Restoring disturbed and over-exploited ecosystems is important to mitigate human pressures on natural ecosystems. China has launched an ambitious national ecosystem restoration program called Grain to Green Program (GTGP) over the last decade. By using remote sensing techniques and ecosystem modelling, we quantitatively evaluated the changes in ecosystem carbon sequestration since China's GTGP program during period of 2000–2008. It was found the NPP and NEP in this region had steadily increased after the initiative of the GTGP program, and a total of 96.1 Tg of additional carbon had been sequestered during that period. Changes in soil carbon storage were lagged behind and thus insignificant over the period, but was expected to follow in the coming decades. As a result, the Loess Plateau ecosystem had shifted from a net carbon source in 2000 to a net carbon sink in 2008. The carbon sequestration efficiency was constrained by precipitation, and appropriate choices of restoration types (trees, shrubs, and grasses) in accordance to local climate are critical for achieving the best benefit/cost efficiency.

Accurately quantifying evapotranspiration (ET) is essential for modelling regional-scale ecosystem water balances. This study assembled an ET data set estimated from eddy flux and sapflow measurements for 13 ecosystems across a large climatic and management gradient from the United States, China, and Australia. Our objectives were to determine the relationships among monthly measured actual ET (ET), calculated FAO-56 grass reference ET (ET o ), measured precipitation (P), and leaf area index (LAI)-one associated key parameter of ecosystem structure. Results showed that the growing season ET from wet forests was generally higher than ET o while those from grasslands or woodlands in the arid and semi-arid regions were lower than ET o . Second, growing season ET was found to be converged to within š10% of P for most of the ecosystems examined. Therefore, our study suggested that soil water storage in the nongrowing season was important in influencing ET and water yield during the growing season. Lastly, monthly LAI, P, and ET o together explained about 85% of the variability of monthly ET. We concluded that the three variables LAI, P, and ET o , which were increasingly available from remote sensing products and weather station networks, could be used for estimating monthly regional ET dynamics with a reasonable accuracy. Such an empirical model has the potential to project the effects of climate and land management on water resources and carbon sequestration when integrated with ecosystem models.

Water is a limiting factor and significant driving force for ecosystem processes in arid and semi-arid areas. Knowledge of plant water uptake pattern is indispensable for understanding soil-plant interactions and species coexistence. The 'Grain for Green' project that started in 1999 in the Loess Plateau of China has led to large scale vegetation change. However, little is known about the water uptake patterns of the main plant species that inhabit in this region. In this study, the seasonal variations in water uptake patterns of three representative plant species, Stipa bungeana, Artemisia gmelinii and Vitex negundo, that are widely distributed in the semi-arid area of the Loess Plateau, were identified by using dual stable isotopes of δH and δO in plant and soil water coupled with a Bayesian mixing model MixSIAR. The soil water at the 0-120cm depth contributed 79.54±6.05% and 79.94±8.81% of the total water uptake of S. bungeana and A. gmelinii, respectively, in the growing season. The 0-40cm soil contributed the most water in July (74.20±15.20%), and the largest proportion of water (33.10±15.20%) was derived from 120-300cm soils in August for A. gmelinii. However, V. negundo obtained water predominantly from surface soil horizons (0-40cm) and then switched to deep soil layers (120-300cm) as the season progressed. This suggested that V. negundo has a greater degree of ecological plasticity as it could explore water sources from deeper soils as the water stress increased. This capacity can mainly be attributed to its functionally dimorphic root system. V. negundo may have a competitive advantage when encountering short-term drought. The ecological plasticity of plant water use needs to be considered in plant species selection and ecological management and restoration of the arid and semi-arid ecosystems in the Loess Plateau.

a b s t r a c t a r t i c l e i n f oSoil erosion is a critical environmental problem of the Loess Plateau, China. As an important project for soil and water conservation in the semi-arid environment, the Grain-for-Green extensively transformed a wide range of farmland into vegetated land after the 1980s. Yet, the effects of vegetation restoration on soil erosion reduction are not well understood. In this study, we monitored runoff and sediment yield at sites restored with six typical restoration vegetation types including shrubs (Armeniaca sibirica, Spiraea pubescens and Artemisia coparia), grasses (Andropogon), and shrub-grass-compounds (Andropogon and A. coparia) in the Loess Plateau. We employed structural equation modelling (SEM) to systematically analyze the relative effects of precipitation and vegetation on soil erosion. The results showed that the runoff and sediment yield at the grasslands were significantly higher than other cover types. The shrub cover had the strongest soil conservation capacity of all restoration vegetation. SEM results showed varying impacts of precipitation (i.e., total amount and erosive rainfall intensity) on runoff and soil erosion under different vegetation types owing to differences in canopy structure and surface litter layer. Our study quantitatively revealed the interactive effects of precipitation and vegetation on runoff and sediment, which may be beneficial to conserving available water and soil resources in the semiarid environment.

Trade-off is defined as a situation where one ecosystem service (ES) increases while another decreases. In a broader sense, trade-off also refers to unidirectional changes with uneven paces or rates in ESs. Although trade-off analysis for multiple ESs is more integral for ecosystem assessment and management, studies regarding trade-offs are rare in the literature, especially at the landscape scale or across large environmental gradients. Here, we evaluated the co-variations of multiple ESs of black locust (Robinia pseudoacacia) plantations along a precipitation gradient (400-650 mm) on the Loess Plateau using a quantitative trade-off approach. The multiple ESs had complex relationships, with significant regional variations along the gradient. Aboveground carbon, soil organic carbon (SOC), soil total nitrogen (STN), and soil water content (SWC) showed increasing trends with precipitation, but understory plant diversity (UPD) did not. The highest trade-offs were between UPD and SWC and the lowest trade-offs were between SOC and STN among all of the ES pairs. The differences in the trade-offs of varied ES combinations could be the result of unique competition relationships, mass allocation strategies, and time lags. Stand age appeared to be another critical variable in determining the values of ESs and their trade-offs along the precipitation gradient. The decreasing SWC with stand age indicated that the gaining of the other ESs was at the cost of SWC consumption. Because multiple ESs and their trade-offs exhibit high spatial variations across the landscape, spatially explicit management is needed to maintain the benefits while mitigating negative impacts in this water-limited landscape.

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