Xuguang Tang

How is water-use efficiency of terrestrial ecosystems distributed and changing on Earth?

A better understanding of ecosystem water-use efficiency (WUE) will help us improve ecosystem management for mitigation as well as adaption to global hydrological change. Here, long-term flux tower observations of productivity and evapotranspiration allow us to detect a consistent latitudinal trend in WUE, rising from the subtropics to the northern high-latitudes. The trend peaks at approximately 51°N, and then declines toward higher latitudes. These ground-based observations are consistent with global-scale estimates of WUE. Global analysis of WUE reveals existence of strong regional variations that correspond to global climate patterns. The latitudinal trends of global WUE for Earths major plant functional types reveal two peaks in the Northern Hemisphere not detected by ground-based measurements. One peak is located at 20° ~ 30°N and the other extends a little farther north than 51°N. Finally, long-term spatiotemporal trend analysis using satellite-based remote sensing data reveals that land-cover and land-use change in recent years has led to a decline in global WUE. Our study provides a new framework for global research on the interactions between carbon and water cycles as well as responses to natural and human impacts.

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.

SPOT5 multi-spectral (MS) and panchromatic (PAN) image fusion using an improved wavelet method based on local algorithm

Remote sensing image fusion is an effective way to extract a large volume of data from multi-source images. However, traditional image fusion methods cannot meet the requirements of applications because they can lose spatial information or distort spectral characteristics. In this paper, a new wavelet method based on a local algorithm is presented. The proposed method fuses multi-spectral (MS) and panchromatic (PAN) images to improve spatial information and preserve spectral characteristics. The main advantage of the new fusion method is the exploitation of the dependency between neighboring pixels. SPOT5 MS and PAN images were employed to execute the fusion methods. To compare with the new method, the principal component analysis (PCA), wavelet transformation, and PCA-based wavelet (PCA+W) image fusion methods were selected. Qualitative and quantitative analyses and classification accuracy assessment were conducted to evaluate the performance of the fusion methods. The results demonstrate that the new wavelet method based on a local algorithm is better than traditional image fusion methods. The new fusion method can achieve a wide range of balance between high spatial resolution retention and spectral characteristic preservation; thus, the new method is suitable for different applications.

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.

Monitoring Loss and Recovery of Salt Marshes in the Liao River Delta, China

ABSTRACT Jia, M.; Wang, Z.; Liu, D.; Ren, C.; Tang, X., and Dong, Z., 2015. Monitoring loss and recovery of salt marsh in the Liao River Delta, China. Coastal salt marsh plays an important role in the aquatic food web and the export of nutrients to coastal waters. The salt marshes in the Liao River Delta of China, dominated by Suaeda heteroptera, experienced a dramatic loss in the 1990s and then recovered in the 2000s. This study investigates these changes of salt marsh using a time series of Landsat Thematic Mapper (TM) images acquired in 1988, 1995, 2000, 2004, 2007, and 2009. The classification tree method was used on these TM images to extract S. heteroptera, and an interactive self-organizing data analysis algorithm was used to determine other land cover types. The conversions between salt marsh and other land cover types were described with conversion matrices. The classification results show that, during 1998–2004, salt marsh decreased dramatically at an average rate of 662.68 ha/y. However, during the period 2004–2009, salt marsh recovered gradually at a rate of 115.51 ha/y. The conversion matrix indicates that, from 1988 to 2004, a large area of former salt marsh was directly replaced by man-made landscape types, such as reed field (5111 ha), aquaculture pond (2655 ha), reservoir (1720 ha), and paddy field (729 ha). In contrast, the result for the period from 2004 to 2009 shows that salt marshes were recovered by the conversion of some areas of former barren beaches and river back to salt marsh. Driving forces analysis suggests that salt marsh dynamics were mainly caused by human activities, with the secondary drivers being climatic warming and dry conditions.

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.

Tracking Ecosystem Water Use Efficiency of Cropland by Exclusive Use of MODIS EVI Data

One of the most important linkages that couple terrestrial carbon and water cycles is ecosystem water use efficiency (WUE), which is relevant to the reasonable utilization of water resources and farming practices. Eddy covariance techniques provide an opportunity to monitor the variability in WUE and can be integrated with Moderate Resolution Imaging Spectroradiometer (MODIS) observations. Scaling up in situ observations from flux tower sites to large areas remains challenging and few studies have been reported on direct estimation of WUE from remotely-sensed data. This study examined the main environmental factors driving the variability in WUE of corn/soybean croplands, and revealed the prominent role of solar radiation and temperature. Time-series of MODIS-derived enhanced vegetation indices (EVI), which are proxies for the plant responses to environmental controls, were also strongly correlated with ecosystem WUE, thereby implying great potential for remote quantification. Further, both performance of the indirect MODIS-derived WUE from gross primary productivity (GPP) and evapotranspiration (ET), and the direct estimates by exclusive use of MODIS EVI data were evaluated using tower-based measurements. The results showed that ecosystem WUE were overpredicted at the beginning and ending of crop-growth periods and severely underestimated during the peak periods by the indirect estimates from MODIS products, which was mainly attributed to the error source from MODIS GPP. However, a simple empirical model that is solely based on MODIS EVI data performed rather well to capture the seasonal variations in WUE, especially for the growing periods of croplands. Independent validation at different sites indicates the method has potential for broad application.

