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Terrestrial Carbon Cycle

A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Biogeosciences Remote Sensing".

Deadline for manuscript submissions: closed (30 June 2019) | Viewed by 54593

Special Issue Editors


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Guest Editor
School of Geography & Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada
Interests: earth observation; global change ecology; phenology; remote sensing of vegetation; terrestrial carbon cycle

E-Mail Website
Guest Editor
Department of Geography and Planning, University of Toronto, 100 St. George Street, Toronto, ON M5S 3G3, Canada
Interests: remote sensing of vegetation; terrestrial carbon cycle; photosynthetic trait mapping; phenology; ecophysiology

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Guest Editor
Department of Biogeochemical Integration, Max Planck Institute for Biogeochemistry, Hans Knoll Strasse 10, Jena, Germany
Interests: carbon cycle; phenology; sun-induced fluorescence; carbon-water interactions

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Guest Editor
Directorate D – Sustainable Resources - Bio-Economy Unit, Joint Research Centre, European Commission, Via Enrico Fermi 2749, TP261, 26A/010B, I-21027 Ispra (VA), Italy
Interests: carbon cycle; terrestrial ecosystem response to climate; sun-induced fluorescence; land use dynamics in Earth system; phenology

Special Issue Information

Dear Colleagues,

The terrestrial carbon cycle is controlled not only by photosynthesis, but also by respiration, carbon allocation, disturbance and rates of carbon turnover. However, these processes remain difficult to measure and challenging to model. As terrestrial ecosystem carbon cycle models become increasingly sophisticated, the level of uncertainty has also increased, as more mechanisms have been incorporated into the models. Therefore, spatially explicit quantification of terrestrial carbon budget remains uncertain. In this issue, we welcome contributions that make use of legacy or modern remote sensing observations to improve the characterisation of terrestrial carbon cycle processes. We particularly welcome novel remote sensing techniques and applications, such as chlorophyll fluorescence, CO2 flux observations, and photosynthetic trait mapping and their integration into mechanistic models to better understand carbon cycle processes. Model-data integration and observational studies at leaf, plant, field, regional and global scales are also welcome. Potential topics for research and review articles that make use of remote sensing observations include, but are not limited to:

  • Photosynthesis
  • Respiration
  • Net ecosystem productivity
  • Photosynthetic trait mapping
  • Satellite observation of terrestrial CO2 flux
  • Terrestrial ecosystem carbon cycle
  • Terrestrial ecosystem and climate change
  • Dynamic global vegetation models
  • Leaf-to-canopy photosynthesis scaling
  • Model-data integration for carbon cycle modelling
  • Terrestrial remote sensing in Earth system models
  • Application of chlorophyll fluorescence for photosynthesis mapping
Dr. Alemu Gonsamo
Dr. Holly Croft
Dr. Mirco Migliavacca
Dr. Gregory Duveiller
Guest Editors

Manuscript Submission Information

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Keywords

  • Carbon cycle
  • Photosynthesis
  • Respiration
  • Net ecosystem productivity
  • Leaf-to-canopy photosynthesis scaling
  • Model-data integration for carbon cycle modelling
  • Terrestrial CO2 flux observation

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Published Papers (7 papers)

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Research

12 pages, 2441 KiB  
Article
Does Earlier and Increased Spring Plant Growth Lead to Reduced Summer Soil Moisture and Plant Growth on Landscapes Typical of Tundra-Taiga Interface?
by Alemu Gonsamo, Michael T. Ter-Mikaelian, Jing M. Chen and Jiaxin Chen
Remote Sens. 2019, 11(17), 1989; https://doi.org/10.3390/rs11171989 - 23 Aug 2019
Cited by 19 | Viewed by 5245
Abstract
Over the past four decades, satellite observations have shown intensified global greening. At the same time, widespread browning and reversal of or stalled greening have been reported at high latitudes. One of the main reasons for this browning/lack of greening is thought to [...] Read more.
Over the past four decades, satellite observations have shown intensified global greening. At the same time, widespread browning and reversal of or stalled greening have been reported at high latitudes. One of the main reasons for this browning/lack of greening is thought to be warming-induced water stress, i.e., soil moisture depletion caused by earlier spring growth and increased summer evapotranspiration. To investigate these phenomena, we use MODIS collection 6, Global Inventory Modeling and Mapping Studies third-generation (GIMMS) normalized difference vegetation index (NDVI3g), and Global Land Evaporation Amsterdam Model (GLEAM) satellite-based root-zone soil moisture data. The study area was the Far North of Ontario (FNO), 453,788 km2 of heterogeneous landscape typical of the tundra-taiga interface, consisting of unmanaged boreal forests growing on mineral and peat soils, wetlands, and the most southerly area of tundra. The results indicate that the increased plant growth in spring leads to decreased summer growth. Lower summer soil moisture is related to increased spring plant growth in areas with lower soil moisture content. We also found that earlier start of growing season leads to decreased summer and peak season maximum plant growth. In conclusion, increased spring plant growth and earlier start of growing season deplete summer soil moisture and decrease the overall summer plant growth even in temperature-limited high latitude ecosystems. Our findings contribute to evolving understanding of changes in vegetation dynamics in relation to climate in northern high latitude terrestrial ecosystems. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>A 30 m land cover map and images of landscape characteristics typical of ecosystems of the Far North of Ontario. Land cover data courtesy of Ontario Ministry of Natural Resources and Forestry, Version 1.4 (year 2014), Far North Land Cover; landscape scenes courtesy of Google Maps (<a href="http://www.google.ca/maps" target="_blank">www.google.ca/maps</a>).</p>
Full article ">Figure 2
<p>Comparison between summer-time data-model estimate of root-zone soil moisture (GLEAM) and satellite observed Climate Change Initiative (CCI) soil moisture, both in m<sup>3</sup>/m<sup>3</sup>. The scatter points include all 0.25° × 0.25° Latitude-Longitude grid cells which have June–August datasets between 1982 and 2015.</p>
Full article ">Figure 3
<p>Density plot of linear slopes between the spring and summer greenness from MODIS normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) for 2000–2017 per land cover classes. The dashed vertical lines indicate the mean value of slope while the solid vertical lines indicate the slope value of zero, the latter added for visual clarity. Mean of linear slope is significantly different from zero in all cases (<span class="html-italic">p</span> &lt; 0.05, two-tailed one-sample <span class="html-italic">t</span>-test).</p>
Full article ">Figure 4
<p>Linear slopes of summer greenness with spring greenness (<b>a</b>) and start of growing season (SOS) (<b>b</b>) along soil moisture (m<sup>3</sup>/m<sup>3</sup>) classes. Linear slopes of summer greenness with spring greenness (<b>c</b>) and SOS) (<b>d</b>) along tree cover classes. VI is vegetation index (i.e., NDVI or EVI). Error bars represent one standard deviation of spatial slopes in each soil moisture or tree cover class. The linear slopes are computed at native spatial resolutions of Global Inventory Modeling and Mapping Studies third-generation (GIMMS) and MODIS data. Mean of linear slope is significantly different from zero in all cases (<span class="html-italic">p</span> &lt; 0.05, two-tailed one-sample <span class="html-italic">t</span>-test).</p>
Full article ">Figure 5
<p>Linear slopes of start of season (SOS) with maximum productivity obtained from GIMMS NDVI3g and MODIS NDVI plotted along the long-term mean summer soil moisture (m<sup>3</sup>/m<sup>3</sup>) (<b>a</b>) and tree cover (<b>b</b>) classes. VI is vegetation index (i.e., NDVI or EVI). Error bars represent one standard deviation of spatial slopes in each soil moisture or tree cover class. The linear slopes are computed at native spatial resolutions of GIMMS and MODIS data. Maximum productivity values are obtained from peak of season day (i.e., POSvalue) using the curve-fitting algorithm and directly from maximum annual value of the raw data (i.e., max). Mean of linear slope is significantly different from zero in all cases (<span class="html-italic">p</span> &lt; 0.05, two-tailed one-sample <span class="html-italic">t</span>-test) except indicated as NS (not significant) otherwise.</p>
Full article ">Figure 6
<p>Spatial distributions of the summer soil moisture correlation against the spring GIMMS (<b>a</b>), summer GIMMS (<b>b</b>), spring MODIS (<b>c</b>) and summer MODIS (<b>d</b>) NDVI. All coloured pixels for (<b>a</b>–<b>d</b>) are significantly correlated at 90% confidence level (one-tailed). The relationships of spatial trends were computed after projecting all datasets into 0.25° × 0.25° Latitude-Longitude grids. Long-term (1980−2016) mean summer soil moisture is shown in (<b>e</b>).</p>
Full article ">
29 pages, 9733 KiB  
Article
Quantifying the Impacts of Land-Use and Climate on Carbon Fluxes Using Satellite Data across Texas, U.S.
by Ram L. Ray, Ademola Ibironke, Raghava Kommalapati and Ali Fares
Remote Sens. 2019, 11(14), 1733; https://doi.org/10.3390/rs11141733 - 23 Jul 2019
Cited by 8 | Viewed by 6206
Abstract
Climate change and variability, soil types and soil characteristics, animal and microbial communities, and photosynthetic plants are the major components of the ecosystem that affect carbon sequestration potential of any location. This study used NASA’s Soil Moisture Active Passive (SMAP) Level 4 carbon [...] Read more.
Climate change and variability, soil types and soil characteristics, animal and microbial communities, and photosynthetic plants are the major components of the ecosystem that affect carbon sequestration potential of any location. This study used NASA’s Soil Moisture Active Passive (SMAP) Level 4 carbon products, gross primary productivity (GPP), and net ecosystem exchange (NEE) to quantify their spatial and temporal variabilities for selected terrestrial ecosystems across Texas during the 2015–2018 study period. These SMAP carbon products are available at 9 km spatial resolution on a daily basis. The ten selected SMAP grids are located in seven climate zones and dominated by five major land uses (developed, crop, forest, pasture, and shrub). Results showed CO2 emissions and uptake were affected by land-use and climatic conditions across Texas. It was also observed that climatic conditions had more impact on CO2 emissions and uptake than land-use in this state. On average, South Central Plains and East Central Texas Plains ecoregions of East Texas and Western Gulf Coastal Plain ecoregion of Upper Coast climate zones showed higher GPP flux and potential carbon emissions and uptake than other climate zones across the state, whereas shrubland on the Trans Pecos climate zone showed lower GPP flux and carbon emissions/uptake. Comparison of GPP and NEE distribution maps between 2015 and 2018 confirmed substantial changes in carbon emissions and uptake across Texas. These results suggest that SMAP carbon products can be used to study the terrestrial carbon cycle at regional to global scales. Overall, this study helps to understand the impacts of climate, land-use, and ecosystem dynamics on the terrestrial carbon cycle. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
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Figure 1

