Afforestation and Reforestation: Drivers, Dynamics, and Impacts

Afforestation/reforestation (or forestation) has been implemented worldwide as an effective measure towards sustainable ecosystem services and addresses global environmental problems such as climate change. The conversion of grasslands, croplands, shrublands, or bare lands to forests can dramatically alter forest water, energy, and carbon cycles and, thus, ecosystem services (e.g., carbon sequestration, soil erosion control, and water quality improvement). Large-scale afforestation/reforestation is typically driven by policies and, in turn, can also have substantial socioeconomic impacts. To enable success, forestation endeavors require novel approaches that involve a series of complex processes and interdisciplinary sciences. For example, exotic or fast-growing tree species are often used to improve soil conditions of degraded lands or maximize productivity, and it often takes a long time to understand and quantify the consequences of such practices at watershed or regional scales. Maintaining the sustainability of man-made forests is becoming increasingly challenging under a changing environment and disturbance regime changes such as wildland fires, urbanization, drought, air pollution, climate change, and socioeconomic change. Therefore, this Special Issue focuses on case studies of the drivers, dynamics, and impacts of afforestation/reforestation at regional, national, or global scales. These new studies provide an update on the scientific advances related to forestation. This information is urgently needed by land managers and policy makers to better manage forest resources in today’s rapidly changing environments

Impact of Land Use and Climate Change on Hydrological Ecosystem Services (Water Supply) in the Dryland Area of the Middle Reaches of the Yellow River

