Yizhao Chen

Spatio-temporal dynamics of vegetation coverage and its relationship with climate factors in Inner Mongolia, China

The vegetation coverage dynamics and its relationship with climate factors on different spatial and temporal scales in Inner Mongolia during 2001–2010 were analyzed based on MODIS-NDVI data and climate data. The results indicated that vegetation coverage in Inner Mongolia showed obvious longitudinal zonality, increasing from west to east across the region with a change rate of 0.2/10°N. During 2001–2010, the mean vegetation coverage was 0.57, 0.4 and 0.16 in forest, grassland and desert biome, respectively, exhibiting evident spatial heterogeneities. Totally, vegetation coverage had a slight increasing trend during the study period. Across Inner Mongolia, the area of which the vegetation coverage showed extremely significant and significant increase accounted for 11.25% and 29.13% of the area of whole region, respectively, while the area of which the vegetation coverage showed extremely significant and significant decrease accounted for 7.65% and 26.61%, respectively. On inter-annual time scale, precipitation was the dominant driving force of vegetation coverage for the whole region. On inter-monthly scale, the change of vegetation coverage was consistent with both the change of temperature and precipitation, implying that the vegetation growth within a year is more sensitive to the combined effects of water and heat rather than either single climate factor. The vegetation coverage in forest biome was mainly driven by temperature on both inter-annual and inter-monthly scales, while that in desert biome was mainly influenced by precipitation on both the two temporal scales. In grassland biome, the yearly vegetation coverage had a better correlation with precipitation, while the monthly vegetation coverage was influenced by both temperature and precipitation. In grassland biome, the impacts of precipitation on monthly vegetation coverage showed time-delay effects.

Vegetation dynamics and its driving forces from climate change and human activities in the Three-River Source Region, China from 1982 to 2012

The Three-River Source Region (TRSR), a region with key importance to the ecological security of China, has undergone climate changes and a shift in human activities driven by a series of ecological restoration projects in recent decades. To reveal the spatiotemporal dynamics of vegetation dynamics and calculate the contributions of driving factors in the TRSR across different periods from 1982 to 2012, net primary productivity (NPP) estimated using the Carnegie-Ames-Stanford approach model was used to assess the status of vegetation. The actual effects of different climatic variation trends on interannual variation in NPP were analyzed. Furthermore, the relationships of NPP with different climate factors and human activities were analyzed quantitatively. Results showed the following: from 1982 to 2012, the average NPP in the study area was 187.37gcm(-2)yr(-1). The average NPP exhibited a fluctuation but presented a generally increasing trend over the 31-year study period, with an increase rate of 1.31gcm(-2)yr(-2). During the entire study period, the average contributions of temperature, precipitation, and solar radiation to NPP interannual variation over the entire region were 0.58, 0.73, and 0.09gcm(-2)yr(-2), respectively. Radiation was the climate factor with the greatest influence on NPP interannual variation. The factor that restricted NPP increase changed from temperature and radiation to precipitation. The average contributions of climate change and human activities to NPP interannual variation were 1.40gcm(-2)yr(-2) and -0.08gcm(-2)yr(-2), respectively. From 1982 to 2000, the general climate conditions were favorable to vegetation recovery, whereas human activities had a weaker negative impact on vegetation growth. From 2001 to 2012, climate conditions began to have a negative impact on vegetation growth, whereas human activities made a favorable impact on vegetation recovery.

