Browsing by Author "Fu, Bojie"
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Item A global synthesis of transpiration rate and evapotranspiration partitioning in the shrub ecosystems(Elsevier, 2022-03) Gao, Guangyao; Wang, Di; Zha, Tianshan; Wang, Lixin; Fu, Bojie; Earth and Environmental Sciences, School of ScienceTranspiration (T) is a fundamental process in understanding the ecophysiology of plants, and it is the dominant component of evapotranspiration (ET) in the terrestrial water cycle. Although previous studies have examined T characteristics of shrub ecosystems in some regions, global-scale synthesis that integrates the spatial variations of T, ET and ratio of T to ET (T/ET) and the associated influences of bio-/abiotic factors in the shrub ecosystems is currently lacking. In this study, we synthesized and analyzed T rate, ET rate and T/ET of the shrub ecosystems from the peer-reviewed articles using field observations around the world. These studies were mainly distributed in drylands with aridity index (ratio of precipitation to potential ET) < 0.65, which accounted for 86.4% of the study locations. Globally, the mean daily T and ET rates of shrubs were 1.5 ± 1.0 mm d−1 and 2.4 ± 0.8 mm d−1, with coefficient of variation of 63.2% and 36.2% among the study locations, respectively. Mean T/ET of the shrubs over the growing season was 0.54 ± 0.14, which was generally lower compared with forest, grassland and cropland ecosystems. The T rate of shrubs was positively related to shrub age, shrub height, leaf area index, and vegetation coverage (p < 0.05), and the effects of biotic factors on T rate were stronger compared with abiotic factors. The ET rate of shrubs was positively related to aridity index, long-term annual mean precipitation, mean soil water content, as well as shrub height and vegetation coverage (p < 0.05). By contrast, the effects of biotic factors on variations of shrub T/ET were weaker than those of abiotic factors, and the T/ET of shrubs was negatively related to aridity index, long-term annual mean precipitation and mean soil water content, but positively related to latitude (p < 0.05). This study is an important supplement of our knowledge gap in terrestrial water cycle, and the findings suggest that T accounted for about half of the water into atmosphere from shrub ecosystems, and the variations of T rate of shrubs were mainly controlled by biotic factors, whereas ET rate and T/ET was mainly affected by abiotic factors.Item African dryland ecosystem changes controlled by soil water(Wiley, 2019-08) Wei, Fangli; Wang, Shuai; Fu, Bojie; Wang, Lixin; Liu, Yi Y.; Li, Yan; Earth Sciences, School of ScienceMonitoring long‐term vegetation dynamics in African drylands is of great importance for both ecosystem degradation studies and carbon‐cycle modelling. Here, we exploited the complementary use of optical and passive microwave satellite data— normalized difference vegetation index (NDVI) and vegetation optical depth (VOD)—to provide new insights of ecosystem changes in African drylands. During 1993–2012, 54% of African drylands experienced a significant increase of VOD, mainly located in southern Africa and west and central Africa, with an average rate of increase of (1.2 ± 2.7) × 10−3 yr−1. However, a significant decreasing NDVI was observed over 43% of the African drylands, in particular in western Niger and eastern Africa, with an average browning rate of (−0.13 ± 1.5) × 10−3 yr−1. The contrasting vegetation trends (increasing VOD and decreasing NDVI) were largely caused by an increase in the relative proportion of the woody component of the vegetation, as a result of the prevailing woody encroachment in African drylands during the study period. Soil water emerges as the dominant driver of ecosystem changes in African drylands, in particular in arid and semiarid areas. This is evidenced by a strong spatio‐temporal correlation between soil water and vegetation, where soil water changes explain about 48% of vegetation variations. This study emphasizes the potential of utilizing multiple satellite products with different strengths in monitoring different characteristics of ecosystems to evaluate ecosystem changes and reveal the underlying mechanisms of the observed changes.Item Age-related water use characteristics of Robinia pseudoacacia on the Loess Plateau(Elsevier, 2021-05) Wang, Jian; Fu, Bojie; Jiao, Lei; Lu, Nan; Li, Jianye; Chen, Weiliang; Wang, Lixin; Earth Sciences, School of ScienceUnderstanding water use characteristics of revegetation species is crucial for evaluating plant adaptability and guiding the sustainability of vegetation restoration in semiarid regions. Ecological restoration projects