Driven by many factors, the water supply services (streamflow and groundwater) of many rivers in the dryland area of China have declined significantly. This aggravates the inherent severe water shortages and results in increased severity in the water use conflicts that are threatening sustainable development in the region. Innovative strategies towards more water-efficient land management are vital for enhancing water quantity to ensure water supply security. A key step in the successful development and implementation of such measures is to understand the response of hydrological processes and related services to changes in land management and climate. To this end, it was decided to investigate these processes and responses in the upper reaches of the Jing River (Jinghe), an important meso-scale watershed in the middle reaches of the Yellow River on the Loess Plateau (NW China). It has been shown that vegetation restoration efforts (planting trees and grass) are effective in controlling soil erosion on the Loess Plateau. Shifts in land cover/use lead to modifications of soil physical properties. Yet, it remains unclear if the hydraulic properties have also been improved by vegetation restoration. A better understanding of how vegetation restoration alters soil structure and related soil hydraulic properties, such as water conductivity and soil water storage capacity, is necessary. Three adjacent sites, with comparable soil texture, soil type, and topography but contrasting land cover (Black locust forest, grassland, and cropland), were investigated in a small catchment in the upstream Jinghe watershed (near Jingchuan, Gansu province). Seasonal variations of soil hydraulic properties in topsoil and subsoil were examined. Results revealed that the type of land use had a significant impact on field-saturated, near-saturated hydraulic conductivity, and soil water characteristics. Specifically, conversion from cropland to grass or forests promotes infiltration capacity as a result of increased saturated hydraulic conductivity, air capacity, and macroporosity. Moreover, conversion from cropland to forest tends to promote the formation of mesopores that increase soil water storage capacity. Tillage in cropland temporarily created well-structured topsoil, but also compacted subsoil, as indicated by low subsoil saturated hydraulic conductivity, air capacity, and plant available water capacity. An impact of land cover conversion on unsaturated hydraulic conductivities was not identified, indicating that changes in land cover do not affect functional meso- and microporosity. Changes in soil hydraulic properties and associated hydrological processes and services due to soil conservation efforts need to be considered, should soil conservation measures be implemented in water-limited regions for sustaining adequate water supply. To differentiate between the impacts of land management and climate change on streamflow, the variation of annual streamflow, precipitation, potential evapotranspiration, and climatic water balance in a small catchment of the upstream Jinghe watershed (near Pingliang, Gansu province) was examined during the period of 1955 – 2004. During this time the relative contributions of changes in land management and climate to the reduction of streamflow were estimated. A statistically significant decreasing trend of -1.14 mm y-1 in annual streamflow was detected. Furthermore, an abrupt streamflow reduction due to afforestation and construction of terraces and check-dams was identified around 1980. Remarkably, 74% of the total reduction in mean annual streamflow can be attributed to the soil conservation measures. Among various conservation measures, streamflow could be considerably reduced by afforestation and terracing (including damland creation), due to their low contribution to water yield. In contrast, slope farmland and grassland can maintain a certain level of water supply services due to higher runoff coefficients. According to a meta-analysis of the published studies on the Loess Plateau, the impact of changes in land management on annual streamflow appears to diminish with increasing catchment size while the impact of climate change appears uniform across space. This means that there is a dependency between the catchment size and the response of hydrological processes to environmental change. At least at the local scale, it appears that well-considered land management may help to ensure the water supply services. Due to limited surface water availability, groundwater is an essential water source for supporting ecosystem and socio-economic development in the dryland region. However, the groundwater process is susceptible and vulnerable to changes in climate and landscape (i.e., land cover and form) that in turn can result in profound adverse consequences on water supply services in water-limited regions. In addition, an improved understanding of the response of groundwater related processes to natural and artificial disturbances is likely to ensure more secure and more sustainable governance and management of such regions, as well as better options for adapting to climate change. Yet, this topic has seldom been researched, especially in areas that have already experienced large-scale alteration in landscape and are located in dryland regions, such as the Loess Plateau. Therefore, an investigation of the baseflow variation along the landscape change was conducted. The average annual baseflow has significantly decreased at catchment scale during the period of 1962 – 2002 without any obvious significant change in climate. At decadal scale, the reduction accounts for approximately 9% in the 1970s, 48% in the 1980s, and 92% in the 1990s, while the baseflow index declines averaging 5%, 16% and 67%, respectively. All of the monthly baseflow levels dropped at varying rates except in January, among which July was the most severe in terms of both magnitude (-4.17) and slope (-0.09 mm y-1). In perspective of landscape change, landform change (terrace and check-dam) tends to reduce baseflow by reallocation of surface fluxes and retention for crop growth causing limited deep drainage in other areas. Land cover change (i.e., afforestation) reduced the baseflow to a larger extent by enhanced evapotranspiration and thus hampered deep drainage as suggested by the soil moisture measurement underneath. The study indicates that knowledge about baseflow formation on catchment scale needs further improvement. Integrated soil conservation and water management for optimizing landscape structure and function in order to balance soil (erosion) and water (supply) related hydrological ecosystem services is vital. The governing processes to the changes of water-supply-services-related hydrological process (e.g., streamflow) are assumed to be different across space. To this end, the factors controlling streamflow were investigated on both a small and large scale. Streamflow in small catchments was found to be mainly controlled by precipitation and land cover type. On a larger scale, evaporative demand was found to be another additional major driving force. Hydrological modeling is a frequently used tool for the assessment of impacts of land use and climate change on water balance and water fluxes. However, application of the Soil and Water Assessment Tool (SWAT) model in the upstream Jinghe watershed was unsuccessful due to difficulties in calibration. The inability of the SWAT model to take the influence of terraces on steep slopes into consideration and the method how to calculate lateral flow were the main reasons for unsatisfactory calibration, at least for the current version of SWAT used in this study. Alternatively, Budyko’s frameworks were applied to predict the annual and long-term streamflow. However, the effect of changes in land management (e.g., afforestation) on streamflow could not be assessed due to a lack of vegetation factors. Therefore, an empirical analysis tool was derived based on an existing relationship for estimation. This method was found to be the most effective in reproducing the annual and long-term streamflow. The incorporation of temporal changes in land cover and form in the approach enables the estimation of the possible impact of soil conservation measures (e.g., afforestation or terracing). The importance of adaptive land management strategies for mitigating water shortage and securing the water supply services on the Loess Plateau was highlighted. A cross-sectoral view of the multiple services offered by managed ecosystems at different spatial scales under changing environments needs to be integrated to improve adaptive land management policy. In a water limited environment, such as the Loess Plateau, multiple ecosystem services including hydrological services need to be balanced with minimum trade-offs. This can only be achieved when management is based on a holistic understanding of the interdependencies among various ecosystem services and how they might change under alternative land management

Assessment of the Effects of Riparian Buffer Zones on Water Quality in the Jinghe Catchment Using the AnnAGNPS and REMM Models

