Chinese Science Bulletin

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Springer

Response of soil heat-water processes to vegetation

cover on the typical permafrost and seasonally frozen

soil in the headwaters of the Yangtze and Yellow Rivers

HU HongChang1, WANG GenXu1,2, WANG YiBo1, LIU GuangSheng1, LI TaiBing1 & REN DongXing1

1

College of Resources and Environment, Lanzhou University, Lanzhou 730000, China;

2

Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China

The response of soil temperature and moisture to vegetative cover in the active layer of permafrost and

seasonally frozen soil were assessed and compared. Soil temperature and moisture, under a range of

vegetation covers (92%, 65% and 30%) in the permafrost and vegetation covers (95%, 70% 80%,

40% 50% and 10%) in the seasonally frozen soil, were measured on a daily basis. A decline in vege-

tation cover led to a decrease in the integral of freezing depth of active permafrost layer, but an in-

crease in seasonally frozen soil. The maximum invasion depth and duration of the negative isotherm

during the frozen period and of the positive isotherm during the non-frozen period clearly increased

when vegetation cover declined. With a reduction of vegetation cover, the soil moisture in the active

layer of the permafrost decreased for depths of 0.20 0.60 m, but increased for depths of 0.60 0.80 m,

while for seasonally frozen soil, soil moisture of the entire profile (0.10 1.20 m) increased. Variation in

vegetation cover alters soil heat-water processes, but the response to it is different between permafrost

and seasonally frozen soil.

heat-water processes, vegetation cover, response, headwaters of the Yangtze and Yellow Rivers

The Qinghai-Tibet Plateau, which houses the greatest distribution of soil temperature (heat) and moisture (dry

portion of high altitude permafrost in the world[1], is ex- vs. wet), thus constituting an extremely important com-

ponent in the energy and water cycles of the soil-

tremely sensitive to global climate change. It is an indi-

atmosphere system[10 12]. A lot of monitoring about the

cator region and a magnifier of climate change in

China[2], and its energy and water cycles play a very heat-water processes have been developed in the Qing-

hai-Tibet Plateau[13 16] in recent years, and studies show

important role in the formation and evolution of the

Asian monsoon system[3]. Since the 1980s, the Qing- the response of freezing-thawing processes to surface

feature[17 19] especially on the vegetation[1,20 23] is obvi-

hai-Tibet Plateau s ecosystem has seriously degenerated

under the influence of global climatic change and an- ous. The relation between vegetation and frozen soil, es-

GEOGRAPHY

thropogenic activities. This degradation mainly mani- pecially the influence of different vegetation covers on

fested itself in the loss of vegetation, glacial ablation, soil heat-water processes, has become the evident index

of global climate change[24]. However, the study of soil

permafrost degradation and the disappearance of wet-

lands and lakes[4 6]. It also led to the acute changes in

Received May 26, 2008; accepted October 6, 2008; published online December 14, 2008

soil physical and chemical characteristics[7,8], the hydro- doi: 10.1007/s11434-008-0532-x

Corresponding author (email: abpiy7@r.postjobfree.com)

logic cycle, and in soil heat-water processes and carbon Supported jointly by National Natural Science Foundation of China (Grant Nos.

cycling[9]. 40730634 and 90151003), National Basic Research Program of China (Grant No.

2007CB11504), and Hundred Talent Scholar Foundation, Chinese Academy of

Soil heat-water processes reflect the spatiotemporal Sciences (2004)

www.scichina.com csb.scichina.com www.springerlink.com Chinese Science Bulletin April 2009 vol. 54 no. 7 1225-1233

heat-water processes under different vegetation covers is fecture, Qinghai Province), located in the Yangtze and

rare, and there is very few pioneer research work on the Yellow Rivers headwater region, were selected as study

mechanism and representation of the vegetation influ- areas (Figure 1).

ence on the soil heat-water processes because of the The Fenghuoshan district is located in the Beiluhe

shortage of the sufficient system observation. watershed of the Yangtze headwaters and represents a

The purpose of this study is to document the response typical permafrost region. The elevation ranges from

of soil temperature and moisture to the degree of vegeta- 4680 to 5360 m, and the annual mean air temperature,

tion cover in the active layer of permafrost and season- precipitation, evaporation, maximum snow depth and

ally frozen soil, their differences, and how the degrada- relative humidity are 5.2, 290.9 mm, 1316.9 mm, 14

tion of vegetation affects heat-water processes. cm and 57% respectively, the annual mean ground tem-

perature is 1.5 to 3.0, the main frozen soil depth

1 Materials and methods

ranges from 50 to 120 m and permafrost table is in the

1.1 Study area range of 0.8 2.5 m. The mean daily air temperature and

precipitation in 2005 and 2006 are presented in Figure

The Fenghuoshan (Qumalai County, Yushu Tibetan

2(a). The main soil type is alpine meadow soil, the main

Autonomous Prefecture, Qinghai Province) and Jianshe

plant species are Kobresia pygmaea, K. humilis, K.