have been implemented for decades in degraded ecosystems, achieving significant changes in vegetation cover. However, water use characteristics of the main tree species at different ages remain poorly understood in such systems. We investigated water use characteristics of Robinia pseudoacacia in plantations of different stand-age (18 and 30 years). The species is the most widely planted tree in revegetation efforts on the Loess Plateau. The δ2H and δ18O of xylem and soil water within 500 cm of the soil surface and the δ13C values of plant leaves were measured during two consecutive hydrological years. The results showed that that water uptake proportions from across the soil columns changed in 18-yr R. pseudoacacia between a drier (2016) and wetter year (2017). In contrast, shallow soil water was largely comparable in a stand of 30-yr R. pseudoacacia in 2016 and 2017, and similarly the pattern of water uptake by roots from the middle and deep soil column was comparable. However, leaf-level water use efficiency (WUEi) of trees in the older plantation was higher during the wetter year, thereby partly alleviating a low infiltration to precipitation ratio. These findings suggest that different stand-age plantation trees have distinct water use characteristics and display different responses to variations in precipitation. Older plantation trees respond to increased water availability by increasing WUEi instead of switching water sources. This means that stand-age is an essential factor to be considered in ecological restoration management, which can enhance the effectiveness of vegetation restoration strategies. The study indicates useful input from research to management throughout the continuity of restoration effort.Item Current and future carbon stocks of natural forests in China(Elsevier, 2022-05-01) Chen, Shiyin; Lu, Nan; Fu, Bojie; Wang, Shuai; Deng, Lei; Wang, Lixin; Earth and Environmental Sciences, School of ScienceNatural regeneration of forests is the most cost-effective and most technically straightforward strategy to mitigate climate change. Natural forests account for 71% of China’s forested area, but their carbon stocks and sequestration potentials remain unclear. Here, we compiled data from 762 natural forest sites across China and found that natural forests had a carbon stock of 9.40 ± 1.45 Pg C in 2010. When naturally regenerated to the over-mature stage, existing natural forests can additionally sequestrate 8.67 ± 6.93 Pg C in the next two centuries, accounting for 48% of the carbon carrying capacity of the natural forest ecosystem in China, i.e. 18.07 ± 6.78 Pg C. Future carbon sequestration potential in natural forests is dominated by the tree layer at 6.88 ± 6.87 Pg, followed by the shrub layer at 1.02 ± 0.55 Pg, floor layer at 0.72 ± 0.74 Pg and herb layer at 0.05 ± 0.10 Pg. The natural forests are expected to achieve 70% of their future carbon sequestration potential by 2100. We also note that assisted regeneration via tree planting can play a significant role in natural forest restoration, as the carbon densities of natural and planted forests are rarely significantly different at the same age under 60 years old. Therefore, the preservation and expansion of natural forests is the key strategy for achieving long-term carbon sequestration.Item Divergent trends of ecosystem-scale photosynthetic efficiency between arid and humid lands across the globe(Wiley, 2022-09) Wei, Fangli; Wang, Shuai; Fu, Bojie; Wang, Lanhui; Zhang, Wenmin; Wang, Lixin; Pan, Ning; Fansholt, Rasmus; Earth and Environmental Sciences, School of ScienceAim Widespread greening and an increasing global terrestrial carbon sink over recent decades have been reported. However, the spatio-temporal relationships between vegetation greenness and productivity and the factors influencing this relationship remain unclear. We define a new metric of ecosystem-scale photosynthetic efficiency (EPE) to analyse its spatio-temporal pattern and investigate how potential drivers regulate the greenness–productivity relationship. Location Global. Time period From 2001 to 2016. Major taxa studied Global terrestrial ecosystems. Methods This study used global datasets of leaf area index (LAI) and solar-induced fluorescence (SIF) as proxies of vegetation greenness and ecosystem productivity, respectively, to propose a new metric of SIF/LAI, representing ecosystem-scale photosynthetic efficiency (EPE). We identified the spatial pattern and dynamics of EPE and examined factors influencing EPE. Results The results showed a weaker increase in productivity compared with the global greening rate from 2001 to 2016, suggesting a decline in EPE at the global scale. This decline