The linkages with fires, vegetation composition and human activity in response to climate changes in the Chinese Loess Plateau during the Holocene

Holocene paleo-records of the Chinese Loess Plateau loess-soil profiles were used to reconstruct wildfire patterns and landscape evolution. We examine black carbon and charcoal influx, combined with the Magnetic susceptibility, delta C-13 values of soil organic matter, pollen counts and other paleo-environmental proxies to discuss interactions with biomass-climate during the Holocene. The history of fires from the charcoal and black carbon (BC, char and soot) influx at the two sites demonstrates a transition from climate-controlled low amplitude variations with peaks during the Early and Middle Holocene (11-3.1kyearsB.P.) to higher amplitude variability in fire occurrence decoupled from climate and tied to human activities during the Late Holocene (3.1-0kyearsB.P.). The difference in fire patterns was attributed to regional effective moisture and human land use over the entre Loess Plateau; meanwhile, fire activities observed during the Holocene are consistent with variations in vegetation composition inferred from delta C-13 values in soil organic matter, pollen counts, and paleoclimate proxies. Regional wildfires rarely occurred on the desert steppe dominated by a weedy C-3 taxon (Artemisia, Compositae, and Chenopodiaceae dominated)during the late glacial period. A limited biomass would not meet fire propagation in the extreme colder and drier environment of the Loess Plateau during those periods, though. As the climate became ameliorated during the early Holocene, there was an increasing biomass and a sufficient contribution do to high fuel accumulation from C-4 taxon (Gramineae). As the middle Holocene progressed toward warmer and wetter conditions, fire events were less frequent on the steppe and forest-steppe (e.g. expansion of trees C-3,C- Quercus, Corylus) of the Loess Plateau. Subsequently, the number of local and regional fire events have largely increased with the colder and drier climate conditions (e.g. expansion of C-3 weedy), which have been decoupling with intensive anthropogenic burning for farming since the past 3kyr. These data suggests that the regional fire patterns vary strongly along environmental gradients in the effective moisture and regional fuel availability as well as the spatial and temporal distributions of Neolithic burning practices over the Loess Plateau in response to the weakening East Asian monsoon during the Holocene

INVESTIGATING THE IMPACTS OF ANTHROPOGENIC AND CLIMATIC CHANGES ON THE STEPPE ECOSYSTEM IN CHINA’S LOESS PLATEAU AND THE MIXED-GRASS PRAIRIE REGION IN SOUTHWEST OKLAHOMA, USA

Grassland ecosystems occupy approximately 40% of the earth’s terrestrial area and represent one of most important ecosystems on Earth in terms of its impacts on global food supply, carbon sequestration and maintaining biodiversity. Grassland ecosystems are very sensitive to disturbances caused by either climatic or anthropogenic changes such as changes in precipitation regimes or management practices. The objective of this dissertation is to investigate the impacts imposed by grassland restoration activities and changes in precipitation anomalies on the steppe in China’s Loess Plateau and the mixed-grass prairie in southwest Oklahoma. In chapter two, I analyzed how large-scale vegetation conservation programs affected the grassland dynamics in China’s Loess Plateau by combining remotely sensed data with socio-economic statistics. The results of this study showed that the impact of vegetation conservation programs on vegetation change in the Loess Plateau is twofold. On the one hand, vegetation conservation programs target marginal lands. Thus, significant vegetation increases due to cropland conversion and afforestation can be found in these regions. On the other hand, intensified agricultural production can be found in croplands with suitable topography and well-established irrigation systems which were not enrolled in conservation programs to offset the agricultural production loss caused by vegetation conservation programs elsewhere. In chapter three, I demonstrated a new methodology on mapping the historical distribution of grassland species in southwest Oklahoma based on the Random Forest classification algorithm. In this study, elevation, soil pH and soil clay content were found to be significant variables for predicting the distribution of C3 and C4 grassland species. With the mapped distribution of grassland species between 1981 and 2010, in chapter four, I examined the relationship between changes in precipitation anomalies and the dynamics of relative abundance of C3 and C4 grassland species in southwest Oklahoma. In this study, significant decreases of C3/C4 ratio were identified in pasture/hay fields due to the increases in C4 abundance resulting from the decreases of sparsely vegetated area between 2005 and 2010. I suspect that the increase in C4 abundance was a drought adaptation strategy adopted by ranchers. Because C4 species are more tolerant of drought conditions and thus can help to maintain stable forage/hay production when negative precipitation anomalies prevailed during the growing season of C3 species

