Junbang Wang

Comparison of gross primary productivity derived from GIMMS NDVI3g, GIMMS, and MODIS in Southeast Asia

Gross primary production (GPP) plays an important role in the net ecosystem exchange of CO2 between the atmosphere and terrestrial ecosystems. It is particularly important to monitor GPP in Southeast Asia because of increasing rates of tropical forest

Effects of grassland restoration programs on ecosystems in arid and semiarid China.

We explored the ecological effects of grassland restoration programs using satellite imagery and field plots sampling data and analyzing the patterns and mechanisms of land cover change and vegetation activities in arid and semiarid China during the period from 1982 to 2008. The grassland cover in the 1980s, 2000 and 2005 was compared before and after the restoration programs. The variability of net primary production (NPP) and rain use efficiency (RUE) were analyzed as indicators of vegetation productivity. Our study showed that changes in grassland cover were closely related to the relative area of farmland, with increases in grassland being caused by returning farmland to grassland and decreases being caused by reclamation for agriculture. The results of NPP and RUE measurements over the past 30 years showed systematic increases in the area of grassland in most regions, especially from 2000 to 2008. This fact was reflected by intensified vegetation activity and cannot be completely explained by the warmer and wetter climate, which suggested a contribution from restored, ungrazed grasslands. Our analysis indicates that both vegetation activity and grassland cover increased in regions in which grassland and rangeland restoration programs were implemented.

Analysis of net primary productivity of terrestrial vegetation on the Qinghai-Tibet Plateau, based on MODIS remote sensing data

GLO-PEM is driven by soil moisture data of AMSR-E and PAR (Photosynthetically active radiation) which is retrieved from MODIS atmospheric data product in this paper. Using remote sensing data can overcome uncertainty brought from interpolation of precipitation and PAR. Comparing with observed radiation data, PAR retrieved by remote sensing is in high accuracy in this study. RMSE is 9 and 19.68 W m−2 and R2 is 0.89 and 0.67 respectively. As a result of GLO-PEM, annual total amount of NPP of Qinghai-Tibet Plateau is 0.37 Pg C a−1 in 2005–2008. There is a significant linear relationship between field and simulated NPP. Determination coefficient reached 0.93. NPP is decrease from southeast to northwest in the Qinghai-Tibet Plateau. NPP changes from 0 to 1500 g C m−2 a−1. There is different limit factors of vegetation growth in west and east plateau. In the west of 450 mm rainfall line, the limit factors is precipitation. In the east of 450 mm rainfall line, temperature is the dominated factor of vegetation growth.

Assessing spatiotemporal variation of drought in China and its impact on agriculture during 1982–2011 by using PDSI indices and agriculture drought survey data

Inspired by concerns of the effects of a warming climate, drought variation and its impacts have gained much attention in China. Arguments about Chinas drought persist and little work has utilized agricultural drought survey area to evaluate the impact of natural drought on agriculture. Based on a newly revised self-calibrating Palmer Drought Severity Index (PDSI) model driven with air-relative-humidity-based two-source (ARTS) E0 (PDSIARTS; Yan et al., 2014), spatial and temporal variations of drought were analyzed for 1982–2011 in China, which indicates that there was nonsignificant change of drought over this interval but with an extreme drought event happened in 2000–2001. However, using air temperature (Ta)-based Thornthwaite potential evaporation (EP_Th) and Penman-Monteith potential evaporation (EP_PM) to drive the PDSI model, their corresponding PDSITh and PDSIPM all gave a significant drying trend for 1982–2011. This suggests that PDSI model was sensitive to EP parameterization in China. Annual drought-covered area from agriculture survey was initially adopted to evaluate impact of PDSI drought on agriculture in China during 1982–2011. The results indicate that PDSIARTS drought area (defined as PDSIARTS < −0.5) correlated well with the agriculture drought-covered area and PDSIARTS successfully detected the extreme agriculture drought in 2000–2001 during 1982–2011, i.e., climate factors dominated the interannual changes of agriculture drought area, while PDSITh and PDSIPM drought areas had no relationship with the agriculture drought-covered area and overestimated the uptrend of agriculture drought This study highlights the importance of coupling PDSI with drought survey data in evaluating the impact of natural drought on agriculture.

