min Yue

Spectral indices for estimating ecological indicators of karst rocky desertification

Karst rocky desertification is a process of land desertification associated with human disturbance of the fragile eco-geological setting of karst ecosystems. The fractional cover of photosynthetic vegetation (PV), non-photosynthetic vegetation (NPV), bare soil and exposed bedrock are key ecological indicators of the extent and degree of land degradation in karst regions. In this study, field spectral-reflectance measurements were used to develop a karst rocky desertification synthesis index (KRDSI) based on unique spectral features observed in non-vegetation land-cover types (NPV, bare soil and exposed bedrock) and were used to estimate the fractional cover of NPV, bare soil and exposed bedrock. Compared with linear spectral unmixing (LSU) using a tied-spectrum transform, the KRDSI is more consistent with the field measurement of non-vegetation land-cover fractions. This study indicates that ecological indicators of karst rocky desertification can be extracted relatively simply with the combination of vegetation indices and KRDSI values.

Development of new remote sensing methods for mapping green vegetation and exposed bedrock fractions within heterogeneous landscapes

Karst rocky desertification is a process of land desertification associated with human disturbance of the fragile karst ecosystems. The fractional cover of photosynthetic vegetation (PV) and exposed bedrock (Rock) are the main land-surface symptoms of karst rocky desertification. In this study, we explored a new methodology for quantifying PV and Rock by remote sensing. To reduce the effects of the high heterogeneity of karst landscapes on vegetation information extraction, a whole image was segmented into relatively homogeneous subsets and then the PV was estimated using a normalized difference vegetation index spectral mixture analysis (NDVI-SMA) model. The percentage of Rock was estimated using a karst rocky desertification synthesis index (KRDSI) and lignin cellulose absorption index (LCA). The results showed that the heterogeneity of a complex landscape is a major factor in the uncertainty of PV retrievals. The fractional cover of PV can be accurately estimated by the proposed method, but might be underestimated using NDVI and overestimated using the SMA-NDVI model. The bedrock fractions can be rapidly and objectively estimated with Hyperion or simulated Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery. Compared with multispectral images, hyperspectral images could be used to estimate PV and Rock more accurately. Our findings indicate that PV and Rock can be directly and efficiently quantified using remote sensing techniques within heterogeneous landscapes.

Exploring the relationship between vegetation spectra and eco-geo-environmental conditions in karst region, Southwest China

Remote sensing of local environmental conditions is not accessible if substrates are covered with vegetation. This study explored the relationship between vegetation spectra and karst eco-geo-environmental conditions. Hyperspectral remote sensing techniques showed that there were significant differences between spectral features of vegetation mainly distributed in karst and non-karst regions, and combination of 1,300- to 2,500-nm reflectance and 400- to 680-nm first-derivative spectra could delineate karst and non-karst vegetation groups. Canonical correspondence analysis (CCA) successfully assessed to what extent the variation of vegetation spectral features can be explained by associated eco-geo-environmental variables, and it was found that soil moisture and calcium carbonate contents had the most significant effects on vegetation spectral features in karst region. Our study indicates that vegetation spectra is tightly linked to eco-geo-environmental conditions and CCA is an effective means of studying the relationship between vegetation spectral features and eco-geo-environmental variables. Employing a combination of spectral and spatial analysis, it is anticipated that hyperspectral imagery can be used in interpreting or mapping eco-geo-environmental conditions covered with vegetation in karst region.

Spatiotemporal Variation of Karst Ecosystem Service Values and Its Correlation with Environmental Factors in Northwest Guangxi, China