Spatiotemporal variations in vegetation cover on the Loess Plateau, China, between 1982 and 2013: possible causes and potential impacts

Vegetation is a key component of the ecosystem and plays an important role in water retention and resistance to soil erosion. In this study, we used a multiyear normalized difference vegetation index (NDVI) dataset (1982-2013) and corresponding datasets for observed climatic variables to analyze changes in the NDVI at both temporal and spatial scales. The relationships between NDVI, climate change, and human activities were also investigated. The annual average NDVI showed an upward trend over the 32-year study period, especially in the center of the Loess Plateau. NDVI variations lagged behind monthly temperature changes by approximately 1 month. The contribution of human activities to variations in NDVI has become increasingly significant in recent years, with human activities responsible for 30.4% of the change in NDVI during the period 2001-2013. The increased vegetation coverage has reduced soil erosion on the Loess Plateau in recent years. It is suggested that natural restoration of vegetation is the most effective measure for control of erosion; engineering measures that promote this should feature in the future governance of the Loess Plateau

Peri-urbanization may vary with vegetation restoration: A large scale regional analysis

Over that past decade, ecological restoration practices have expanded globally. However, the effectiveness of ecological restoration depends on the complex interactions of various natural and socioeconomic factors, about which there is limited scientific understanding and thus provides an important research frontier. This paper analyzed the relationship between regional scale vegetation restoration and the process of urbanization using the Loess Plateau of China as a case study. This region has experienced both rapid urbanization and a high number of vegetation restoration activities. Urbanization and vegetation restoration can be considered as extremes on the spectrum of environment preservation activities. Three separate spatial correlation analyses between urbanization and vegetation restoration were identified, resulting in: 1) insignificant correlations in saturated urban areas; 2) significant negative correlations in peri-urban areas; and 3) significant positive correlations in undeveloped areas. The relationship between urbanization and vegetation restoration is thus stage-dependent. Impacts of urbanization on vegetation degradation has improved but has not been fully addressed by large scale vegetation restoration. Regardless of whether the county or grid scale is used, peri-urbanization was found to be the critical factor affecting the effectiveness of vegetation restoration over both time and space. Therefore, peri-urbanized areas are viewed as priorities for improving the coupling of urban development and vegetation restoration

Contributions of natural and human factors to increases in vegetation productivity in China

Increasing trends in vegetation productivity have been identified for the last three decades for many regions in the northern hemisphere including China. Multiple natural and human factors are possibly responsible for the increases in vegetation productivity, while their relative contributions remain unclear. Here we analyzed the long-term trends in vegetation productivity in China using the satellite-derived normalized difference vegetation index (NDVI) and assessed the relationships of NDVI with a suite of natural (air temperature, precipitation, photosynthetically active radiation (PAR), atmospheric carbon dioxide (CO2) concentrations, and nitrogen (N) deposition) and human (afforestation and improved agricultural management practices) factors. Overall, China exhibited an increasing trend in vegetation productivity with an increase of 2.7%. At the provincial scale, eleven provinces exhibited significant increases in vegetation productivity, and the majority of these provinces are located within the northern half of the country. At the national scale, annual air temperature was most closely related to NDVI and explained 36.8% of the variance in NDVI, followed by afforestation (25.5%) and crop yield (15.8%). Altogether, temperature, total forest plantation area, and crop yield explained 78.1% of the variance in vegetation productivity at the national scale, while precipitation, PAR, atmospheric CO2 concentrations, and N deposition made no significant contribution to the increases in vegetation productivity. At the provincial scale, each factor explained a part of the variance in NDVI for some provinces, and the increases in NDVI for many provinces could be attributed to the combined effects of multiple factors. Crop yield and PAR were correlated with NDVI for more provinces than were other factors, indicating that both elevated crop yield resulting from improved agricultural management practices and increasing diffuse radiation were more important than other factors in increasing vegetation productivity at the provincial scale. The relative effects of the natural and human factors on vegetation productivity varied with spatial scale. The true contributions of multiple factors can be obscured by the correlation among these variables, and it is essential to examine the contribution of each factor while controlling for other factors. Future changes in climate and human activities will likely have larger influences on vegetation productivity in China