Figure 1
<p>(<b>a</b>) Climate zones and their corresponding land-use, and (<b>b</b>) ecological regions and climate zones in the study area. White crosses are the locations of ten select Soil Moisture Active Passive (SMAP) grids. The inset figure (9 km × 9 km, not in scale) is a selected SMAP grid, covering Prairie View A&amp;M University (PVAMU) research farm, to evaluate SMAP Gross Primary Productivity (GPP) and Net Ecosystem Exchange (NEE).</p>
Full article ">Figure 2
<p>Cumulative distribution of precipitation (<b>a</b>), and average annual distribution of temperature across Texas (<b>b</b>) using precipitation and temperature data from 2001–2018. The cumulative precipitation plot for each climate zone was developed using National Climatic Data Center (NCDC) daily precipitation data, whereas the annual average temperature (calculated using NCDC daily temperature data) was used to develop the spatial average temperature map across Texas. The x-axis of the cumulative distribution of the precipitation plot includes a fraction of time, which means 0.5 represents 50% of the time (9 years for this plot), and 1.0 represents 100% of the time (18 years for this plot).</p>
Full article ">Figure 3
<p>Daily gross primary productivity (GPP) and NEE at Upper Coast (UC) and High Plains (HP) for cropland during 2015 and 2016 to select the critical month of each year to develop spatial distribution maps. A sequence of time series plots was developed to identify the sensitive months when GPP and NEE values were changing or abruptly changed during the study period. Based on the changes in seasonal distributions of GPP and NEE, six different months (Apr, May, Jun, Aug, Sep, and Dec) of 2015 and 2018 were used to develop spatial distribution maps of GPP and NEE.</p>
Full article ">Figure 4
<p>Daily and monthly SMAP and in-situ GPP and NEE at PVAMU Research Farm (2016–2018). The unit of RMSE is the same as of GPP and NEE.</p>
Full article ">Figure 5
<p>Distributions of monthly GPP in each climate zone (2015–2018).</p>
Full article ">Figure 6
<p>Distributions of monthly NEE in each climate zone (2015–2018).</p>
Full article ">Figure 7
<p>Spatial distribution of GPP across Texas for selected months of 2015 and 2018. White crosses are the location of selected SMAP grids.</p>
Full article ">Figure 8
<p>Spatial distribution of NEE across Texas for selected months of 2015 and 2018. White crosses are the location of selected SMAP grids.</p>
Full article ">Figure 9
<p>Annual distribution of GPP (<b>a</b>–<b>c</b>) and NEE (<b>d</b>–<b>f</b>) across Texas. White crosses are the location of selected SMAP grids.</p>
Full article ">Figure 10
<p>Temporal distributions of monthly precipitation, GPP (<b>a</b>–<b>e</b>) and NEE (<b>f</b>–<b>j</b>) of five major land-use categories: Cropland (crop), forestland (forest), pastureland (pasture), shrubland (shrub) and developed land (developed) at each selected climate zone in Texas. Note: HP = High Plains; LRP = Low Rolling Plains; ET = East Texas; EP = Edwards Plateau; SC = South Central; TP = Trans Pecos; UC = Upper Coast; NC = North Central; S = Southern; LV = Lower Valley.</p>
Full article ">Figure 11
<p>Box and Whisker plots of monthly (<b>a</b>) GPP distributions and (<b>b</b>) NEE distributions for five major land uses at each selected climate zone in the state of Texas. Note: C = cropland, F = forestland, P = pastureland, S = shrubland, and D = developed land. The white circles represent the mean, the solid horizontal lines represent the median, and gray circles represent outliers. Upper horizontal line = maximum, lower horizontal line= minimum, top of the box = upper quartile, bottom of the box = lower quartile, upper quartile to maximum = upper whisker, and the lower quartile to minimum = lower whisker.</p>
Full article ">Figure 11 Cont.
<p>Box and Whisker plots of monthly (<b>a</b>) GPP distributions and (<b>b</b>) NEE distributions for five major land uses at each selected climate zone in the state of Texas. Note: C = cropland, F = forestland, P = pastureland, S = shrubland, and D = developed land. The white circles represent the mean, the solid horizontal lines represent the median, and gray circles represent outliers. Upper horizontal line = maximum, lower horizontal line= minimum, top of the box = upper quartile, bottom of the box = lower quartile, upper quartile to maximum = upper whisker, and the lower quartile to minimum = lower whisker.</p>
Full article ">
27 pages, 4841 KiB  
Article
Effects of the Temporal Aggregation and Meteorological Conditions on the Parameter Robustness of OCO-2 SIF-Based and LUE-Based GPP Models for Croplands
by Xiaofeng Lin, Baozhang Chen, Huifang Zhang, Fei Wang, Jing Chen, Lifeng Guo and Yawen Kong
Remote Sens. 2019, 11(11), 1328; https://doi.org/10.3390/rs11111328 - 3 Jun 2019
Cited by 7 | Viewed by 3699
Abstract
Global retrieval of solar-induced chlorophyll fluorescence (SIF) using remote sensing by means of satellites has been developed rapidly in recent years. Exploring how SIF could improve the characterization of photosynthesis and its role in the land surface carbon cycle has gradually become a [...] Read more.
Global retrieval of solar-induced chlorophyll fluorescence (SIF) using remote sensing by means of satellites has been developed rapidly in recent years. Exploring how SIF could improve the characterization of photosynthesis and its role in the land surface carbon cycle has gradually become a very important and active area. However, compared with other gross primary production (GPP) models, the robustness of the parameterization of the SIF model under different circumstances has rarely been investigated. In this study, we examined and compared the effects of temporal aggregation and meteorological conditions on the stability of model parameters for the SIF model ( ε / S I F yield ), the one-leaf light-use efficiency (SL-LUE) model ( ε max ), and the two-leaf LUE (TL-LUE) model ( ε msu and ε msh ). The three models were parameterized based on a maize–wheat rotation eddy-covariance flux tower data in Yucheng, Shandong Province, China by using the Metropolis–Hasting algorithm. The results showed that the values of the ε / S I F yield and ε max were similarly robust and considerably more stable than ε msu and ε msh for all temporal aggregation levels. Under different meteorological conditions, all the parameters showed a certain degree of fluctuation and were most affected at the mid-day scale, followed by the monthly scale and finally at the daily scale. Nonetheless, the averaged coefficient of variation ( C V ) of ε / S I F yield was relatively small (15.0%) and was obviously lower than ε max ( C V = 27.0%), ε msu ( C V = 43.2%), and ε msh ( C V = 53.1%). Furthermore, the SIF model’s performance for estimating GPP was better than that of the SL-LUE model and was comparable to that of the TL-LUE model. This study indicates that, compared with the LUE-based models, the SIF-based model without climate-dependence is a good predictor of GPP and its parameter is more likely to converge for different temporal aggregation levels and under varying environmental restrictions in croplands. We suggest that more flux tower data should be used for further validation of parameter convergence in other vegetation types. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
Show Figures

Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Effects of varying view zenith angle (VZA) on OCO-2 Glint SIF observations and the gross primary production (GPP)–SIF correlation at the mid-day timescale: (<b>a</b>) the differences between SIF with and without VZA restrictions: VZA ≤ <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>20</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>; <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>20</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math> &lt; VZA ≤ <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>40</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>; and VZA &gt; <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>40</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>; (<b>b</b>) the relationships of ground GPP and SIF under various intervals of VZAs: VZA ≤ <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>20</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>; <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>20</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math> &lt; VZA ≤ <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>40</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>; VZA &gt; <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>40</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>; and no restriction (<math display="inline"><semantics> <mrow> <msup> <mn>0</mn> <mo>∘</mo> </msup> </mrow> </semantics></math> &lt; VZA ≤ <math display="inline"><semantics> <mrow> <msup> <mrow> <mn>50</mn> </mrow> <mo>∘</mo> </msup> </mrow> </semantics></math>). The data points are averages of the sounding for each point in the time referring to <a href="#remotesensing-11-01328-t001" class="html-table">Table 1</a> and the error bar represents ±1<math display="inline"><semantics> <mi mathvariant="sans-serif">σ</mi> </semantics></math> statistical uncertainty estimations of OCO-2 SIF.</p>
Full article ">Figure 2
<p>The consistency of OCO-2 SIF in Glint and Nadir observation modes with the tower derived SIF: (<b>a</b>) Seasonal dynamics of tower gross primary production (GPP), normalized difference vegetation index (NDVI<sub>BRDF</sub>), Vogelmann red edge index (VOG<sub>BRDF</sub>), tower sun-induced chlorophyll inflorescence (SIF), and OCO-2 SIF; (<b>b</b>) the relationship between OCO-2 SIF in Glint and Nadir modes and the tower SIF at the CN-YuC site over the 2015 growth period of summer maize. There are only six data points on Glint mode and seven data points on Nadir mode for both satellite overpassing and tower observing at the same time. The missing observations from 17 August 2015 to 1 September 2015 were due to an equipment failure.</p>
Full article ">Figure 3
<p>Fitting results of parameters for different models on different timescales in 2015. The columns from left to right show the mid-day, daily, and monthly timescales, respectively. The rows from top to bottom indicate the SIF model (<math display="inline"><semantics> <mrow> <mi>G</mi> <mi>P</mi> <mi>P</mi> <mo>≈</mo> <mrow> <mi>ε</mi> <mo>/</mo> <mrow> <mi>S</mi> <mi>I</mi> <msub> <mi>F</mi> <mrow> <mi>yield</mi> </mrow> </msub> </mrow> </mrow> <mo>×</mo> <mi>S</mi> <mi>I</mi> <mi>F</mi> </mrow> </semantics></math>), the one-leaf light-use efficiency (SL-LUE) model (<math display="inline"><semantics> <mrow> <mi>G</mi> <mi>P</mi> <mi>P</mi> <mo>=</mo> <msub> <mi>ε</mi> <mrow> <mi>max</mi> </mrow> </msub> <mo>×</mo> <mi>A</mi> <mi>P</mi> <mi>A</mi> <mi>R</mi> <mo>×</mo> <mi>f</mi> <mrow> <mo>(</mo> <mrow> <mi>V</mi> <mi>P</mi> <mi>D</mi> </mrow> <mo>)</mo> </mrow> <mo>×</mo> <mi>g</mi> <mrow> <mo>(</mo> <mrow> <msub> <mi>T</mi> <mi>a</mi> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </semantics></math>), and the two-leaf LUE (TL-LUE) model (<math display="inline"><semantics> <mrow> <mi>G</mi> <mi>P</mi> <mi>P</mi> <mo>=</mo> <msub> <mi>ε</mi> <mrow> <mi>msu</mi> </mrow> </msub> <mo>×</mo> <mi>A</mi> <mi>P</mi> <mi>A</mi> <msub> <mi>R</mi> <mrow> <mi>su</mi> </mrow> </msub> <mo>×</mo> <mi>f</mi> <mrow> <mo>(</mo> <mrow> <mi>V</mi> <mi>P</mi> <mi>D</mi> </mrow> <mo>)</mo> </mrow> <mo>×</mo> <mi>g</mi> <mrow> <mo>(</mo> <mrow> <msub> <mi>T</mi> <mi mathvariant="normal">a</mi> </msub> </mrow> <mo>)</mo> </mrow> <mo>+</mo> <msub> <mi>ε</mi> <mrow> <mi>msh</mi> </mrow> </msub> <mo>×</mo> <mi>A</mi> <mi>P</mi> <mi>A</mi> <msub> <mi>R</mi> <mrow> <mi>sh</mi> </mrow> </msub> <mo>×</mo> <mi>f</mi> <mrow> <mo>(</mo> <mrow> <mi>V</mi> <mi>P</mi> <mi>D</mi> </mrow> <mo>)</mo> </mrow> <mo>×</mo> <mi>g</mi> <mrow> <mo>(</mo> <mrow> <msub> <mi>T</mi> <mi mathvariant="normal">a</mi> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </semantics></math>), respectively. There are 39 data points for mid-day and daily time scales and 12 data points for monthly time scale. The unit of tower GPP is gC m<sup>−2</sup> day<sup>−1</sup>, the unit of OCO-2 SIF is W m<sup>−2</sup> <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m<sup>−1</sup> sr<sup>−1</sup>, and the unit of APAR × f(VPD) × g(T) is MJ m<sup>−2</sup> day<sup>−1</sup> (1 MJ = 10<sup>6</sup> J).</p>
Full article ">Figure 4
<p>Seasonal dynamics of normalized meteorological conditions (scaled within 0–1), including normalized meteorological condition index (ECI), air temperature, VPD, and APAR at the (<b>a</b>) mid-day timescale, (<b>b</b>) daily timescale, and (<b>c</b>) monthly timescale. The solid grey lines refer to the threshold value of ECI values. The whole canopies are prone to experiencing exposure to excess light when the ECI is greater than 0.8 compared with when the ECI is lower.</p>
Full article ">Figure 5