Village (Dari County, Guoluo Tibetan Autonomous Pre-

Figure 1 Location of the study area.

Figure 2 Daily air temperature and precipitation of the Dari (a) and Fenghuoshan (b) district in 2005 and 2006.

1226 HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7 1225-1233

ARTICLES

capilifolia, etc. Two series of soil moisture and temperature profiles

The Dari district represents a region of seasonally were developed for all observation plots, and their heat-

frozen soil. The elevation ranges from 4040 to 4520 m, water processes were monitored systematically. Thermal

and the annual mean air temperature, precipitation and resistance sensors with digital multimeters (Fluke 180

evaporation are 0.22, 546.6 mm and 1219.3 mm re- series, Fluke Co. USA), developed by the State Key

Laboratory of Frozen Soil Engineering, served to measure

spectively, the maximum seasonal frozen depth is in the

soil temperatures ranging from 40 to 50, with a

range of 1.2 2.0 m, the mean daily air temperature and

precision of 0.02 [25].

precipitation in 2005 and 2006 are presented in Figure

2(b). The main soil type is alpine bush and meadow soil, Soil moisture was measured by Frequency Domain

and the main plant species are Salix oritrepha, Potentilla Reflectometry (FDR), using a calibrated soil moisture

fruticosa, Poa pratensis, and K. pygmaea, etc. sensor equipped with a Theta-probe (Holland Eijkelamp

Co.). Based on differences in the soil s dielectric con-

1.2 Measurement methods and data collection

stant with moisture content, the signal was converted to

a millivolt signal; its accuracy was 2%[25,26].

At the Fenghuoshan site, three observation plots were

constructed at common slope angle (30 ) and aspect At the Fenghuoshan site, thermal resistance sensors

(half negative slop), with different levels of vegetation were buried at depths of 0.20, 0.30, 0.40, 0.55, 0.65,

cover (92%, 65% and 30%), which were represented as 0.85 and 1.20 m, and soil moisture sensors were buried

non-degraded, moderately degraded and seriously de- at depths of 0.20, 0.40, 0.65 and 1.20 m. Soil temperature

graded alpine Kobresia meadow, respectively[8]. The soil and moisture content were monitored from August 1,

physicochemical properties of the observation plots are 2005 to September 30, 2006. The data for soil moisture

presented in Table 1. from the observation plot with 65% vegetation cover

At the Dari site, similar plots, with levels of vegeta- were not used. At the Dari site, thermal resistance sen-

tion cover of 95%, 40 50%, 70 80%, and 10% rep- sors were buried at depths of 0.10, 0.30, 0.50, 0.80, 1.00

resented non-degraded and moderately degraded alpine and 1.50 m, and soil moisture sensors were buried at

Kobresia meadow, slightly degraded alpine scrub depths of 0.10, 0.30, 0.50 and 0.80 cm. Soil temperature

meadow grassland, and severely degraded black soil and moisture content were monitored from November

patches, respectively[25]. The soil physicochemical prop- 15, 2005 to November 15, 2006. The data were meas-

erties of the observation plots are presented in Table 2. ured on a daily basis.

Table 1 Soil physiochemical properties in observation plots at different levels of vegetation cover in the Fenghuoshan permafrost soil district

Bulk density

Vegetation cover Granularity Granularity Organic matter

Total N Total P Total K Depth (cm)

