or wood harvest. Declining forest area threatens biodiversity at all levels. Considering trees, more than 5000 species from 180 countries are currently threatened with
extinction (IUCN 2019). Given that 15 of the 25 global ‘biodiversity hotspots’
identified by Myers et al. (2000) represent tropical forests, deforestation in the
tropics has by far the most dramatic consequences for global biodiversity. All
tropical ‘hotspots’ once covered about 12.5% of the Earth’s land surface, but their
remaining area now amounts to 1.4%, indicating a 88% loss of the former area
(MEA 2005). If current rates of tropical rainforest clear-cuts remain unchanged,
Pimm and Raven (2000) predict that species extinction rates achieve a maximum by
2060, with decadal losses of nearly 5% of the total species richness of tropical
forests.
Shifts in forest cover (across biomes) in turn will feedback on important functions
of forest ecosystems. For example, analyses of (Alkama and Cescatti 2016) showed
that forest losses amplify the diurnal temperature variation and increase the mean
and maximum air temperature, with the largest signal in arid zones, followed by
temperate, tropical, and boreal zones. The authors further demonstrated that variations of forest cover (related to the decade 2003–2012) generated a mean biophysical
warming on land corresponding to about 18% of the global biogeochemical signal
due to CO 2 emission from land-use change.
The term “overexploitation” summarizes all forms of unsustainable use of forest
area (e.g. wood harvest exceeding natural regrowth), including failure in forest
management measures due to missing or inappropriate management plans.
Overexploitation or management failure result in habitat and finally diversity losses
(Stork et al. 1997), but the underlying mechanisms are manifold. Despite the
complexity of processes, it is clear that overexploitation is a result of many direct
or indirect drivers, with specific combinations of drivers varying between countries
or localities (MEA 2005). Intensively managed forests, for example, suffer from a
lack of sites that remain undisturbed in the long term and therefore often are missing
an ‘ecological continuity’ being important for the course of undisturbed ecological
processes and the establishment of many stenotopic forest species (Maes et al. 2019;
also cf. Sect. 4.2). ‘Legacy effects’ of forest management in turn may increase a
forest’s sensitivity to climate extremes (Mausolf et al. 2018a). Forest management
shortens the life cycle of trees (in most cases more than 50%), reduces the formation
of dead wood (i.e. missing old trees, missing dead wood), and impairs the textural
diversity by preventing the establishment of developmental phases typical of forest
ecosystems (e.g. missing terminal or decay phase, reduced structural diversity in
space and time). Since many forest species (particularly fungi or xylobiontic species)
depend on the presence of old or dead trees, they often failure to develop stable
populations in managed forests (Fichtner et al. 2015; Heilmann and Christensen
2004; Moning and Muller 2009). Moreover, the establishment of monocultures or
stands with non-native tree species may negatively affect both forest diversity and
productivity, particularly at sites with high natural tree diversity (Huang et al. 2018;
cf. Sect. 5.3).
During the last 30 years, forest ecosystems have been increasingly subject to
climate change (MEA 2005; IPCC 2013). Because climate change alters (and will
Forest Ecosystems: A Functional and Biodiversity Perspective
389
extinction (IUCN 2019). Given that 15 of the 25 global ‘biodiversity hotspots’
identified by Myers et al. (2000) represent tropical forests, deforestation in the
tropics has by far the most dramatic consequences for global biodiversity. All
tropical ‘hotspots’ once covered about 12.5% of the Earth’s land surface, but their
remaining area now amounts to 1.4%, indicating a 88% loss of the former area
(MEA 2005). If current rates of tropical rainforest clear-cuts remain unchanged,
Pimm and Raven (2000) predict that species extinction rates achieve a maximum by
2060, with decadal losses of nearly 5% of the total species richness of tropical
forests.
Shifts in forest cover (across biomes) in turn will feedback on important functions
of forest ecosystems. For example, analyses of (Alkama and Cescatti 2016) showed
that forest losses amplify the diurnal temperature variation and increase the mean
and maximum air temperature, with the largest signal in arid zones, followed by
temperate, tropical, and boreal zones. The authors further demonstrated that variations of forest cover (related to the decade 2003–2012) generated a mean biophysical
warming on land corresponding to about 18% of the global biogeochemical signal
due to CO 2 emission from land-use change.
The term “overexploitation” summarizes all forms of unsustainable use of forest
area (e.g. wood harvest exceeding natural regrowth), including failure in forest
management measures due to missing or inappropriate management plans.
Overexploitation or management failure result in habitat and finally diversity losses
(Stork et al. 1997), but the underlying mechanisms are manifold. Despite the
complexity of processes, it is clear that overexploitation is a result of many direct
or indirect drivers, with specific combinations of drivers varying between countries
or localities (MEA 2005). Intensively managed forests, for example, suffer from a
lack of sites that remain undisturbed in the long term and therefore often are missing
an ‘ecological continuity’ being important for the course of undisturbed ecological
processes and the establishment of many stenotopic forest species (Maes et al. 2019;
also cf. Sect. 4.2). ‘Legacy effects’ of forest management in turn may increase a
forest’s sensitivity to climate extremes (Mausolf et al. 2018a). Forest management
shortens the life cycle of trees (in most cases more than 50%), reduces the formation
of dead wood (i.e. missing old trees, missing dead wood), and impairs the textural
diversity by preventing the establishment of developmental phases typical of forest
ecosystems (e.g. missing terminal or decay phase, reduced structural diversity in
space and time). Since many forest species (particularly fungi or xylobiontic species)
depend on the presence of old or dead trees, they often failure to develop stable
populations in managed forests (Fichtner et al. 2015; Heilmann and Christensen
2004; Moning and Muller 2009). Moreover, the establishment of monocultures or
stands with non-native tree species may negatively affect both forest diversity and
productivity, particularly at sites with high natural tree diversity (Huang et al. 2018;
cf. Sect. 5.3).
During the last 30 years, forest ecosystems have been increasingly subject to
climate change (MEA 2005; IPCC 2013). Because climate change alters (and will
Forest Ecosystems: A Functional and Biodiversity Perspective
389