Xibao Xu

Impacts of China's Three Gorges Dam Project on net primary productivity in the reservoir area

Chinas Three Gorges Dam Project (TGP) is the worlds largest hydroelectric power project, and as a consequence the reservoir area is at risk of ecological degradation. This study uses net primary productivity (NPP) as an important indicator of the reservoir ecosystems productivity to estimate the impacts of the TGP in the local resettlement region of the Three Gorges Reservoir Area (TGRA) over the 2000-2010 period. The modeling method is based upon the Carnegie Ames Stanford Approach (CASA) terrestrial carbon model and uses Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing data for modeling simulation. The results demonstrate that total NPP in the resettlement region decreased by 8.0% (632.8Gg) from 2000 to 2010. The impact of the TGP on NPP is mainly mediated by land-use change brought about by the large-scale inundation of land and subsequent massive resettlement of both rural and urban residents. Nearby resettlement, land inundation, and relocation of old urban centers and affiliated urban dwellers are responsible for 54.3%, 28.0%, and 5.8% respectively of total NPP reduction in the resettlement region over the study period. The major national ecological projects implemented in the TGRA since 1998 have played a key role in offsetting the negative impacts of the TGP on NPP in the region.

Ecological risk assessment of ecosystem services in the Taihu Lake Basin of China from 1985 to 2020

There are tremendous theoretical, methodological and policy challenges in evaluating the impact of land-use change on the degradation of ecosystem services (ES) at the regional scale. This study addresses these challenges by developing an interdisciplinary methodology based on the Procedure for Ecological Tiered Assessment of Risk (PETAR). This novel methodology integrates ecological models with a land-use change model. This study quantifies the multi-dimensional degradation risks of ES in the Taihu Lake Basin (TLB) of China from 1985 to 2020. Four key ES related to water purification, water quantity adjustment, carbon sequestration and grain production are selected. The study employs models of Denitrification-Decomposition (DNDC), Soil-Water-Atmosphere-Plant (SWAP), Biome-BGC and Agro-ecological Zoning (AEZ) for assimilations. Land-use changes by 2020 were projected using a geographically weighted multinomial logit-cellular automata (GWML-CA) model. The results show that rapid land-use change has posed a great degradation risk of ES in the region in 1985-2020. Slightly less than two-thirds of the basin experienced degradation of ES over the 1985-2010 period, and about 12% of the basin will continue to experience degradation until 2020. Hot spots with severe deterioration in 2010-2020 are projected to be centered around some small and less developed cities in the region. Regulating accelerated urban sprawl and population growth, reinforcing current environmental programs, and establishing monitoring systems for observing dynamics of regional ES are suggested as practical counter-measures.

Urban household carbon emission and contributing factors in the Yangtze River Delta, China.

Carbon reduction at the household level is an integral part of carbon mitigation. This study analyses the characteristics, effects, contributing factors and policies for urban household carbon emissions in the Yangtze River Delta of China. Primary data was collected through structured questionnaire surveys in three cities in the region – Nanjing, Ningbo, and Changzhou in 2011. The survey data was first used to estimate the magnitude of household carbon emissions in different urban contexts. It then examined how, and to what extent, each set of demographic, economic, behavioral/cognitive and spatial factors influence carbon emissions at the household level. The average of urban household carbon emissions in the region was estimated to be 5.96 tonnes CO2 in 2010. Energy consumption, daily commuting, garbage disposal and long-distance travel accounted for 51.2%, 21.3%, 16.0% and 11.5% of the total emission, respectively. Regulating rapidly growing car-holdings of urban households, stabilizing population growth, and transiting residents’ low-carbon awareness to household behavior in energy saving and other spheres of consumption in the context of rapid population aging and the growing middle income class are suggested as critical measures for carbon mitigation among urban households in the Yangtze River Delta.

Observations of reconnection exhausts associated with large-scale current sheets within a complex ICME at 1 AU

During 26-27 November 2000 a complex interplanetary coronal mass ejection, composed of four flux ropes, was detected by Wind and ACE at 1 AU. We identify two Petschek-like exhaust events within the interiors of the second and third flux ropes, respectively. In the first event, Wind and ACE detected an exhaust at the same side from the reconnection site, which was associated with a large-scale bifurcated current sheet with a spatial width of similar to 10,000 ion inertial lengths and the magnetic shear was 155 degrees. In the second event, the two spacecraft observed the oppositely directed exhausts from a single reconnection X line. The exhausts were also related to a large-scale current sheet with a spatial width of similar to 3000 ion inertial lengths and a shear angle of about 135 degrees. The two exhaust events resulted from fast and quasi-stationary reconnection. The related current sheets were both flat on the scale of a few hundred Earth radii and located close to the centers of subflux ropes. The decrease of radial expansion speed of each flux rope might account for the formation of the two current sheets. Reconnections at the centers of flux ropes may change the entire topology of the flux ropes and may fragment them into smaller ones.