in EPE indicates a disproportionate increase in terrestrial productivity against the widespread greening. When stratified into areas following an aridity gradient, we found that EPE overall showed upward trends in arid and semi-arid areas, and downward trends in dry sub-humid and humid regions. The EPE was controlled primarily by soil moisture, which promoted or constrained the EPE in xeric and mesic ecosystems, respectively. Moreover, the increase in short vegetation cover and atmospheric water demand contributed positively or negatively to EPE changes in xeric and mesic ecosystems, respectively. Main conclusions Our study shows that greening of the Earth is associated with decreasing EPE, revealing that current rates of carbon sequestration do not increase proportionally to greening of the Earth and highlighting that soil moisture is a key controller of EPE. These results help to reduce the uncertainties in future climate change impacts on vegetation dynamics, thus having implications for sustainable ecosystem management and climate change mitigation.Item Drylands contribute disproportionately to observed global productivity increases(Elsevier, 2023-01-30) Wang, Shuai; Fu, Bojie; Wei, Fangli; Piao, Shilong; Maestre, Fernando T.; Wang, Lixin; Jiao, Wenzhe; Liu, Yanxu; Li, Yan; Li, Changjia; Zhao, Wenwu; Earth and Environmental Sciences, School of ScienceDrylands cover about 40% of the terrestrial surface and are sensitive to climate change, but their relative contributions to global vegetation greening and productivity increase in recent decades are still poorly known. Here, by integrating satellite data and biosphere modeling, we showed that drylands contributed more to global gross primary productivity (GPP) increase (65% ± 16%) than to Earth greening (33% ± 15%) observed during 1982–2015. The enhanced productivity per unit leaf area, i.e., light-use efficiency (LUE), was the mechanism behind this pattern. We also found that LUE was more sensitive to soil moisture than to atmospheric vapor pressure deficit (VPD) in drylands, while the opposite was observed (i.e., LUE was more sensitive to VPD) in humid areas. Our findings suggest the importance of using different moisture stress metrics in projecting the vegetation productivity changes of dry versus humid regions and highlight the prominent role of drylands as key controllers of the global carbon cycle.Item Enhanced coupling of light use efficiency and water use efficiency in arid and semi-arid environments(Wiley, 2021) Gao, Dexin; Wang, Shuai; Wang, Lixin; Li, Zidong; Pan, Ning; Liu, Yanxu; Fu, Bojie; Earth Sciences, School of ScienceBoth light use efficiency (LUE) and water use efficiency (WUE) play essential roles in ecosystem production. The extent to which ecosystem production is affected by the coupling between LUE and WUE remains unclear. In this study, we used data from flux measurements and weather stations in the Heihe River Basin, China, along a strong climatic gradient to quantify the relationship between LUE and WUE. Variations of LUE explained up to 85% of the variations of WUE. The contribution of LUE to WUE increased with increasing water stress. Pearson’s correlation coefficient between LUE and WUE increased from −0.12 to +0.63 with decreasing LUE. The coordination between LUE and WUE was essential to explaining the insensitive of WUE and GPP with increasing water stress. These results suggest that ecosystems enhance the coordination between light and water use when water stress is high. They enhance our understanding of the drought resilience of ecosystems and reduce uncertainties associated with the carbon cycle in drylands.Item Quantitative synthesis on the ecosystem services of cover crops(Elsevier, 2018-10) Daryanto, Stefani; Fu, Bojie; Wang, Lixin; Jacinthe, Pierre-André; Zhao, Wenwu; Earth Sciences, School of ScienceThe maintenance of soil health in agro-ecosystems is essential for sustaining agricultural productivity. Through its positive impacts on various soil physical and biological processes, cover cropping can be an important component of sustainable agricultural production systems. However, the practice of cover cropping can be complex, and possible trade-offs between the benefits and side effects of cover crops have not been examined. To evaluate these benefits and potential trade-offs, we quantitatively synthesized different ecosystem services provided by cover crops (e.g., erosion control, water quality regulation, soil moisture retention, accumulation of soil organic matter and microbial biomass, greenhouse gas (GHG) emission, weed and pest control, as well as yield of the subsequent cash crop) using data from previous publications. We used a