The simulation of gross primary production (GPP) at various spatial and temporal scales remains a major challenge for quantifying the global carbon cycle. We developed a light use efficiency model, ...called EC-LUE, driven by only four variables: normalized difference vegetation index (NDVI), photosynthetically active radiation (PAR), air temperature, and the Bowen ratio of sensible to latent heat flux. The EC-LUE model may have the most potential to adequately address the spatial and temporal dynamics of GPP because its parameters (i.e., the potential light use efficiency and optimal plant growth temperature) are invariant across the various land cover types. However, the application of the previous EC-LUE model was hampered by poor prediction of Bowen ratio at the large spatial scale. In this study, we substituted the Bowen ratio with the ratio of evapotranspiration (ET) to net radiation, and revised the RS-PM (Remote Sensing-Penman Monteith) model for quantifying ET. Fifty-four eddy covariance towers, including various ecosystem types, were selected to calibrate and validate the revised RS-PM and EC-LUE models. The revised RS-PM model explained 82% and 68% of the observed variations of ET for all the calibration and validation sites, respectively. Using estimated ET as input, the EC-LUE model performed well in calibration and validation sites, explaining 75% and 61% of the observed GPP variation for calibration and validation sites respectively.
Global patterns of ET and GPP at a spatial resolution of 0.5° latitude by 0.6° longitude during the years 2000–2003 were determined using the global MERRA dataset (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate Resolution Imaging Spectroradiometer). The global estimates of ET and GPP agreed well with the other global models from the literature, with the highest ET and GPP over tropical forests and the lowest values in dry and high latitude areas. However, comparisons with observed GPP at eddy flux towers showed significant underestimation of ET and GPP due to lower net radiation of MERRA dataset. Applying a procedure to correct the systematic errors of global meteorological data would improve global estimates of GPP and ET. The revised RS-PM and EC-LUE models will provide the alternative approaches making it possible to map ET and GPP over large areas because (1) the model parameters are invariant across various land cover types and (2) all driving forces of the models may be derived from remote sensing data or existing climate observation networks.
•Dependence of eddy covariance flux measurement on landscape heterogeneity was tested.•Haralick texture features were used to describe landscape heterogeneity.•Negative effects were found when ...modelling CO2 fluxes with texture features under stable and unstable conditions.•Opportunities exist for integrating high-resolution remote sensing data in the correction procedure of EC flux tower estimates.
Flux measurements based on aerodynamic principles (e.g., eddy covariance method, or EC) assume the vegetation and landform within the footprint must be flat, large, and homogeneous, although very rarely do flux tower sites meet such requirements. Here, using two long term EC tower sites in Mongolian grasslands, we test a hypothesis that the magnitude and variation of EC flux measurements are partially dependent of landscape heterogeneity. We define landscape heterogeneity as the spatial composition and distribution of components within the footprint of a flux tower, which was quantified using high resolution WorldView-2 images to extract vegetation texture features (Contrast, Dissimilarity and Entropy). Bayesian analysis was performed to model the linkage of landscape heterogeneity with EC fluxes of CO2 in 8 intercardinal directions by dissecting it into 5 distances. We found that higher levels of landscape heterogeneity have an impact on flux measurements, especially under stable conditions. Specifically, a total of 24 Bayesian models based on the EVI-derived texture features passed the Gelman and Rubin convergence diagnostic statistic test. A positive relationship is shown between the percentage of footprint cover (Footprint%sector) and landscape texture features (Contrast and Dissimilarity), with the footprint cover acting as a function of heterogeneity under stable conditions. Negative effects were found when modeling CO2 flux (Fcsector) with Contrast under stable and unstable conditions, and with Dissimilarity and Entropy under stable conditions. With increases in high resolution remote sensing images and UAV technology (e.g., LiDAR), the results and approaches outlined in this study highlight new frontiers and opportunities for the FLUXNET community to integrate flux measurements and high-resolution remote sensing data, promoting a new generation of footprint models, and exploring the cohesive connections between flux measurements and the underlying processes (e.g., soil, physiological, ecosystem processes).
•Gs was the primary biophysical factor regulating the ecosystem EF.•Cloudiness reduced the EF of this riparian plantation.•Water use strategy significantly altered by cloudy sky conditions.•Ecosystem ...ET exceeded rainfall, which would exacerbate regional drought.