Driving Factors of Land Change in China’s Loess Plateau: Quantification Using Geographically Weighted Regression and Management Implications

Land change is a key topic in research on global environmental change, and the restoration of degraded land is the core component of the global Land Degradation Neutrality target under the UN 2030 Agenda for Sustainable Development. In this study, remote-sensing-derived land-use data were used to characterize the land-change processes in China’s Loess Plateau, which is experiencing large-scale ecological restoration. Geographically Weighted Regression was applied to capture the spatiotemporal variations in land change and driving-force relationships. First, we explored land-use change in the Loess Plateau for the period 1990–2015. Grassland, cropland and forestland were dominant land cover in the region, with a total percentage area of 88%. The region experienced dramatic land-use transitions during the study period: degraded grassland and wetland, expansion of cropland and built-up land and weak restoration of forestland during 1990–2000; and increases in grassland, built-up land, forestland and wetland, concurrent with shrinking cropland during 2000–2015. A Geographically Weighted Regression (GWR) analysis revealed altitude to be the common dominant factor associated with the four major land-use types (forestland, grassland, cropland and built-up land). Altitude and slope were found to be positively associated with forestland, while being negatively associated with cropland in the high, steep central region. For both forestland and grassland, temperature and precipitation behaved in a similar manner, with a positive hotspot in the northwest. Altitude, slope and distance to road were all negatively associated with built-up land across the region. The GWR captured the spatial non-stationarity on different socioeconomic driving forces. Spatial heterogeneity and temporal variation of the impact of socioeconomic drivers indicate that the ecological restoration projects positively affected the region’s greening trend with hotspots in the center and west, and also improved farmer well-being. Notably, urban population showed undesired effects, expressed in accelerating grassland degradation in central and western regions for 1990–2000, hindering forestland and grassland restoration in the south during 2000–2015, and highlighting the long-term sustainability of the vegetation restoration progress. Such local results have the potential to provide a methodological contribution (e.g., nesting local-level approaches, i.e., GWR, within land system research) and spatially explicit evidence for context-related and proactive land management (e.g., balancing urbanization and ecological restoration processes and advancing agricultural development and rural welfare improvement)

Bringing ancient loess critical zones into a new era of sustainable development goals

Critical Zone Observatories (CZOs) have been established initially in natural environments to monitor CZ processes. A new generation of CZOs has been extended to human-modified landscapes to address the impacts of climate change and human-caused actions such as erosion, droughts, floods, and water resource pollution. This review focuses on numerous plot, field, and regional scale studies conducted in the CZO facilities distributed across the China Loess Plateau (CLP). The CLP CZO features the world's largest and deepest loess deposits, highly disturbed by human activities, and consists of a longitudinal series of monitoring sites. This observation system consists of plot, slope, watershed, and regional observatories and is promoted by large-scale comprehensive experiments to achieve multiscale observations. Deep soil boreholes, hydro-geophysical tools, multiple tracers-based techniques, proximal and remote sensing techniques, and automatic monitoring equipment are implemented to monitor CZ processes. Observation and modeling of critical hydrological and biogeochemical processes (e.g., water, nutrients, carbon, and microbial activities) in land surface and deep loess deposits across CLP CZOs have unveiled crucial insights into human-environment interactions and sustainability challenges. Large-scale ecological efforts such as revegetation and engineering such as check dam construction have effectively mitigated flood and soil erosion while enhancing deep soil carbon sequestration. However, these interventions can yield both benefits and drawbacks, impacting deep soil water, groundwater recharge, and agricultural production. Converting arable cropland to orchards for increased income has raised nitrate accumulation in the deep vadose zone, posing a risk of groundwater pollution. These findings, combined with the CZ data, have identified knowledge exchange opportunities to unravel diverse factors within the relations of agriculture, ecosystem, and environment. These could directly improve local livelihoods and eco-environmental conditions by optimizing land use and management practices, increasing water use efficiency, and reducing fertilizer application. These efforts contribute towards Sustainable Development Goals (SDGs) and environmental policies. Overall, studies within the CLP have provided significant scientific advancements and guidance on managing CZ processes and services with regional SDGs, that may be transferable to other highly disturbed regions of the world