In this investigation we analyzed the spatiotemporal variation of ecosystem service values (ESVs) and its correlation with numerous environmental factors (EFs) for the karst region of Northwest Guangxi, China, from 1985 to 2005 using remote sensing, geographic information systems (GIS) and statistical techniques. The results indicate that historically ESVs for this karst region decreased from 1985 (109.652 billion Yuan) to 1990 (88.789 billion Yuan) and then increased at the turn of the twenty-first century. However, the ESVs in both 2000 (103.384 billion Yuan) and 2005 (106.257 billion Yuan) never achieved the level recorded in 1985. The total of nutrient cycling, organic production and gas regulation combined were 72.69, 64.57, 70.18 and 72.10% of ESVs in 1985, 1990, 2000 and 2005, respectively. In contrast, the ESVs of water conservation, soil reservation, recreation and culture were determined to be relatively low contributing only 17.44, 23.82, 19.26 and 24.76% of total ESVs, respectively, during these four years. With regards to the spatial distribution of ESVs, larger values were recorded in the west and smaller ones recorded in the east. The most significant factors that were deemed to influence ESVs are annual rainfall, per capita cropland, slope and vegetation coverage. Annual rainfall and slope exert a negative force, whereas per capita cropland and vegetation coverage exert a positive force on ESVs. The results of the study would suggest that ecosystem conditions of this important karst region have been improved as the result of the implementation of rocky desertification control policies.

Assessing Future Vegetation Trends and Restoration Prospects in the Karst Regions of Southwest China

To alleviate the severe rocky desertification and improve the ecological conditions in Southwest China, the national and local Chinese governments have implemented a series of Ecological Restoration Projects since the late 1990s. In this context, remote sensing can be a valuable tool for conservation management by monitoring vegetation dynamics, projecting the persistence of vegetation trends and identifying areas of interest for upcoming restoration measures. In this study, we use MODIS satellite time series (2001–2013) and the Hurst exponent to classify the study area (Guizhou and Guangxi Provinces) according to the persistence of future vegetation trends (positive, anti-persistent positive, negative, anti-persistent negative, stable or uncertain). The persistence of trends is interrelated with terrain conditions (elevation and slope angle) and results in an index providing information on the restoration prospects and associated uncertainty of different terrain classes found in the study area. The results show that 69% of the observed trends are persistent beyond 2013, with 57% being stable, 10% positive, 5% anti-persistent positive, 3% negative, 1% anti-persistent negative and 24% uncertain. Most negative development is found in areas of high anthropogenic influence (low elevation and slope), as compared to areas of rough terrain. We further show that the uncertainty increases with the elevation and slope angle, and areas characterized by both high elevation and slope angle need special attention to prevent degradation. Whereas areas with a low elevation and slope angle appear to be less susceptible and relevant for restoration efforts (also having a high uncertainty), we identify large areas of medium elevation and slope where positive future trends are likely to happen if adequate measures are utilized. The proposed framework of this analysis has been proven to work well for assessing restoration prospects in the study area, and due to the generic design, the method is expected to be applicable for other areas of complex landscapes in the world to explore future trends of vegetation.

Deriving vegetation fraction information for the alpine grassland on the Tibetan plateau using in situ spectral data

Abstract Vegetation fraction (VF) is the indispensable factor involved in the assessment of land degradation in the inclement climate condition and harsh natural environment. Based on the analysis of an in situ spectral dataset of alpine grasslands on the Tibetan plateau, we assessed the performance of 28 widely used vegetation indices (VIs) and a spectral mixture analysis (SMA) model applied on the analytical spectral device and simulated enhanced thematic mapper (ETM)+ and Huan Jing (HJ)-1 data to select a method for retrieving VF there. The results show that simple VIs are competent for extracting VF information, and VIs with an extra blue band involved will produce a better performance. However, involvement of too many more bands does not yield much higher accuracy, indicated by the fact that hyperspectral VIs are not superior to multispectral ones in our case. The SMA model provides an acceptable accuracy as well but lower than that of VI regression. In addition, the normalized difference vegetation index (NDVI) values of vegetation and soil, generally, as the key parameter in the widely used NDVI-SMA model is obtained, and this would benefit the application of this model to derive VF of alpine grasslands on the Tibetan plateau with minimal or no need for field work support.