Cloudiness is an important environmental variable that affects the total and proportion of diffuse radiation reaching the ground and thereby the rate of plant carbon assimilation. However, its regulations on ecosystem energy partitioning and water use strategy are not well understood, particularly for riparian ecosystems. We used the eddy covariance technique and micrometeorological sensors to measure the energy fluxes and environmental conditions for a poplar plantation adjacent to the Chaobai River in North China during the growing seasons (April–October) in 2014 and 2015. We found that canopy conductance (Gs) was the primary biophysical factor regulating the ecosystem energy partitioning, while vapor pressure deficit (VPD) did not impose significant effects on evaporative fraction (EF). Cloudiness suppressed EF primarily due to the stomatal closure caused by the decrease in direct radiation (Rdir). Furthermore, the ratio of stomatal sensitivity (m) and reference conductance (Gsref) was 0.36 and 0.48 mol m−2 s−1 ln(kPa)−1 during clear sky and cloudy sky conditions, respectively. These results indicated that this poplar plantation with an anisohydric behavior weakened stomatal control on water loss under clear skies by avoiding leaf burn arising from higher direct sunlight and temperature. Finally, the mean Priestley–Taylor coefficient (α) and EF was 1.01 and 0.61 across two growth periods, respectively, and ecosystem evapotranspiration (ET) exceeded rainfall, even in rainy year, suggesting that a certain amount of groundwater might be consumed by this riparian poplar plantation, which would exacerbate regional drought.
Land surface temperature over the past decades has shown a faster warming trend during the night than during the day. Extremely low night temperatures have occurred frequently due to the influence of ...land-sea thermal difference, topography and climate change. This asymmetric night temperature change is expected to affect plant ecophysiology and growth, as the plant carbon consumption processes could be affected more than the assimilation processes because photosynthesis in most plants occurs during the daytime whereas plant respiration occurs throughout the day. The effects of high night temperature (HNT) and low night temperature (LNT) on plant ecophysiological and growing processes and how the effects vary among different plant functional types (PFTs) have not been analyzed extensively. In this meta-analysis, we examined the effect of HNT and LNT on plant physiology and growth across different PFTs and experimental settings. Plant species were grouped according to their photosynthetic pathways (C
, C
, and CAM), growth forms (herbaceous, woody), and economic purposes (crop, non-crop). We found that HNT and LNT both had a negative effect on plant yield, but the effect of HNT on plant yield was primarily related to a reduction in biomass allocation to reproduction organs and the effect of LNT on plant yield was more related to a negative effect on total biomass. Leaf growth was stimulated at HNT and suppressed at LNT. HNT accelerated plants ecophysiological processes, including photosynthesis and dark respiration, while LNT slowed these processes. Overall, the results showed that the effects of night temperature on plant physiology and growth varied between HNT and LNT, among the response variables and PFTs, and depended on the magnitude of temperature change and experimental design. These findings suggest complexities and challenges in seeking general patterns of terrestrial plant growth in HNT and LNT. The PFT specific responses of plants are critical for obtaining credible predictions of the changes in crop production, plant community structure, vegetation dynamics, biodiversity, and ecosystem functioning of terrestrial biomes when asymmetric night temperature change continues.
•Seven light use efficiency models were compared at global eddy covariance towers.•Performance of seven models differed substantially among ecosystem types.•It is needed to improve LUE models by ...integrating impacts of diffuse radiation and reliable water stress equations.
Simulating gross primary productivity (GPP) of terrestrial ecosystems has been a major challenge in quantifying the global carbon cycle. Many different light use efficiency (LUE) models have been developed recently, but our understanding of the relative merits of different models remains limited. Using CO2 flux measurements from multiple eddy covariance sites, we here compared and assessed major algorithms and performance of seven LUE models (CASA, CFix, CFlux, EC-LUE, MODIS, VPM and VPRM). Comparison between simulated GPP and estimated GPP from flux measurements showed that model performance differed substantially among ecosystem types. In general, most models performed better in capturing the temporal changes and magnitude of GPP in deciduous broadleaf forests and mixed forests than in evergreen broadleaf forests and shrublands. Six of the seven LUE models significantly underestimated GPP during cloudy days because the impacts of diffuse radiation on light use efficiency were ignored in the models. CFlux and EC-LUE exhibited the lowest root mean square error among all models at 80% and 75% of the sites, respectively. Moreover, these two models showed better performance than others in simulating interannual variability of GPP. Two pairwise comparisons revealed that the seven models differed substantially in algorithms describing the environmental regulations, particularly water stress, on GPP. This analysis highlights the need to improve representation of the impacts of diffuse radiation and water stress in the LUE models.
