Research progress in cold region wetlands, China

YaYi Tan, Xuan Wang , ZhiFeng Yang, YuLi Wang

Key Laboratory for Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China

Research progress in cold region wetlands, China

YaYi Tan, Xuan Wang*, ZhiFeng Yang, YuLi Wang

Key Laboratory for Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China

China has some of the most abundant wetland resources in the world. Cold region wetlands cover more than 60% of the total natural wetlands in China and play an indispensable role in global climate regulation, water holding, uptake and emission of greenhouse gases, and biodiversity conservation. Because cold region wetlands are sensitive to climatic and environmental changes, it is important for ecological conservation and environmental management to summarize and analyze current research progress on these specific ecosystems. This paper reviews the focus of present studies on the typical cold region wetlands in the northeast region and the Qinghai-Tibet Plateau of China from several aspects as follows: types and distribution, responses of permafrost to climatic changes, uptake and emission of greenhouse gas, eco-hydrological processes, and vegetation succession. Our conclusions are: global warming has a long-term and serious impact on cold region permafrost; emission of greenhouse gases has great temporal and spatial heterogeneity; and hygrophytes in the cold region wetlands have been generally replaced by xerophytes,although it is still unclear whether the vegetation diversity index has increased or decreased. Based on this review, some key topics for future study are recommended as follows: (1) the response of degeneration of cold region permafrost at various spatial and temporal scales; (2) prediction of wetland degeneration tendency by coupling weather, soil, and hydrological models; (3) evaluation of carbon storage; (4) the actual response mechanisms of greenhouse gases to climatic changes; and (5) development of water requirement calculation methods tailored to the unique ecosystems of cold region wetlands.

cold region wetlands; China; progress; review

1. Introduction

Wetlands, praised as "kidneys of the Earth" are macro-ecosystems on a par with forests and oceans. They have played an irreplaceable role in regional ecological balance, biodiversity conservation, and rare species protection(Tanget al., 2008). China has some of the most substantial wetland resources in the world, ranking fourth after Canada,Russia, and the United States (Zhao, 1999; Sunet al., 2006).Globally, wetlands are widespread, being distributed from cold temperate zones to tropics, from coasts to inland regions, and from plains to highlands. Cold region wetlands,one of the most important types, are always located in low-temperature environments with permafrost and seasonal frozen soil. Almost 60% of China’s natural wetlands are located in cold regions. Therefore, cold region wetlands occupy an important position in China, being mainly distributed in cold temperate zones, north of temperate zones and high-altitude areas (Bullock and Acreman, 2003). Furthermore, several cold region wetlands are the main means of water conservation in riverhead areas, having essential functions in climate regulation, water storage, and providing habitats for rare wild animals and plants. Thus, cold region wetlands have extremely high value in ecological protection.

However, in recent decades, the structure and function of wetland ecosystems have significantly changed with global warming and human disturbance. Water resources of rivers and lakes are in great decline and vegetation degeneration is accelerating (Schneider and Eugster, 2007; Wanget al.,2007a; Yanget al., 2008). Due to the simple components and trophic structure of cold region wetland ecosystems,such ecosystems severely degrade in response to climate variation and environmental changes (Peng, 2008).

In order to effectively restrain the deterioration of cold region wetlands, and to possibly restore those that have been degraded, researchers and environmental management departments have increasingly focused on these specific ecosystems. Early studies mainly addressed certain singular aspects of theoretical research or exploration, such as biological characteristics, wild animal and plant resources, or environmental quality (Yang and Wang, 2006). Gradually,however, researches have been extended to more comprehensive exploration of multiple ecosystem aspects. Therefore, the objective of this study is to summarize and analyze the recent research progress and the existing problems of cold region wetlands, and then recommend some directions for future study.

2. Research progress

2.1. Types and distribution of cold region wetlands

The distribution of wetlands mainly depends on the conditions of water and temperature, which are influenced by the changes of latitude as well as azonal factors including geology, physiognomy, and drainage conditions. Thus, the types and distribution of wetlands vary with latitude as climatic zones, which also present distinct regional characteristics. In China, cold region wetlands are mainly located in the northeast region and the Tibetan Plateau region (Table 1).The northeast region, lying on the east edge of the Eurasia continent, is the largest wetland with the greatest ecological diversity in China. The average temperature in this region ranges from approximately 3 °C to 6 °C at 42°N and from-1 °C to -3 °C at 46°N. There exist large areas of permafrost and seasonal frozen soil. The cool and moist environment leads to the formation and growth of wetlands with considerable meadow soil and boggy soil. The most important wetland types in this region are forest, shrubs, grass, and moss. Based on different physiognomy landscape and climatic characteristics, Maet al. (2007) divided the wetlands of the northeast China region into seven parts: the humid forest wetland region of the cold temperate zone north of the Daxing’anling Mountains; the sub-humid forest-meadow steppe wetland region of the temperate zone in the Sanjiang Plain; the semiarid meadow steppe wetland region of the temperate zone in the Songnen Plain; the humid forest wetland region of the temperate zone in the Xiaoxing’anling and Changbai Mountains; the semiarid typical grassland wetland region of the temperate zone in the Hulunbeier Prairie; the semiarid typical grassland wetland region of the temperate zone in the Keerqin Prairie; and the sub-humid meadow steppe wetland region of the warm temperate zone in the lower-middle reaches of the Liao River and the Liaodong Bay.

