(central Europe, the USA, South America) (Simmons et al. 2010; IPCC 2013). The
regional and temporal differences in changes can be large both in magnitude and
direction, as indicated, e.g. by Fatichi et al. (2015), reporting a declining trend in RH
from January to June but an increasing trend in RH from July to August over the
recent decades 1981–2011 in central Europe (Switzerland). The increase in RH is
closely associated with increasing precipitation and cloud cover, with decreasing
atmospheric pressure (Boé and Terray 2014; Fatichi et al. 2015) and the whole
hydrological cycle (Willett et al. 2007). Atmospheric water vapour has an important
role as a greenhouse gas absorbing solar heat and holding it in the troposphere, with
a positive feedback on the global warming (Dessler et al. 2013). VPD is closely
related to plant productivity; VPD explains more of the variation in the primary
production of vegetation than soil temperature, air temperature or soil moisture does
(Eamus et al. 2013; Fu and Shen 2016). Therefore, air humidity is an important
climatic variable that should be taken into account when estimating the climate
change impacts on plant production (Zhang et al. 2017a).
Info Box
There are several definitions and units describing the atmospheric water
vapour content, i.e. air humidity. Relative humidity (RH, %) and vapour
pressure deficit (VPD, Pa) are the most commonly used definitions. RH
describes the actual water vapour content relative to maximum water vapour
content of air at a certain temperature. Relative humidity of 100% means that
the maximum water vapour holding capacity of air is reached, and at this
saturation point (also called a dew point), water condensates onto solid
surfaces or into fog and clouds and precipitates. Humidity can be expressed
as VPD, describing the difference between the actual water vapour pressure
and the maximum vapour pressure at the saturation point at a certain temperature. Increasing RH decreases VPD and atmospheric evaporative demand,
which drives water evaporation from soil and plants, i.e. evapotranspiration.
Evapotranspiration rate depends also on temperature, solar radiation and wind
speed and is closely related to plant functioning. Absolute humidity is the
actual water vapour content of air, whereas specific humidity is the ratio of the
mass of water vapour to the total mass of air. Specific humidity follows closely
Clausius-Clapeyron equation: the maximum water vapour capacity of air
increases exponentially with temperature. Therefore, it is predicted that specific humidity will increase in the future due to global warming, especially at
the northern latitudes (IPCC 2013).
High RH ¼ low VPD
Low RH ¼ high VPD
Northern Forest Trees Under Increasing Atmospheric Humidity
319
regional and temporal differences in changes can be large both in magnitude and
direction, as indicated, e.g. by Fatichi et al. (2015), reporting a declining trend in RH
from January to June but an increasing trend in RH from July to August over the
recent decades 1981–2011 in central Europe (Switzerland). The increase in RH is
closely associated with increasing precipitation and cloud cover, with decreasing
atmospheric pressure (Boé and Terray 2014; Fatichi et al. 2015) and the whole
hydrological cycle (Willett et al. 2007). Atmospheric water vapour has an important
role as a greenhouse gas absorbing solar heat and holding it in the troposphere, with
a positive feedback on the global warming (Dessler et al. 2013). VPD is closely
related to plant productivity; VPD explains more of the variation in the primary
production of vegetation than soil temperature, air temperature or soil moisture does
(Eamus et al. 2013; Fu and Shen 2016). Therefore, air humidity is an important
climatic variable that should be taken into account when estimating the climate
change impacts on plant production (Zhang et al. 2017a).
Info Box
There are several definitions and units describing the atmospheric water
vapour content, i.e. air humidity. Relative humidity (RH, %) and vapour
pressure deficit (VPD, Pa) are the most commonly used definitions. RH
describes the actual water vapour content relative to maximum water vapour
content of air at a certain temperature. Relative humidity of 100% means that
the maximum water vapour holding capacity of air is reached, and at this
saturation point (also called a dew point), water condensates onto solid
surfaces or into fog and clouds and precipitates. Humidity can be expressed
as VPD, describing the difference between the actual water vapour pressure
and the maximum vapour pressure at the saturation point at a certain temperature. Increasing RH decreases VPD and atmospheric evaporative demand,
which drives water evaporation from soil and plants, i.e. evapotranspiration.
Evapotranspiration rate depends also on temperature, solar radiation and wind
speed and is closely related to plant functioning. Absolute humidity is the
actual water vapour content of air, whereas specific humidity is the ratio of the
mass of water vapour to the total mass of air. Specific humidity follows closely
Clausius-Clapeyron equation: the maximum water vapour capacity of air
increases exponentially with temperature. Therefore, it is predicted that specific humidity will increase in the future due to global warming, especially at
the northern latitudes (IPCC 2013).
High RH ¼ low VPD
Low RH ¼ high VPD
Northern Forest Trees Under Increasing Atmospheric Humidity
319