<p>The fluctuations in the parameters of the SIF model and the LUE models for a range of meteorological conditions, where 0.8 is the threshold value at which ECI values are fitted separately. The columns from left to right show the mid-day, daily, and monthly timescales, respectively. The rows from top to bottom indicate the SIF model (<math display="inline"><semantics> <mrow> <mi>G</mi> <mi>P</mi> <mi>P</mi> <mo>≈</mo> <mrow> <mi>ε</mi> <mo>/</mo> <mrow> <mi>S</mi> <mi>I</mi> <msub> <mi>F</mi> <mrow> <mi>yield</mi> </mrow> </msub> </mrow> </mrow> <mo>×</mo> <mi>S</mi> <mi>I</mi> <mi>F</mi> </mrow> </semantics></math>), SL-LUE model (<math display="inline"><semantics> <mrow> <mi>G</mi> <mi>P</mi> <mi>P</mi> <mo>=</mo> <msub> <mi>ε</mi> <mrow> <mi>max</mi> </mrow> </msub> <mo>×</mo> <mi>A</mi> <mi>P</mi> <mi>A</mi> <mi>R</mi> <mo>×</mo> <mi>f</mi> <mrow> <mo>(</mo> <mrow> <mi>V</mi> <mi>P</mi> <mi>D</mi> </mrow> <mo>)</mo> </mrow> <mo>×</mo> <mi>g</mi> <mrow> <mo>(</mo> <mrow> <msub> <mi>T</mi> <mi mathvariant="normal">a</mi> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </semantics></math>), and TL-LUE model (<math display="inline"><semantics> <mrow> <mi>G</mi> <mi>P</mi> <mi>P</mi> <mo>=</mo> <msub> <mi>ε</mi> <mrow> <mi>msu</mi> </mrow> </msub> <mo>×</mo> <mi>A</mi> <mi>P</mi> <mi>A</mi> <msub> <mi>R</mi> <mrow> <mi>su</mi> </mrow> </msub> <mo>×</mo> <mi>f</mi> <mrow> <mo>(</mo> <mrow> <mi>V</mi> <mi>P</mi> <mi>D</mi> </mrow> <mo>)</mo> </mrow> <mo>×</mo> <mi>g</mi> <mrow> <mo>(</mo> <mrow> <msub> <mi>T</mi> <mi mathvariant="normal">a</mi> </msub> </mrow> <mo>)</mo> </mrow> <mo>+</mo> <msub> <mi>ε</mi> <mrow> <mi>msh</mi> </mrow> </msub> <mo>×</mo> <mi>A</mi> <mi>P</mi> <mi>A</mi> <msub> <mi>R</mi> <mrow> <mi>sh</mi> </mrow> </msub> <mo>×</mo> <mi>f</mi> <mrow> <mo>(</mo> <mrow> <mi>V</mi> <mi>P</mi> <mi>D</mi> </mrow> <mo>)</mo> </mrow> <mo>×</mo> <mi>g</mi> <mrow> <mo>(</mo> <mrow> <msub> <mi>T</mi> <mi mathvariant="normal">a</mi> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </semantics></math>), respectively. The unit of tower GPP is gC m<sup>−2</sup> day<sup>−1</sup>, the unit of OCO-2 SIF is W m<sup>−2</sup> <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m<sup>−1</sup> sr<sup>−1</sup>, and the unit of APAR × f(VPD) × g(T) is MJ m<sup>−2</sup> day<sup>−1</sup> (1 MJ = 10<sup>6</sup> J).</p>
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<p>Validation of the SIF model, the SL-LUE model, and the TL-LUE model on the mid-day, daily, and monthly timescales in 2016, respectively. The panels (<b>a</b>–<b>c</b>): the seasonal variations of simulated and tower observed GPP. The panels (<b>d</b>–<b>f</b>): comparison of the GPP simulated by the SIF model (red lines), SL-LUE (green lines), and TL-LUE (blue lines).</p>
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<p>Relationship between SIF yield (SIF/APAR, <math display="inline"><semantics> <mrow> <mi>S</mi> <mi>I</mi> <msub> <mi>F</mi> <mrow> <mi>y</mi> <mi>i</mi> <mi>e</mi> <mi>l</mi> <mi>d</mi> </mrow> </msub> </mrow> </semantics></math>) and light-use efficiency (<math display="inline"><semantics> <mi>ε</mi> </semantics></math>) at the satellite overpass time throughout 2015 at the CN-YuC cropland site in northern China, where 0.8 is the threshold value at which ECI values are fit separately for the mid-day, daily, and monthly timescales, respectively. The whole canopies are prone to experiencing exposure to excess light when the ECI is greater than 0.8 compared with when the ECI is lower.</p>
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<p>Scatterplots of the solar-induced chlorophyll fluorescence (W m<sup>−2</sup> <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m<sup>−1</sup> sr<sup>−1</sup>) and sunlit/shaded/total leaf area index (LAI<sub>su</sub>, LAI<sub>sh</sub>, LAI, m<sup>2</sup>/m<sup>2</sup>) on the mid-day, daily, and monthly timescales, respectively.</p>
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25 pages, 5529 KiB  
Article
Evaluating the Effectiveness of Using Vegetation Indices Based on Red-Edge Reflectance from Sentinel-2 to Estimate Gross Primary Productivity
by Shangrong Lin, Jing Li, Qinhuo Liu, Longhui Li, Jing Zhao and Wentao Yu
Remote Sens. 2019, 11(11), 1303; https://doi.org/10.3390/rs11111303 - 31 May 2019
Cited by 66 | Viewed by 8633
Abstract
Gross primary productivity (GPP) is the most important component of terrestrial carbon flux. Red-edge (680–780 nm) reflectance is sensitive to leaf chlorophyll content, which is directly correlated with photosynthesis as the pigment pool, and it has the potential to improve GPP estimation. The [...] Read more.
Gross primary productivity (GPP) is the most important component of terrestrial carbon flux. Red-edge (680–780 nm) reflectance is sensitive to leaf chlorophyll content, which is directly correlated with photosynthesis as the pigment pool, and it has the potential to improve GPP estimation. The European Space Agency (ESA) Sentinel-2A and B satellites provide red-edge bands at 20-m spatial resolution on a five-day revisit period, which can be used for global estimation of GPP. Previous studies focused mostly on improving cropland GPP estimation using red-edge bands. In this study, we firstly evaluated the relationship between eight vegetation indices (VIs) retrieved from Sentinel-2 imagery in association with incident photosynthetic active radiation (PARin) and carbon flux tower GPP (GPPEC) across three forest and two grassland sites in Australia. We derived a time series of five red-edge VIs and three non-red-edge VIs over the CO2 flux tower footprints at 16-day time intervals and compared both temporal and spatial variations. The results showed that the relationship between the red-edge index (CIr, ρ 783 ρ 705 1 ) multiplied by PARin and GPPEC had the highest correlation (R2 = 0.77, root-mean-square error (RMSE) = 0.81 gC∙m−2∙day−1) at the two grassland sites. The CIr also showed consistency (rRMSE defined as RMSE/mean GPP, lower than 0.25) across forest and grassland sites. The high spatial resolution of the Sentinel-2 data provided more detailed information to adequately characterize the GPP variance at spatially heterogeneous areas. The high revisit period of Sentinel-2 exhibited temporal variance in GPP at the grassland sites; however, at forest sites, the flux-tower-based GPP variance could not be fully tracked by the limited satellite images. These results suggest that the high-spatial-resolution red-edge index from Sentinel-2 can improve large-scale spatio-temporal GPP assessments. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
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Figure 1