Spatio-Temporal Variation and Impact Factors for Vegetation Carbon Sequestration and Oxygen Production Based on Rocky Desertification Control in the Karst Region of Southwest China

The Grain to Green Program (GTGP) and eco-environmental emigration have been employed to alleviate poverty and control rocky desertification in the Southwest China Karst region. Carbon sequestration and oxygen production (CSOP) is used to indicate major ecological changes, because they involve complex processes of material circulation and energy flow. Using remote sensing images and weather records, the spatiotemporal variation of CSOP was analyzed in a typical karst region of northwest Guangxi, China, during 2000–2010 to determine the effects of the Chinese government’s ecological rehabilitation initiatives implemented in 1999. An increase with substantial annual change and a significant increase (20.94%, p < 0.05) in variation were found from 2000 to 2010. CSOP had a highly clustered distribution in 2010 and was correlated with precipitation and temperature (9.18% and 8.96%, respectively, p < 0.05). CSOP was significantly suppressed by human activities (p < 0.01, r = −0.102) but was consistent with the intensity of GTGP (43.80% positive). The power spectrum of CSOP was consistent with that of the gross domestic product. These results indicate that ecological services were improved by rocky desertification control in a typical karst region. The results may provide information to evaluate the efficiency of ecological reconstruction projects.

Spectral characteristics of alpine grassland and their changes responding to grassland degradation on the Tibetan Plateau

Remote sensing is used as the indispensable technology in alpine grassland degradation assessment especially at regional scale on the Tibetan Plateau. However, the lack of field spectral data, as the foundation of remote sensing, due to the formidable natural and climate conditions hinders the understanding of spectral characteristics of alpine grassland and their degradation assessment. In this study, spectral characteristics of alpine grasslands and their changes responding to degradation were explored. The results showed that the main spectral characteristics for discriminating the dominant species of alpine meadow (Kobresia littledalei and Kobresia pygmaea), alpine steppe (Stipa purpurea) and desert (Potentilla fruticosa) are spectral features of chlorophyll, cellulose and water which are related to their growth form, plant inclination and residue of withered leaf sheaths. The spectral curves of alpine meadow have a much smaller variety over the whole spectral region compared to those of alpine steppe and desert which generally have weaker chlorophyll and water absorption features and more noticeable non-vegetation features. Different grassland degradation processes exhibit different patterns of spectral characteristics change due to the species composition, vegetation succession, vegetation coverage and soil background. Grassland degradation can happen without obvious vegetation coverage reduction or even with an increment of NDVI (normalized difference vegetation index). Therefore, the assessment of grassland degradation cannot be fulfilled well using single vegetation index or spectral feature. The combination of several vegetation indices or hyperspectral remote sensing along with the priori knowledge is needed in order to perform the assessment more accurately in further studies.

Satellite‐Observed Major Greening and Biomass Increase in South China Karst During Recent Decade

Above-ground vegetation biomass is one of the major carbon sinks and provides both provisioning (e.g., forestry products) and regulating ecosystem services (by sequestering carbon). Continuing deforestation and climate change threaten this natural resource but can effectively be countered by national conservation policies. Here we present time series (1999–2017) derived from complementary satellite systems to describe a phenomenon of global significance: the greening of South China Karst. We find a major increase in growing season vegetation cover from 69% in 1999 to 81% in 2017 occurring over ~1.4 million km. Over 1999–2012, we report one of the globally largest increases in biomass to occur in the South China Karst region (on average +4% over 0.9 million km), which accounts for ~5% of the global areas characterized with increases in biomass. These increases in southern China’s vegetation have occurred despite a decline in rainfall ( 8%) and soil moisture ( 5%) between 1999 and 2012 and are derived from effects of forestry and conservation activities at an unprecedented spatial scale in human history (~20,000 km yr 1 since 2002). These findings have major implications for the provisioning of ecosystem services not only for the Chinese karst ecosystem (e.g., carbon storage, water filtration, and timber production) but also for the study of global carbon cycles.

Wetland change in Wangning during 2002 and 2013

The study investigated wetland change in Wanning. For this purpose, three high resolution SPOT images recorded in 2002, 2007 and 2013, respectively, were classified. The results indicated that there were little change in wetland types during 2002 and 2013. The coastal waters, culture pond and river were the main wetland types. The natural wetland trended to decline. The ditch had the largest net increase and the reservoir shrank the most. There was a dramatic increase of culture pond plaques, which making the landscape more fragmentized. The coastal waters and the land had a lot change with other wetland types. The area change in Wanning was mainly composed of the transition between the land and culture pond.