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The hydrothermal relationship between vegetation and seasonal frozen soil is one of the key research contents in the fields of permafrost ecological environment, hydrology and climate change in alpine mountainous areas. Based on the monitoring data of air temperature, precipitation and soil hydrothermal conditions at the depth of 0–5 m from site TS-04 (with high vegetation coverage) and site TS-05 (with low vegetation coverage) in the alpine grassland of the Tianshan Mountains, this study compared and analyzed the characteristics of freezing-thawing process, temperature and moisture changes of seasonal frozen soil with different vegetation coverage. The results show that the maximum seasonal freezing depth of the two sites is almost comparable, but site TS-04 has a smaller freezing and thawing rate, and a shorter duration of freeze-thaw at all depths. TS-04 also has a smaller annual range of surface temperature and ground-air temperature difference. The analysis indicates that vegetation acts as a thermal buffer and has a good thermal insulation effect on the ground surface. Site TS-04 had high unfrozen water content in the unfrozen period and the water content increased with depth, while the unfrozen water content was low in site TS-05. In addition, the thresholds of soil water content response to rainfall events at 5 cm depth of site TS-04 and site TS-05 were 5 and 11 mm precipitation respectively, which indicated that the high vegetation coverage is conducive to rainwater infiltration, and the underlying soil of the site has a faster response to rainfall events.
Many factors affect the soil hydrothermal state in the alpine ecosystem. The coupling of various factors (e.g., air temperature, precipitation, soil properties) results in variations in the subsurface soil at different temporal-spatial scales (
The variations in subsurface soil hydrothermal processes in alpine regions can lead to anomalies in the land–air energy and water balance, which can significantly affect surface hydrological processes, the ecological environment, carbon–nitrogen cycles, and climate systems in the Qinghai-Tibet Plateau and its surrounding areas (
The Tianshan Mountains are the largest mountain range in Central Asia, where permafrost, seasonal frozen soil, and alpine steppe are prominent in high altitude areas. The Tianshan Mountains and the Qinghai-Tibet Plateau feature different permafrost types. The permafrost areas of the Qinghai-Tibet Plateau have high-plain permafrost, while the permafrost areas of the Tianshan Mountains have mountain-type permafrost. At present, domestic and foreign scholars have done a lot of research on the soil hydrothermal characteristics of the Qinghai Tibet Plateau, but the soil hydrothermal state in the Tianshan area is not yet clear. In the mountainous permafrost region of the Tianshan Mountains, how vegetation impacts the soil water and heat conditions (such as soil freeze–thaw and soil moisture phase changes) is still poorly understood. In this study, we used a large amount of field monitoring data from typical alpine grassland ecosystems in the hinterland of the Tianshan Mountains to study the differences of soil hydrothermal dynamic responses to vegetation coverage changes in typical seasonal frozen regions.
The study area is located in the Kuixiandaban area, Tianshan Mountains, Xinjiang Province. This area has a typical continental dry climate (
Location of the study area and observation sites.
The elevations of monitoring points TS-04 (42.97°N, 86.84°E) and TS-05 (42.94°N, 86.84°E) are 3,145 and 2,949 m, respectively. Both stations are located on the southern slope of the watershed between the north and south slopes of the Tianshan Mountains within the source area of the Ulastai River. The TS-04 site is located on the alluvial and diluvial platform in the Wusite Valley, 3.2 km from the Wusite Railway Bridge. On the east and west sides of the site, the rivers flowing southwest and southeast meet at 100 m south of the site, so the water conditions of the site are good. According to the drilling data, the main types and ranges of soil textures at the TS-04 site are black brown clay (0–0.3 m), fine sand (0.3–0.7 m), rock (0.7–1.3 m), sand stone (1.3–2 m), sand (2–3 m); the TS-05 site has fine sandy soil (0–0.3 m), clay (0.3–0.6 m), fine sand (0.6–1.2 m), and coarse sand with gravel (1.2–3 m). Field investigations show that the two study sites are not in the permafrost area, which is typically frozen seasonally.
The vegetation types of both sites are alpine steppe. The dominant plant species are
The research period spans September 2017 to August 2018. The main observations include air temperature, snow depth, precipitation, soil temperature, and water content (
List of measurements used in this study.
