XiaoLi Chang, ShaoPeng Yu, HuiJun Jin, YanLin Zhang

1. State Key Laboratory of Frozen Soils Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

2. Department of Geography, Harbin College, Harbin, Heilongjiang 150086, China

Vegetation impact on the thermal regimes of the active layer and near-surface permafrost in the Greater Hinggan Mountains, Northeastern China

XiaoLi Chang1*, ShaoPeng Yu2, HuiJun Jin1, YanLin Zhang1

1. State Key Laboratory of Frozen Soils Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

2. Department of Geography, Harbin College, Harbin, Heilongjiang 150086, China

The ground temperature and active layer are greatly influenced by vegetation in the Greater Hinggan Mountains in Northeastern China. However, vegetation, as a complex system, is difficult to separate the influence of its different components on the ground thermal regime. In this paper, four vegetation types, including aLarix dahurica–Ledum palustrevar.dilatatum–Bryumforest (P1), aL. dahurica–Betula fruticosaforest (P2), aL. dahurica–Carex tatoforest (P3) in the China Forest Ecological Research Network Station in Genhe, and aCarex tatoswamp (P4) at the permafrost observation site in Yitulihe, have been selected to study and compare their seasonal and annual influence on the ground thermal regime. Results show that the vegetation insulates the ground resulting in a relatively high ground temperature variability in theCarex tatoswamp where there are no tree stands and shrubs when compared with three forested vegetation types present in the area. Vegetation thickness, structure, and coverage are the most important factors that determine the insulating properties of the vegetation. In particular, the growth of ground cover, its water-holding capacity and ability to intercept snow exert a significant effect on the degree of insulation of the soil under the same vegetation.

vegetation types; permafrost; ground thermal regime; Greater Hinggan Mountains

1 Introduction

Vegetative cover affects the heat exchange between the boundary of the atmosphere and the lithosphere (Tyrtikov, 1959). It dampens the impact of air temperature and exerts an insulating effect on the soil thermal regime (Shur and Jorgenson, 2007). Vegetation shields the soil from maximum penetration of heat by shading, decreasing air circulation, retaining moisture in and just above the soil, and intercepting rain (Benninghoff, 1952), and also decreases air current velocities within its stratum, and thus impedes heat radiation from the soil to the cold air (Benninghoff, 1952; Wang, 1999). As vegetation is a dynamic factor, its influence on the ground thermal regime vary with time and space (Brown, 1963). Even within the vegetation complex itself, some components have more influence than others (Bakalin and Vetrova, 2008; Cannone and Guglielmin, 2009). For example, it is often difficult to delineate the boundary between trees and ground cover, and to separate the influence of these two components of a forest, although each may have adifferent effect (Changet al., 2011). Therefore, an effort should be made to understand the ground thermal variation with vegetation dynamics, and to separate the influence of the different components of vegetation on the ground thermal regime.

Vegetation and the underlying permafrost are important environmental components in the Greater Hinggan Mountains in Northeastern China. The influence of vegetation on permafrost and its degradation has recently been observed in this region (Jinet al., 2007; Heet al., 2009; Weiet al., 2011), but the processes involved in the interaction between vegetation and the ground thermal regime have only recently been investigated (Sun, 2000; Sunet al., 2008; Changet al., 2011). The insulating effects provided by different vegetation types or different components of vegetation on the permafrost thermal regime are still poorly known in the Greater Hinggan Mountains of Northeastern China (Wang, 1999).