Figure 1
<p>False-color composite images of the study area at each site (band 8 for the red channel, band 4 for the green channel, and band 3 for the blue channel) in January 2018. The region in each subfigure is a 3 × 3-km footprint range. The yellow box in the middle of the image is the carbon flux footprint region. CUM—Cumberland Plain; WOM—Wombat Forest; TUM—Tumbarumba; RIG—Riggs Creek; YNC—Yanco.</p>
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<p>Cloud cover fractions of Sentinel-2 images of the coordinate tile at each site. The <span class="html-italic">y</span>-axis is the month tag in (YYYYMM; Y = year, M = month) since the launch of Sentinel-2A, and the <span class="html-italic">x</span>-axis is the day of that month. The color bar represents the percentage of clouds in the image.</p>
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<p>The number of effective Sentinel-2 images obtained during the 16-day interval. An effective image here is defined as an image with less than 10% cloud cover. The legend on the top left represents the abbreviation for each site. The bottom <span class="html-italic">x</span>-axis is the date, and the top <span class="html-italic">x</span>-axis is the season in Australia. The pink dashed line is the launch date of Sentinel-2B.</p>
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<p>Time series of gross primary productivity (GPP) estimated by vegetation indices (VIs) and GPP<sub>EC</sub> for 16-day intervals at five sites (<b>a</b>–<b>e</b>). The solid circles are the VIs derived from satellite-based reflectance, whereas the hollow circles are the VIs derived from GPP<sub>VI</sub>, which were gap-filled using the Savitzky–Golay filter. The unit of GPP is gC∙m<sup>−2</sup>∙day<sup>−1</sup>.</p>
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<p>The relationship between daily GPP<sub>CIr</sub> and GPP<sub>EC</sub> under different percentages of cloud cover. The dates with daily clear sky index (CSI) &gt; 0.8 (blue circles) are defined as clear days, whereas CSI &lt; 0.8 (red squares) indicates cloudy days.</p>
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<p>Seasonal change in GPP<sub>EC</sub>, incident photosynthetic active radiation (PARin), and red-edge chlorophyll index (CIr) during the research period (PAR in MJ∙day<sup>−1</sup>, GPP in gC∙m<sup>−2</sup>∙day<sup>−1</sup>).</p>
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<p>Hierarchical normalized vegetation index of CIr and enhanced vegetation index (EVI) near flux towers. All of the VI values in the 3 × 3-km spatial window were normalized from 0 to 1 and then classified by the different levels of standard deviation. The subfigures show the CIr and EVI values from 1 January to 10 January 2018, under conditions of no cloud cover. The spatial resolution is 30 m.</p>
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<p>Spatial differences in GPP in the footprint region of each site. We set the footprint as a 3 × 3-km region. The GPP was mapped by using the rebuilt CIr over a 16-day interval coupled with the PAR coefficient at each site, as shown in <a href="#remotesensing-11-01303-t003" class="html-table">Table 3</a> and <a href="#remotesensing-11-01303-t004" class="html-table">Table 4</a>. The GPP variance is the CV of the GPP in each pixel (standard deviation (STD)/mean value) throughout the research period. The pixels in gray are the regions without rebuilt VIs or non-vegetation components.</p>
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<p>Temporal trends in the coefficient of variance (CV = STD/mean value) of GPP at the five research sites. The solid points in subfigure (<b>a</b>) are the mean GPP values in the footprint regions on different dates, whereas the shaded areas represent one standard deviation around the mean prediction. Each curve in subfigure (<b>b</b>) is the CV of GPP on each date.</p>
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<p>Temporal trends in GPP spatial distribution at the YNC (a1–a8) and CUM (b1–b8) sites. The GPP estimation of each subfigure is based on the CIr–PARin–GPP relationship shown in <a href="#remotesensing-11-01303-t003" class="html-table">Table 3</a> and <a href="#remotesensing-11-01303-t004" class="html-table">Table 4</a>.</p>
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<p>Cross-site comparison of GPP<sub>EC</sub> and GPP modeling results, including MOD17A2H product (presented as Sitename<sub>MOD</sub>), GPP<sub>EVI</sub> (presented as Sitename<sub>EVI</sub>), and GPP<sub>CIr</sub> (presented as Sitename<sub>CIr</sub>). The GPP<sub>EVI</sub> and GPP<sub>CIr</sub> across selected EBF sites were estimated by 1.93 × EVI × PAR + 2.45 and 0.32 × CIr × PAR + 2.56, respectively. The GPP<sub>EVI</sub> and GPP<sub>CIr</sub> across selected grassland sites were estimated by 1.33 × EVI × PAR − 0.5 and 0.31 × CIr × PAR + 0.05, respectively. The units of GPP<sub>EC</sub>, modeled GPP, and RMSE are gC∙m<sup>−2</sup>∙day<sup>−1</sup>. The black line in the middle is the 1:1 line.</p>
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22 pages, 4601 KiB  
Article
Improved Modeling of Gross Primary Productivity of Alpine Grasslands on the Tibetan Plateau Using the Biome-BGC Model
by Yongfa You, Siyuan Wang, Yuanxu Ma, Xiaoyue Wang and Weihua Liu
Remote Sens. 2019, 11(11), 1287; https://doi.org/10.3390/rs11111287 - 30 May 2019
Cited by 35 | Viewed by 5684
Abstract
The ability of process-based biogeochemical models in estimating the gross primary productivity (GPP) of alpine vegetation is largely hampered by the poor representation of phenology and insufficient calibration of model parameters. The development of remote sensing technology and the eddy covariance (EC) technique [...] Read more.
The ability of process-based biogeochemical models in estimating the gross primary productivity (GPP) of alpine vegetation is largely hampered by the poor representation of phenology and insufficient calibration of model parameters. The development of remote sensing technology and the eddy covariance (EC) technique has made it possible to overcome this dilemma. In this study, we have incorporated remotely sensed phenology into the Biome-BGC model and calibrated its parameters to improve the modeling of GPP of alpine grasslands on the Tibetan Plateau (TP). Specifically, we first used the remotely sensed phenology to modify the original meteorological-based phenology module in the Biome-BGC to better prescribe the phenological states within the model. Then, based on the GPP derived from EC measurements, we combined the global sensitivity analysis method and the simulated annealing optimization algorithm to effectively calibrate the ecophysiological parameters of the Biome-BGC model. Finally, we simulated the GPP of alpine grasslands on the TP from 1982 to 2015 based on the Biome-BGC model after a phenology module modification and parameter calibration. The results indicate that the improved Biome-BGC model effectively overcomes the limitations of the original Biome-BGC model and is able to reproduce the seasonal dynamics and magnitude of GPP in alpine grasslands. Meanwhile, the simulated results also reveal that the GPP of alpine grasslands on the TP has increased significantly from 1982 to 2015 and shows a large spatial heterogeneity, with a mean of 289.8 gC/m2/yr or 305.8 TgC/yr. Our study demonstrates that the incorporation of remotely sensed phenology into the Biome-BGC model and the use of EC measurements to calibrate model parameters can effectively overcome the limitations of its application in alpine grassland ecosystems, which is important for detecting trends in vegetation productivity. This approach could also be upscaled to regional and global scales. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