•Aliens established across green spaces with greater proportions in artificial ones.•Native and alien species richness always had positive relationships.•Urbanization had distinct effects on flora of ...artificial and near-natural habitats.•Establishment of aliens caused biotic homogenization in artificial habitats.•Biotic differentiation in near-natural habitats was preserved by protecting natives.
Shenzhen, a new city existed for less than three decades, experienced dramatic biological invasions of alien plants resulting from the construction of urban green spaces. In an investigation of plant species from 390 plots in 186 sites across five main types of green space (public parks, forests, vegetation corridors, residential and industrial area), a total of 474 plant species, including 221 alien species (46.6%), were recorded. Our study witnessed the spread and establishment of alien species across all urban green spaces and revealed significantly greater proportions of alien species in artificial green spaces than in natural ones. Furthermore, we found the positive relationship between native and alien species richness existed across all the green spaces and was particularly prevalent in the artificial ones. Additionally, successful establishment of alien species instigated by anthropogenic disturbances and most frequent species assemblages caused biotic homogenization in the artificial habitats. In contrast, biotic differentiation in the near-natural habitats was still preserved due to a greater level of protection for native species, which showed some resistance against the establishment of alien species. Therefore, urbanization was proven distinct effects on the flora of artificial and near-natural habitats, coexisting in the new city.
Vegetation in Northeast China (NEC) has faced dual challenges posed by climate change and human activities. However, the factors dominating vegetation development and their contribution remain ...unclear. In this study, we conducted a comprehensive evaluation of the response of vegetation in different land cover types, climate regions, and time scales to water availability from 1990 to 2018 based on the relationship between normalized difference vegetation index (NDVI) and the standardized precipitation evapotranspiration index (SPEI). The effects of human activities and climate change on vegetation development were quantitatively evaluated using the residual analysis method. We found that the area percentage with positive correlation between NDVI and SPEI increased with time scales. NDVI of grass, sparse vegetation, rain-fed crop, and built-up land as well as sub-humid and semi-arid areas (drylands) correlated positively with SPEI, and the correlations increased with time scales. The negatively correlated area was concentrated in humid areas or areas covered by forests and shrubs. Vegetation water surplus in humid areas weakens with warming, and vegetation water constraints in drylands enhance. Moreover, potential evapotranspiration had an overall negative effect on vegetation, and precipitation was a controlling factor for vegetation development in semi-arid areas. A total of 53% of the total area in NEC showed a trend of improvement, which is mainly attributed to human activities (93%), especially through the implementation of ecological restoration projects in NEC. The relative role of human activities and climate change in vegetation degradation areas were 56% and 44%, respectively. Our findings highlight that the government should more explicitly consider the spatiotemporal heterogeneity of the influence of human activities and water availability on vegetation under changing climate and improve the resilience of regional water resources. The relative proportions and roles map of climate change and human activities in vegetation change areas provide a basis for government to formulate local-based management policies.
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•Reviews on tectonics and paleoenvironment of the Junggar Basin are present.•An updated chronostratigraphic framework is provided for Permian Junggar Basin.•Dry climate and ...evaporative environment prevailed during deposition of source rocks.•Stable salinity stratification and limited dilution promoted organic matter burial.•Organic carbon burial in lakes may act as a significant carbon sink in Permian.
