4. Discussion
4.1. Similar air temperature
vs. different ground temperatures
There were no significant differences in daily air temperatures between
the two sites,
presumably
due to their geographic proximity and mixing of air in the study area
(Table 1). In contrast, differences in ground temperatures were
significant (Figures
6b
and 7). The annual mean difference in Ts or
Tns between sites was over 1.3
°C,
and the ground at the sunny slope
was significantly warmer (1-2 °C) than the shady slope.
There was little
diurnal
variation in Ts at the shady slope
compared
to the sunny slope (Figure 12). The
daily
amplitude of Ts was usually ~5 °C
at the shady slope, with a
maximum
value of ~10 °C in the warm season (Figures 12f-i), and
a minimum value of ~2 °C in the cold
season
(Figures
12j-l). In contrast, the daily amplitude of Ts was
usually 10-15 °C
at
sunny slope, with a maximum close to 20 °C in the warm season and a
minimum of ~5 °C in the cold season. The lower
Ts throughout the day at the shady slope in the cold
season indicate that there is little available heat to offset surface
losses at night. As a result the surface offset
(ΔTs=Ts-Ta)
was significantly lower at the shady slope than that at the sunny slope
(Figure 13).
The surface offset
(ΔTs)
is a function of the surface energy balance and is controlled by factors
such as vegetation, snow cover, moisture availability, and topography
(Eaton et al. 2001; Beltrami and Kellman 2003). The mean surface offset
in 2016-19 was
3.54±0.07
°C
at
the sunny slope and 1.91±0.12 °C at the shady slope. A
~1.5
°C
difference in ΔTs further highlights the warmer ground
surface conditions at the sunny slope.
The results indicate that permafrost degradation should consider
differences in surface temperature caused by microenvironment, and
particularly the slope aspect in mountainous areas. In this study slope
aspect was associated with a difference in annual mean surface
temperature of about 1-2 °C, and similar studies have reported that the
difference in mean ground surface temperature reached 4.6 °C at Tianshan
Mountain, China (Cheng 2003) and 3-4 °C in northern Mongolia
(Munkhjargal et al. 2020). The data will help inform realistic boundary
conditions for modelling permafrost degradation in mountain permafrost
areas.
4.2 Seasonal dynamics of water and
thermal at
surface
Our observations of relations between Ta and
Ts were similar to studies at flat sites in Beiluhe
basin, QTP (Lin et al. 2019).
With
the increase in air temperature in the middle of January, the ground
began to warm, but with a lag of about one week.
Tsneared 0 °C in the middle of April, after which there was a rapid
increase in soil moisture induced by downward thawing.
Ts reached the maximum value in July and August, at
which time the sunny slope was much warmer than the
shady
slope
(Figure
12g and h). The soil moisture content fluctuated drastically with the
arrival of monsoon rains in May to October.
The higher Ts at
the sunny slope may increase the surface evapotranspiration, resulting
in lower soil moisture content at the sunny slope than at the shady
slope.
The
higher soil moisture during the entire thawing period was the main
control on the energy budget. Though measurements of soil heat flux (G)
were attempted in this study, the instruments failed, but G has been
shown to be correlated with Rn in Beiluhe Basin (You et
al. 2017). The Rn continued to increase and reached
their maxima in July-August when the solar altitude is the
highest
(Figure 14), which was
synchronized
with variations in Ts.
Rnin November-January maintained low values with a ~350
W·m-2 of peak value, less than half the maximum value
in May-August (Figure 10).
When
the surface soil froze and vegetation withered in early October, soil
moisture content dropped rapidly. The variation in UR and ULR showed
strong seasonality, but the albedo did not increase rapidly in winter.
The
freeze-thaw
cycle of the wet surface soil at the shady slope may trigger a drastic
energy change in permafrost regions, which is different from the
alternations correlated with the onset of the summer.
The
results are helpful for understanding the dynamic process of surface
water thermal energy in mountain permafrost regions.
4.3 Effect of slope aspect on regional
permafrost
environment
The observational results show the air temperatures were similar at both
sites
(Figure 6a). However,
the
difference of slope direction resulted in different ground temperatures
during the freezing and thawing periods at both sites. It is generally
recognized that net radiation (Rn) on sunny slopes is
greater than on shady slopes. However, in this study Rnat shady slope was slightly
higher
(~13 W·m-2) than on the sunny slope.
The shortwave downward radiation (DR) at the sunny slope was much
greater than at the shady slope. Higher albedo at the sunny slope
resulted in higher upward shortwave radiation (UR).
The differences caused by the slope direction were apparent in soil
moisture measurements. The sunny slope was much warmer,
increasing
the potential evaporation and resulting in drier
soils
(Figure 8). The complex interactions between Ta,
Ts, and surface soil water retention and heat exchange
between the land surface and the lower atmosphere.
Other
partitions in the energy balance (e.g. sensible heat flux (H), latent
heat flux (LE)) were not calculated, but a dramatic increase in LE with
the rapid increase in soil moisture in thawing period was previously
observed on QTP
(You
et al. 2017). Studies have shown that the LE was the main portion of
energy budget in summer (Wang et al. 2005; Chen et al. 2009; Ma et al.