(g cm 3) >0.5 mm 0.8 1.15 93.3 1.10 0.26 0.065 1.74

0 10

92 1.3 1.60 93.6 1.63 0.25 0.050 1.74

10 20

1.3 11.65 61.8 1.38 0.17 0.058 1.71

20 40

0.9 1.45 93.2 1.91 0.34 0.051 1.75

0 10

65 1.1 2.85 92.9 1.04 0.20 0.061 1.77

10 20

1.2 7.20 72.4 1.10 0.19 0.061 1.80

20 40

1.2 4.95 93.1 0.66 0.17 0.056 1.83

0 10

30 1.5 11.30 85.4 0.25 0.13 0.064 1.74

10 20

1.4 17.95 61.2 0.43 0.04 0.055 1.85

20 40

GEOGRAPHY

Table 2 Soil physiochemical properties in observation plots at different levels of vegetation cover in the Dari seasonally frozen soil district

Soil moisture (0 10 cm)

Vegetation cover

Bulk density (g cm 3) Organic matter Total N Vegetation type Weight TDR 95 1.528 62.21 41.90 1.33 0.26

Kobresia

1.291 43.18 33.47 1.91 0.32

70 80 Saiix oritrepha

1.102 43.73 45.51 1.10 0.17

40 50 Kobresia

10 Black soil 1.546 25.86 28.93 1.02 0.07

HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7-122*-****-****

(from mid-October to late December), when soil profile

1.3 Methods of analysis

is in the process of freezing; the frozen stage (from early

Soil temperature reflects soil heat condition. Two pa- January to late April), when the soil profile is com-

rameters were introduced: maximum invasion depth of pletely frozen; the thawing process stage (from early

an isotherm and integral of freezing depth. The former, a May to early July), when soil profile is in the process of

reflection of the state of soil heat, is the maximum depth thawing; and the non-frozen stage (from mid-July to the

to which a temperature exists in soil, and represents the beginning of the next freezing process stage), when the

maximum depth of this isotherm in an isoline chart. The soil profile is completely melted[27].

latter, reflecting freezing time and depth of soil, is the 2.1.1 Response of soil temperature to vegetation cov-

integral of freezing depth over time, the area encom- ers in active layers of a permafrost soil. The active

passed by the 0 isotherm. Soil water conditions are layers of the Fenghuoshan permafrost site s three obser-

reflected by the unfrozen water at different soil depths, vation plots began to freeze in mid-October, when the

describing the variation in soil moisture by the use of vegetation covers had little effect. However, the process

soil moisture and scatter diagram of contour map. and duration of freezing varied with the level of vegeta-

tion cover. An obvious zonation in the freezing profiles

2 Experimental results and comparative of plots differing in their level of cover led to the analy-

analysis of sites sis of the soil freezing process in three layers. The first

boundary was roughly 0.60 m below the soil surface,

2.1 Fenghuoshan permafrost region

and the second depth increased with decreasing vegeta-

The freeze-thaw period of active layers in the Fenghu- tion cover. The zonation became less distinct as vegeta-

oshan permafrost region occurred between October and tion cover declined (Figure 3), while soil freezing was

hastened and the duration of freezing became short[28].

June of the next year. The analysis of soil water and heat

processes was divided into four stages based on freeze- In a frozen condition, the full soil profile s tempera-

thaw state of the active layers: the freezing process stage ture declined in concert with vegetation cover, while the

Figure 3 Soil temperature isotherms under different levels of vegetation cover in the active layer of a permafrost soil (2005-08-01

2006-09-30).

1228 HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7 1225-1233

ARTICLES

1.20 m, and least at 0.20 m. Later in the freezing stage,

maximum invasion depth and duration of negative iso-

therms increase. The maximum invasion depths of the soil moisture across the profile under 92% cover ranged

10 isotherm were 0.55, 0.60 and 0.70 m under vege- between 10% and 15%, compared to only 5% under

30% cover (Figure 4(a))[29].

tation covers of 92%, 65% and 30%, respectively.

During the permafrost s frozen stage minimum soil

The onset of soil thawing advanced with a decline in

moisture under 92% cover was about 6% at a depth of

vegetation cover: thawing onsets of mid-June, early June

0.60 m; whereas under 30% cover, soil moisture de-

and late May date were noted for the 92%, 65% and

creased with depth, showing a minimum of about 2% at

30% cover plots, respectively, with a roughly 10-day

a depth of 1.20 m (Figure 5).

interval between each. The process and duration of

During the thawing process, soil moisture changed

thawing clearly differed with differing levels of vegeta-

sharply, under 92% cover the station of maximum soil

tion cover. Fewer than 92% and 65% cover thawing took

moisture shifted from 0.20 m at the beginning to 0.40 m

about 50 days to reach a depth of 1.20 m; however, in

comparison, under 30% cover thawing took only 30 at the end of thawing process. By contrast, under 30%

days to reach the same depth (Figure 3). cover the maximum soil moisture at the depth of 0.40 m

At the non-frozen stage, the maximum invasion depth was at the early stage, while the soil moisture at 1.20 m

and duration of positive isotherms differed according to rose quickly to a maximum at the end. These results

the level of vegetation cover. The maximum invasion concurred with the distribution of soil moisture under

different vegetation covers at the freezing stage (Figure

depth and duration of 5 isotherms under 92% and

4(b))[30].