Modelling carbon fluxes of different forests by coupling a remote-sensing model with an ecosystem process model

A new model (GLOPEM-CEVSA) to determine terrestrial carbon budgets was developed by coupling remote sensing with ecosystem process simulation, and was validated with reference to the carbon fluxes of three forests. MODIS FPAR (MOD15A2 product) was applied together with meteorological data on flux towers. The seasonal variances of modelled gross primary production and ecosystem respiration were significantly correlated with observed values (correlation coefficient, r > 0.9). The seasonal dynamics of the modelled net ecosystem production over the plant-growth season showed significant agreement with observed values with a r range of 0.64 to 0.87. This work demonstrates the potential of GLOPEM-CEVSA to quantify the spatial patterns and temporal dynamics of terrestrial ecosystem carbon sources and sinks with consideration of the spatial heterogeneity of ecosystems based on remote sensing.

Advances in experimental methods for root system architecture and root development

Plant roots play important roles in acquisition of water and nutrients, storage, anchoring, transport, and symbiosis with soil microorganisms, thus quantitative researches on root developmental processes are essential to understand root functions and root turnover in ecosystems, and at the same time such researches are the most difficult because roots are hidden underground. Therefore, how to investigate efficiently root functions and root dynamics is the core aspect in underground ecology. In this article, we reviewed some experimental methods used in root researches on root development and root system architecture, and summarized the advantages and shortages of these methods. Based on the analyses, we proposed three new ways to more understand root processes: (1) new experimental materials for root development; (2) a new observatory system comprised of multiple components, including many observatory windows installed in field, analysis software, and automatic data transport devices; (3) new techniques used to analyze quantitatively functional roots.

Comparison of Huanjing and Landsat satellite remote sensing of the spatial heterogeneity of Qinghai-Tibet alpine grassland

Remote sensing is widely applied in the study of terrestrial primary production and the global carbon cycle. The researches on the spatial heterogeneity in images with different sensors and resolutions would improve the application of remote sensing. In this study two sites on alpine meadow grassland in Qinghai, China, which have distinct fractal vegetation cover, were used to test and analyze differences between Normalized Difference Vegetation Index (NDVI) and enhanced vegetation index (EVI) derived from the Huanjing (HJ) and Landsat Thematic Mapper (TM) sensors. The results showed that: 1) NDVI estimated from HJ were smaller than the corresponding values from TM at the two sites whereas EVI were almost the same for the two sensors. 2) The overall variance represented by HJ data was consistently about half of that of Landsat TM although their nominal pixel size is approximately 30m for both sensors. The overall variance from EVI is greater than that from NDVI. The difference of the range between the two sensors is about 6 pixels at 30m resolution. The difference of the range in which there is not more corrective between two vegetation indices is about 1 pixel. 3) The sill decreased when pixel size increased from 30m to 1km, and then decreased very quickly when pixel size is changed to 250m from 30m or 90m but slowly when changed from 250m to 500m. HJ can capture this spatial heterogeneity to some extent and this study provides foundations for the use of the sensor for validation of net primary productivity estimates obtained from ecosystem process models.

