DISCUSSION
Warming reduces dew formation and changes its seasonal
patterns
Using three distinct measurement methods, our study showed that warming
significantly reduces the dew amount (Fig. 1), which may have
substantial impacts on plant growth, especially during the dry period in
the alpine and dryland ecosystems (Stone 1957, Benasher et al. 2010,
Rodney et al. 2013). Warming can reduce dew formation in two ways: by
hindering dew condensation and shortening dew retention. Warming can
hinder the dew condensation processes by decreasing the air humidity and
increasing evaporation (Oliveira 2013, Li et al. 2018). Additionally,
warming changes the air temperature, dew point temperature and dew point
depression (Fig. 2b), which makes it more difficult for the air
temperature to approach the dew point temperature (Beysens 1995, Jacobs
et al. 2006). Warming can also accelerate the dew evaporation process
(Xiao et al, 2013). Dew droplets lasted for a shorter period of time
under warmer temperatures, which also led to a lower dew duration or
amount (Xu and Tang 2015).
In this study, we found that warming not only reduces dew formation but
also changes its seasonal variation (Fig. 2). Therefore, plants growing
under water stress would have higher risks of not surviving under
warming conditions (Rodney et al. 2013, Tomaszkiewicz et al. 2017).
Overall, under the rapidly changing climate, changes in dew formation
should be considered an important environmental factor and should not be
neglected in arid and cold regions.
Dew formation varied among different functional groups under
ambient and warming
conditions
Functional groups create different microenvironments and have different
water use strategies to influence the dew production and absorption by
plants (Zhuang & Zhao 2017, Wang et al. 2019). Environmental
conditions, such as temperature, relative humidity and wind speed,
change due to various micromorphological features and distribution
patterns among different functional groups (Agam & Berliner 2006),
affecting dew formation and duration (Ninari & Berliner 2002). Our
results showed that different functional groups had different degrees of
dew formation, consistent with our expectations.
To date, few studies have investigated how biotic factors (e.g., plant
traits and functional groups) affect dew formation. Here, we examined
the effects of plant traits (i.e., plant height and aboveground biomass)
on dew formation in different plant functional groups (sedges, forbs and
grasses) and found that sedges and forbs with shorter heights are
associated with less dew than grasses with taller heights under natural
conditions (Fig. 3). Because under ambient conditions, the upper canopy
air temperature is lower at night due to this area receiving less
land-surface radiation, dew formation occurs earlier in higher leaves,
such as those of grasses (Zhang et al. 2009, Wang et al. 2017). In
addition, the dominant taller species (Stipa aliena , Elymus
nutans , and Helictotrichon tibeticum ) usually have more
aboveground biomass (Konrad et al. 2015, Ma et al. 2017) than shorter
species, which can facilitate dew formation and retention (Pan et al.
2016). Additionally, the dew water stored within a
dense
canopy
can be preserved for a longer period of time through the reduction in
evaporation (Xiao et al, 2013).
Under warming conditions, the aboveground biomass and plant height
increased, and the community composition changed with a higher
prevalence of grass in the alpine ecosystems (Liu et al. 2018). Such
changes should be beneficial for dew formation based on our findings
under ambient conditions (i.e., results from the control plots, Fig.
4a). However, a substantial reduction in dew formation was observed
under the warming treatments (Fig. 1 and Fig. 2). In addition, we found
that warming resulted in a lower dew amount on taller plants, in
contrast to the results under ambient conditions (Fig. 4). Warming
changed the relationship between plant height and dew amount in both
direct and indirect ways. Warming directly affected the air temperature
profile and made dew formation more difficult (Wolkovich et al. 2012).
In this case, the taller plants had less dew formation because
artificial infrared heating made the temperature of the taller canopy
higher than that of the lower canopy (Xiao et al, 2013). Warming
indirectly caused the soil moisture to evaporate more quickly during the
night (Tomaszkiewicz et al. 2015, Li et al. 2018). Therefore, the
shorter plants experienced more dew collection than the higher plants
during the night under warming conditions. Clearly, warming influenced
the dew formation on plants and changed the ecosystem processes compared
with those under natural conditions.
Infrared heater warming system reduces dew formation: an
overlooked factor in climate change
studies
There have been many studies about the response of ecosystem processes
to climate change using various artificial warming methods in dry
ecosystems (Kimball et al. 2018, Song et al. 2019, Korell et al. 2019),
but the possible impacts from the differences between artificial and
natural warming on the experimental results have often been overlooked.
Our results showed that artificial warming (with an infrared heater
warming system) affects dew formation, which likely affects ecosystem
processes (Liu et al. 2016). However, it is worth noting that natural
climate warming and the infrared heater warming system differ in terms
of their heat-dissipating pathways (Korell et al. 2019). Artificial
warming generates more heat radiation in the air and drier
micro-environments than natural warming (Liu et al. 2018). This
difference will affect a number of ecosystem processes and is often
overlooked across simulated climate change experiments. Warming makes
plants grow taller (Liu et al. 2018), but taller plants produced less
dew under warming in our study (Fig. 3). This indicates that the dew
formation was significantly reduced under the experimental warming
conditions. In addition, the relationship between dew formation and
plant height changed being positively correlated under the control
treatment to being negatively correlated under the warming treatment
(Fig. 4). For such cases, the conclusions of the impacts of warming
obtained by artificial warming experiments may deviate from the actual
impacts of warming on ecosystem processes. Under future climate warming,
the changes in water condensation will also have an especially profound
impact on the ecosystem patterns and processes in dryland ecosystems (Li
et al. 2018, Wang et al. 2017). Therefore, we suggest that the impact of
experimental warming on dew formation should be considered an important
environmental factor affecting ecosystems processes during climate
warming.