under drought or heat stress. Transcriptomic analysis under combined drought and
heat stress demonstrates that low VPD enhances the heat-specific gene responses and
alleviates drought-related responses. This is in accordance with the reduced transpirational cooling under low VPD. High relative humidity can mitigate the negative
effects of salt stress and improve the ion homeostasis, as seen in mangrove trees
(Rhizophora apiculata and R. stylosa), melon (Cucumis melo) cultivars, Jatropha
curcas and cucumber (Ball et al. 1997; An et al. 2002; Rodrigues et al. 2016;
Shibuya et al. 2017).
3 High Humidity Experiments with Forest Trees and Forest
Ecosystems
While the responses of trees to increasing temperature and CO 2 are already well
documented (e.g. Saxe et al. 2001; Ainsworth and Long 2005; Leakey et al. 2009),
the impact of changes in atmospheric humidity on forest trees and ecosystems is
much less studied. The increase in daytime VPD is shown to affect transpiration and
forest net primary production more than the temperature (Eamus et al. 2013).
Evidence is accumulating that also decreasing tendency in VPD during the growing
season will have an impact on the growth of northern forests, evapotranspiration,
ecosystem functions and global carbon dynamics.
Leaves of forest trees are frequently wetted by rain, dew or fog. The condensation
of water onto the leaf surface impairs stomatal functioning, gas exchange and
photosynthesis (Ishibashi and Terashima 1995). High humidity and prolonged
water retention on the leaf surface promote fungal pathogen attacks (Sturrock et al.
2011). In addition, a high influx of phytotoxic air pollutants through the open
stomata under low VPD may have harmful impact on trees. For example, in sugar
maple (Acer saccharum) and white ash (Fraxinus americana), photosynthesis was
more repressed when sulphur dioxide (SO 2 ) and ozone (O 3 ) fumigations were
conducted under low VPD compared with high VPD (Carlson 1979). Similarly,
SO 2 fumigation under low VPD caused stronger growth inhibition, leaf necrosis and
leaf abscission in paper birch (Betula papyrifera) compared with fumigation under
high VPD (Norby and Kozlowski 1982). Thus, increasing air humidity due to
climate change can increase the susceptibility of trees to fungal diseases and air
pollutants.
Forest trees can experience low or high VPD over long time periods: for days,
weeks or months. Therefore, long-term experiments are required to study the
ecophysiological responses to VPD changes. A special experimental facility for
Free Air Humidity Manipulation (FAHM) was established in Estonia to investigate
the long-term effects of increasing air humidity on tree performance and on the
functioning of the deciduous forest ecosystem (Kupper et al. 2011). The first studies
on the effects of low VPD with several tree species showed a general decrease in leaf
and canopy transpiration rates (Meinzer 2003; Hölscher et al. 2005; Kupper et al.
Northern Forest Trees Under Increasing Atmospheric Humidity
323
heat stress demonstrates that low VPD enhances the heat-specific gene responses and
alleviates drought-related responses. This is in accordance with the reduced transpirational cooling under low VPD. High relative humidity can mitigate the negative
effects of salt stress and improve the ion homeostasis, as seen in mangrove trees
(Rhizophora apiculata and R. stylosa), melon (Cucumis melo) cultivars, Jatropha
curcas and cucumber (Ball et al. 1997; An et al. 2002; Rodrigues et al. 2016;
Shibuya et al. 2017).
3 High Humidity Experiments with Forest Trees and Forest
Ecosystems
While the responses of trees to increasing temperature and CO 2 are already well
documented (e.g. Saxe et al. 2001; Ainsworth and Long 2005; Leakey et al. 2009),
the impact of changes in atmospheric humidity on forest trees and ecosystems is
much less studied. The increase in daytime VPD is shown to affect transpiration and
forest net primary production more than the temperature (Eamus et al. 2013).
Evidence is accumulating that also decreasing tendency in VPD during the growing
season will have an impact on the growth of northern forests, evapotranspiration,
ecosystem functions and global carbon dynamics.
Leaves of forest trees are frequently wetted by rain, dew or fog. The condensation
of water onto the leaf surface impairs stomatal functioning, gas exchange and
photosynthesis (Ishibashi and Terashima 1995). High humidity and prolonged
water retention on the leaf surface promote fungal pathogen attacks (Sturrock et al.
2011). In addition, a high influx of phytotoxic air pollutants through the open
stomata under low VPD may have harmful impact on trees. For example, in sugar
maple (Acer saccharum) and white ash (Fraxinus americana), photosynthesis was
more repressed when sulphur dioxide (SO 2 ) and ozone (O 3 ) fumigations were
conducted under low VPD compared with high VPD (Carlson 1979). Similarly,
SO 2 fumigation under low VPD caused stronger growth inhibition, leaf necrosis and
leaf abscission in paper birch (Betula papyrifera) compared with fumigation under
high VPD (Norby and Kozlowski 1982). Thus, increasing air humidity due to
climate change can increase the susceptibility of trees to fungal diseases and air
pollutants.
Forest trees can experience low or high VPD over long time periods: for days,
weeks or months. Therefore, long-term experiments are required to study the
ecophysiological responses to VPD changes. A special experimental facility for
Free Air Humidity Manipulation (FAHM) was established in Estonia to investigate
the long-term effects of increasing air humidity on tree performance and on the
functioning of the deciduous forest ecosystem (Kupper et al. 2011). The first studies
on the effects of low VPD with several tree species showed a general decrease in leaf
and canopy transpiration rates (Meinzer 2003; Hölscher et al. 2005; Kupper et al.
Northern Forest Trees Under Increasing Atmospheric Humidity
323