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<p>Spatial distributions of grassland subtypes and elevations on the Tibetan Plateau (TP). (<b>a</b>) Grassland subtypes; (<b>b</b>) Elevation.</p>
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<p>The flow chart of this study. EC, eddy covariance; GPP, gross primary productivity.</p>
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<p>Spatial distribution of phenological indicators of alpine grasslands on the TP. (<b>a</b>) The start of growing season (SOS); (<b>b</b>) The end of growing season (EOS); (<b>c</b>) The length of growing season (LOS).</p>
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<p>Comparisons between GPP derived from EC measurements and GPP simulated by the version of V0, V1, and V2 for the calibration year. (<b>a</b>) Haibei Station (alpine meadow); (<b>b</b>) Damxung Station (alpine steppe).</p>
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<p>Comparisons between GPP derived from EC measurements and GPP simulated by the version of V0, V1, and V2 for the calibration year. (<b>a</b>) Haibei Station (alpine meadow); (<b>b</b>) Damxung Station (alpine steppe).</p>
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<p>Comparisons between NEE measured by EC technique and NEE simulated by the version of V0, V1, and V2 for the calibration year. (<b>a</b>) Haibei Station (alpine meadow); (<b>b</b>) Damxung Station (alpine steppe).</p>
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<p>Comparisons between total ecosystem respiration (TER) derived from EC measurements and TER simulated by the version of V0, V1, and V2 for the calibration year. (<b>a</b>) Haibei Station (alpine meadow); (<b>b</b>) Damxung Station (alpine steppe).</p>
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<p>Spatial distribution of mean and standard deviation of GPP of alpine grasslands on the TP from 1982 to 2015. (<b>a</b>) Mean annual GPP; (<b>b</b>) Standard deviation of GPP.</p>
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<p>Spatial distribution of trends and significance levels of GPP of alpine grasslands on the TP from 1982 to 2015. (<b>a</b>) Trends of GPP; (<b>b</b>) Significance levels of GPP trends.</p>
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<p>Interannual variations of different factors of alpine grasslands on the TP from 1982 to 2015. (<b>a</b>) Mean annual GPP; (<b>b</b>) Mean annual precipitation; (<b>c</b>) Mean annual temperature.</p>
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16 pages, 3900 KiB  
Article
Underestimates of Grassland Gross Primary Production in MODIS Standard Products
by Xiaoyan Zhu, Yanyan Pei, Zhaopei Zheng, Jinwei Dong, Yao Zhang, Junbang Wang, Lajiao Chen, Russell B. Doughty, Geli Zhang and Xiangming Xiao
Remote Sens. 2018, 10(11), 1771; https://doi.org/10.3390/rs10111771 - 8 Nov 2018
Cited by 41 | Viewed by 5174
Abstract
As the biggest carbon flux of terrestrial ecosystems from photosynthesis, gross primary productivity (GPP) is an important indicator in understanding the carbon cycle and biogeochemical process of terrestrial ecosystems. Despite advances in remote sensing-based GPP modeling, spatial and temporal variations of GPP are [...] Read more.
As the biggest carbon flux of terrestrial ecosystems from photosynthesis, gross primary productivity (GPP) is an important indicator in understanding the carbon cycle and biogeochemical process of terrestrial ecosystems. Despite advances in remote sensing-based GPP modeling, spatial and temporal variations of GPP are still uncertain especially under extreme climate conditions such as droughts. As the only official products of global spatially explicit GPP, MOD17A2H (GPPMOD) has been widely used to assess the variations of carbon uptake of terrestrial ecosystems. However, systematic assessment of its performance has rarely been conducted especially for the grassland ecosystems where inter-annual variability is high. Based on a collection of GPP datasets (GPPEC) from a global network of eddy covariance towers (FluxNet), we compared GPPMOD and GPPEC at all FluxNet grassland sites with more than five years of observations. We evaluated the performance and robustness of GPPMOD in different grassland biomes (tropical, temperate, and alpine) by using a bootstrapping method for calculating 95% confident intervals (CI) for the linear regression slope, coefficients of determination (R2), and root mean square errors (RMSE). We found that GPPMOD generally underestimated GPP by about 34% across all biomes despite a significant relationship (R2 = 0.66 (CI, 0.63–0.69), RMSE = 2.46 (2.33–2.58) g Cm−2 day−1) for the three grassland biomes. GPPMOD had varied performances with R2 values of 0.72 (0.68–0.75) (temperate), 0.64 (0.59–0.68) (alpine), and 0.40 (0.27–0.52) (tropical). Thus, GPPMOD performed better in low GPP situations (e.g., temperate grassland type), which further indicated that GPPMOD underestimated GPP. The underestimation of GPP could be partly attributed to the biased maximum light use efficiency (εmax) values of different grassland biomes. The uncertainty of the fraction of absorbed photosynthetically active radiation (FPAR) and the water scalar based on the vapor pressure deficit (VPD) could have other reasons for the underestimation. Therefore, more accurate estimates of GPP for different grassland biomes should consider improvements in εmax, FPAR, and the VPD scalar. Our results suggest that the community should be cautious when using MODIS GPP products to examine spatial and temporal variations of carbon fluxes. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
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<p>Distribution of 15 FLUXNET sites selected with more than five years of observations.</p>
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<p>Seasonal variations of GPP<sub>MOD</sub> and observed GPP<sub>EC</sub> at all study sites.</p>
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<p>The relationship between GPP<sub>MOD</sub> and GPP<sub>EC</sub> for all sites. The short-dashed line is a 1:1 line. The unit of RMSE was g C m<sup>−2</sup> day<sup>−1</sup>.</p>
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<p>The relationship between GPP<sub>MOD</sub> and GPP<sub>EC</sub> for all sites. The short-dashed line is a 1:1 line. The unit of RMSE was g C m<sup>−2</sup> day<sup>−1</sup>.</p>
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<p>The relationships between GPP<sub>EC</sub> and GPP<sub>MOD</sub> for the temperate, tropical, and alpine grassland biomes. The short-dashed line is a 1:1 line. The unit of RMSE was g C m<sup>−2</sup> day<sup>−1</sup>.</p>
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<p>The relationship between GPP<sub>MOD</sub> and GPP<sub>EC</sub> for all sites with the <span class="html-italic">ε<sub>max</sub></span> (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </mrow> </semantics></math>-BPLUT) used in the MOD17 GPP algorithm was replaced by the estimated <span class="html-italic">ε<sub>max</sub></span> (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </mrow> </semantics></math>-EST). The unit of RMSE was g C m<sup>−2</sup> day<sup>−1</sup>.</p>
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<p>The relationships between W<sub>scalar</sub> of MOD17A2H GPP products and GPP<sub>EC</sub>, respectively. The solid line is a linear regression. The short-dashed line is a 1:1 line.</p>
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38 pages, 9570 KiB  
Article