Table 1 Distribution of major cold region wetlands in China

The Tibetan Plateau cold region wetlands are specialized ecological types that are resulted from long-term adaptation to the Plateau’s cold climate. The unique plateau climate,seasonal snowmelt, and permafrost jointly promote the formation and growth of wetlands. The principal wetland types in the region are marsh, peatland, and lake (Table 2), and they are located on river sides, lake sides, and basins with low-permeability soil; on slope rock with overflowing phreatic water; and on edges of mountain snow. Some wetland types are also distributed on the edges of discontinuous permafrost (Sun, 1996). Based on the topography and landscape features, Baiet al. (2004) divided the Tibetan Plateau cold region wetlands into three classes according to the Ramsar Convention (Table 2).

The regional characteristics of cold region wetlands distribution are influenced by the natural factors of topography and physiognomy, and are also greatly affected by human activities. For example, due to digging of ditches to drain water from wetlands for grazing, the Zoige wetland on the Tibetan Plateau has been in a water loss condition since the 1970s; from the 1960s to the 1990s, the wetland area in this region has been reduced by 15.5×104hm2(Shenet al., 2003).In addition, Liu and Li (2005) demonstrated that changes in the underlying surface caused by drainage projects had seriously reduced the seasonal runoff of the Sanjiang Plain.These researches proved that changes to the drainage conditions of wetlands eventually lead to changes in the types and distribution of cold region wetlands.

2.2. Responses of permafrost to climate changes

Climate is one of the important factors that control the formation and maintenance of wetlands, because changes of moisture and heat directly affect the structure and function of wetland ecosystems. Many studies have shown that temperature increased and precipitation declined in wetlands due to the effects of global warming in cold regions (Liet al.,2003; Zhaoet al., 2007). In cold region wetlands, the warming trend directly causes degeneration of permafrost by breaking the surface heat balance of permafrost, moisture,and the distribution of permafrost. Heet al. (2009) found that over the past few decades, significant permafrost degeneration has occurred in Xing’anling. This phenomenon has been mainly evidenced by decline of permafrost tables,temperature changes, expansion of melting zones, and the disappearance of insular permafrost. Those researchers also pointed out that climate warming was the most important natural factor that caused permafrost degeneration.

Table 2 Types and distribution of wetlands in the Tibetan Plateau

Jinet al. (2006) showed that degeneration of permafrost greatly lowered velocity and strength of heat transmission downward, and the degree of luffing depends on the thermal conductivity of rock. Accordingly, secular permafrost will continue degenerating in the next 40-50 years. This indicates that climate has had a profound influence on cold region wetlands. In addition, Jinet al. (2008) pointed out that responses of permafrost to climate changes had great spatial heterogeneity and a "hot collection" effect was the leading factor.

In conclusion, the influence of global warming on cold region wetlands is the hot issue at present. In order to investigate the mechanisms involved, additional studies on the responses of permafrost to climate changes at various temporal scales should be carried out in the future.

2.3. Sources and sinks of greenhouse gases

Although wetlands account for only 5% of the global surface area, they play a crucial role in the emission of global greenhouse gases (Matthews and Fung, 1987). The stable impermeability of permafrost forms a strong reduction environment. Moreover, low temperature inhibits the decomposition of organic matter. Thus, a large amount of gases can be sequestered in wetlands. Bridges (1978) stated that most of the wetlands in the world are carbon sinks.Zhang Fet al. (2008) indicated that cold wetlands in the northeast region of the Tibetan Plateau could absorb 230.16 gCO2/m2in the growing season (from May to September),but emit about 546.18 gCO2/m2in the other months of the year. This may be because the melting of ice and snow in the spring releases greenhouse gases, which makes cold wetlands transform carbon sinks to carbon sources.