Changing patterns and determinants of natural capital in the Yangtze River Delta of China 2000-2010

Natural capital (NC) is crucial to human existence and human well-being. Evaluating ecosystem services on a regional scale has presented tremendous theoretical, methodological and policy challenges. This study addresses the challenges by developing an interdisciplinary methodology, based on expert knowledge, and by focusing on the Yangtze River Delta of China. It evaluates the stock of NC, analyzes the characteristics of, and identifies the key drivers for, spatial and temporal change in NC in the deltaic region from 2000 to 2010. A main contribution is the novel incorporation of remote sensing data that explains the dynamics of the spatio-temporal change in land use and a set of ecosystem service indicators derived from it. The study focuses on key indicators for key ecosystem services related to carbon sequestration, grain production and water supply. The indictors reflect the spatial heterogeneity of NC across diverse ecosystems in the region. Each indicator builds on land use configuration and land use composition information derived from 250 m 16-day MODIS and Landsat TM remote sensing data for 2000 and 2010, with adjustment parameters being constructed. The regional evaluation shows an overall degradation of ecosystem services, reducing total NC by 10.4% (or 8.44 billion yuan) in 2000-2010. The spatial distribution of NC exhibits a declining pattern from the south to the north of the delta. At the city level, 15 out of 16 major cities in the region have experienced dramatic loss of NC, and this pattern is significantly correlated with rapid urbanization, population growth and industrialization. Land use/land cover change and deteriorating water quality are dominant factors causing NC depletion, while increased grain productivity and environmental policies help offset the NC losses. Outcomes of this research are useful to policy makers to mitigate the declines in NC through balancing the growth between economy and population.

A comprehensive assessment of MODIS-derived GPP for forest ecosystems using the site-level FLUXNET database

Accurate and continuous monitoring of forest production is critical for quantifying the dynamics of regional-to-global carbon cycles. MOD17A2 provides high frequency observations of terrestrial gross primary productivity (GPP) and is widely used to evaluate the spatiotemporal variability and responses to changing climate. However, the effectiveness of the Moderate Resolution Imaging Spectroradiometer (MODIS) in measuring GPP is directly constrained by the large uncertainties in the modeling process, specifically for complicated and extensive forest ecosystems. Although there have been plenty of studies to verify the MODIS GPP product with ground-based measurements covering a range of biome types, few have comprehensively validated the performance of MODIS estimates (C5.5) for diverse forests. Thus, this study examined the degree of correspondence between the MODIS-derived GPP and the EC-measured GPP at seasonal and interannual time scales for the main forest ecosystems, encompassing evergreen broadleaf forest (EBF), evergreen needleleaf forest (ENF), deciduous broadleaf forest (DBF), and mixed forest (MF) relying on 16 flux towers with a total dataset of 68 site-years. Overall, the site-specific evaluation of multi-year mean annual GPP estimates indicates that the current MODIS product works more significantly for DBF and MF, less for ENF, and least for EBF. Except for the tropical forest, MODIS estimates could capture the broad trends of GPP at an 8-day time scale for the other sites. At the seasonal time scale, the highest performance was observed in ENF, followed by MF and DBF, and the least performance was observed in EBF. Trend analyses also revealed the weak performance in EBF and DBF. This study suggested that current MODIS GPP estimates still need to improve the quality of different upstream inputs in addition to the algorithm for accurately quantifying forest production.

Changing land use and its impact on the habitat suitability for wintering Anseriformes in China's Poyang Lake region.

As an internationally important wetland for migratory waterbirds, Chinas Poyang Lake region has experienced substantial changes in land use during the past two decades owing to climate change and anthropogenic disturbances. Recent dam constructions on the Yangtze River and its tributaries for agriculture and hydroelectric power exert strong effects on the hydrological regimes of this lake. However, few studies have investigated how the land-use changes through time affect the habitat suitability for wintering Anseriformes-the largest community in this region. Thus, it is necessary to timely monitor changes in the habitat quality and understand the potential factors that alter it. In this study, three periods (1995, 2005 and 2014) of typical environmental indicators that have direct impacts on foraging and resting for the Anserformes, including proximity to water (density of lakes, rivers and ponds), human disturbances (density of residences and various road networks), preferred land cover types and food availability (NDVI), are integrated to develop a habitat suitability index model for habitat mapping. The results indicate that long-term lake shrinkage in low-water periods led to greatly expanded wetlands in these years, which provided more suitable habitat for migratory waterfowl. The amount of highly suitable habitat in 2014 was nearly twice as much as in 1995. Recent survey data from 1997 to 2013 also revealed an increase in the population size, and confirmed the improvement of habitat suitability in the Poyang Lake region. Spatial analysis revealed that land use changes contributed most to the improved habitat coverage between 1995 and 2014. However, the relative significances of these transformations for highly suitable and moderately suitable habitats are strikingly different. Increases in wetland and paddy field area are the main reasons for explaining these improvements, respectively. The framework model proposed in this study will help governments to evaluate habitat conservation and restoration for protecting waterbirds in a spatially explicit way.