The China Forest Ecological Research Network (CFERN) Station in Genhe (50°56′N, 121°30′E; 805 m a.s.l.) and Yitulihe Permafrost Observatory (YPO) (50°38′N, 121°33′E; 720 m a.s.l.) are both located on the western flanks of the Greater Hinggan Mountains in Northeastern China (Figure 1). As a typical forest ecological research station in cold temperate regions, CFERN Genhe Ecological Station, founded in 1991, is the northernmost in China at present and rich in wildlife species (Zhou, 2003). YPO, about 24 km south of Genhe city in the Inner Mongolia Autonomous Region of China, is in an undisturbed meadow on the first terrace of the northern bank of the Yitulihe River. Soil temperature at YPO was observed scattered from early 1980s to 2008 (Jinet al., 2007), but systematically since 2009. Therefore, CFERN Genhe Ecological Station and YPO provide a unique opportunity to examine the thermal regime of permafrost covered by different types of vegetation by comparing and contrasting their respective temperature records. This paper aims to provide more information on the following topics:

· Analysis of the seasonal and annual variation of ground temperatures in the study area beneath different vegetation types under similar microclimates;

· Determine the insulating impacts of different vegetation types on the Hinggan permafrost.

Figure 1 Study site (a), location of ground temperature monitoring plots (P1–P4) (a, b), and the longitudinal profile of P1, P3 and P4 (c) in the Greater Hinggan Mountains in Northeastern China

2 Study region and monitoring

CFERN Genhe Ecological Station and YPO are characterized by a cold temperate continental monsoon climate. Based on meteorological records from January 1, 2001 to December 31, 2010 provided by the Genhe Meteorological Station, mean annual air temperature is ?3.0 °C and mean monthly air temperature vary from ?31.7 °C (January 2001) to 19.5 °C (July 2002) during 2001 and 2010. The extreme maximum mean daily air temperature (MDAT) (27.4 °C; June 27, 2010) and minimum MDAT (?41.3 °C; January 10, 2001) were also recorded by the Genhe Meteorological Station. Annual precipitation ranges from 350 to 550 mm, with snowfall contributing 12%–20% of the annual total. Snow usually accumulates on the ground in October and disappears in March of the following year. The maximum depth of the snow pack ranges from 10 to 40 cm for the last ten years.

Both CFERN Genhe Ecological Station and YPO have a brown coniferous forest soil, with approximately 10 cm of humus overlain by about 8 cm of litter. Permafrost was discontinuously distributed in this region, with the areal extent approximately 60%–75% (Jinet al., 2007). According to Luet al. (1993), mean annual ground temperature (MAGT) was inferred to be?1.3 °C at the depth of 14 m in Genhe from ground temperatures at shallower depths during 1958 and 1965, and ?0.8 °C at the depth of 13 m observed at YPO during 1966 and 1979, respectively. Furthermore, the permafrost base was estimated to be around 40 m at YPO. Base on ground temperatures at shallow depths observed from January 1, 2009 to May 20, 2011, the active layer varies from 0.5–2.0 m in this region.

The vegetation in the experimental area at the CFERN Genhe Ecological Station is primarilyLarix gmeliniiforest which can be subdivided into various types based on the dominant understory and ground cover.Three representative plots were selected for long-term ground temperature monitoring, including aLarix dahurica–Ledum palustrevar.dilatatum–Bryumforest (P1), aL. dahurica–Betula fruticosaforest (P2), and aL. dahurica–Carex tatoforest (P3). The plot at the permafrost observation site in Yitulihe consists of aCarex tatoswamp (P4), and was also selected for this investigation. The major difference between P1 and P2 is that the ground cover consists ofBryumandCare tato, respectively, with similar larch stands and shrubs. The main distinction between P3 and P4 is absence of larch stands and shrubs in P4 although their ground cover is almost the same. The location and detailed characteristics of the four investigated plots are listed in table 1. Soil moisture in volume was measured by a HydroSense portable soil moisture meter in the first 20 cm of soil of these plots in September 2010.

At each plot in this study region, ground temperatures were monitored by thermistors installed at various depths (0, 10, 20, 50, 80 and 160 cm) in each borehole. All the thermistors, made by the State Key Laboratory of Frozen Soils Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, have an accuracy of ±0.05 °C for ?30 to +30 °C but ±0.1 °C for ?45 to ?30 °C or +30 to +50 °C. The data were recorded manually once a week on Saturday between 10:00 and 12:00 with a multi-meter Fluke 189?. Weather monitoring was recorded from June 7, 2008 to August 26, 2010 every hour using a HoboPro? datalogger manufactured by Onset. Monitoring data included: solar radiation, air temperature, wind speed/direction, relative humidity (RH) and rainfall.