Observational elements | Instrument type | Accuracy |
---|---|---|
Air temperature and relative humidity | HMP155A-L, Vaisala | ± 0.2°C; 2%RH |
Wind speed and wind direction | 05103, R. M. Young | ± 0.3 m/s; ± 3° |
Atmospheric pressure | CS106, Vaisala | ± 1.5 hPa |
Percipitation | T-200B, Vaisala | ± 0.1 mm |
Snow depth | SR50A, Campbell | ± 1 cm |
Soil heat flux | HFP01-10, HUKSEFLUX | 50 μV/W·m-2 |
Soil temperature | SKLFSE-TS | ± 0.05°C |
Unfrozen soil water | CS616, Campbell | ± 2% |
According to the accuracy of the rain gauge, when the precipitation was greater than 0.2 mm within 6 h, a new precipitation–soil moisture response process started. Six hours represents the interval between two consecutive precipitation periods; generally 6–8 h is used in the literature (
Diurnal changes in soil temperature may cause soil moisture to increase at a rate of 0.1% every 30 min during the day, so we defined a 0.2% increase in soil moisture within 30 min as the response threshold of soil moisture to precipitation (
The freeze–thaw process of soil can be divided into four stages, regardless of the soil salinity and texture, according to the daily maximum and minimum temperatures of the soil: thawed period, Tmin>0°C; frozen period, Tmax<0°C; freezing period, Tmax >0°C and Tmin <0°C; thawing period, Tmax >0°C and Tmin <0°C. The freezing and thawing periods are also called the daily freeze–thaw cycle (
The meteorological conditions of the two stations are very similar, and the trend in the data is nearly the same (
Comparison of meteorological data between TS-04 and TS-05 stations. (WS, Wind Speed; Ta, Air temperature; RH, Relative humidity).
Soil heat flux is an important thermal index of soil, which can directly reflect the transmission state of soil heat. When the ambient temperature is greater than the soil temperature, the heat passes into the soil from the environment under the action of the temperature gradient, and the soil heat flux is positive. In contrast, when the soil heat flux is negative, the soil releases heat to the environment. The study of heat exchange between soils elucidates the conditions of heat absorption and release in soil and the change of heat storage, which is directly related to the distribution of energy in the atmosphere and soil. These processes also play an important role in the study of climate change.
Monthly mean changes of air temperature (Ta) and 5 cm soil heat flux (G5cm) at TS-04 and TS-05 sites.
At the TS-04 site (
Daily freezing-thawing cycles occurred at 5∼80 cm of TS-04 and TS-05 sites (2017.9.1–2018.9.1).
The time of soil freeze-thaw process at different depths.
Site | Soil depth/cm | Thawed | Freezing | Frozen | Thawing | ||||
---|---|---|---|---|---|---|---|---|---|
Starting and ending time/year-month-day | Duration/day | Starting and ending time/year-month-day | Duration/day | Starting and ending time/year-month-day | Duration/day | Starting and ending time/year-month-day | Duration/day | ||
TS-04 | 5 | 2017/9/1–2017/10/11 | 178 | 2017/10/12–2017/10/21 | 10 | 2017/10/22–2018/3/20 | 150 | 2018/3/21–2018/4/16 | 27 |
2018/4/17–2018/8/31 | |||||||||
20 | 2017/9/1–2017/11/11 | 201 | 2017/11/12–2017/11/19 | 8 | 2017/11/20–2018/4/5 | 137 | 2018/4/6–2018/4/24 | 19 | |
2018/4/25–2018/8/31 | |||||||||
40 | 2017/9/1–2017/12/4 | 207 | 2017/12/5–2017/12/6 | 2 | 2017/12/7–2018/4/22 | 137 | 2018/4/23–2018/5/11 | 19 | |
2018/5/12–2017/8/31 | |||||||||
80 | 2017/9/1–2017/12/27 | 204 | 2017/12/28 | 1 | 2017/12/29–2018/6/4 | 158 | 2018/6/5–2018/6/6 | 2 | |
2018/6/7–2017/8/31 | |||||||||
TS-05 | 5 | 2017/9/1–2017/10/7 | 175 | 2017/10/8–2017/11/15 | 39 | 2017/11/16–2018/2/23 | 100 | 2018/2/24–2018/4/15 | 51 |
2018/4/16–2018/8/31 | |||||||||
20 | 2017/9/1–2017/11/11 | 231 | 2017/11/12 | 1 | 2017/11/13–2018/3/21 | 129 | 2018/3/22–2018/3/25 | 4 | |
2018/3/26–2018/8/31 | |||||||||
40 | 2017/9/1–2017/11/20 | 233 | 2017/11/21 | 1 | 2017/11/22–2018/3/26 | 125 | 2018/3/27–2018/4/1 | 6 | |
2018/4/2–2017/8/31 | |||||||||
80 | 2017/9/1–2017/12/16 | 253 | 2017/12/17 | 1 | 2017/12/18–2018/4/6 | 110 | 2018/4/7 | 1 | |
2018/4/8–2017/8/31 |
At the TS-05 site, the depth of the daily soil freeze–thaw cycle is less than that at the TS-04 site. The daily soil freeze–thaw cycle at TS-05 almost only occurs at the 5-cm layer (
The monitoring results of a complete freeze–thaw cycle were analyzed in
Soil temperature contour map of two sites:
For TS-05 in
Vegetation is the interface of water and heat exchange between Earth and air and plays an important role in energy transfer; the difference in air temperatures can reflect the heat insulation effect of vegetation.
