Influence of land use and management histories on vegetation structure and ecosystem services of paddy fields and surrounding wetlands in the lower of Tumen River Basin, Northeast China

概要

Chapter 1 General Introduction
Wetlands are among the world’s most productive and valuable ecosystems. They are important habitats for a variety of flora and fauna, and are vital ecosystems that provide diverse services. They not only are essential for ecological functions such as maintaining biodiversity, controlling floods, and removing pollutants, but also serve necessary economic functions in rice and fish production, transport, and hydropower energy. Despite this, they have suffered a loss of 54%–57% of their area worldwide, which continues under pressures from agriculture, urban expansion, industrialization, and resource extraction. Conservation and restoration of wetland ecosystems is important issues for conservation biodiversity and sustainable use of ecosystem services. Abandoned paddy fields provide opportunities to restore wetlands and serve as substitute habitats for wetland species. They may be good sites for restoration of wetlands and conservation of wetland habitats. Effective protection and restoration of damaged and degraded wetlands is becoming more and more urgent.

In this study, we focus on the lower of Tumen River, Northeast China where the wetlands decreased dramatically because of land use change. Our objective was to reveal influence of the land use and management histories on vegetation structure and ecosystem services of paddy fields and surrounding wetlands, analyze the effects of environmental factors on the restoration process and identifies the key factors for wetland restoration affecting historical land use.

Chapter 2 Effects of land use changes on ecosystem services based on InVEST model at the regional scale
Despite increasing awareness in the importance of wetlands for biodiversity conservation and ecosystem services, the area of wetland is sharply decreasing due to land uses including agricultural development. The impact of agricultural land uses on those ecosystem services can be significantly vary among different farming practices, including paddy fields and dry farmlands, however, little studies have actually evaluated potential intensity of tradeoffs among different land use types. Therefore, understanding the effects of land cover changes on ecosystem services is essential for ecosystem management and environmental protection at the regional scale in the lower of Tumen River Basin that has undergone marked changes.

We used the models in InVEST to estimate the changes of habitat quality and carbon regulation services of the lower of Tumen River Basin in the Northeast China during the last 30 years. Firstly, to obtain land use and land cover date we used satellite imagery to produce the maps in 1986 and 2015. For the main land cover changes, the important change was a decline in wetland and an increase in woodland. Secondly, to study the effects of land use pattern change on habitat quality, InVEST habitat quality model was applied to calculate the contributions of habitat threats (rural construction area, main roads and agriculture land) to habitat degradation, and to assess habitat quality changes between 1986 and 2015.The results indicate that habitat quality in the lower of the Tumen River Basin is heterogeneously distributed. Wetlands loss had the largest effect on habitat degradation. Finally, we combined the maps with the table of carbon pools of each LULC category and estimated changes in the carbon storage by using InVEST carbon storage model. We only considered carbon storage without sequestration due to the high degree of uncertainty in estimating sequestration. Carbon storage data in terrestrial areas collected from relevant previous studies and national reports in China. The results indicate that the carbon storage of wetland the study area decreased 73.1*103 Mg C, which is huge enough to offset potential gain in the development of woodlands in hilly areas. Change detection analysis in land use and land cover revealed wetland turned into cropland and water body and those changes were major causes the continual decreases in carbon storage.

Chapter 3 Vegetation dynamics of abandoned paddy fields and surrounding wetlands in the lower of Tumen River Basin, Northeast China
On the temperate lowland plain of the lower Tumen River, agricultural development has converted most marshland into paddy fields. However, the locations of old paddy fields in the lowland temperate zone, where the vegetation structure is dominated by herbs adapted to seasonally wet or waterlogged conditions, are poorly known, and the impact of land use history on marshland diversity and shifts in plant functional groups has been scantly researched.

To reveal the vegetation dynamics of abandoned paddy fields and surrounding wetlands dominated by seasonal herbaceous communities under natural succession, we used a chronosequence approach to investigate herbaceous wetland communities in different recovery phases (15 years), as well as natural wetland as a reference. We assessed their ecological characteristics, species composition and diversity to determine how they change during natural succession. Plant species composition and dominance in the abandoned fields changed markedly during natural secondary succession. Initially, the annual weeds Echinochloa crus-galli and Bidens tripartita were dominant. Later, communities gradually became dominated first by Polygonum thunbergii and then by tussock-forming Carex rostrata. Species diversity was higher in abandoned fields than in natural wetlands and decreased with the time since abandonment. The partition of β-diversity components revealed that replacement was the prominent process structuring plant communities in paddy field at different times since abandonment. Our results suggest that with the successional time, especially 15 years since abandonment, the species composition and diversity was becoming similar to the natural wetland, however, there were differences in plant functional groups. We found that grass and sedge species rather than forbs were identified as key factors affecting succession of abandoned paddy fields in the lower of Tumen River Basin.

Chapter 4 Effects of soil and water factors on vegetation characteristics in abandoned paddy fields and surrounding wetlands
Relationship between environmental factors and vegetation is important to manage degraded ecosystem and also helps to determine the main factors which can effect on vegetation succession patterns and processes. We used the canonical correspondence analysis (CCA) to conduct the corresponding analysis of the community structure of plant species and soil physical and chemical properties, to explore the dominate environmental factors affecting the plant community structure in wetlands. The results revealed the explanatory power of the water factors was relatively low than soil factors. The soil organic matter and available N were good explanatory variables, and they were positively correlated with the time since abandonment. The available N, soil organic matter and pH in soil tended to increase with the time since abandonment of paddy fields. The results suggested that nutrient contents in soil tended to be increase with the progress of vegetation restoration and established vegetation were changed as nutrient contents were changed. On the other hand, among the water factors, the total N, NO3-N, total P, NPOC exhibited a high explanatory power of the vegetation changes. The explanatory power of the water factors was relatively low than soil factors. NO3-N and total P contents in water tended to decrease with the successional stage of abandoned paddy fields. The total N was increased with developmental stage of abandoned paddy fields.NO3-N and total P contents in water tended to decrease with the successional stage of abandoned paddy fields. On the contrary, the total N in water was increased with developmental stage of abandoned paddy fields. Results showed there were no clear results on water purification. It may be considered the water purifying efficiencies of natural wetlands have great differences between each other, by reason of the wetland location, loading time of water purifying, and surrounding site conditions. In a word, the results not only showed that the environmental factors played an important role in the vegetation community process, but also revealed that the vegetation community in turn played an important role in ecosystem services such like soil restoration and water quality purification in some degree.

Chapter 5 General discussion
By combining the results presented in previous Chapters, I had a general discussion based on the link between chapter 3 and chapter 4. The study used a combined GIS - field sampling approach to describe land use patterns and ecosystem services and also include detailed data on the species composition of abandoned paddy fields and surrounding wetlands. Restoration of degraded ecosystem in an attempt to recover weakened function by using appropriate scientific theory and the method for restoration is different according to degree of disturbance. This study provides a scientific evidence to restore wetland and abandoned paddy fields as a successful compensation for wetland loss and degradation in the lower of Tumen River Basin. In doing so, it discusses the implications for wetland restoration, and provides key recommendations to improve wetland restoration and management.