3 Results and analyses

3.1 Inter-site variation

3.1.1 Variation at plots P1 and P2

At the CFERN Genhe Ecological Station, the MAGT is always higher at P1 than that at P2 for all depths (Table 2), with the difference increasing with depth from 1.9 °C at 0 cm to 4.4 °C at 160 cm. The two plots also differ in mean seasonal ground temperatures. In this paper, it has to be mentioned that spring usually occurs in March, April and May, summer for June to August, autumn for September to November and winter for December to the following February, which is in accordance with the vegetation growth. During the spring season, average ground surface temperature (GST) at P1 (?0.6 °C) is the same as at P2. However, the comparable mean ground temperatures of the two plots differ by more than 2 °C at 10 cm, increasing to 7.4 °C at 160 cm. During the summer season, average GST of P1 (15.1 °C) is 0.6 °C higher than of P2 (14.5 °C). At depths of 80 and 160 cm, ground temperatures are positive at P1 in summer and autumn, but negative at P2 (Table 2). During the winter season, mean ground temperatures are considerably higher at P1 (about 6.3 °C) than that at P2.

The patterns of ground temperature for both P1 and P2 (Figure 2) are similar and follow the patterns of air temperature at depths of 0, 10 and 20 cm. However, below 20 cm, the ground temperature wave is gradually damped out with depth, and shows a delayed response to fluctuations in air temperature. The correlation coefficients of the ground and air temperatures are similar between the two plots (Table 3), but the correlation decreases with depth of measurement. As depth increases the relation between air temperature and ground temperature becomes weaker at both sites, but P2 shows a stronger relation to air temperature than P1 at all depths.

Figure 2 Ground temperatures for P1 and P2 at different depths and air temperature recorded between December 29, 2008 and May 20, 2011 (Meteorological data after September 2010 was lost because of equipment malfunction)

Table 3 Correlation coefficients between air and ground temperatures at the CFERN Genhe Ecological Station and the YPO during the period from January 3, 2009 to September 17, 2010

3.1.2 Variation at plots P3 and P4

The MAGT of P3 at the CFERN Genhe Ecological Station is about 2 °C lower than P4 at YPO for each measured depth (Table 4). The range of variation at depth for each site is 0.9 °C. Mean seasonal ground temperatures at P3 and P4 are somewhat different from that of P1 and P2. During the spring season, mean ground temperatures of P3 are lower than that of P4 (Table 4), with the difference ranging from 3.1 to 3.9 °C in the upper 160 cm. During the summer season, the two plots display the maximum difference in ground temperatures. For example, the comparable mean ground temperatures of the two plots differ by 6.9 °C at the ground surface, 6.7 °C at 10 cm, 6.5 °C at 20 cm and 4.3 °C at 50 cm. During the autumn season, the mean ground temperatures of P4 are about 0.6–1.3 °C higher than that of P3 at various depths. However, it is colder at P4 than that at P3 in the upper 50 cm depth (3.5 °C lower for P4 than P3 at ground surface, 2.4 °C at 10 cm and 1.5 °C at 20 cm respectively), but warmer below that depth in winter (Table 4).

The ground temperature (Figure 3) shows the same relationship to P1 and P2, but below 20 cm, the ground temperature wave is gradually damped out. When comparing ground temperature with air temperature, the same relationships are observed at P3 and P4, where the correlation coefficient between air temperature and GST (0.87) at P3 is weaker than that at P4 (0.96), and the same applies for other depths (Table 3). A stronger correlation is observed between air and ground temperatures at P4 even at the depth of 80 cm (0.70), compared with 0.58 at the depth of 50 cm at P3 (Table 3). This suggests that air temperature has a greater impact on ground temperature at P4 than that at P3.

