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1. Dynamic global vegetation models underestimate net CO 2 flux mean and inter-annual variability in dryland ecosystems

   https://doi.org/10.1088/1748-9326/ac1a38  

   MacBean, Natasha ; Scott, Russell L ; Biederman, Joel A ; Peylin, Philippe ; Kolb, Thomas ; Litvak, Marcy E ; Krishnan, Praveena ; Meyers, Tilden P ; Arora, Vivek K ; Bastrikov, Vladislav ; et al ( August 2021 , Environmental Research Letters)

   Abstract Despite their sparse vegetation, dryland regions exert a huge influence over global biogeochemical cycles because they cover more than 40% of the world surface (Schimel 2010 Science 327 418–9). It is thought that drylands dominate the inter-annual variability (IAV) and long-term trend in the global carbon (C) cycle (Poulter et al 2014 Nature 509 600–3, Ahlstrom et al 2015 Science 348 895–9, Zhang et al 2018 Glob. Change Biol . 24 3954–68). Projections of the global land C sink therefore rely on accurate representation of dryland C cycle processes; however, the dynamic global vegetation models (DGVMs) used in future projections have rarely been evaluated against dryland C flux data. Here, we carried out an evaluation of 14 DGVMs (TRENDY v7) against net ecosystem exchange (NEE) data from 12 dryland flux sites in the southwestern US encompassing a range of ecosystem types (forests, shrub- and grasslands). We find that all the models underestimate both mean annual C uptake/release as well as the magnitude of NEE IAV, suggesting that improvements in representing dryland regions may improve global C cycle projections. Across all models, the sensitivity and timing of ecosystem C uptake to plant available moisture was at fault. Spring biases in gross primary production (GPP) dominate the underestimate of mean annual NEE, whereas models’ lack of GPP response to water availability in both spring and summer monsoon are responsible for inability to capture NEE IAV. Errors in GPP moisture sensitivity at high elevation forested sites were more prominent during the spring, while errors at the low elevation shrub and grass-dominated sites were more important during the monsoon. We propose a range of hypotheses for why model GPP does not respond sufficiently to changing water availability that can serve as a guide for future dryland DGVM developments. Our analysis suggests that improvements in modeling C cycle processes across more than a quarter of the Earth’s land surface could be achieved by addressing the moisture sensitivity of dryland C uptake. 

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2. Contrasting ecosystem CO 2 fluxes of inland and coastal wetlands: a meta‐analysis of eddy covariance data

   https://doi.org/10.1111/gcb.13424  

   Lu, Weizhi ; Xiao, Jingfeng ; Liu, Fang ; Zhang, Yue ; Liu, Chang'an ; Lin, Guanghui ( August 2016 , Global Change Biology)

   Wetlands play an important role in regulating the atmospheric carbon dioxide (CO₂) concentrations and thus affecting the climate. However, there is still lack of quantitative evaluation of such a role across different wetland types, especially at the global scale. Here, we conducted a meta‐analysis to compare ecosystemCO₂fluxes among various types of wetlands using a global database compiled from the literature. This database consists of 143 site‐years of eddy covariance data from 22 inland wetland and 21 coastal wetland sites across the globe. Coastal wetlands had higher annual gross primary productivity (GPP), ecosystem respiration (R_e), and net ecosystem productivity (NEP) than inland wetlands. On a per unit area basis, coastal wetlands provided largeCO₂sinks, while inland wetlands provided smallCO₂sinks or were nearlyCO₂neutral. The annualCO₂sink strength was 93.15 and 208.37 g C m⁻²for inland and coastal wetlands, respectively. AnnualCO₂fluxes were mainly regulated by mean annual temperature (MAT) and mean annual precipitation (MAP). For coastal and inland wetlands combined,MATandMAPexplained 71%, 54%, and 57% of the variations inGPP,R_e, andNEP, respectively. TheCO₂fluxes of wetlands were also related to leaf area index (LAI). TheCO₂fluxes also varied with water table depth (WTD), although the effects ofWTDwere not statistically significant.NEPwas jointly determined byGPPandR_efor both inland and coastal wetlands. However, theNEP/R_eandNEP/GPPratios exhibited little variability for inland wetlands and decreased for coastal wetlands with increasing latitude. The contrasting ofCO₂fluxes between inland and coastal wetlands globally can improve our understanding of the roles of wetlands in the global C cycle. Our results also have implications for informing wetland management and climate change policymaking, for example, the efforts being made by international organizations and enterprises to restore coastal wetlands for enhancing blue carbon sinks.

    

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3. Chlorophyll fluorescence tracks seasonal variations of photosynthesis from leaf to canopy in a temperate forest

   https://doi.org/10.1111/gcb.13590  

   Yang, Hualei ; Yang, Xi ; Zhang, Yongguang ; Heskel, Mary A. ; Lu, Xiaoliang ; Munger, J. William ; Sun, Shucun ; Tang, Jianwu ( January 2017 , Global Change Biology)