Direct discharge of sediments and agricultural chemicals into rivers is a major areal source for surface water pollution. This study was conducted to evaluate the impact on water quality by converting riverside crop fields to riparian buffer zones at river basin scale. The Jinghe catchment of the Weihe River basin in China, a heavily cultivated watershed in the middle reach of the Yellow River basin, was selected for this study. The Annualized Agricultural Non-Point Source Pollution Model (AnnAGNPS) was used to simulate water and sediment loadings from upland fields. These generated upland loadings then were used as inputs to run the Riparian Ecosystem Management Model (REMM) for assessing the impact of riparian buffer zones on reducing sediment concentration and nutrients in surface runoff. The whole Jinghe catchment was divided into 4290 homogeneous drainage areas by AnnAGNPS, and both models were run for each drainage area for one year. The annual output data for each drainage area were spatially integrated over the whole Jinghe catchment. Five designs of the riparian buffer with different zone numbers, zone widths and vegetation types were tested. Annual totals of water inflow, sediment yields and dissolved nitrogen in surface runoff into zones 3, 2, and 1, and those out of zone 1 for the whole Jinghe catchment were calculated, compared and analyzed for all the five designs. The analysis indicated that the sediment was reduced by 85.7% to 90.8% and the dissolved nitrogen in surface runoff was reduced by 85.4% to 91.9% over the whole catchment. Our study has provided a helpful way in using the field-scale REMM model in conjunction with the well accepted AnnAGNPS model in evaluating ecological engineering projects at river basin scale. The results of this study are anticipated useful in guiding the land use changes in the Yellow River basin in China for improving surface water quality by building riverside riparian buffer zones

Habitat quality assessment and multi-scenario prediction of the Gansu-Qinghai section of the Yellow River Basin based on the FLUS-InVEST model

Research on the impact of land use change on regional habitat quality, in various future scenarios, can effectively aid planning and decision-making for sustainable development at a regional level. The study conducted its research in the Gansu-Qinghai Yellow River section and used ArcGIS and a land use transfer matrix to analyze the spatiotemporal pattern of land use and land cover changes. The study assessed the changes in habitat quality in the Gansu-Qinghai Yellow River region between 1990 and 2020, using the Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) model, by evaluating the gains and losses. Simultaneously, 15 elements of the natural economy were chosen and examined for their temporal and spatial impact on habitat quality using the random forest model and spatially weighted regression model. To forecast land use changes in the Gansu-Qinghai Yellow River section for 2030, the Future Land Use Simulation Model (FLUS) model was utilized and a series of four scenarios (cultivated land protection scenario, ecological protection scenario, natural development scenario, and rapid development scenario) were employed. The research results indicate that over 70% of the Gansu-Qinghai Yellow River is occupied by grasslands, and only a small portion of the area, about 0.22%, is developed for construction purposes. The quality of the habitat in the Gansu-Qinghai Yellow River had a minor drop between 1990 and 2020, followed by an improvement. Habitat quality changes are primarily attributed to improvements, with variations across different areas, i.e., enhanced in the east and reduced in the central and western parts. The habitat quality of the Gansu-Qinghai Yellow River has improved in all four scenarios compared to 2020, as evidenced by the decrease in low-value habitats and increase in high-value areas. The ecological protection scenario has the highest average habitat quality value. These research results can be used to support policy development and ecological restoration initiatives in the Gansu-Qinghai Yellow River

Intra-annual sediment dynamic assessment in the Wei River Basin, China, using the AIC functional-structural connectivity index

Hydrological and sediment dynamics have changed considerably on the Chinese Loess Plateau during the last six decades due to large scale land use changes and numerous water regulation actions. Understanding the mechanism of sediment transport change and its effects is of great importance to food and environmental security. Numerical approaches are useful to map and assess spatio-temporal patterns in sediment dynamics. This study evaluates monthly and annual sediment connectivity in the Wei River Basin (134,800 km2) at the basin and sub-basin scales using the aggregated index of sediment connectivity (AIC). For the first time, this index is applied on this relatively large regional scale. The two objectives were to (1) evaluate the performance of the AIC at the regional scale, addressing substantial differences among areas, and (2) analyze how each AIC sub-factor co-determines the monthly sediment and connectivity patterns. Results show that AIC has strong or moderate positive correlation with sediment yield from 15 out of 23 stations in the Wei and Jing sub-basin. The Jing sub-basin has the highest sediment connectivity due to degraded vegetation, while the Beiluo sub-basin has the lowest sediment connectivity on average due to better ecological restoration. Within the year, sediment connectivity is highest in April and lowest in January, due to the rainfall regime and intra-annual land cover variations. Among the AIC factors, the rainfall factor has the highest effect on sediment connectivity, implying that functional connectivity (graded by rainfall and soil cover) determines sediment dynamics more than structural connectivity (mainly determined by topography and soil permeability). This study provides one of the first large-scale estimates of spatial and temporal sediment connectivity from hillslopes to river stream and including large reservoirs, which can be further employed to implement regional ecological construction works and environmental catchment management.info:eu-repo/semantics/publishedVersio

Temporal and spatial change of habitat quality and its driving forces: The case of Tacheng region, China