2009). The lower surface temperature at the shady slope reduces heat
input to the boundary layer and decreases the temperature lapse rate in
the lower atmosphere
(Otterman
1974). Although the Rn at the shady slope was slightly
higher (~13 W/m2) than the sunny
slope, the ground surface temperature was lower due to the high soil
heat capacity.
The presence of permafrost supports the development of alpine meadow
ecosystems (Jin et al. 2009; Wang et al. 2009), and is strongly related
to heat insulation resulting from hydrothermal conditions in the active
layer. Soil moisture content decreases with increasing active layer
thickness for the slope in the study. Permafrost warming and degradation
could potentially lead to a reduction in soil moisture and soil nutrient
content, resulting in vegetation degradation and possible
desertification over the QTP
(Xue
et al. 2009).
4.4 Effect of slope aspect onsoil texture and
SOM
A high frequency of
freeze-thaw
cycles is effective for mechanical weathering near-surface bedrock or
coarse-grained soil, leading to spalling of relatively small rock
fragments (Matsuoka 1994), though lack of moisture may considerably
reduce the number of effective freeze-thaw cycles (e.g. Prick 2003).
The
freeze-thaw cycle frequency is proportional to the mechanical weathering
rate of surface materials. The soil texture at surface of sunny slope
was finer with greater than 50% silt and clay combined (Figure 4),
which may be due to a higher freeze-thaw frequency and weathering rate.
In
addition, soil freezing is accompanied by the migration of moisture to
the freezing front (Everett 1961), further driving the movement of fine
soil particles. When the frozen soil is thawing, fine particles also
move as moisture seeps downslope. As a result, each freeze-thaw cycle
will cause soil redistribution that affects soil texture. In general,
fine particles may move downslope more at the sunny slope due to the
high frequency of freeze-thaw cycles (Figure 4a).
Alluvial material coupled with the low temperatures and low
decomposition rates in permafrost regions favors the accumulation of SOM
(Dorfer et al. 2013).
Vegetation
conditions and pedogenesis are usually regarded as the dominant
controlling factors for SOM accumulation (Jobbagy and Jackson 2000). In
the present study, SOM accumulation was much greater at the shady slope
than that at the sunny slope, in contrast with the degree of vegetation
coverage. We hypothesize that pedogenesis had an important effect on the
SOM distribution since the major distinction between the two sites is
the different slope aspect, and
the
vegetation coverage was relatively low at both sites (sunny slope:
16.7%; shady slope: 7.9%). Higher SOM content is expected in the upper
soils at the two sites (Figure 5) because the fallen litter on the
ground surface and the turnover of fine roots are considered to be the
dominant SOM inputs to natural ecosystems (Zollinger et al. 2013). The
observations of SOM were similar to those at Eboling Mountain,
northeastern QTP (Mu et al. 2016). SOM content increased with depth in
the upper permafrost (3-5 m) at the two sites, which may be because
ground ice in this depth range contributes to SOM stabilization
(Jorgenson and Shur, 2009; Grosse et al. 2011).
High
SOM is associated with high proportions of fine soil particles as they
tend to stabilize and retain more organic matter (Figure 4).
Moreover,
the humid soil at the shady slope promotes
microbial
activities that are beneficial to SOM formation (Mu et al. 2016). Thus,
greater SOM accumulated at the shady slope, particularly in the top
portion of the active layer.
4.5 Effect of factors on the
variations within
sites
There
are clear differences in ground temperature (Ts,
Tps, and Tg) between the two sites, and
in maximum seasonal thaw depth, which differed by ~1.0 m
in 2016-19. These differences are related to the effects of local
factors in the study areas. The slope aspect is undoubtedly one of the
key factors (Lin et al. 2019). In addition, vegetation can strongly
control the depth of seasonal thawing in mountainous areas (Wang et al.
2009;
Shiklomanov
et al. 2010; Shiklomanov and Nelson 2013; Chang et al. 2014a; Wang et
al. 2017;
Keuper
et al 2017). The greater leaf area of vegetation at the sunny slope can
reflect and blocks solar radiation, reducing the amount of net radiation
to the ground (Fisher et al. 2016). This can have an important effect on
surface temperature variations (Chang et al. 2012). The well-vegetated
surface is covered with an insulative organic layer, which has a net
cooling effect on the ground surface (Cannone and Guglielmin 2009) and
slows the rate of change in the active layer temperature (Fisher et al.
2016). In the alpine meadow regions on QTP vegetation can also slow down
the response of permafrost to climate warming through the greater water
retention capacity of its root zone (Wang et al. 2009). Variation in
vegetation cover will also indirectly affect the active layer thermal
regime by affecting soil properties and snow cover (Chang et al. 2014b;
Fisher et al. 2016). Though the plant heights (<15 cm) and
coverage (8-16%) are relatively low, variations in vegetation
conditions still likely impact the thermal regime of the active layer
and near-surface permafrost at the study sites (Lin et al. 2019).
Seasonal thaw depths are also related to the soil thermal properties,
which depend on soil density, porosity, and texture (Pang et al. 2011;
Li et al. 2018). Variations in soil moisture caused by the infiltration
of summer precipitation may reduce the seasonal amplitude of soil
temperatures, decrease mean annual soil temperatures, and thin active
layers (Luo et al. 2020). For the shady slope, a cooling effect may thus
be caused by liquid water that accumulated and persisted within the
active layer.