BG3.12

Solar-induced chlorophyll fluorescence (SIF) has been shown as a promising approach for the estimation of gross primary productivity (GPP), but whether SIF is merely a function of canopy structure or also contains precious physiological information, is presently heavily discussed. In this study, the SIF-GPP relationship was quantified at a Pinus sylvestris forest (Mezyk, Poland) during a series of short-term cold spells throughout the spring awakening to investigate the potential of SIF as a proxy for GPP during this period characterized by cold stress. GPP was inferred from the net ecosystem CO₂ exchange measured by the eddy covariance technique. Canopy-scale SIF was measured using a high-resolution spectrometer system and retrieved via spectral fitting (SFM) algorithms. Active leaf-scale chlorophyll fluorescence measurements were conducted on seven branches using an automated field-deployable fluorometer system. Our results demonstrate a clear difference in GPP and the utilization of chlorophyll-absorbed energy between cold spell and warm days. At short, sub-daily time scales, the correlation between SIF and GPP was minor, but increased significantly when observed over extended temporal periods, when SIF exhibited a seasonal pattern that was more closely aligned with the GPP. Furthermore, the strong relationship between non-photochemical quenching (NPQ) and the photochemical reflectance index (PRI) shows good potential to better estimate GPP when integrated in the SIF-GPP model, as the integration of PRI overall increased the relation between SIF and GPP.

How to cite: Schwarz, M., Sakowska, K., Ziemblińska, K., Dukat, P., Urbaniak, M., Olejnik, J., Hammerle, A., and Wohlfahrt, G.: Chlorophyll fluorescence-gross primary productivity relationships during the spring awakening of an evergreen needleleaf forest, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3094, https://doi.org/10.5194/egusphere-egu21-3094, 2021.

During the regular seasonal drought conditions in our semi-arid pine forest, soil water content decrease below the 16% threshold of no transpirable soil water availability, and VPD increase to values of >5 kPa. Soil drought in one forest plot was eliminated by using supplemental drip irrigation during summer. We used automatic branch chambers to measure CO₂/H₂O exchange together with laser-based COS exchange, and retrieving canopy sun induced fluorescence (SIF) using a high-resolution spectrometer above the canopy that was moved between the control and irrigated plots on a weekly basis. Using these research tools, we investigated the ecophysiological response (including rates of gas exchange, conductances, and photochemical response) of the mature pine trees to the differential effects of soil and atmospheric droughts. Leaf relative uptake (LRU) ratio of COS to CO₂ fluxes was used to constrain estimates of carbon assimilation (An) and changes in the An/gl ratio (where gl is leaf conductance), and leaf COS and H₂O exchange fluxes were also used to partition leaf conductance. We will report on the first seasonal cycle of COS and CO₂ fluxes, LRU and SIF_A of mature pine trees under field conditions in the semi-arid forest, which includes the summer dry stress period, the recovery during the transition to the winter wet season, and the spring peak activity period.

How to cite: Qubaja, R., Stern, R., Oz, I., Cochavi, A., Muller, J., and Yakir, D.: Combining branch-scale COS/CO2 exchange and canopy-scale SIF measurements to disentangle the effects of high VPD and low soil moisture in mature pine trees under field conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6430, https://doi.org/10.5194/egusphere-egu21-6430, 2021.

Sun Induced Chlorophyll Fluorescence (SIF) is considered as a good proxy for photosynthesis given its closer link to the photosynthetic light reactions compared to remote sensing vegetation indices typically used for ecosystem productivity modelling (eg. NDVI). Satellite-based SIF shows significant linear relationships with gross primary production (GPP) from in-situ measurements across sites, biomes and seasons. While SIF can be considered a good proxy for large scale spatial and seasonal variability in GPP, much of the SIF-GPP co-variance can be explained by their common dependence on the absorbed photosynthetically active radiation. Whether SIF can be an equally good proxy for interannual variability in GPP especially during periods of vegetation stress (drought/heat) is, so far, not clear.