3.2 Ground thermal regimes

On the basis of data records spanning 2.5-year at the CFERN Genhe Ecological Station and YPO, the thermal regimes of soils in the active layer and near-surface permafrost of P1 with P2 and P3 with P4 were compared for assessing the impacts of vegetative coverage.

Based on the geothermal contour map of these plots, in P1 of theL. dahurica–Ledum palustrevar.dilatatum–Bryumforest, the permafrost table is greater than 240 cm in the fall of 2009 but about 200 cm in 2010. The reason is thatBryumis easily removed when drilling machines went into this plot in 2008 because ofBryum’s loose structure and lack of roots, and did not recovered until 2010. In P2 of theBetula fruticosa–L. dahuricaforest, the permafrost table lay at around 65 cm both in the fall of 2009 and 2010 (Figure 4). Carex tato, the main ground cover plant in P2–P4, firmly stretches into the ground with a clustered fibrous root system. It can hardly be destroyed except above ground structures, and recovered rapidly again in the following years, leaving less influence on ground thermal regime. In P3 of theL. dahurica–Carex tatoforest, the active layer thickness is no more than 55 cm in the fall of 2009 and 2010 (Figure 4). In P4 of theCarex tatoswamp, the active layer thickness is about 110 cm in the same years (Table 4).

Based on the thermal profile of these plots, there is a striking difference during the winter between P1 and P2, P3 and P4 (Figure 4). Ground above the near-surface permafrost at P1 is totally warmer than P2 throughout the year, with less intense thermal gradients in summer but just the opposite in winter in comparison with P2. As to P3 and P4, ground temperatures above the near-surface permafrost at P3 is lower than P4 in summer but higher in winter, but the thermal gradients in the upper 160 cm is more intense at P4 than P3 throughout the year. It is probable that thermal gradients and the amplitude of ground temperature variation are very dissimilar among these plots due to different vegetative cover.

Table 4 Mean annual and seasonal ground temperatures (°C) for different depths (cm) at P3 and P4 in 2010

Figure 3 Ground temperatures for P3 and P4 at different depths, and air temperature recorded between January 3, 2009 to May 20, 2011 (Meteorological data after September 2010 was lost because of equipment malfunction)

Figure 4 Ground temperature fields for P1–P4 between January 3, 2009 to May 20, 2011 (bold line is the 0 °C isotherm)

4 Discussion

Due to the insulating effect of vegetation, the results show that in each plot, as expected, air temperature and the amplitude of its fluctuation are higher than the ground temperature under the same climatic conditions. We also observed that ground temperatures and their amplitude of fluctuation differ for each study plot due to differences in vegetation type, coverage and structure.

4.1 Variation at plots P1 and P2

There can be a large contrast in ground temperatures between closely spaced locations owing to difference in microrelief, vegetation, and soil characteristics at each site (Kleneet al., 2001). Our investigation shows that, at the CFERN Genhe Ecological Station, microrelief and soil characteristics at P1 and P2 are almost the same, within a distance of 600 m. As mentioned previously, the larch stands and shrubs of P1 are similar to that of P2, whereas the ground cover is quite different (Table 1), resulting in different insulating effects.

Plots P1 and P2 exhibit differences in average ground temperature and amplitude of fluctuations for each season. The greatest difference in ground temperatures between P1 and P2 occurs in winter, and could be related to differences in ground cover.