65% cover were 0.60 m and 50 days, but exceeded 1.20

During the non-frozen stage, the permafrost active

m and 60 days under 30% cover (Figure 3). The maxi-

layer s maximum soil moisture under 92% cover oc-

mum invasion depth and duration of relatively high

curred at 0.40 m, compared to 1.20 m under 30% cover.

temperature isotherms increased gradually with de-

creasing vegetation cover, and caused the thickening of The active layer s minimum soil moisture occurred at

the active layer of the permafrost. 0.60 m. As vegetation cover declined, soil moisture

2.1.2 Response of soil moisture to vegetation covers in above and below 0.60 m declined and increased, respec-

tively[31].

active layer of a permafrost soil. Under 92% cover,

moisture in the soil profile during the permafrost s early

2.2 Dari seasonally frozen soil region

freezing stage peaked at 0.40 m from the surface, and

The freeze-thaw period in the Dari seasonally frozen soil

remained high at 0.20 m and 0.40 m from the surface,

region occurred from November to May of the next year.

compared to the remainder of the profile. Comparatively,

Similar to the Fenghuoshan permafrost region s soils,

fewer than 30% cover, soil moisture was greatest at

GEOGRAPHY

Figure 4 Variation in soil moisture in the active layer of a permafrost soil at the freezing stage (a) and the thawing stage (b) under different

levels of vegetation cover.

HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7-122*-****-****

Figure 5 Soil moisture isolines under vegetation cover (92% and 30%) in the active layer of a permafrost soil (2005-08-01

2006-09-30).

Figure 6 Soil temperature isotherms under different levels of vegetation cover in a seasonally frozen soil (2005-11-15 2006-11-15).

the freeze-thaw period was divided into four stages: 40% 50% and 10% cover. As vegetation cover de-

freezing process (mid-November to late December); clined, the curvature rate of the freezing process de-

frozen (early January to late March); thawing process creased, and the freezing process was quicker, the time

(early April to early May); non-frozen (mid-May to next for the 0 isotherm to reach a depth of 1.50 m was

year s freezing process stage)[27]. shorter. At 95% cover the freezing front only reached its

2.2.1 Response of soil temperature to vegetation covers deepest point (1.40 m) in early April, while at 10%

in seasonally frozen soil. Isotherm charts (Figure 8) cover the soil profile was frozen down to 1.50 m by

showed that the onset of freezing advanced as vegetation mid-January (Figure 6).

The duration and maximum invasion depth of iso-

cover declined. At 95% vegetation cover freezing had

therms clearly increased as vegetation cover declined.

not begun on November 15, but was noted at a depth of

These changes were most significant for isotherms from

0.10 m on November 22. Comparatively, on November

5 to 0 ; isotherms under 5 showed limited changes

15 the depth of freezing had already reached 0.25, 0.46

with changing levels of cover. The lowest soil tempera-

and 0.58 m, respectively, in the plots with 70% 80%,

1230 HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7 1225-1233

ARTICLES

ture under 95% cover was roughly 5, but about 7 depths for 95%, 70% 80%, 40% 50% and 10% cover

under lesser cover. Vegetation serves as thermal insula- was 1.05, 0.60, 0.35 and 0.15 m, respectively, while its

tion for soil at the frozen stage, but with the reduction of duration was 70, 60, 30 and 20 days, respectively (Fig-

vegetation cover as a result of ecological degradation, ure 6). Excluding the high cover plots, maximum inva-

the cover s capacity for thermal insulation is lost even sion depth of isotherms under 10 increased with de-

under slight degradation of vegetation. clining vegetation cover. For covers of 70% 80%,

The onset of thawing under 95% cover occurred in

40% 50% and 10% the maximum invasion depth of

early April, roughly one month earlier than under lesser

the 9 isotherm was 0.85, 0.90 and 1.10 m, respec-

cover. Under the lower vegetation cover, the onset of

tively, and that of the 8 isotherm was 1.25, 1.30 and

thawing occurred as early as the first days of May, and

>1.50 m. The 95% cover plot s temperatures across full

as it advanced, the bidirectional thawing process gradu-

soil profile were higher than those with lesser cover,

ally developed into an unidirectional one, the thawing

which was consistent with an upward shift in the thaw-

process being faster and shorter.

ing process.

