nutrient for plant growth, high N loads may affect ecosystem functions such as
biomass production and the complex interplays within and between tree species, but
also their susceptibility to abiotic and biotic stressors such as drought, frost events
and herbivory (Bobbink et al. 2010; Hess et al. 2018). This pertains to strongly
N-limited sites with conservative N cycles in particular (e.g. forests at acidic or
podzolic sites), because many species typical of these sites are physiologically
adapted to low N availability, for example due to high N use efficiency or mycorrhizal associations (Aerts 1999; Phoenix et al. 2012). Besides the direct effects of N
deposition on forest species competition and performance, atmospheric N deposition
has been shown to interact with drivers of climate change, e.g. increasing temperature or drought events. Taking European beech forest ecosystems as example, Hess
et al. (2018) demonstrated that high rates of airborne N loads significantly increases
the trees’ sensitivity to increasing annual mean temperatures (antagonistic effects on
tree-ring width), possibly due to N deposition-induced fine root dieback, decreasing
mycorrhizal colonization or shifts in biomass allocation patterns (i.e. increasing
aboveground, but decreasing belowground biomass allocation; Agren and Franklin
2003). Effects of N deposition may also interfere with increasing CO 2 concentrations
in the atmosphere. Many experiments with leaves, shoots, and tree seedlings indicated a significant increase of productivity due to “CO 2 fertilization”, but these
effects on forests may be saturated within a short time span (Scholes et al. 1999).
This conclusion is supported by experimental data, according to which “fertilization” effects due to increasing CO 2 levels are low (Bader et al. 2013; Korner 2003).
In some areas, losses in biodiversity are also driven by invasive tree or shrub
species. Many woody species have spread from planting sites, and some are now
among the most widespread and damaging of invasive organisms (Richardson and
Rejmanek 2011). Across the globe, the authors identified 434 tree species and
317 shrub species as being “invasive outside their natural range”, and both further
spreads and impacts of these species on biodiversity and ecosystem functioning is
strongly accelerating (Rejmanek and Richardson 2013). Invasive woody species
constitute a particular threat in North America (212 species), the Pacific Islands
(208), Australia (203), Southern Africa (178), and Europe (134), indicating that
these neobiota affect forest ecosystems across biomes. In a case study conducted in
riparian forests invaded by Eucalyptus species, Tererai et al. (2013) found that forest
species richness, diversity and structural attributes (e.g. height, relative cover and
mean basal area) of native species decreased consistently along an invasion gradient.
These findings indicate the importance of native tree species to be used in the context
of afforestation projects (cf. Chap. 5.3).
4 A Functional-Based Perspective on Forest Ecosystems
4.1 Biodiversity and Forest Ecosystem Functioning
During the last two decades experimental and observational studies provided ample
evidence for positive links between biodiversity and ecosystem functions, such as
Forest Ecosystems: A Functional and Biodiversity Perspective
391
biomass production and the complex interplays within and between tree species, but
also their susceptibility to abiotic and biotic stressors such as drought, frost events
and herbivory (Bobbink et al. 2010; Hess et al. 2018). This pertains to strongly
N-limited sites with conservative N cycles in particular (e.g. forests at acidic or
podzolic sites), because many species typical of these sites are physiologically
adapted to low N availability, for example due to high N use efficiency or mycorrhizal associations (Aerts 1999; Phoenix et al. 2012). Besides the direct effects of N
deposition on forest species competition and performance, atmospheric N deposition
has been shown to interact with drivers of climate change, e.g. increasing temperature or drought events. Taking European beech forest ecosystems as example, Hess
et al. (2018) demonstrated that high rates of airborne N loads significantly increases
the trees’ sensitivity to increasing annual mean temperatures (antagonistic effects on
tree-ring width), possibly due to N deposition-induced fine root dieback, decreasing
mycorrhizal colonization or shifts in biomass allocation patterns (i.e. increasing
aboveground, but decreasing belowground biomass allocation; Agren and Franklin
2003). Effects of N deposition may also interfere with increasing CO 2 concentrations
in the atmosphere. Many experiments with leaves, shoots, and tree seedlings indicated a significant increase of productivity due to “CO 2 fertilization”, but these
effects on forests may be saturated within a short time span (Scholes et al. 1999).
This conclusion is supported by experimental data, according to which “fertilization” effects due to increasing CO 2 levels are low (Bader et al. 2013; Korner 2003).
In some areas, losses in biodiversity are also driven by invasive tree or shrub
species. Many woody species have spread from planting sites, and some are now
among the most widespread and damaging of invasive organisms (Richardson and
Rejmanek 2011). Across the globe, the authors identified 434 tree species and
317 shrub species as being “invasive outside their natural range”, and both further
spreads and impacts of these species on biodiversity and ecosystem functioning is
strongly accelerating (Rejmanek and Richardson 2013). Invasive woody species
constitute a particular threat in North America (212 species), the Pacific Islands
(208), Australia (203), Southern Africa (178), and Europe (134), indicating that
these neobiota affect forest ecosystems across biomes. In a case study conducted in
riparian forests invaded by Eucalyptus species, Tererai et al. (2013) found that forest
species richness, diversity and structural attributes (e.g. height, relative cover and
mean basal area) of native species decreased consistently along an invasion gradient.
These findings indicate the importance of native tree species to be used in the context
of afforestation projects (cf. Chap. 5.3).
4 A Functional-Based Perspective on Forest Ecosystems
4.1 Biodiversity and Forest Ecosystem Functioning
During the last two decades experimental and observational studies provided ample
evidence for positive links between biodiversity and ecosystem functions, such as
Forest Ecosystems: A Functional and Biodiversity Perspective
391