Estimation of Terrestrial Global Gross Primary Production (GPP) with Satellite Data-Driven Models and Eddy Covariance Flux Data
by Joanna Joiner, Yasuko Yoshida, Yao Zhang, Gregory Duveiller, Martin Jung, Alexei Lyapustin, Yujie Wang and Compton J. Tucker
Remote Sens. 2018, 10(9), 1346; https://doi.org/10.3390/rs10091346 - 23 Aug 2018
Cited by 145 | Viewed by 18393
Abstract
We estimate global terrestrial gross primary production (GPP) based on models that use satellite data within a simplified light-use efficiency framework that does not rely upon other meteorological inputs. Satellite-based geometry-adjusted reflectances are from the MODerate-resolution Imaging Spectroradiometer (MODIS) and provide information about [...] Read more.
We estimate global terrestrial gross primary production (GPP) based on models that use satellite data within a simplified light-use efficiency framework that does not rely upon other meteorological inputs. Satellite-based geometry-adjusted reflectances are from the MODerate-resolution Imaging Spectroradiometer (MODIS) and provide information about vegetation structure and chlorophyll content at both high temporal (daily to monthly) and spatial (∼1 km) resolution. We use satellite-derived solar-induced fluorescence (SIF) to identify regions of high productivity crops and also evaluate the use of downscaled SIF to estimate GPP. We calibrate a set of our satellite-based models with GPP estimates from a subset of distributed eddy covariance flux towers (FLUXNET 2015). The results of the trained models are evaluated using an independent subset of FLUXNET 2015 GPP data. We show that variations in light-use efficiency (LUE) with incident PAR are important and can be easily incorporated into the models. Unlike many LUE-based models, our satellite-based GPP estimates do not use an explicit parameterization of LUE that reduces its value from the potential maximum under limiting conditions such as temperature and water stress. Even without the parameterized downward regulation, our simplified models are shown to perform as well as or better than state-of-the-art satellite data-driven products that incorporate such parameterizations. A significant fraction of both spatial and temporal variability in GPP across plant functional types can be accounted for using our satellite-based models. Our results provide an annual GPP value of ∼140 Pg C year - 1 for 2007 that is within the range of a compilation of observation-based, model, and hybrid results, but is higher than some previous satellite observation-based estimates. Full article
(This article belongs to the Special Issue Terrestrial Carbon Cycle)
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<p>Locations and vegetation types of the FLUXNET 2015 flux tower sites used in this study. DBF: deciduous broadleaf forest; MF: mixed forest; ENF: evergreen needleleaf Forest; EBF: evergreen broadleaf forest; CRO: cropland; OSH: open shrubland; SAV: savannah; CSH: closed shrubland; GRA: grassland; WET: wetland; WSA: woody savannah; SNO: snow-covered.</p>
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<p>Flow of data used in training and evaluating simple satellite-based GPP models. SIF, solar-induced fluorescence; GOME-2, Global Ozone Monitoring Experiment 2; VPM, Vegetation Photosynthesis Model; LUE, light-use efficiency; FLUXCOM-RS, machine learning upscaling of flux data using remote sensing (RS) data.</p>
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<p>Two-dimensional histograms (density or heat map) of the monthly-averaged and scaled (<b>a</b>) SIF*, (<b>b</b>) NDVI<math display="inline"><semantics> <mrow> <msup> <mrow/> <mo>′</mo> </msup> <mo>×</mo> </mrow> </semantics></math> SW<math display="inline"><semantics> <msub> <mrow/> <mi>TOA</mi> </msub> </semantics></math> and (<b>c</b>) NIR<math display="inline"><semantics> <mrow> <msub> <mrow/> <mi>V</mi> </msub> <mo>×</mo> </mrow> </semantics></math> SW<math display="inline"><semantics> <msub> <mrow/> <mi>TOA</mi> </msub> </semantics></math>, all at approximately 0.05<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution, versus GPP from the FLUXNET 2015 dataset. The colors represent the number of individual data points in a particular bin. Single points within a bin are represented as a dot rather than a color-filled box. Data are from 47 individual sites with 2065 individual data points for years 2007–2013. The statistics for the fit are listed in the lower right corner with the linear fit at the top. The solid line is the 1:1 line. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>. MEF, model efficiency factor.</p>
Full article ">Figure 4
<p>Similar to <a href="#remotesensing-10-01346-f003" class="html-fig">Figure 3</a>, but showing two-dimensional histograms of the eight-day averaged results for (<b>a</b>) two-band, (<b>b</b>) seven-band and (<b>c</b>) NDVI<math display="inline"><semantics> <msup> <mrow/> <mo>′</mo> </msup> </semantics></math>-based GPP estimates at 0.00833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution, versus GPP from the FLUXNET 2015 dataset. Data are from 64 individual sites with 13,592 individual observations. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 5
<p>Locations where SIF and NDVI data have been flagged as high productivity, requiring a different slope between GPP and NDVI. Blue diamonds represent the flux tower locations used to define the flagging criteria.</p>
Full article ">Figure 6
<p>Similar to <a href="#remotesensing-10-01346-f004" class="html-fig">Figure 4</a> (same data sample), but showing two-dimensional histograms of eight-day averaged GPP estimates with the (<b>a</b>) FluxSat-7, (<b>b</b>) NIR<math display="inline"><semantics> <msub> <mrow/> <mi>V</mi> </msub> </semantics></math>-based, and (<b>c</b>) FluxSat-N models using the highest spatial resolution MCD43D dataset (0.0083<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>) versus collocated FLUXNET 2015 GPP estimates. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 7
<p>Similar to <a href="#remotesensing-10-01346-f004" class="html-fig">Figure 4</a> (same data sample), but showing two-dimensional histograms of eight-day averaged satellite datasets (<b>a</b>) FluxSat-7, (<b>b</b>) NIR<math display="inline"><semantics> <msub> <mrow/> <mi>V</mi> </msub> </semantics></math>-based, and (<b>c</b>) FLUXCOM-RS at a lower spatial resolution (0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>) versus collocated FLUXNET 2015 GPP estimates. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 8
<p>Similar to <a href="#remotesensing-10-01346-f007" class="html-fig">Figure 7</a>, but showing two-dimensional histograms of monthly averaged GPP estimates with the (<b>a</b>) FluxSat-7, (<b>b</b>) NIR<math display="inline"><semantics> <msub> <mrow/> <mi>V</mi> </msub> </semantics></math>-based and (<b>c</b>) VPM models, with VPM at 0.05<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> spatial resolution and others at 0.083<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution versus collocated FLUXNET 2015 GPP estimates. Results are for 64 sites and 3702 individual data points. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 9
<p>Similar to <a href="#remotesensing-10-01346-f007" class="html-fig">Figure 7</a>, but showing two-dimensional histograms of the normalized GPP interannual variations in eight-day averages at 0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution computed using (<b>a</b>) FluxSat-7, (<b>b</b>) NIR<math display="inline"><semantics> <msub> <mrow/> <mi>V</mi> </msub> </semantics></math>-based, and (<b>c</b>) FLUXCOM-RS versus FLUXNET 2015 GPP. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 10