In recent years, climate changes and human disturbance have had increasingly serious effects on cold region wetlands. More and more researches have demonstrated that cold region wetlands are significant sources of greenhouse gases (Hirotaet al., 2004; Haoet al., 2007; Zhang Fet al.,2008). Due to lack of sufficient data on cold region wetlands,some researchers have only given preliminary estimates of greenhouse gases emitted from wetlands. For example, Jinet al. (1999) estimated that the emission of CH4from the wetlands in the northeast region was about 0.68 Tg within a year and the maximum volume in the Tibetan Plateau was about 0.7-0.9 Tg. In comparison, the total emission of CH4from natural wetlands in China was about 1.7-2.0 Tg, which further proves that cold region wetlands are significant sources of CH4. The emission of CH4from natural freshwater wetlands all over the world has been estimated to vary from 100 to 200 Tg, which indicates that cold region wetlands have strong emission potential. During the next 200 years, the melting of permafrost could make the content of CH4in the atmosphere increase 2-25 Tg per year (Jin and Cheng,1997).

The emission of greenhouse gases in cold region wetlands varies significantly with time and space. Different spatial and temporal features lead to different edaphic conditions, hydrologic situations, vegetation types, and meteorological conditions, all of which affect the emission of greenhouse gases. Chenet al. (2009) documented apparent spatiotemporal heterogeneity of CH4emission in the Zoige wetland based on the CH4fluxes measured in three wetland landscapes. A special diurnal variation pattern of CH4emission was observed: there were two emission peaks—the minor peak occurred at 06:00 and the major one at 15:00.Furthermore, there was a significant correlation between this change and soil temperature.

In contrast, results obtained by Dinget al. (2004)showed that temperature had little impact on the daily variation of CH4emission. Also, Duanet al. (2007) demonstrated that the daily variations of CH4emission were different in various phenological seasons. Van der Natet al. (1998) indicated that the effect of temperature on the daily variation was less than 35%, which suggests that photosynthetic radiation had a greater influence on the CH4emission than did temperature. On the other hand, the seasonal trend of greenhouse gases emission tended to be more typical. The peak values of CH4emission appeared during growth periods and there was little emission in winter (Jin and Cheng,1997; Duanet al., 2007; Chenet al., 2009). There was a significant correlation between these seasonal CH4emissions and soil or matrix temperature.

The seasonal variation of CO2was found to be the same as that of CH4in a study on the functions of cold region wetland ecosystems in the northeast region of the Tibetan Plateau (Zhang Fet al., 2008). Several reasons accounted for the spatial variation of CO2emission, including edaphic conditions, hydrologic situations, vegetation types, and human disturbance. These factors always interacted intimately or had a combined influence. Chenet al. (2009) indicated that emission of CH4in lakeside wetlands was greater than that in broad valley wetlands, which in turn was greater than that in riverside wetlands. This was caused by vegetation species differences as well as various edaphic conditions and hydrologic situations in each region.

The most influential types of human disturbance were reclamation (conversion of marsh to cropland) and grazing.Reclamation usually reduces waterlogging, which might reduce the emission of CH4significantly but increase that of N2O slightly (Haoet al., 2007). Grazing has been shown to reduce the biomass in the Tibetan Plateau by 85%, which weakens the assimilation of vegetation and also influences the function of gas transmission of aquatic plants; consequently, this reduces the absorption of CO2but increases the emission of CH4(Mitsuruet al., 2005).

However, there have been few accurate estimates of the effects of climate changes on the sources and sinks of greenhouse gases in cold region wetlands. Zhanget al. (2007)developed a model of carbon cycling and predicted that the organic carbon density in the Sanjiang Plain wetland would increase under the scenario of doubled CO2concentration and less than 2.5 °C increase of temperature. However, according to their model, more-warming scenarios would not be conducive to the carbon accumulation in wetland ecosystems. These results revealed the influence of climate changes on carbon sources and sinks in wetlands to some extent, but this study only focused on the effect of climate warming on the carbon fixation and mineralization and decomposition of organic carbon. It did not consider the effects of emission variation of greenhouse gases resulting from melting of permafrost. Therefore, the actual response mechanism of climate changes on the greenhouse gases emission in cold region wetlands has not yet been clarified.

To sum up, cold region wetlands have an important influence on the global emission of greenhouse gases, and there may exist a positive feedback mechanism between the effects of those gases and global warming which has not been clarified thus far. Also, precise estimates of the carbon content in cold region wetlands are not yet available. Therefore, future studies of the warming model on sources and sinks of greenhouse gases should be further improved to explain the possible response mechanism.