Dynamic changes of habitats in China’s typical national nature reserves on spatial and temporal scales

Until 2015, China had established 2740 nature reserves with a total area of 1.47 million km2, covering 14.8% of China’s terrestrial land surface. Based on remote sensing inversion, ecological model simulation and spatial analysis methods, we analyzed the spatial and temporal variations of fractional vegetation coverage (FVC), net primary production (NPP), and human disturbance (HD) in habitats of typical national nature reserves (NNRs) during the first 15 years of the 21st century from 2000 to 2015. And then the three indicators were compared between different NNR types and varied climate zones. The results showed that (1) the average 5-year FVC of NNRs increased from 36.3% to 37.1%, and it improved in all types of NNRs to some extent. The annual average FVC increased by 0.11%, 0.84%, 0.21%, 0.09%, 0.11% and 0.08% in NNRs of forest ecosystem, plain meadow, inland wetland, desert ecosystem, wild animal and wild plant, respectively. (2) The NPP annually increased by 2.06 g·m-2, 1.23 g·m-2, 0.28 g·m-2 and 0.4 g·m-2 in NNRs of plain meadow, inland wetland, desert ecosystem and wild animal, respectively. However, it decreased by 3.45 g·m-2 and 2.35 g·m-2 in NNRs of forest ecosystem and wild plant respectively. (3) In the past 15 years, besides the slight decreases in the NNRs located at the Qinghai-Tibet Plateau and the south subtropical zone, HD enhanced in most of NNRs, especially HD in the warm temperate humid zone increased from 4.7% to 5.35%.

Linking the benefits of ecosystem services to sustainable spatial planning of ecological conservation strategies

The maintenance and improvement of ecosystem services on the Tibet Plateau are critical for national ecological security in China and are core objectives of ecological conservation in this region. In this paper, ecosystem service benefits of the Tibet Ecological Conservation Project were comprehensively assessed by estimating and mapping the spatiotemporal variation patterns of critical ecosystem services on the Tibet Plateau from 2000 to 2015. Furthermore, we linked the benefit assessment to the sustainable spatial planning of future ecological conservation strategies. Comparing the 8 years before and after the project, the water retention and carbon sink services of the forest, grassland and wetland ecosystems were slightly increased after the project, and the ecosystem sand fixation service has been steadily enhanced. The increasing forage supply service of grassland significantly reduced the grassland carrying pressure and eased the conflict between grassland and livestock. However, enhanced rainfall erosivity occurred due to increased rainfall, and root-layer soils could not recover in a short period of time, both factors have led to a decline in soil conservation service. The warm and humid climate is beneficial for the restoration of ecosystems on the Tibet Plateau, and the implementation of the Tibet Ecological Conservation Project has had a positive effect on the local improvement of ecosystem services. A new spatial planning strategy for ecological conservation was introduced and aims to establish a comprehensive, nationwide system to protect important natural ecosystems and wildlife, and to promote the sustainable use of natural resources.

Long-term monitoring of citrus orchard dynamics using time-series Landsat data: a case study in southern China

ABSTRACT Citrus orchard planting is a typical land-use change process that can impact terrestrial ecosystem services both locally and globally. Long-term monitoring of citrus orchard dynamics is critical for understanding its change patterns as well as the potential driving factors. Satellite remote-sensing imagery has been a primary data source for this purpose. However, most previous studies with multi-year intervals only captured some, but not all detailed information on citrus orchard expansion. In this study, we developed a framework for mapping annual citrus orchard extent and track its long-term dynamics in Xunwu County, China, using the historical Landsat repository from 1990 to 2016. The results suggested that the average overall accuracy of original annual mapping was 87.73%, and its performance was significantly improved after the temporal filtering approach (91.46%). Several features (e.g. elevation, slope, normalized difference vegetation index) played more important roles in citrus orchard identification. With the achieved annual mapping layers, we found a rapid citrus orchard expansion trend during the study interval (i.e. from 22.18 to 697.21 km2). Moreover, this expansion process was unevenly distributed in time. Spatially, emerging citrus orchards were primarily transformed from forests and croplands and mainly distributed in areas with elevations from 200 to 500 m and slopes range from 5° to 20°. This study demonstrated the potential of mapping citrus orchard dynamics at a higher temporal frequency with remote-sensing time-series, which can contribute to providing reference for sustainable land-use policy.