Sensitivity of near real-time MODIS gross primary productivity in terrestrial forest based on eddy covariance measurements

As an important product of Moderate Resolution Imaging Spectroradiometer (MODIS), MOD17A2 provides dramatic improvements in our ability to accurately and continuously monitor global terrestrial primary production, which is also significant in effort to advance scientific research and eco-environmental management. Over the past decades, forests have moderated climate change by sequestrating about one-quarter of the carbon emitted by human activities through fossil fuels burning and land use/land cover change. Thus, the carbon uptake by forests reduces the rate at which carbon accumulates in the atmosphere. However, the sensitivity of near real-time MODIS gross primary productivity (GPP) product is directly constrained by uncertainties in the modeling process, especially in complicated forest ecosystems. Although there have been plenty of studies to verify MODIS GPP with ground-based measurements using the eddy covariance (EC) technique, few have comprehensively validated the performance of MODIS estimates (Collection 5) across diverse forest types. Therefore, the present study examined the degree of correspondence between MODIS-derived GPP and EC-measured GPP at seasonal and interannual time scales for the main forest ecosystems, including evergreen broadleaf forest (EBF), evergreen needleleaf forest (ENF), deciduous broadleaf forest (DBF), and mixed forest (MF) relying on 16 flux towers with a total of 68 site-year datasets. Overall, site-specific evaluation of multi-year mean annual GPP estimates indicates that the current MOD17A2 product works highly effectively for MF and DBF, moderately effectively for ENF, and ineffectively for EBF. Except for tropical forest, MODIS estimates could capture the broad trends of GPP at 8-day time scale for all other sites surveyed. On the annual time scale, the best performance was observed in MF, followed by ENF, DBF, and EBF. Trend analyses also revealed the poor performance of MODIS GPP product in EBF and DBF. Thus, improvements in the sensitivity of MOD17A2 to forest productivity require continued efforts.

Remotely Monitoring Ecosystem Water Use Efficiency of Grassland and Cropland in China’s Arid and Semi-Arid Regions with MODIS Data

Scarce water resources are available in the arid and semi-arid areas of Northwest China, where significant water-related challenges will be faced in the coming decades. Quantitative evaluations of the spatio-temporal dynamics in ecosystem water use efficiency (WUE), as well as the underlying environmental controls, are crucial for predicting future climate change impacts on ecosystem carbon-water interactions and agricultural production. However, these questions remain poorly understood in this typical region. By means of continuous eddy covariance (EC) measurements and time-series MODIS data, this study revealed the distinct seasonal cycles in gross primary productivity (GPP), evapotranspiration (ET), and WUE for both grassland and cropland ecosystems, and the dominant climate factors performed jointly by temperature and precipitation. The MODIS WUE estimates from GPP and ET products can capture the broad trend in WUE variability of grassland, but with large biases for maize cropland, which was mainly ascribed to large uncertainties resulting from both GPP and ET algorithms. Given the excellent biophysical performance of the MODIS-derived enhanced vegetation index (EVI), a new greenness model (GR) was proposed to track the eight-day changes in ecosystem WUE. Seasonal variations and the scatterplots between EC-based WUE and the estimates from time-series EVI data (WUEGR) also certified its prediction accuracy with R2 and RMSE of both grassland and cropland ecosystems over 0.90 and less than 0.30 g kg−1, respectively. The application of the GR model to regional scales in the near future will provide accurate WUE information to support water resource management in dry regions around the world.

Potential and environmental control of carbon sequestration in major ecosystems across arid and semi-arid regions in China

With the continuous expansion of drylands in the context of global climate change, governments have implemented a series of greening policies such as afforestation, to reduce ecological degradation. However, owing to historical conditions and technical constraints, few attempts have been made to quantitatively assess the differences in carbon sequestration capacity and the associated environmental controls among major ecosystems in the arid and semi-arid areas. Based on six flux towers located in northwestern China measuring the carbon fluxes in a maize (Zea mays L.) cropland, alpine meadow, wetland, swamp meadow, Tamarix, and gobi desert, this work revealed that all ecosystems sequestered CO2 at various magnitudes. The cropland had the highest carbon uptake, followed by the alpine meadow, swamp meadow, wetland and Tamarix, respectively. Distinct seasonal dynamics in carbon sequestration were observed across these ecosystems with the peak values in summertime, whereas the gobi desert exhibited as a weak carbon sink with considerable fluctuations around the year. In this water-limited region, soil water content instead of rainfall, is expected to be the primary environmental control on the land-atmosphere carbon fluxes, and regarded as a key linkage between hydrologic and ecologic processes. Therefore, not only the appropriate vegetation types, but also the water availability controlled by the local climatic constraints and soil characteristics, should be addressed in order to identify management strategies for ecological restoration in the dry areas.