2.4. Eco-hydrological processes

Wetland hydrology exerts an important control on the structure and function of wetland ecosystems by interacting with other environmental components (Figure 1). Increasing research emphasis on eco-hydrological processes is revealing the interrelationship among vegetation, frost, soil, and runoff. Friendet al. (1997) and Jorgensonet al. (2001) indicated that the hydrologic cycle and ecosystem balance in Alaska and the east Siberia regions have greatly changed in recent times. Numerous studies have focused on the significant effect of vegetation degeneration and melting permafrost in cold region wetlands. Research on the forested peat plateau at Scotty Creek, a small wetland-dominated discontinuous permafrost basin in the Northwest Territories, Canada, showed that most runoff occurred in response to snowmelt inputs (Wrightet al., 2008). During the study periods,the melting of ground ice was also a significant source of water which was largely retained in soil storage. However,high ice content and poor drainage in soil could lead to loss of water generated from melting of excessive ice (Wrightet al., 2008). Liu Het al. (2008) provided evidence of temporal heterogeneity in soil moisture of different vegetations and showed that the higher interception of rainfall is probably responsible for the higher temporal variability of soil moisture content in shrubland and surface grass.

In the past 40 years, river flows in the headwater zone of the Yangtze Basin have continuously degraded under climate warming, while the occurrence of abnormal flood flows (more than 550 m3/s) has increased. This is an integrated effect of vegetation and permafrost variation (Wanget al., 2007b). Thus, it can be concluded that vegetation types as well as vegetation coverage play an important role in the formation and maintenance of river flows in cold regions.

Hydrological conditions of cold region wetlands have a great influence upon vegetation types and distribution, and the formation of permafrost. Wuet al. (2005) pointed out significant differences in submerging frequency, submerging duration, and submerging period of different vegetation belts in the Beigu Mountain wetland, which controlled the distribution of the wetland vegetation. Wanget al. (2008) found that the values of morphological indicators and chlorophyll content decreased as the water level increased under the variable water regime. In wetlands, water conditions not only directly affect the growth of plants, but also indirectly influence the composition of plant communities and soil characteristics. Furthermore, wetland vegetation and the peat horizon have unique characteristics of thermal diode, which result in the thermal offsets. Thus, they are useful for the maintenance and growth of permafrost (Jinet al., 2008).Permafrost can also prevent water from evaporating, thereby accumulating more water on the surface, which inhabits aerobic bacterial decomposition and promotes the accumulation of the peat horizon. In this way the existence of permafrost ensures the continuity of vegetation growth in wetlands. Such a symbiotic connection (Sunet al., 2008) reveals the relationships between hydrology and permafrost degeneration in cold region wetlands.

Figure 1 The interaction mechanism between wetland hydrology and other components of ecosystems (Zhang G et al., 2008)

By analyzing the interrelationship among the three major elements of wetlands (water, soil, and vegetation), we conclude that the degeneration of cold region wetlands thus far can be attributed to the problem of water supply. Thus, how to meet the necessary water requirements in cold regions is an important task for future researches on ecological and hydrological processes.

2.5. Succession and degradation of vegetation

In the frigid climates of cold region wetlands, vegetation generally has a narrow survival niche that is extremely sensitive to habitat fluctuation (Lianget al., 2007). With the environment of cold region wetlands getting drier, researches on the number of vegetation types in micro-scale are no longer the primary focus. Instead, the direction of vegetation succession and patterns of species diversity in macro-scale have become the research hotspots. Surface water, climate, soil, micro-topography, and other factors affect vegetation succession. These factors can change micro-habitats, which then induce the variation and transport of wetlands vegetation types accordingly (Zhang Tet al., 2008).Tian (2005) conducted a field survey in the Zoige wetland on the Tibetan Plateau and found that the ecological succession series of plant communities in different wetland habitats was as follows: aquatic vegetation → moor vegetation →swampy prata vegetation → meadow vegetation, and its horizontal patterns were parallel distribution (near oxbow lakes or stream banks) and concentric circles (near depressions). The vertical distribution regularity was as follows:aquatic vegetation → moor vegetation → swampy prata vegetation → subalpine meadow → subalpine shrub, from the center of the lake to the hilltop.

Adopting spatial series instead of time series, Houet al.(2009) studied the wetlands vegetation variation in the Shouqu wetlands of the Yellow River, and found that the natural degradation process of wetlands was from marsh to swampy prata vegetation, and then cold region vegetation to grassland meadow; the dominant species succeeded toward mesosere and xerophytic species.