   Accurate estimation of terrestrial gross primary productivity (GPP) remains a challenge despite its importance in the global carbon cycle. Chlorophyll fluorescence (ChlF) has been recently adopted to understand photosynthesis and its response to the environment, particularly with remote sensing data. However, it remains unclear how ChlF and photosynthesis are linked at different spatial scales across the growing season. We examined seasonal relationships between ChlF and photosynthesis at the leaf, canopy, and ecosystem scales and explored how leaf‐level ChlF was linked with canopy‐scale solar‐induced chlorophyll fluorescence (SIF) in a temperate deciduous forest at Harvard Forest, Massachusetts,USA. Our results show that ChlF captured the seasonal variations of photosynthesis with significant linear relationships between ChlF and photosynthesis across the growing season over different spatial scales (R² = 0.73, 0.77, and 0.86 at leaf, canopy, and satellite scales, respectively;P < 0.0001). We developed a model to estimateGPPfrom the tower‐based measurement ofSIFand leaf‐level ChlF parameters. The estimation ofGPPfrom this model agreed well with flux tower observations ofGPP(R² = 0.68;P < 0.0001), demonstrating the potential ofSIFfor modelingGPP. At the leaf scale, we found that leafF_q’/F_m’, the fraction of absorbed photons that are used for photochemistry for a light‐adapted measurement from a pulse amplitude modulation fluorometer, was the best leaf fluorescence parameter to correlate with canopySIFyield (SIF/APAR,R² = 0.79;P < 0.0001). We also found that canopySIFandSIF‐derivedGPP(GPP_SIF) were strongly correlated to leaf‐level biochemistry and canopy structure, including chlorophyll content (R² = 0.65 for canopyGPP_SIFand chlorophyll content;P < 0.0001), leaf area index (LAI) (R² = 0.35 for canopyGPP_SIFandLAI;P < 0.0001), and normalized difference vegetation index (NDVI) (R² = 0.36 for canopyGPP_SIFandNDVI;P < 0.0001). Our results suggest that ChlF can be a powerful tool to track photosynthetic rates at leaf, canopy, and ecosystem scales.

    

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4. Interannual and Seasonal Drivers of Carbon Cycle Variability Represented by the Community Earth System Model (CESM2)

   https://doi.org/10.1029/2021GB007034  

   Wieder, William R. ; Butterfield, Zachary ; Lindsay, Keith ; Lombardozzi, Danica L. ; Keppel‐Aleks, Gretchen ( September 2021 , Global Biogeochemical Cycles)

   Earth system models are intended to make long‐term projections, but they can be evaluated at interannual and seasonal time scales. Although the Community Earth System Model (CESM2) showed improvements in a number of terrestrial carbon cycle benchmarks, relative to its predecessor, our analysis suggests that the interannual variability (IAV) in net terrestrial carbon fluxes did not show similar improvements. The model simulated low IAV of net ecosystem production (NEP), resulting in a weaker than observed sensitivity of the carbon cycle to climate variability. Low IAV in net fluxes likely resulted from low variability in gross primary productivity (GPP)—especially in the tropics—and a high covariation between GPP and ecosystem respiration. Although lower than observed, the IAV of NEP had significant climate sensitivities, with positive NEP anomalies associated with warmer and drier conditions in high latitudes, and with wetter and cooler conditions in mid and low latitudes. We identified two dominant modes of seasonal variability in carbon cycle flux anomalies in our fully coupled CESM2 simulations that are characterized by seasonal amplification and redistribution of ecosystem fluxes. Seasonal amplification of net and gross carbon fluxes showed climate sensitivities mirroring those of annual fluxes. Seasonal redistribution of carbon fluxes is initiated by springtime temperature anomalies, but subsequently negative feedbacks in soil moisture during the summer and fall result in net annual carbon losses from land. These modes of variability are also seen in satellite proxies of GPP, suggesting that CESM2 appropriately represents regional sensitivities of photosynthesis to climate variability on seasonal time scales.

    

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5. The Global Influence of Cloud Optical Thickness on Terrestrial Carbon Uptake

   https://doi.org/10.1175/EI-D-17-0035.1  

   Zhu, Peiyun ; Cheng, Susan J. ; Butterfield, Zachary ; Keppel-Aleks, Gretchen ; Steiner, Allison L. ( September 2019 , Earth Interactions)

   Clouds can modify terrestrial productivity by reducing total surface radiation and increasing diffuse radiation, which may be more evenly distributed through plant canopies and increase ecosystem carbon uptake (the “diffuse fertilization effect”). Previous work at ecosystem-level observational towers demonstrated that diffuse photosynthetically active radiation (PAR; 400–700 nm) increases with cloud optical thickness (COT) until a COT of approximately 10, defined here as the “low-COT regime.” To identify whether the low-COT regime also influences carbon uptake on broader spatial and longer temporal time scales, we use global, monthly data to investigate the influence of COT on carbon uptake in three land-cover types: shrublands, forests, and croplands. While there are limitations in global gross primary production (GPP) products, global COT data derived from Moderate Resolution Imaging Spectroradiometer (MODIS) reveal that during the growing season tropical and subtropical regions more frequently experience a monthly low-COT regime (>20% of the time) than other regions of the globe. Contrary to ecosystem-level studies, comparisons of monthly COT with monthly satellite-derived solar-induced chlorophyll fluorescence and modeled GPP indicate that, although carbon uptake generally increases with COT under the low-COT regime, the correlations between COT and carbon uptake are insignificant (p > 0.05) in shrublands, forests, and croplands at regional scales. When scaled globally, vegetated regions under the low-COT regime account for only 4.9% of global mean annual GPP, suggesting that clouds and their diffuse fertilization effect become less significant drivers of terrestrial carbon uptake at broader spatial and temporal scales.

    

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