the leaf wettability, which in turn affects water retention on the leaf surface. Leaf
wetting exposes the leaves for fungal pathogen attacks. Susceptibility to herbivores
and pathogens is further enhanced by the lower glandular trichome density, because
the trichomes are exuding defence compounds to the leaf surface. In the first study
years in the FAHM experiment, fungal damages were more frequent in the birch
stands in the humidified plots than in the plots with ambient VPD (Sellin et al.
2017a). The chamber study also revealed that high N availability can mitigate the
effects of low VPD on cuticular wax chemistry by affecting the biosynthesis of wax
compounds and genetic regulation of some key enzymes in the triterpenoid pathway.
4 Future Perspectives
The results obtained by FAHM experiment described above indicate that atmospheric humidity is a highly important climatic factor that affects growth, leaf
and root morphology, stomatal function, water relations, stem wood properties and
mineral nutrition of northern forest trees, as demonstrated with silver birch and
hybrid aspen. These results are in line with the previous results with different plant
species but give a more comprehensive picture of responses at the whole-tree and
ecosystem level. The species-specific responses under low VPD were indicated as a
more pronounced growth reduction in hybrid aspen compared with silver birch.
Birch showed more efficient adjustment of the leaf and root morphology and
hydraulic architecture that could compensate for the reduced transpirational flux,
mineral nutrient acquisition and photosynthetic capacity under decreased VPD. Low
VPD triggered the readjustment of primary carbon and nitrogen metabolism and
altered the leaf surface properties. The experimental knowledge of responses
and acclimation capacity of birch to several climate change factors, and the recent
sequencing of silver birch genome (Salojärvi et al. 2017) with several candidate
genes for adaptation to different environmental variables (such as temperature and
precipitation), makes silver birch an attractive model for forest biotechnology and
breeding. During the last decades, Populus species have been extensively studied for
acclimation and adaptation, for example, for dry and wet conditions (Street et al.
2006). Both species are considered typical pioneer species, able to quickly colonize
disturbed sites (e.g. Worrell 1995).
The FAHM study showed the importance of long-term experiments, because the
responses to low VPD were dependent on the prevailing weather conditions. Further,
several indirect effects of low VPD through altered soil processes (moisture,
microbiota, nitrification) and the competitive status of trees became apparent. High
levels of antioxidants and phenolic compounds under low VPD indicate that trees
employed defence against stress factors such as nutrient limitation, soil hypoxia and
exposure to fungal attacks. The high contents of carbohydrates and phenolics in the
green leaves and leaf litter with the reduced foliage biomass and mineral nutrient
content can further affect the decomposition rates and carbon and nutrient cycling of
the forest ecosystems.
330
E. Oksanen et al.
wetting exposes the leaves for fungal pathogen attacks. Susceptibility to herbivores
and pathogens is further enhanced by the lower glandular trichome density, because
the trichomes are exuding defence compounds to the leaf surface. In the first study
years in the FAHM experiment, fungal damages were more frequent in the birch
stands in the humidified plots than in the plots with ambient VPD (Sellin et al.
2017a). The chamber study also revealed that high N availability can mitigate the
effects of low VPD on cuticular wax chemistry by affecting the biosynthesis of wax
compounds and genetic regulation of some key enzymes in the triterpenoid pathway.
4 Future Perspectives
The results obtained by FAHM experiment described above indicate that atmospheric humidity is a highly important climatic factor that affects growth, leaf
and root morphology, stomatal function, water relations, stem wood properties and
mineral nutrition of northern forest trees, as demonstrated with silver birch and
hybrid aspen. These results are in line with the previous results with different plant
species but give a more comprehensive picture of responses at the whole-tree and
ecosystem level. The species-specific responses under low VPD were indicated as a
more pronounced growth reduction in hybrid aspen compared with silver birch.
Birch showed more efficient adjustment of the leaf and root morphology and
hydraulic architecture that could compensate for the reduced transpirational flux,
mineral nutrient acquisition and photosynthetic capacity under decreased VPD. Low
VPD triggered the readjustment of primary carbon and nitrogen metabolism and
altered the leaf surface properties. The experimental knowledge of responses
and acclimation capacity of birch to several climate change factors, and the recent
sequencing of silver birch genome (Salojärvi et al. 2017) with several candidate
genes for adaptation to different environmental variables (such as temperature and
precipitation), makes silver birch an attractive model for forest biotechnology and
breeding. During the last decades, Populus species have been extensively studied for
acclimation and adaptation, for example, for dry and wet conditions (Street et al.
2006). Both species are considered typical pioneer species, able to quickly colonize
disturbed sites (e.g. Worrell 1995).
The FAHM study showed the importance of long-term experiments, because the
responses to low VPD were dependent on the prevailing weather conditions. Further,
several indirect effects of low VPD through altered soil processes (moisture,
microbiota, nitrification) and the competitive status of trees became apparent. High
levels of antioxidants and phenolic compounds under low VPD indicate that trees
employed defence against stress factors such as nutrient limitation, soil hypoxia and
exposure to fungal attacks. The high contents of carbohydrates and phenolics in the
green leaves and leaf litter with the reduced foliage biomass and mineral nutrient
content can further affect the decomposition rates and carbon and nutrient cycling of
the forest ecosystems.
330
E. Oksanen et al.