Moreover, researches on the vegetation succession in other cold region wetlands (Zhuet al., 2003; Lianget al.,2007; Liu Xet al., 2008; Xinget al., 2008) showed similar trends, that is, the native marsh vegetation and meadow vegetation types, which adapted to cold and humid habitats,were being gradually degraded. They were replaced by typical meadow or shrub types, the majority of which were cold mesosere and xerophytic plants. However, Louet al.(2006) pointed out that as wetlands vegetation succeeded toward xerophytic plants, the Simpson index, the Shannon-Wiener index, and the Pielou evenness index ofCarex lasiocarpaandCalamagrostis angustifolia(which were distributed most widely in the Sanjiang Plain) clearly fluctuated. Also, the area ofCarex lasiocarpacommunity decreased. They concluded that the species diversity of vegetation in the Sanjiang Plain was decreasing.

The above conclusions about whole-wetlands species diversity are drawn from various analyses of natural dominant species diversity. However, Xiaoet al. (2008) formed a different opinion that cold region wetlands vegetation diversity and dominance index increased with xerarch succession.In their study, the native marsh lived under water throughout the year, so that the total soil nutrients were enclosed, resulting in weak absorption and utilization. In such conditions only a few adaptive species could grow and become the particular community, and thus vegetation diversity would decrease. However, if flooding conditions gradually disappear, this suggests that cold region wetlands would succeed from marsh toward meadow, which would strengthen the mineralization of organic matter. When this occurs, individual or interspecies vegetations compete for the limited soil resource, and this affects the formation and dynamics of vegetation community species (Bartet al., 2001). These competitions restrain the dominant species’ development so that more species can coexist and species diversity can increase (Patrick and Strawbridge, 1963).

This conclusion has been supported by many studies.May (1972) studied some marsh and coastal wetlands vegetation communities and found that systems would become more stable as the flora became single species; he interpreted this as showing that an increase in species diversity may simply be a signal of wetlands degradation. Houet al.(2009) found that as the importance of constructive species decreased, their dominance in communities would be gradually diminished, while the ability of companion species to obtain the needed resources would increase accordingly.This provided opportunities for noxious weeds to intrude,which made the percentage of noxious weeds and thus vegetation diversity of the community increased. This result was consistent with a similar study in the Haibei wetlands of the Qilian Mountains (Liet al., 2003) and in the northern frost zone of the Qinghai-Tibetan Plateau wetlands (Guoet al., 2007).

However, the direction of succession and species diversity variation is determined by many factors. Many aspects,such as vegetation succession of different landscape types at the macro-scale, the relationship between their diversity changes and water gradients, soil fertility patterns, as well as climate change must be further studied to reveal the mechanisms of vegetation succession and degradation.

3. Directions for future research

(1) As an important part of natural ecosystems in cold region wetlands, permafrost plays an indispensable role in retaining moisture and regulating wetland environments. At present, global warming is having a strong impact on the existence and development of wetland ecosystems in cold regions. Therefore, the focus of future researches should be to clarify the response of permafrost in cold region wetlands to climate changes. In-depth research directions should include: the differences of responses at different time and space scales, and coupling weather, soil, and hydrological models to predict the trend of permafrost degradation.

(2) Cold region wetlands have important potential for absorption and emission of greenhouse gases, and their carbon sources and sinks may directly affect the global carbon balance. There have been some recent studies on the dynamic laws of greenhouse gas emissions, but there is still a lack of exact conclusions about greenhouse gas emission responses to global warming. Therefore, in-depth research directions should include: assessment of the carbon storage in cold region wetlands, and simulation of the response between climate changes and greenhouse gas fluxes.

(3) Water balance is the key link in the study of drying and degrading wetlands, especially regarding the annual distribution of permafrost in cold region wetlands. It is very important to study the changes of water in order to maintain the health and stability of cold region wetland ecosystems.Although there are many existing researches on ecological water requirements of wetlands, very few of them address water requirement calculation methods tailored to the special wetland ecosystems in cold regions. Therefore, as a precondition for determining the ecological water requirement calculation methods appropriate for cold region wetlands, research on the relationship between water conditions and ecosystem health, and the mechanisms involved, should be strengthened.

This project is supported by the National Key Technologies R&D Program (No. 2007BAC18B01) and the Program for Changjiang Scholars and the Innovative Research Team in University (No. IRT0809). The authors would like to extend special thanks to the reviewers and editors for their constructive comments and suggestions in improving the quality of this paper.

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10.3724/SP.J.1226.2011.00441

*Correspondence to: Dr. Xuan Wang, Professor of School of Environment, Beijing Normal University. No.19, Xinjiekouwai Street, Beijing 100875, China. Tel: +86-10-58800830; Email: wangx@bnu.edu.cn

24 April 2011 Accepted: 2 July 2011