Grassland dynamics in response to climate change and human activities in Inner Mongolia, China between 1985 and 2009

China’s grassland has been undergoing rapid changes in the recent past owing to increased climate variability and a shift in grassland management strategy driven by a series of ecological restoration projects. This study investigated the spatio-temporal dynamics of Inner Mongolia grassland, the main grassland region in China and part of the Eurasia Steppe, to detect the interactive nature of climate, ecosystems and society. Land-use and landscape patterns for the period from 1985 to 2009 were analysed based on TM- and MODIS-derived land-use data. Net Primary Productivity (NPP) estimated by using the Carnegie-Ames-Stanford Approach model was used to assess the growth status of grassland. Furthermore, the factors related to the dynamics of grassland were analysed from the perspectives of two driving factors, climate change and human activities. The results indicated that higher temperatures and lower precipitation may generally have contributed to grassland desertification, particularly in arid regions. During the period from 1985 to 2000, a higher human population and an increase in livestock numbers were the major driving forces responsible for the consistent decrease in NPP and a relatively fragmented landscape. From 2000 to 2009, the implementation of effective ecological restoration projects has arrested the grassland deterioration in some ecologically fragile regions. However, a rapid growth of livestock numbers has sparked new degradation onnon-degraded or lightly degraded grassland, which was initially neglected by these projects. In spite of some achievement in grassland restoration, China should take further steps to develop sustainable management practices for climate adaptation and economic development to bring lasting benefits.

Assessing the spatiotemporal variation in distribution, extent and NPP of terrestrial ecosystems in response to climate change from 1911 to 2000.

To assess the variation in distribution, extent, and NPP of global natural vegetation in response to climate change in the period 1911–2000 and to provide a feasible method for climate change research in regions where historical data is difficult to obtain. In this research, variations in spatiotemporal distributions of global potential natural vegetation (PNV) from 1911 to 2000 were analyzed with the comprehensive sequential classification system (CSCS) and net primary production (NPP) of different ecosystems was evaluated with the synthetic model to determine the effect of climate change on the terrestrial ecosystems. The results showed that consistently rising global temperature and altered precipitation patterns had exerted strong influence on spatiotemporal distribution and productivities of terrestrial ecosystems, especially in the mid/high latitudes. Ecosystems in temperate zones expanded and desert area decreased as a consequence of climate variations. The vegetation that decreased the most was cold desert (18.79%), while the maximum increase (10.31%) was recorded in savanna. Additionally, the area of tundra and alpine steppe reduced significantly (5.43%) and were forced northward due to significant ascending temperature in the northern hemisphere. The global terrestrial ecosystems productivities increased by 2.09%, most of which was attributed to savanna (6.04%), tropical forest (0.99%), and temperate forest (5.49%). Most NPP losses were found in cold desert (27.33%). NPP increases displayed a latitudinal distribution. The NPP of tropical zones amounted to more than a half of total NPP, with an estimated increase of 1.32%. The increase in northern temperate zone was the second highest with 3.55%. Global NPP showed a significant positive correlation with mean annual precipitation in comparison with mean annual temperature and biological temperature. In general, effects of climate change on terrestrial ecosystems were deep and profound in 1911–2000, especially in the latter half of the period.

Application of a normalized difference impervious index NDII to extract urban impervious surface features based on Landsat TM images

The ecological conditions of urban areas have been deteriorating in some aspects due to population growth and increasing expansion, with significant effects on human health. Impervious surface areas are an important indicator of urban ecological environmental change, therefore quickly and accurately estimating impervious surface areas is essential to monitoring the urban dynamics of change and human activities and their effects on urban environmental quality. Currently, few methods that are applied in estimating urban impervious surfaces are capable of providing results quickly and accurately. Accordingly, this study proposes a new index, named the normalized difference impervious index (NDII), based on Landsat TM images, which uses the visible (red, green, and blue) and thermal bands. The index was used to extract the impervious surface areas of Nanjing city, Jiangsu Province, China, and we assume that the average value of five times strict supervised classification is the true value of impervious surfaces. A combination of red and thermal bands extracted the impervious surfaces with a producer’s accuracy of 86.9%, a user’s accuracy of 84.6%, an overall accuracy of 91.4%, and a kappa coefficient of 0.8. The accuracy is 87.7% validated by high-resolution images. This method can rapidly extract urban impervious surface areas with promising accuracy.