During the non-frozen period maximum invasion

2.2.2 The response of soil moisture to vegetation cov-

depth and duration of warm isotherms ( 10 ) de-

ers in seasonally frozen soil. The soil profile s maxi-

creased with declining vegetation cover. In seasonally

mum soil moisture content during the early freezing

frozen soil the 10 isotherm s maximum invasion

GEOGRAPHY

Figure 7 Variation in soil moisture at different levels of vegetation cover in seasonally frozen soil at the freezing stage (a) and the

thawing stage (b).

HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7-122*-****-****

Figure 8 Soil moisture isolines under different levels of vegetation cover in a seasonally frozen soil (2005-11-15 2006-11-15).

process was 50%, 45%, 35% and 30%, respectively, un- and greatest at a depth of 0.80 m. The maximum soil

der vegetation covers of 95%, 70% 80%, 40% 50% moisture under 95%, 70% 80%, 40% 50% and 10%

and 10%. Under 95% cover maximum soil moisture oc- cover were 50%, 45%, 40% and 30%, respectively (Fig-

curred at depths of 0.10 and 0.30 m, compared to 0.80 m ure 7(b)). During the thawing process soil water is re-

under lesser cover. With declining cover, soil moisture leased by the melting of frozen soil water. Soil freezing

isolines became more and more sparse as reduced cover blocks evaporation, thus reducing the loss of soil moisture.

had influenced soil moisture at earlier freeze-thaw stages. The soil moisture profile late in the thawing process was

similar to that early in the freezing process[31].

The decline in soil moisture across the entire soil profile

with reduced vegetation cover was highlighted by the At the non-frozen stage soil moisture was greatest at a

depth of 0.80 m and lowest at a depth of 0.30 m (Figure

fact that soil moisture under 95% and 70% 80% cover

8). The soil moisture across the entire soil profile de-

was about 10% 20%, but was only 5% 10% under

creased with declining vegetation cover.

40% 50% and 10% cover (Figure 7(a)).

In the frozen stage, soil moisture increased with in-

3 Conclusions

creasing soil depth, and increasing vegetation cover

(1) The freeze-thaw process was sped up by the reduc-

(Figure 8). The presence of vegetation decreased the

tion in vegetation cover, with the date of onset of freez-

absorption of solar radiation by soil, decreased the sub-

ing for the seasonally frozen soil and of onset of thawing

limation of ice and the evaporation of liquid water, im-

for the permafrost soil being clearly earlier. With de-

peded the reduction of soil temperature, and increased

the liquid water content of soil[15]. clining cover the integral of freezing depth for the sea-

sonally frozen soil increased, but decreased for the per-

Soil moisture across the entire soil profile increased

mafrost soil.

during the thawing process (Figure 8). Under all four lev-

(2) The maximum invasion depth and duration of the

els of cover soil moisture was lowest at a depth of 0.30 m,

1232 HU HongChang et al. Chinese Science Bulletin April 2009 vol. 54 no. 7 1225-1233

ARTICLES

the profile of the active layer of permafrost. Compara-

negative isotherm for the frozen stage and of the positive

tively, in the seasonally frozen soil the two high mois-

isotherm for non-frozen stage increased with declining

ture layers occurred at depths of 0.10 and 0.80 m, and a

vegetation cover, however, positive isotherms ( 10 )

low moisture layer at a depth of 0.30 m.

of seasonally frozen soil at the non-frozen stage de-

(4) With a decline in vegetation cover, the permafrost

creased with declining vegetation cover, whereas the

soil moisture decreased in the top (0.2 0.60 m) of soil

influence of negative isotherms ( 5 ) for frozen

profile, but increased at greater depths. Comparatively, a

soils is not obvious.

decline in cover of the seasonally frozen soil resulted in

(3) There were two high soil moisture layers (0.40

a decrease in soil moisture over the entire soil profile.

and 1.20 m depths) and a low moisture layer (0.70 m) in

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