Three Gorges Project: effects of resettlement on nutrient balance of the agroecosystems in the reservoir area

This paper reports on the effects of human resettlement on the nutrient balance of the agroecosystems in Three Gorges Reservoir Area (TGRA) of China. The analysis used is the OECD (Organisation for Economic Co-operation and Development) ‘Soil Surface Nitrogen Balance Model’ and agricultural statistical data for the county level in 1985–2005. Spatial and temporal changes of nutrient balance and the impacts of resettlement on such changes were examined. The results demonstrate that rural resettlement has significantly increased soil surface nitrogen and phosphorous surplus since 2000. The structural transformation of agricultural activities from grain production to horticulture or forestry should be encouraged, and more people may need to be moved out of the TGRA to reduce nutrient water pollution.

How do disturbances and climate effects on carbon and water fluxes differ between multi-aged and even-aged coniferous forests?

Disturbances and climatic changes significantly affect forest ecosystem productivity, water use efficiency (WUE) and carbon (C) flux dynamics. A deep understanding of terrestrial feedbacks to such effects and recovery mechanisms in forests across contrasting climatic regimes is essential to predict future regional/global C and water budgets, which are also closely related to the potential forest management decisions. However, the resilience of multi-aged and even-aged forests to disturbances has been debated for >60years because of technical measurement constraints. Here we evaluated 62site-years of eddy covariance measurements of net ecosystem production (NEP), evapotranspiration (ET), the estimates of gross primary productivity (GPP), ecosystem respiration (Re) and ecosystem-level WUE, as well as the relationships with environmental controls in three chronosequences of multi- and even-aged coniferous forests covering the Mediterranean, temperate and boreal regions. Age-specific dynamics in multi-year mean annual NEP and WUE revealed that forest age is a key variable that determines the sign and magnitude of recovering forest C source-sink strength from disturbances. However, the trends of annual NEP and WUE across succession stages between two stand structures differed substantially. The successional patterns of NEP exhibited an inverted-U trend with age at the two even-aged chronosequences, whereas NEP of the multi-aged chronosequence increased steadily through time. Meanwhile, site-level WUE of even-aged forests decreased gradually from young to mature, whereas an apparent increase occurred for the same forest age in multi-aged stands. Compared with even-aged forests, multi-aged forests sequestered more CO2 with forest age and maintained a relatively higher WUE in the later succession periods. With regard to the available flux measurements in this study, these behaviors are independent of tree species, stand ages and climate conditions. We also found that distinctly different environmental factors controlled forest C and water fluxes under three climatic regimes. Typical weather events such as temperature anomalies or drying-wetting cycles severely affected forest functions. Particularly, a summer drought in the boreal forest resulted in an increased NEP owing to a considerable decrease in Re, but at the cost of greater water loss from deeper groundwater resources. These findings will provide important implications for forest management strategies to mitigate global climate change.

Impacts of Land-Use Changes on Net Ecosystem Production in the Taihu Lake Basin of China from 1985 to 2010

Land-use changes play a major role in determining sources and sinks of carbon at regional and global scales. This study employs a modified BIOME-BGC model to examine the changes in the spatio-temporal pattern of net ecosystem production (NEP) in the Taihu Lake Basin of China during 1985-2010 and the extent to which land-use change impacted NEP. The model is calibrated with observed NEP at three flux sites for three dominant land-use types in the Basin including cropland, evergreen needleleaf forest, and mixed forest. Two simulations are conducted to distinguish the net effects of land-use change and increasing atmospheric concentrations of CO2 and nitrogen deposition on NEP. The study estimates that NEP in the Basin decreased by 9.8% (1.57 TgC) from 1985 to 2010, showing an overall downward trend. The NEP distribution exhibits an apparent spatial heterogeneity at the municipal level. Land-use changes during 1985-2010 reduced the regional NEP (3.21 Tg C in year 2010) by 19.9% compared to its 1985 level, while the increasing atmospheric CO2 concentrations and nitrogen deposition compensated for a half of the total carbon loss. Critical measures for regulating rapid urban expansion and population growth and reinforcing environment protection programs are recommended to increase the regional carbon sink.