nutrients and the higher number of the living parenchyma cells (Tullus et al. 2014).
Impaired leaf nutrient status and soil hypoxia caused by increased soil wetness led
to the retardation of foliar development (leaf area and mass), which was clearly
demonstrated at leaf, tree and stand levels in both species, having a direct negative
impact on productivity (Sellin et al. 2017a). High humidity reduced bud size and
primordia growth in silver birch, but these changes did not translate into reduced leaf
area in the current-year shoots (Kukk et al. 2015). Nevertheless, changes in leaf
morphogenesis in response to high humidity can have long-term effects on the crown
development, including a trend towards increased branchiness (Sellin et al. 2015).
Humidification stimulated bud break in aspen (Kukk et al. 2015) and delayed leaf
fall in birch (Godbold et al. 2014). The prolonged leaf lifespan enables trees to
photosynthesize and to acquire and resorb mineral nutrients for a longer time, which
may compensate for the reduced photosynthetic capacity and mineral nutrient status.
The root systems of birch trees showed structural and morphological adjustments
in response to high air humidity (Rosenvald et al. 2014; Godbold et al. 2014).
Increased root hydraulic capacity (Sellin et al. 2015), greater fine-root biomass,
higher root-specific area and higher turnover rate may be considered to mitigate
low nutrient gaining due to reduced transpirational flux under elevated humidity.
Acclimation of root morphological traits of silver birch in humidified plots depends
on the type of the understory vegetation (Parts et al. 2013). For example, specific
root length and density were very high in the humidified plots with the earlysuccessional grass understory. Silver birch displayed a better acclimation capacity
to high humidity conditions than hybrid aspen did. One of the explanations for this
could be the natural variation in silver birch stands and thereby higher plasticity
compared with monoclonal hybrid aspens (Rosenvald et al. 2014).
3.3 Metabolic Responses in the FAHM Experiment
Leaves play a key role in plant responses to variable environmental conditions. The
foliar metabolite profiles of trees grown under ambient and reduced VPD condition
were analysed by gas chromatography-mass spectrometry (GC-MS) (Lihavainen
et al. 2016a). There was a shift in the metabolite profiles of aspen and birch leaves in
response to humidification treatment. The changes in metabolite levels were subtle,
and there were significant differences in 28 metabolites in birch and 6 metabolites in
aspen (Lihavainen et al. 2016a). Metabolite responses were largely similar in both
species but stronger in birch. The differences appeared in carbohydrates, amino
acids, ascorbate metabolites and secondary metabolites. The high starch content in
the leaves indicated an increased carbon storage activity at the expense of biomass
accumulation, sugar transport and respiration processes. This was supported by the
reduced dark respiration rate of the second-generation stump sprouts of hybrid aspen
in the humidified plots (Rohula et al. 2017).
Changes in N assimilation were observed in amino acid profiles, in particular as a
lower glutamine-to-glutamate ratio in the low VPD conditions, reflecting a reduced
Northern Forest Trees Under Increasing Atmospheric Humidity
327
Impaired leaf nutrient status and soil hypoxia caused by increased soil wetness led
to the retardation of foliar development (leaf area and mass), which was clearly
demonstrated at leaf, tree and stand levels in both species, having a direct negative
impact on productivity (Sellin et al. 2017a). High humidity reduced bud size and
primordia growth in silver birch, but these changes did not translate into reduced leaf
area in the current-year shoots (Kukk et al. 2015). Nevertheless, changes in leaf
morphogenesis in response to high humidity can have long-term effects on the crown
development, including a trend towards increased branchiness (Sellin et al. 2015).
Humidification stimulated bud break in aspen (Kukk et al. 2015) and delayed leaf
fall in birch (Godbold et al. 2014). The prolonged leaf lifespan enables trees to
photosynthesize and to acquire and resorb mineral nutrients for a longer time, which
may compensate for the reduced photosynthetic capacity and mineral nutrient status.
The root systems of birch trees showed structural and morphological adjustments
in response to high air humidity (Rosenvald et al. 2014; Godbold et al. 2014).
Increased root hydraulic capacity (Sellin et al. 2015), greater fine-root biomass,
higher root-specific area and higher turnover rate may be considered to mitigate
low nutrient gaining due to reduced transpirational flux under elevated humidity.
Acclimation of root morphological traits of silver birch in humidified plots depends
on the type of the understory vegetation (Parts et al. 2013). For example, specific
root length and density were very high in the humidified plots with the earlysuccessional grass understory. Silver birch displayed a better acclimation capacity
to high humidity conditions than hybrid aspen did. One of the explanations for this
could be the natural variation in silver birch stands and thereby higher plasticity
compared with monoclonal hybrid aspens (Rosenvald et al. 2014).
3.3 Metabolic Responses in the FAHM Experiment
Leaves play a key role in plant responses to variable environmental conditions. The
foliar metabolite profiles of trees grown under ambient and reduced VPD condition
were analysed by gas chromatography-mass spectrometry (GC-MS) (Lihavainen
et al. 2016a). There was a shift in the metabolite profiles of aspen and birch leaves in
response to humidification treatment. The changes in metabolite levels were subtle,
and there were significant differences in 28 metabolites in birch and 6 metabolites in
aspen (Lihavainen et al. 2016a). Metabolite responses were largely similar in both
species but stronger in birch. The differences appeared in carbohydrates, amino
acids, ascorbate metabolites and secondary metabolites. The high starch content in
the leaves indicated an increased carbon storage activity at the expense of biomass
accumulation, sugar transport and respiration processes. This was supported by the
reduced dark respiration rate of the second-generation stump sprouts of hybrid aspen
in the humidified plots (Rohula et al. 2017).
Changes in N assimilation were observed in amino acid profiles, in particular as a
lower glutamine-to-glutamate ratio in the low VPD conditions, reflecting a reduced
Northern Forest Trees Under Increasing Atmospheric Humidity
327