The Junggar Basin in northwestern China is a typical “walled” sedimentary basin that has a prolonged basin evolution history from the Paleozoic to Cenozoic and is rich in petroleum resources. Permian strata reach thousands of meters in thickness and have been intensively studied for their record of regional tectonics and paleoenvironments. They have also been exploited for oil and gas for the past several decades. In this paper, we summarize perspectives on tectonic evolution, stratigraphic framework, sedimentary environments, and the paleoclimate of the Permian Junggar Basin and the adjacent Turpan Basin. Further, we discuss the mechanism and consequences of massive organic carbon burial. Based on a compilation of published radiometric dating datasets, an updated chronostratigraphic framework was proposed for the Junggar Basin. This framework constrains the ages of two main petroleum source intervals, the Fengcheng Formation and the Lucaogou and equivalent formations, as Early Permian and Middle Permian, respectively. Although a global warming and drying trend was present throughout the Permian Period, the regional climate over the Junggar Basin was drier in Early Permian and more humid in the Middle-Late Permian with short-term climatic fluctuations. During the deposition of the two petroleum source intervals, dry and temperate climate prevailed and evaporative lacustrine environment dominated. Organic matter enrichment was promoted by stable salinity stratification and limited dilution by inorganic sediments, leading to one of the richest lacustrine source rock deposits in the world. The average organic carbon burial rates were 11.2 gC/m2/yr for the Lucaogou Formation and 3.5 gC/m2/yr for the Fengcheng Formation, lower than the average organic carbon burial rate in modern lakes. However, considering the large areas of global tectonic lakes in the Permian Period, organic carbon burial in lacustrine environments could act as a significant carbon sink and impact the global carbon cycle and climate evolution.
Quantifying the surface energy fluxes of grazed and ungrazed steppes is essential to understand the roles of grasslands in local and global climate and in land use change. We used paired ...eddy-covariance towers to investigate the effects of grazing on energy balance (EB) components: net radiation (Rn), latent heat (LE), sensible heat (H), and soil heat (G) fluxes adjacent grazed and ungrazed areas in a desert steppe of the Mongolian Plateau for a two-year period (2010-2012). Near 95% of Rn was partitioned as LE and H, whereas the contributions of G and other components of the EB were 5% at an annual scale. H dominated the energy partitioning and shared ~50% of Rn. When comparing the grazed and the ungrazed desert steppe, there was remarkably lower Rn and a lower H, but higher G at the grazed site than at the ungrazed site. Both reduced available energy (Rn - G) and H indicated a "cooling effect" feedback onto the local climate through grazing. Grazing reduced the dry year LE but enhanced the wet year LE. Energy partitioning of LE/Rn was positively correlated with the canopy conductivity, leaf area index, and soil moisture. H/Rn was positively correlated with the vapor pressure deficit but negatively correlated with the soil moisture. Boosted regression tree results showed that LE/Rn was dominated by soil moisture in both years and at both sites, while grazing shifted the H/Rn domination from temperature to soil moisture in the wet year. Grazing not only caused an LE shift between the dry and the wet year, but also triggered a decrease in the H/Rn because of changes in vegetation and soil properties, indicating that the ungrazed area had a greater resistance while the grazed area had a greater sensitivity of EB components to the changing climate.
Gross primary productivity (GPP) quantifies the photosynthetic uptake of carbon by ecosystems and is an important component of the terrestrial carbon cycle. Empirical light use efficiency (LUE) ...models and process-based Farquhar, von Caemmerer, and Berry (FvCB) photosynthetic models are widely used for GPP estimation. In this paper, the MODIS GPP algorithm using the LUE approach and the Boreal Ecosystem Productivity Simulator (BEPS) based on the FvCB model in which a sunlit and shaded leaf separation scheme is evaluated against GPP values derived from eddy-covariance (EC) measurements in a variety of ecosystems. Although the total GPP values simulated using these two models agree within 89% when they are averaged for the conterminous U.S., there are systematic differences between them in terms of their spatial and temporal distribution patterns. The spatial distribution of MODIS GPP therefore differs substantially from that produced by BEPS. These differences may be due to an inherent problem of the LUE modeling approach. When a constant maximum LUE value is used for a biome type, this simplification cannot properly handle the contribution of shaded leaves to the total canopy-level GPP. When GPP is modeled by BEPS as the sum of sunlit and shaded leaf GPP, the problem is minimized, i.e., at the low end, the relative contribution of shaded leaves to GPP is small and at the high end, the relative contribution of shaded leaves is large. Compared with monthly and annual GPP derived from eddy covariance data at 40 tower sites in North America, BEPS performed better than the MODIS GPP algorithm. The difference between MODIS and BEPS GPP widens as with the fraction of shaded leaves increases. The simpler LUE modeling approach should therefore be further improved to reduce this bias issue for effective estimation of regional and temporal GPP distributions.
► MODIS and BEPS agree within 89% for the total annual GPP of continental US. ► MODIS overestimates GPP at the low end and underestimates GPP at the high end. ► Biases of MODIS GPP are positively correlated with the shaded leaf contribution. ► These biases caused considerable distortions in spatio-temporal patterns of GPP.
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