Bryumconsists of dense clusters of plants wherestems have a length of 5–8 cm and roots extend 10–12 cm deep. It turns green when summer arrives, but becomes yellow or yellow brown if the weather cools down at site P1 (Figure 5).Carex tatoforms tussocks about 25–40 cm in height and 50 cm in diameter. It also turns green in early summer and withers in autumn, leaving dry dead leaves covering the ground at site P2 (Figure 5). It has been shown that dry dead leaves have a lower heat conductivity coefficient than living green leaves due to moisture loss (Riseborough and Burn, 1988), resulting in greater annual variation of thermal conductivity ofCarex tatothan that ofBryum. OnceCarex tatobegan to wilt, the thermal conductivity varied as 0.112 W/(m·K), in comparison with 0.071 W/(m·K) forBryumin the same period, which would impede solar radiation absorption of soil in cold seasons. As a result, ground temperatures of P2 exhibit smaller values than that of P1 during the autumn and winter seasons (Figure 2). Moreover, there are usually small pools among the mounds ofCarextatoesat P2, resulting in higher soil moisture (73% at P2 versus 51% at P1) (Table 1) in the warm season and higher ice content in the cold season than that at P1. Therefore, thermal conductivity for site P2 in winter is considerably greater than that for P1, and thus allows more movement of heat from the soil to the cold air, resulting in a greater decrease in winter ground temperatures.

Also, there may be other important factors which contribute to the difference in ground temperatures between P1 and P2 in winter, such as snow accumulation from late autumn to early spring. The forest stands at P1 are more open (75% coverage) than at P2 (80% coverage), leading to lower snow interception at P1 than that of P2 (Rakhmanov, 1957; Jostet al., 2007). According to fieldwork in April 4, 2010, snow depth was on average 44.5 cm at P1, but 39 cm at P2, as a result of greater thermal insulation at P1 than P2 in winter. In addition, more energy is needed to ablate a deeper snow pack in spring.

Figure 5Carex tato(P3) andBryum(P1) in summer and autumn (Photo: XiaoLi Chang)

4.2 Variation at plots P3 and P4

Plot P4 at the permafrost observation site in Yitulihe shows greater variations at depths above 50 cm for both annual and seasonal ground temperatures than at site P3 at the CFERN Genhe Ecological Station. This difference is particularly marked in summer because there are no larch stands and shrubs to shade the ground at Plot 4.

Zhou (2003) demonstrated that 40% of the solar radiation is reflected and absorbed by larch stands and shrubs at P3. The reduced solar radiation intensityunder the canopy of stands and shrubs results in slower warming of the soils at P3. Ground temperature values show that P3 is colder than that of P4 in summer and in autumn, between 0 and 50 cm (Table 4). However, forest cover often decreases air current velocities within its stratum, and thus impedes heat lost from the soil when the air is cooler, such as during the night and during periods of the year when air temperature is colder than the soil (Benninghoff, 1952).

During the winter season, soil moisture at P3 is greater than that of P4, and perhaps latent heat associated with phase change at this site results in higher ground temperatures at shallow depths (above 80 cm). It is known that denser forest stands, especially larch stands and shrubs, could intercept more snow in the winter (Sturm, 1992; Varholaet al., 2010). This would indicate that ground temperatures should be higher at P4 than P3, but the data shows the opposite for depths shallower than 50 cm. In fact, average snow depth at P3 (47.5 cm) is considerably higher than P4 (23 cm) according to fieldwork in April 4, 2010. Thus, there was greater thermal insulation at P3 and ground temperatures were higher in winter. In addition, forest can lower the rate of snow melt (Meagher, 1938) which is confirmed by our survey where the snow cover in the forest often remained months longer than in an open area. This results in slower warming and, therefore, ground temperatures at shallow depths are lower at P3 than P4 in the spring.

