b; Moning and Muller 2009; Moning et al. 2009) and length of
forest management abandonment (Paillet et al. 2010; Alroy 2017; Kaufmann et al.
2018). Ancient forests are therefore priority sites for species conservation (Flensted
et al. 2016; McMullin and Wiersma 2019). Consequently, land-use changes or
intensive forest management of ancient forest sites would contradict biodiversity
conservation. Moreover, forest management should focus on promoting the continuity of habitat structures, species interactions and species composition, which in
turn would benefit synergies among multiple forest ecosystem services (Felipe-Lucia
394
A. Fichtner and W. Härdtle