Grassland coverage inter-annual variation and its coupling relation with hydrothermal factors in China during 1982-2010

GIMMS (Global Inventory Modeling and Mapping Studies) NDVI (Normalised Difference Vegetation Index) from 1982 to 2006 and MODIS (Moderate Resolution Imaging Spectroradiometer) NDVI from 2001 to 2010 were blended to extract the grass coverage and analyze its spatial pattern. The response of grass coverage to climatic variations at annual and monthly time scales was analyzed. Grass coverage distribution had increased from northwest to southeast across China. During 1982–2010, the mean nationwide grass coverage was 34% but exhibited apparent spatial heterogeneity, being the highest (61.4%) in slope grasslands and the lowest (17.1%) in desert grasslands. There was a slight increase of the grass coverage with a rate of 0.17% per year. Increase in slope grasslands coverage was as high as 0.27% per year, while in the plain grasslands and meadows the grass coverage increase was the lowest (being 0.11% per year and 0.1% per year, respectively). Across China, the grass coverage with extremely significant increase (P<0.01) and significant increase (P<0.05) accounted for 46.03% and 11% of the total grassland area, respectively, while those with extremely significant and significant decrease accounted for only 4.1% and 3.24%, respectively. At the annual time scale, there are no significant correlations between grass coverage and annual mean temperature and precipitation. However, the grass coverage was somewhat affected by temperature in alpine and sub-alpine grassland, alpine and sub-alpine meadow, slope grassland and meadow, while grass coverage in desert grassland and plain grassland was more affected by precipitation. At the monthly time-scale, there are significant correlations between grass coverage with both temperature and precipitation, indicating that the grass coverage is more affected by seasonal fluctuations of hydrothermal conditions. Additionally, there is one-month time lag-effect between grass coverage and climate factors for each grassland types.

Grassland Carbon Sequestration Ability in China: A New Perspective from Terrestrial Aridity Zones ☆

ABSTRACT Current climate change (e.g., temperature and precipitation variations) profoundly influences terrestrial vegetation growth and production, ecosystem respiration, and nutrient circulation. Grasslands are sensitive to climate change, and the carbon sequestration ability is closely related to water availability. However, how the terrestrial water budget influences regional carbon sequestration by the grassland ecosystem is still unclear. In this study, we modified a terrestrial biogeochemical model to investigate net ecosystem productivity (NEP) of Chinese grasslands under different aridity index (AI) levels from 1982 to 2008. The results showed that Chinese grasslands acted as a carbon sink of 33.7 TgC. yr-1, with a clear decrease in the spatial distribution from the humid end (near-forest) to the arid end (near-desert). During these 27 years, gross primary productivity (GPP) and net primary productivity (NPP) significantly increased with regional warming over the entire range of the AI, but no significant tendency was found for NEP. Meanwhile, only NPP in the arid zone (AR) and the semiarid zone (SAR) were significantly correlated with mean annual precipitation (MAP), and no significant correlation was found between heterotrophic respiration (Rh and MAP; NPP and Rh were both positively correlated with mean annual temperature (MAT) in all AI zones except for NPP in AR; no significant correlation between NEP and MAP or MAT was found. These results revealed that the grasslands with different AI levels keep different response patterns to temperature and precipitation variations. On the basis of these results, we predicted that the gap of carbon sequestration ability between humid and arid grassland will expand. The total carbon sink in Chinese grasslands will continue to fluctuate, but there is a danger that it might shrink in the future because of a combination of climatic and human factors, although CO2 fertilization and N deposition might partly mitigate this reduction.

Quantitative assessments of water-use efficiency in Temperate Eurasian Steppe along an aridity gradient.