simple indicator (δ), defined as the ratio of an observed variable (i.e., ecosystem service) under cover crop and under fallow condition, to evaluate the impacts of cover crops on a given ecosystem service. Our results showed that cover crops provided beneficial ecosystem services in most cases, except for an increase in GHG emission (δCO2 = 1.46 ± 0.47 and δN2 O = 1.49 ± 1.22; ± SD) and in pest (nematode) incidence (δnematode abundance = 1.29 ± 1.61). It is also important to highlight that, in some cases, tillage could offset the extent of ecosystem service benefits provided by cover crops. Based on this synthesis, we argue that cover crops should be incorporated into modern agricultural practices because of the many environmental benefits they offer, particularly the maintenance of soil and ecosystem health. More importantly, there was generally an increase in cash crop yield with cover cropping (δyield = 1.15 ± 0.75), likely due to improvement in various soil processes. Despite its benefits, the complexity of cover crop management should not be overlooked, and site-specific factors such as climate, soil type, cover crop species and tillage practices must be considered in order to optimize the benefits of cover cropping. In addition to crop yield, detailed economic analyses are needed to calculate the direct (e.g., reduction in the amount of chemical fertilizer) and indirect monetary benefits (e.g., the improvement of soil quality) of cover crops. Such a comprehensive analysis could serve as incentive for producers to integrate cover crops into their management practices.Item Responses and feedbacks of African dryland ecosystems to environmental changes(Elsevier, 2021-02) Wei, Fangli; Wang, Shuai; Brandt, Martin; Fu, Bojie; Meadows, Michael E.; Wang, Lixin; Wang, Lanhui; Tong, Xiaowei; Fensholt, Rasmus; Earth Sciences, School of ScienceDrylands occupy 43% of the African continent and play an important role in the global carbon cycle and in supporting local livelihoods. Understanding how dryland ecosystems respond to environmental changes, both structurally and functionally, is of great significance for sustainable dryland management. In this article, we review the current remote sensing-based knowledge on African dryland ecosystem dynamics and the main drivers of changes. Global CO2 enrichment, changes in rainfall regimes, and a decline in fire activity have collectively driven vegetation greening, woody plant increase and carbon dynamics in African drylands over recent decades, challenging the long-held desertification narrative. Here we also highlight the importance of rainfall–vegetation–fire feedbacks in enhancing dryland ecosystem resilience and predicting future ecosystem responses.Item The vulnerability of ecosystem structure in the semi-arid area revealed by the functional trait networks(Elsevier, 2022-06) Gao, Dexin; Wang, Shuai; Wei, Fangli; Wu, Xutong; Zhou, Sha; Wang, Lixin; Li, Zidong; Chen, Peng; Fu, Bojie; Earth and Environmental Sciences, School of ScienceThe ecosystems were characterized by complex, nonlinear interactions determined by different plant functional traits. The characteristics of the multiple relationships between ecosystem functional traits affected the vulnerability to drought. A three-level network analysis on instead of the network metrics, relationships among inter-components, and essential traits was conducted in dryland ecosystems of China. The new network of functional traits included leaf, root, and biomass components was constructed to simulate different aridity conditions. Results show that the multiple relationships of functional traits that co-regulated ecosystem biomass differ along an aridity gradient. The highest network modularity and degree centrality were observed in the semi-arid ecosystems indicating low integration and high sensitivity of semi-arid ecosystems (269% and 23.7% higher than in dry sub-humid site, and 142% and 51.1% higher than arid sites). The leaf quantity strongly affected the connection between functional traits at the semi-arid zone. The semi-arid areawas found to have relatively low resistance to environmental change because of low integration and high sensitivity of the ecosystem structure at that site. An increase of degree centrality of the root traits and trade-off relationships between roots and leaves indicated greater allocation of resources by vecgetation to underground components by the arid ecosystems to increase water absorption. The study reveals the complex relationships between leaf, root, and biomass components, and the essential traits of the ecosystem. It enhanced understanding of the vulnerability of semi-arid ecosystems to environmental change.