4.3 Ground thermal regime

In our study, ground thermal regimes for P1,L. dahurica–Ledum palustrevar.dilatatum–Bryumforest, and P2,L. dahurica–Betula fruticosaforest, clearly differ because the active layer was greater than 180 cm (Figure 4) at P1, whereas it was only 70 cm at P2. The active layer at P3,L. dahurica–Carex tatoforest, is thinner (about 60 cm) than that at P4 (about 95 cm), in theCarex tatoswamp. It is known that the active layer thickness depends mainly on the summer GST during the thaw period (Romanovsky and Osterkamp, 1997) and the thermal properties of the ground (Cannone and Guglielmin, 2009). Obviously, the summer GST is significantly higher at P1 than P2 (15.1 °C vs. 14.5 °C) and at P4 than P3 (13.8 °C vs. 6.9 °C). At the same time, P1 and P4 have significantly lower water content than P2 and P4 (51% versus 73% and 54% versus 77%, respectively). Williams and Smith (1989) demonstrated that lower water content results in lower thermal conductivity and larger thermal gradients.

The amplitude of ground temperature variation is greater at P1 than P2, and at P4 than P1 due to lower water content and different vegetative cover. In effect, the analysis in the previous sections demonstrated that vegetation is a significant factor affecting ground temperature variation seasonally and annually for regions with similar climate. Correlation coefficients between air and ground temperatures at these plots (Table 3) are higher where there are no trees and shrubs such as P4. However, in the forests at the CFERN Genhe Ecological Station, they decrease with the density of tree coverage. It could be due to different insulating effects of snow cover accumulated at the plots, as vegetation with different coverage and compositions possesses variable snow interception capability (Jostet al., 2007). Correlation coefficients between air and ground temperatures also decrease with depth as the variation in thermal properties of the soils cause a combined lag and damping effect on heat transfer in the ground (Ershov, 1998).

5 Conclusions and future perspectives

The results of this investigation shows that vegetation provides an insulating effect on the GST and shallow ground temperatures in the Greater Hinggan Mountains of Northeastern China, with a relatively high ground temperature variability for theCarex tatoswamp where there are no tree stands and shrubs, and less variation observed for the other three forest types. Vegetation thickness, structure, and coverage are important factors in determining the insulating properties of the vegetation. The growth of ground cover and its water-holding capacity, as well as snow interception capability of forest stands and shrubs also has an impact on the insulating properties even at a site with similar conditions of vegetation type.

Generally, vegetation is subject to local climatic conditions and, at the same time, controls the exchange of energy and water between the atmosphere and the land surface resulting in a modification of the thermal properties of the ground (Shur and Jorgenson, 2007). In Northeastern China’s Greater Hinggan Mountains, a significant warming trend has taken place during the last 50 years (Guoet al., 2003) and there has been micro-environmental modifications by engineering works, construction, deforestation and fire (Jinet al., 2007). All these changes result in a shift from moist to xeric vegetation, which would have a significant impact on the energy balance and the ground thermal regime resulting in deepening active layer and permafrost degradation (Jinet al., 2009), which in turn results in a change in the vegetation coverage and composition (Wanget al., 2000; Lloydet al., 2003; Yanget al., 2010). These feedbacks are complex and vary with time and space. Further research and monitoring will be needed to see if the results are consistent in comparison with other sites with the same vegetation, and to protect the environment and preserve the permafrost.

Acknowledgments:

This study was supported by the Open Fund of the State Key Laboratory of Frozen Soils Engineering (Grant No. SKLFSE200902, SKLFSE-ZT-14 and SKLFSE-ZT-12), and National Natural Science Foundation of China (Grant Nos. 41201066 and J0930003/ J0109). The authors are appreciative of the editors of this journal for their hard work and generous assistance in improving the quality of the paper.

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: Chang XL, Yu SP, Jin HJ,et al.,2014. Vegetation impact on the thermal regimes of the active layer and near-surface permafrost in the Greater Hinggan Mountains, Northeastern China. Sciences in Cold and Arid Regions, 6(5): 0511-0520.

10.3724/SP.J.1226.2014.00511.

March 28, 2014 Accepted: May 30, 2014

*Correspondence to: XiaoLi Chang, Assistant Professor of Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. No. 320, West Donggang Road, Lanzhou, Gansu 730000, China. Tel: +86-931-4967229; E-mail: changxiaoli@lzb.ac.cn