In this study, we evaluate the relationship between satellite-based SIF and in-situ GPP measurements during vegetation stress periods (drought/heat), compared to non-stress periods. GPP is obtained from eddy-covariance measurements from a set of forest sites pre-filtered to ensure homonegeous footprints. SIF is obtained from GOME-2 covering the period 2007-2018. Because of scale mismatch between each site’s footprint (in the order of hundred meters) and the spatial resolution of GOME-2 (ca. 50km), we additionally use SIF from the downscale product from Duveiller et al. 2020 (ca. 5km) and the more recent dataset from TROPOMI (ca. 7 x 3.5 km), covering only the last year of the study period.

We develop a classification of stress periods that is based on both the occurrence of drought/heat extreme events and the presence of photosynthetic downregulation. We then evaluate the relationship between SIF and GPP and their yields, for different plant functional types and at site-level. We evaluate how these relationships vary depending on environmental conditions and in particular for “stress” versus “non-stress” days.

Duveiller, G., Filipponi, F., Walther, S., Köhler, P., Frankenberg, C., Guanter, L., and Cescatti, A.: A spatially downscaled sun-induced fluorescence global product for enhanced monitoring of vegetation productivity, Earth Syst. Sci. Data, 12, 1101–1116, https://doi.org/10.5194/essd-12-1101-2020, 2020.

How to cite: Wieneke, S., Bastos, A., Balzarolo, M., Barrios, J. M., and Janssens, I.: Evaluating the relationship between SIF and GPP under climate extremes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8939, https://doi.org/10.5194/egusphere-egu21-8939, 2021.

Atmospheric aridity and soil drought control vegetation water use and affect the terrestrial water and carbon cycles. Separating these two types of drought is difficult but crucial because they relate to different ecosystem properties and their impacts depend on vegetation types. We examine how well satellite-observed solar-induced chlorophyll fluorescence (SIF) captures the drought responses of taiga and steppe in semiarid areas on the Mongolian plateau, which have experienced a historic drought since the late 1990s. Ten-year records of the GOSAT SIF and the MODIS-band-1 photochemical reflectance index (PRI, also known as chlorophyll/carotenoid index) from Aqua consistently suggest that the taiga is sensitive to vapor-pressure deficit but insensitive to surface-soil drought, and that the opposite is the case for the steppe. The MODIS PRI changes reasonably with temperature and drought stress, and also with canopy shades, which is attributable to the xanthophyll cycle. However the most influential factor on the PRI is leaf area in both vegetations, and temperature is the second in the taiga, indicating the dominant effect of its pigment-pool size. The leaf area index of the taiga has almost similar values and seasonal patterns in each year, while the SIF shows remarkable interannual changes. The SIF yield in the taiga decreases 36–48% on average with the increase of vapor-pressure deficit from 1 kPa to 3 kPa under high-PRI conditions. Almost all detectable information from the SIF yield in the steppe is correlated with PRI on a monthly basis. The SIF in the steppe decreases nonlinearly when the surface-soil-water content fell below ∼0.154 m³ m⁻³, which agrees well with an eddy-covariance result in this region and implies the capability of satellite-based wilting-point estimation. We will further clarify which biological factors affected the observed results of SIF and PRI using the process-based model 'Soil-Canopy-Observation of Photosynthesis and Energy fluxes' (SCOPE) and show the impact of drought on the gross primary production in the past decade.

How to cite: Kiyono, T., Noda, H., Kumagai, T., Oshio, H., Yoshida, Y., Matsunaga, T., and Hikosaka, K.: Detecting the Drought-Response Difference in Semiarid Ecosystems on the Mongolian Plateau: 10-Year Observation Using GOSAT SIF and MODIS PRI, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3583, https://doi.org/10.5194/egusphere-egu21-3583, 2021.

A deep understanding of the responses of terrestrial vegetation to environmental forcing is crucial for understanding the global carbon dynamics, especially under a changing climate. Vegetation responses to stress can manifest first as plant physiological responses, and at later stages through changes in canopy structure. Remote sensing of vegetation has been proven very valuable in providing such understanding. One of the major breakthroughs has been the use of multi and hyper spectral sensors on board satellites that can retrieve Solar Induced Fluorescence (SIF), which is closely linked with plant photosynthesis.

