Scandinavia and large areas of Eurasia. Betula species are among the key species in
the responses of northern ecosystems to climate change. The genome of silver birch
is now assembled (Salojärvi et al. 2017), and it is an attractive target for modern
genomics-based biotechnology, phenotyping and research on resistance traits and
adaptation to climate change. Hybrid aspen is the fastest-growing deciduous tree in
the boreal zone, and it is regarded as one of the most promising species for shortrotation forestry in the northern Europe (Tullus et al. 2012).
In the humidified plots, RH is elevated with a misting pipe system producing fine
water droplets (diameter ~10 μm), leading to on average 7% higher RH in the long
term compared with the ambient level (Fig. 1). The droplet size was optimized to
minimize leaf wetting and temperature decline in the humidified plots. Misting was
started in 2008 and has been applied throughout the growing seasons between May
and October until today (Kupper et al. 2011; Tullus et al. 2012). The first generation
of trees was harvested at the end of 2011 (birch) and 2012 (aspen). The second
generation of birches was planted on the site in 2012 and new aspen saplings
regenerated naturally from the stump sprouts.
3.2 Ecophysiological Responses in the FAHM Experiment
The results from FAHM experiment demonstrate slower tree growth under low VPD
(Fig. 2) both in silver birch and hybrid aspen (Sellin et al. 2017a; Tullus et al. 2017).
However, the radial growth of birch stems was less affected in comparison with the
height growth, leading to lower slenderness (i.e. increased ratio of stem width to
height) under high humidity. Lower slenderness is in line with the carbon allocation
to water-conducting vascular tissues at the expense of photosynthetic leaf area and
increasing proportion of the living parenchyma cells and rays in stem wood relative
to dead xylem elements. These morphological and anatomical changes in the stem
wood tissues favour radial growth to provide mechanical stability and compensate
for the reduced hydraulic conductance but increase the maintenance respiration costs
(due to the increased number of living parenchyma cells) (Sellin et al. 2017a). The
decline of leaf hydraulic conductance was demonstrated for silver birch growing
under low VPD (Sellin et al. 2013). Air humidification reduced the hydraulic
conductance (Sellin et al. 2017b) and changed the xylem anatomy also in hybrid
aspen (Jasińska et al. 2015). Decreased hydraulic capacity becomes a disadvantageous factor for the trees grown under low VPD, limiting leaf water supply and
reducing canopy conductance at least under the conditions of soil water deficit
(Niglas et al. 2014). The effects of elevated humidity on the chemical properties of
the stem wood varied between the tree species (Tullus et al. 2014). In aspen,
cellulose content was higher and hemicellulose content was lower, whereas in
birch, cellulose content was lower under high humidity (Tullus et al. 2014).
Naturally, there was a year-to-year variation in growth responses, due to the
prevailing weather conditions. Humidification reduced the growth rate of aspen and
birch trees during normal rainy seasons (Kupper et al. 2011; Tullus et al. 2012; Sellin
Northern Forest Trees Under Increasing Atmospheric Humidity
325
the responses of northern ecosystems to climate change. The genome of silver birch
is now assembled (Salojärvi et al. 2017), and it is an attractive target for modern
genomics-based biotechnology, phenotyping and research on resistance traits and
adaptation to climate change. Hybrid aspen is the fastest-growing deciduous tree in
the boreal zone, and it is regarded as one of the most promising species for shortrotation forestry in the northern Europe (Tullus et al. 2012).
In the humidified plots, RH is elevated with a misting pipe system producing fine
water droplets (diameter ~10 μm), leading to on average 7% higher RH in the long
term compared with the ambient level (Fig. 1). The droplet size was optimized to
minimize leaf wetting and temperature decline in the humidified plots. Misting was
started in 2008 and has been applied throughout the growing seasons between May
and October until today (Kupper et al. 2011; Tullus et al. 2012). The first generation
of trees was harvested at the end of 2011 (birch) and 2012 (aspen). The second
generation of birches was planted on the site in 2012 and new aspen saplings
regenerated naturally from the stump sprouts.
3.2 Ecophysiological Responses in the FAHM Experiment
The results from FAHM experiment demonstrate slower tree growth under low VPD
(Fig. 2) both in silver birch and hybrid aspen (Sellin et al. 2017a; Tullus et al. 2017).
However, the radial growth of birch stems was less affected in comparison with the
height growth, leading to lower slenderness (i.e. increased ratio of stem width to
height) under high humidity. Lower slenderness is in line with the carbon allocation
to water-conducting vascular tissues at the expense of photosynthetic leaf area and
increasing proportion of the living parenchyma cells and rays in stem wood relative
to dead xylem elements. These morphological and anatomical changes in the stem
wood tissues favour radial growth to provide mechanical stability and compensate
for the reduced hydraulic conductance but increase the maintenance respiration costs
(due to the increased number of living parenchyma cells) (Sellin et al. 2017a). The
decline of leaf hydraulic conductance was demonstrated for silver birch growing
under low VPD (Sellin et al. 2013). Air humidification reduced the hydraulic
conductance (Sellin et al. 2017b) and changed the xylem anatomy also in hybrid
aspen (Jasińska et al. 2015). Decreased hydraulic capacity becomes a disadvantageous factor for the trees grown under low VPD, limiting leaf water supply and
reducing canopy conductance at least under the conditions of soil water deficit
(Niglas et al. 2014). The effects of elevated humidity on the chemical properties of
the stem wood varied between the tree species (Tullus et al. 2014). In aspen,
cellulose content was higher and hemicellulose content was lower, whereas in
birch, cellulose content was lower under high humidity (Tullus et al. 2014).
Naturally, there was a year-to-year variation in growth responses, due to the
prevailing weather conditions. Humidification reduced the growth rate of aspen and
birch trees during normal rainy seasons (Kupper et al. 2011; Tullus et al. 2012; Sellin
Northern Forest Trees Under Increasing Atmospheric Humidity
325