continue to alter) the spatial and temporal patterns of temperature and precipitation—the two most fundamental factors driving distribution and productivity patterns of vegetation—climate change will cause geographical shifts in the ranges of
species, plant communities and vegetation zones (MEA 2005). Aitken et al. (2008)
developed species distribution models, according to which a global redistribution of
trees might be expected in upcoming decades, yet migratory responses necessary to
spatially track climate shifts far exceed the species’ maximum post-glacial migration
rates. In the case of limited migration rates, (long-distance) gene flow can promote
adaptive evolution under novel environmental conditions by increasing genetic
variation for fitness (Kremer et al. 2012). Aitken et al. (2008) hypothesize that
gene flow with preadapted alleles from warmer climates may promote adaptation
and migration at the leading edge, while populations at the rear more likely will face
extinction. However, despite possible range shifts and the adaptive potential of tree
species, climate change affects forest ecosystems not only through direct physiological effects such as modifying photosynthesis and growing season lengths, but also
through indirect effects via shifts in community composition related to species
extinctions and colonizations (i.e. climate change induced shifts in competitive
interactions between tree species; Garcia-Valdes et al. 2018). In a tree species
hotspot in the tropical Andes, Garavito et al. (2015) analyzed the relative impact
of climate change on the extinction risk of 129 tree species endemic to this region.
The authors demonstrated that climate change significantly increases the extinction
risk of 18–20% of the tree species evaluated, depending on the climate change
scenario considered. Strong shifts in tree species composition are also expected for
regions with arid climatic conditions, for example in the Mediterranean area. Analyses of Benito Garzon et al. (2008) conducted for the Iberian Peninsula showed a
notable reduction in the potential distribution of several tree species under all the
IPCC scenarios (e.g. Pinus sylvestris, P. uncinata and Abies alba); temperate tree
species such as Fagus sylvatica and Quercus petraea were also predicted to suffer a
reduction in their range, whereas Mediterranean species appeared to be generally
more capable of migration, and are therefore likely to be less affected (Benito
Garzon et al. 2008). Climate change may not only cause range shifts of a focal
tree species, but also could foster a large-scale fragmentation of species distributions
with consequences for meta-population dynamics and gene flow. For boreal forests
in North America, Murray et al. (2017) demonstrated that climate change directly
alters environmental niche suitability for boreal-obligate species of trees, birds and
mammals, with most species ranges becoming smaller and shifting northward over
time. Importantly, species distributions became increasingly fragmented, as characterized by smaller mean areas and greater isolation of environmentally-suitable
landscape patches (Murray et al. 2017).
Besides climate change, pollution—namely the deposition of reactive forms of
nitrogen (N) from the atmosphere—constitutes a further important driver of biodiversity loss across forest biomes (Galloway et al. 2004; Gruber and Galloway 2008;
IUCN 2019). Airborne N deposition has tripled since the beginning of industrialization (Galloway et al. 2004), resulting in unprecedented impacts on the N status of
many forest ecosystems (De Schrijver et al. 2011). Since N is often the most limiting
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A. Fichtner and W. Härdtle
species, plant communities and vegetation zones (MEA 2005). Aitken et al. (2008)
developed species distribution models, according to which a global redistribution of
trees might be expected in upcoming decades, yet migratory responses necessary to
spatially track climate shifts far exceed the species’ maximum post-glacial migration
rates. In the case of limited migration rates, (long-distance) gene flow can promote
adaptive evolution under novel environmental conditions by increasing genetic
variation for fitness (Kremer et al. 2012). Aitken et al. (2008) hypothesize that
gene flow with preadapted alleles from warmer climates may promote adaptation
and migration at the leading edge, while populations at the rear more likely will face
extinction. However, despite possible range shifts and the adaptive potential of tree
species, climate change affects forest ecosystems not only through direct physiological effects such as modifying photosynthesis and growing season lengths, but also
through indirect effects via shifts in community composition related to species
extinctions and colonizations (i.e. climate change induced shifts in competitive
interactions between tree species; Garcia-Valdes et al. 2018). In a tree species
hotspot in the tropical Andes, Garavito et al. (2015) analyzed the relative impact
of climate change on the extinction risk of 129 tree species endemic to this region.
The authors demonstrated that climate change significantly increases the extinction
risk of 18–20% of the tree species evaluated, depending on the climate change
scenario considered. Strong shifts in tree species composition are also expected for
regions with arid climatic conditions, for example in the Mediterranean area. Analyses of Benito Garzon et al. (2008) conducted for the Iberian Peninsula showed a
notable reduction in the potential distribution of several tree species under all the
IPCC scenarios (e.g. Pinus sylvestris, P. uncinata and Abies alba); temperate tree
species such as Fagus sylvatica and Quercus petraea were also predicted to suffer a
reduction in their range, whereas Mediterranean species appeared to be generally
more capable of migration, and are therefore likely to be less affected (Benito
Garzon et al. 2008). Climate change may not only cause range shifts of a focal
tree species, but also could foster a large-scale fragmentation of species distributions
with consequences for meta-population dynamics and gene flow. For boreal forests
in North America, Murray et al. (2017) demonstrated that climate change directly
alters environmental niche suitability for boreal-obligate species of trees, birds and
mammals, with most species ranges becoming smaller and shifting northward over
time. Importantly, species distributions became increasingly fragmented, as characterized by smaller mean areas and greater isolation of environmentally-suitable
landscape patches (Murray et al. 2017).
Besides climate change, pollution—namely the deposition of reactive forms of
nitrogen (N) from the atmosphere—constitutes a further important driver of biodiversity loss across forest biomes (Galloway et al. 2004; Gruber and Galloway 2008;
IUCN 2019). Airborne N deposition has tripled since the beginning of industrialization (Galloway et al. 2004), resulting in unprecedented impacts on the N status of
many forest ecosystems (De Schrijver et al. 2011). Since N is often the most limiting
390
A. Fichtner and W. Härdtle