Water-use efficiency (WUE), defined as the ratio of net primary productivity (NPP) to evapotranspiration (ET), is an important indicator to represent the trade-off pattern between vegetation productivity and water consumption. Its dynamics under climate change are important to ecohydrology and ecosystem management, especially in the drylands. In this study, we modified and used a late version of Boreal Ecosystem Productivity Simulator (BEPS), to quantify the WUE in the typical dryland ecosystems, Temperate Eurasian Steppe (TES). The Aridity Index (AI) was used to specify the terrestrial water availability condition. The regional results showed that during the period of 1999–2008, the WUE has a clear decreasing trend in the spatial distribution from arid to humid areas. The highest annual average WUE was in dry and semi-humid sub-region (DSH) with 0.88 gC mm-1 and the lowest was in arid sub-region (AR) with 0.22 gC mm-1. A two-stage pattern of WUE was found in TES. That is, WUE would enhance with lower aridity stress, but decline under the humid environment. Over 65% of the region exhibited increasing WUE. This enhancement, however, could not indicate that the grasslands were getting better because the NPP even slightly decreased. It was mainly attributed to the reduction of ET over 70% of the region, which is closely related to the rainfall decrease. The results also suggested a similar negative spatial correlation between the WUE and the mean annual precipitation (MAP) at the driest and the most humid ends. This regional pattern reflected the different roles of water in regulating the terrestrial ecosystems under different aridity levels. This study could facilitate the understanding of the interactions between terrestrial carbon and water cycles, and thus contribute to a sustainable management of nature resources in the dryland ecosystems.

Efficient Water-Saving Irrigation Solution for Direct Seeding Rice under No-Tillage after Cultivating Wheat

In order to explore the water-saving irrigation pattern of direct rice for high yield and excellent quality under no-tillage after wheat, the research was carried out on the experimental farm of Yangzhou University in the year 2005, which selected Wuxiangjing 14 and Lianjiajing 1 as the experimental cultivar. The semi-arid irrigation and general irrigation regime were employed under 3000 thousand basic seedling per hectare with Lianjiajing 1 and wuxiangjing 14. The productivity was similar between the two types of irrigation regime. There was no remarkable difference on the yield components, leaf area index, photosynthesis potential, net assimilation rate, and dry matter at different stage, milling quality and appearance quality, the cooking quality and nutrition quality. But all these aspects in the semi-arid irrigation treatment had a trend of decrease. The rice under semi-arid irrigation regime was strong; their leaves color were light, and the N content of the stem and grain were lower than those of conventional irrigation regime. Semi-arid irrigation saved the amount of water by one third, compared with the general irrigation regime. On the basis of the research, semi-arid irrigation with appropriately improved seeding pattern under no-tillage after wheat could be one of approaches to gain high yield and excellent quality, but the best combination need to be confirmed through further experiments.

Comparative Study on LUCC and CLID of Zhangjiagang, Hanoi and Dehradun in the developing countries of Asia-Pacific region: A real challenge to food security

Three cities with different degrees of urbanization in developing countries of Asia-Pacific region are selected as the study areas, They are Zhangjiagang city in China, Dehradun in India and Hanoi in Vietnam. In this paper, data was collected concluding nearly a decade of remote sensing data, population, economic data and other relevant information. We studied land-use and land-cover change (LUCC) and cultivated land instability degree (CLID) of three cities by the means of spatial analysis and CA model analysis supported by the 3S technology. The results show that: All three cities have significant land use/land cover (LULC) changes under the background of urbanization over the past decade, mainly in the increase of urban land and reduction of agricultural land. The changes of Dehradun is the greatest, Hanoi second, and Zhangjiagang city is the smallest; The CLID value shows the pressure size of transformation for the cultivated land in Zhangjiagang, Dehradun and Hanoi. Among three cities, the pressure of cultivated land in Dehradun is the greatest. Hanoi is the second, the minimum is Zhangjiagang. Facing the changes, rapid urbanization is increasing the risk of food security. Local government should make positive policy to the challenge of decreasing availability of cultivated land and offer unremitting efforts towards the goal of food security.