<p>Similar to <a href="#remotesensing-10-01346-f009" class="html-fig">Figure 9</a>, but showing two-dimensional histograms of the normalized GPP interannual variability in monthly averages at 0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> for (<b>a</b>) FluxSat-7 and (<b>b</b>) NIR<math display="inline"><semantics> <msub> <mrow/> <mi>V</mi> </msub> </semantics></math> and 0.05<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution for (<b>c</b>) VPM versus FLUXNET 2015 GPP. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 11
<p>Scatter diagrams comparing the mean annual GPP for each site for (<b>a</b>) FluxSat-7, (<b>b</b>) NIR<math display="inline"><semantics> <msub> <mrow/> <mi>V</mi> </msub> </semantics></math>-based, and (<b>c</b>) FLUXCOM-RS, all computed at 0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution versus FLUXNET 2015 GPP. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 12
<p>Scatter diagrams of (<b>a</b>) FluxSat-7, (<b>b</b>) NIRV-based, and (<b>c</b>) FluxSat-N (columns) versus GPP from FLUXNET 2015 (all in g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>) for different plant functional types (rows) with the 1:1 line (black line), linear fit (red dashed line and red fit) along with number of observations <span class="html-italic">n</span> and <math display="inline"><semantics> <msup> <mi>r</mi> <mn>2</mn> </msup> </semantics></math>. All satellite-driven estimates use MCD43D reflectances at 0.0083<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> spatial and eight-day temporal resolutions.</p>
Full article ">Figure 13
<p>Similar to <a href="#remotesensing-10-01346-f012" class="html-fig">Figure 12</a>, but for different plant functional types. All units are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure 14
<p>Similar to <a href="#remotesensing-10-01346-f012" class="html-fig">Figure 12</a>, but showing interannual variations normalized by the FLUXNET 2015 climatological ranges (unitless) for different plant functional types.</p>
Full article ">Figure 15
<p>Similar to <a href="#remotesensing-10-01346-f014" class="html-fig">Figure 14</a>, but showing normalized interannual variations (unitless) for different plant functional types.</p>
Full article ">Figure 16
<p>Maps of GPP (eight-day average) estimated with remote sensing data trained on eddy covariance flux tower data for Days 193–200 (<b>left panels</b>) and Days 1–8 (<b>right panels</b>) of 2007; <b>top panels</b>: FluxSat-7, <b>bottom panels</b>: FLUXCOM-RS. Averages are listed in the lower left for all grid boxes (Mean all), along with subsets from the tropics (latitudes &lt; 20<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>, denoted Mean trop), Northern Hemisphere extra tropics (latitudes &gt; 20<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>N, denoted Mean NHET) and Southern Hemisphere extra tropics (latitudes below 20<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>S, denoted Mean SHET).</p>
Full article ">Figure 17
<p>Maps of annual averaged GPP estimated with remote sensing data for the year 2007; <b>left panel</b>: FluxSat-7; <b>right panel</b>: VPM.</p>
Full article ">Figure 18
<p>Evaluation using monthly data at 0.05<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution for independent sites: (<b>a</b>) tropical BR-Sa3 site and (<b>b</b>) sites above 60<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>N, DK-ZaF (WET), FI-Hyy (ENF), FI-Lom (WET) and US-Prr (ENF). In (<b>b</b>), ENF sites are shown as purple (VPM) and green (FluxSat-7) and wetland (WET) sites are shown in blue (VPM) and red (FluxSat-7).</p>
Full article ">Figure A1
<p>Flowchart of all datasets examined in training and evaluating satellite-based GPP models.</p>
Full article ">Figure A2
<p>Probability distribution functions (PDFs) of modeled minus FLUXNET 2015 GPP corresponding to data in <a href="#remotesensing-10-01346-f003" class="html-fig">Figure 3</a> at 0.05<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution, where <span class="html-italic">n</span> is the number of data points, and <span class="html-italic">a</span> and <span class="html-italic">b</span> refer to a linear fit of the data: FLUXNET 2015 GPP = <math display="inline"><semantics> <mrow> <mi>a</mi> <mo>×</mo> </mrow> </semantics></math> satellite data-driven GPP + <span class="html-italic">b</span>. Data are monthly averages from 47 individual sites for years 2007–2013. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A3
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a>, but with eight-day data corresponding to <a href="#remotesensing-10-01346-f004" class="html-fig">Figure 4</a> at 0.00833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution. Data are from 64 individual sites. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A4
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a> (same data sample, eight-day at 0.00833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution at 64 sites), but with data corresponding to <a href="#remotesensing-10-01346-f006" class="html-fig">Figure 6</a>. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A5
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a> (same data sample, eight-day), but with data corresponding to <a href="#remotesensing-10-01346-f007" class="html-fig">Figure 7</a> at lower spatial resolution (0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math>). Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A6
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a>, but with monthly model data at 0.05<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution corresponding to <a href="#remotesensing-10-01346-f008" class="html-fig">Figure 8</a>. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A7
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a>, but with eight-day anomalies corresponding to <a href="#remotesensing-10-01346-f009" class="html-fig">Figure 9</a> at 0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution. Data are from 64 individual sites. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A8
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a>, but with monthly anomaly data corresponding to <a href="#remotesensing-10-01346-f010" class="html-fig">Figure 10</a>. Data are from 64 individual sites. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A9
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a>, but with mean annual data corresponding to <a href="#remotesensing-10-01346-f011" class="html-fig">Figure 11</a> at 0.0833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution. Data are from 42 individual sites. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A10
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a>, but with eight-day data at 0.00833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution corresponding to <a href="#remotesensing-10-01346-f012" class="html-fig">Figure 12</a> and <a href="#remotesensing-10-01346-f013" class="html-fig">Figure 13</a>. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">Figure A11
<p>Similar to <a href="#remotesensing-10-01346-f0A2" class="html-fig">Figure A2</a> but with eight-day data at 0.00833<math display="inline"><semantics> <msup> <mrow/> <mo>∘</mo> </msup> </semantics></math> resolution corresponding to <a href="#remotesensing-10-01346-f014" class="html-fig">Figure 14</a> and <a href="#remotesensing-10-01346-f015" class="html-fig">Figure 15</a>. Units of GPP are g C m<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </semantics></math> d<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics></math>.</p>
Full article ">
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