Ground–air temperature difference generally refers to the difference between air temperature and soil temperature at 5 cm; it can accurately reflect the influence of vegetation on the underlying soil temperature. The soil temperature trend at 5 cm was the same as that of air temperature (
The difference of soil temperature at 5 cm with air temperature:
The variation of soil moisture in the seasonal frozen layer reflects the dry and wet conditions of soil and is an important part of the water cycle in the ground–air system. Through the analysis of unfrozen soil water content during the study period in
Isoline map of soil unfrozen water content in two sites:
Change curves of soil unfrozen water content at different depths in TS-04 site
The content of unfrozen water at TS-05 was analyzed. Based on
In addition to the changes of unfrozen soil water content under freeze–thaw, rainfall also plays an important role in supplementing soil moisture. In the rainfall–soil moisture system, the surface characteristics (especially vegetation) directly affect the soil response time to rainfall and the depth of infiltration. To study the influence of different vegetation coverages on the underlying soil moisture, the response time and depth of soil moisture during rainfall events were selected for analysis. As seen in
Monthly accumulated precipitation and monthly average unfrozen water content of soil at TS-04 site and TS-05 site.
Bubble plots of the intensity of rainfall in the summer and the response of soil moisture at TS-04 and TS-05 sites.
Depth of soil moisture response to rainfall at TS-04 and TS-05 sites.
Vegetation is an important component of terrestrial ecosystems, participating in a variety of physical, chemical and biological processes between the atmosphere and soil (
Envelope of soil temperature under different vegetation coverage.
Vegetation covers the surface and has a large impact on the surface soil. The thermal insulation of vegetation inhibits the temperature change of shallow soil caused by temperature changes. Some studies show that there is a significant negative correlation between vegetation coverage and surface temperature (
The influence of vegetation on soil thermal conditions is multifaceted and complex. Vegetation reduces 1) wind speed near the surface, 2) evaporation of water from the topsoil and 3) the release of heat from the soil to the air. Vegetation can also increase or decrease the heat transfer properties of soil by changing the soil environment. In addition to vegetation, winter snow in Tianshan also affects the thermal conditions of soil. Snow will generally reduce the soil temperature, but a particular snow depth also plays a role in thermal insulation. This paper has not studied the impact of snow on soil thermal conditions in Tianshan, but it is clear that the impact of snow on soil temperature cannot be ignored.
The soil moisture content in frozen soil areas can determine the degree of soil freezing in the cold season. Vegetation is the main interface of soil water exchange with the outside environment, which will have an important impact on the content of the soil water. Generally, soil water retention is controlled by vegetation characteristics and soil properties. The change of vegetation and soil properties will inevitably lead to a change in soil water retention (
Envelope of soil unfrozen water content under different vegetation coverage.
Importantly, vegetation affects soil moisture content through the response of vegetation to precipitation. Plant roots affect the infiltration capacity and rate by indirectly affecting the characteristics of soil structure and changing soil channels. Therefore, the precipitation infiltration rates of soil with different vegetation types and vegetation coverage vary (
Topography also has a great influence on the distribution of soil moisture. The slope of the TS-05 site is steeper than that of the TS-04 site, which is also an important factor affecting the response of soil moisture to rainfall. Sites with steeper slopes have difficulty storing rainwater because most of the rainwater is lost through surface runoff and cannot supplement the soil moisture. This process is also an important explanation of the difference between soil moisture at the two sites.
The results showed that the change in vegetation cover significantly affected the soil freeze–thaw process. Because of the heat insulation of vegetation, the start time of soil freezing–thawing was delayed and the rate of soil freezing–thawing decreased at the site with high vegetation coverage. A smaller vegetation coverage correlates to a shallower and longer duration soil daily freeze–thaw cycle. The duration of daily freeze–thaw cycles during the thawing period was significantly longer than that in the freezing period, and the number of days experiencing a daily freeze–thaw cycle significantly decreased with an increase in the soil depth.
The effect of vegetation is mainly reflected in the winter thermal insulation and summer cooling. A greater vegetation coverage correlates to a larger temperature difference between the ground and air in the cold season, a smaller temperature difference between the ground and air in the warm season, and better insulation of the soil. A higher vegetation coverage correlates to a greater impact of the soil moisture on the soil thermal conditions.
Rainfall easily accumulates in areas with high vegetation coverage. The overall soil moisture content at the site with large vegetation coverage is greater than that at the site with small vegetation coverage.
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
This work was funded by the Independent Project of the State Key Laboratory of Frozen Soils Engineering (Grant No. SKLFSE-ZQ-55).
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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We thank the reviewers for their valuable comments and suggestions that helped to improve the manuscript.