Habitat quality assessment is an important basis for ecological restoration practice. Taking the Tacheng region as an example, the InVEST model was used to evaluate the habitat quality of the Tacheng region in five periods from 2000 to 2020, and analyze the reasons for its changes, to provide theoretical guidance for ecological restoration practice in arid areas. The conclusions were that from 2000 to 2020, the habitat quality in the Tacheng region improved slightly, and the value of the habitat index in the Tacheng region was the highest in 2010, which was 0.577, and then decreased slightly. The habitat quality in the Tacheng region was significantly influenced by land use type conversion and precipitation. The change in land use type directly affected the change in habitat quality. The study region is located in an arid area; the forest land and grassland native to the region have more vegetation communities and genera of species and can be self-sustaining and resilient to disturbance, having high scores for habitat quality. The species of arable land is a monoculture; it cannot be self-sustaining and resilient to disturbance, and though it has high vegetation cover, the value of habitat quality is lower than that of forestland and grassland. The vegetation of unused land is rare, and the ecosystem of unused land is sensitive and vulnerable; the habitat quality scores are very low. The conversion of forest land, grassland, arable land, and unused land would directly affect the value of habitat quality, and conversion was the main factor affecting the change in habitat quality. In addition, precipitation was also an important factor affecting the change in habitat quality in the Tacheng region, which affected the biomass of natural vegetation and then affected the habitat quality. The results provided the temporal and spatial change of habitat quality and its driving forces in the Tacheng region, which helps determine appropriate measures and sites in ecological restoration projects

In situ reconstruction of long-term extreme flooding magnitudes and frequencies based on geological archives

© 2019 Extreme flooding magnitudes and frequencies are essentially related to assessment of risk and reliability in hydrological design. Extreme flooding and its discharge are highly sensitive to regional climate change. Presently, its discharge can be reconstructed by a geological archive or record along the river valley. Two units of typical extreme flooding deposits (EFDs) carrying long-term information preserved in the Holocene loess–palaeosol sequence were found at Xipocun (XPC), which is located in Chengcheng County, China. It is situated in the downstream section of the Beiluohe (hereafter BLH) River. Based on multiple sedimentary proxy indices (grain-size distribution (GSD), magnetic susceptibility (MS), and loss-on-ignition (LOI), etc.), EFDs were interpreted as well-sorted clayey silt in suspension. They were then deposited as a result of riverbank flooding in a stagnant environment during high water level. Through the Optically Stimulated Luminescence (OSL) dating technique and stratigraphic correlations, chronologies of two identified extreme flooding periods were 7600–7400 a B.P. and 3200–3000 a B.P. Two phases of extreme flooding occurrence under climate abnormality scenarios were characterized as having high frequencies of hydrological extremes in river systems. According to simulation and verification using the Slope–Area Method and Hydrologic Engineering Center's River Analysis System (HEC-RAS) model, the extreme flooding discharges at the XPC site were reconstructed between 9625 m 3 /s and 16,635 m 3 /s. A new long-term flooding frequency and peak discharge curve, involved gauged flooding, historical flooding at Zhuangtou station and in situ reconstructed extreme flooding events, was established for the downstream BLH River. The results improve the accuracy of low-frequency flooding risk assessment and provide evidence for predicting the response of fluvial systems to climate instability. Thus, this improves the analysis of the BLH River watershed

Copula-based abrupt variations detection in the relationship of seasonal vegetation-climate in the Jing River Basin, China

Understanding the changing relationships between vegetation coverage and precipitation/temperature (P/T) and then exploring their potential drivers are highly necessary for ecosystem management under the backdrop of a changing environment. The Jing River Basin (JRB), a typical eco-environmentally vulnerable region of the Loess Plateau, was chosen to identify abrupt variations of the relationships between seasonal Normalized Difference Vegetation Index (NDVI) and P/T through a copula-based method. By considering the climatic/large-scale atmospheric circulation patterns and human activities, the potential causes of the non-stationarity of the relationship between NDVI and P/T were revealed. Results indicated that (1) the copula-based framework introduced in this study is more reasonable and reliable than the traditional double-mass curves method in detecting change points of vegetation and climate relationships; (2) generally, no significant change points were identified during 1982-2010 at the 95% confidence level, implying the overall stationary relationship still exists, while the relationships between spring NDVI and P/T, autumn NDVI and P have slightly changed; (3) teleconnection factors (including Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO), Nino 3.4, and sunspots) have a more significant influence on the relationship between seasonal NDVI and P/T than local climatic factors (including potential evapotranspiration and soil moisture); (4) negative human activities (expansion of farmland and urban areas) and positive human activities (Grain For Green program) were also potential factors affecting the relationship between NDVI and P/T. This study provides a new and reliable insight into detecting the non-stationarity of the relationship between NDVI and P/T, which will be beneficial for further revealing the connection between the atmosphere and ecosystems