In this study we assess whether (SIF), as observed by instruments on board of satellites, can adequately capture both of those responses. Using 7 different global SIF products, water and CO2 flux data from 120 eddy-covariance towers and climate reanalysis products, we found a good agreement between the 16-day responses of flux tower observed gross primary productivity (GPP) and SIF to soil moisture and light, and a weaker agreement regarding the responses to temperature and atmospheric humidity. Overall, we found that current satellite SIF responses to environmental stressors mostly reflect structural changes in vegetation structure, and that satellite SIF has limited skill in capturing early stress plant physiological responses except for the light availability response at higher latitudes. While satellite SIF significantly outperforms more traditional vegetation indices (EVI, NDVI), as it does not saturate unlike the latter ones, it does not provide major additional information regarding vegetation physiological responses to either hydrological or atmospheric droughts.

How to cite: Paschalis, A., Gentine, P., Graham, D., and Fatichi, S.: Solar Induced Fluorescence measured from satellite sensors captures mostly structural than physiological responses to environmental stressors, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7321, https://doi.org/10.5194/egusphere-egu21-7321, 2021.

The Eurasian boreal ecosystem acts as a major terrestrial carbon sink in the northern hemisphere. Under changing climatic conditions, it is crucial to monitor biogenic carbon fluxes in this area. The Siberian in-situ CO₂ data are, however, sparse in spatial coverage and limit model-validation there. Satellite observations of CO₂ and Sun-Induced Fluorescence (SIF) can provide essential information to constrain the Eurasian boreal biogenic carbon-cycle and further, to improve carbon cycle inverse models.

In this study, we investigate the Eurasian boreal carbon cycle with satellite observations of the Orbiting Carbon Observatory 2 (OCO-2) and the Greenhouse gase Observing SATellite (GOSAT). We compare the observed carbon cycle dynamics to model data such as provided by CarbonTracker (CT2019, CT-NRT.v2020-1) and find differences in the ppm range. Various sensitivity studies with respect to region selection, sampling biases and model choices are used to consolidate the robustness of the detected pattern. Using SIF and FLUXCOM GPP data, we will show first attempts to attribute the model-measurement differences to uncertainties in biogenic carbon fluxes.

How to cite: Schreiner, L., Grossmann, K., Butz, A., Vardag, S. N., and Schömann, E.-M.: Constraining the Eurasian biogenic boreal carbon-cycle with satellite-SIF, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15375, https://doi.org/10.5194/egusphere-egu21-15375, 2021.

The total amount of CO₂ absorbed and turned into organic matter through photosynthesis can be quantified as Gross Primary Productivity (GPP). Terrestrial vegetation GPP is responsible for offsetting about 30% of carbon dioxide emissions released by anthropogenic activities into the atmosphere. This corresponds to approximately 120 to 170 PgC/year, making terrestrial vegetation both the largest and most uncertain component of the global carbon cycle. Over the last few years, solar- induced chlorophyll fluorescence (SIF) observations from space have emerged as a promising resource for evaluating the spatio-temporal distribution of GPP by terrestrial ecosystems. SIF is an electromagnetic signal emitted by the chlorophyll a of green plants: part of the absorbed energy not used for photosynthesis is emitted at longer wavelengths as a two-peak spectrum roughly covering the 650–850 nm spectral range. The SIF signal responds instantaneously to perturbations in  environmental conditions such as light and water stress, which makes it a direct proxy for photosynthetic activity.

SIF has been estimated at the global scale from several space-borne spectrometers originally intended for atmospheric research because they provide the necessary spectral  resolution and radiometric sensitivity. However, the exploitation of SIF estimates has remained limited by their coarse spatial resolution (GOSAT, GOME-2) or low number of observations (OCO-2). This has been strongly alleviated by the advent of the TROPOspheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor which combines a global continuous spatial sampling with a 5.5 km x 3.5 km pixel size at nadir, a daily  revisit time, a wide spectral coverage and an enormous increase in the number of clear-sky measurements per day in comparison to previous missions, as demonstrated by the first SIF retrievals from TROPOMI derived by Caltech (Köhler et al. 2018).

In this study, we present a new independent SIF product derived from TROPOMI observations using two fitting windows at 743-758 nm (all sky SIF) and 735-758 nm (clear sky SIF). The first one is very robust against atmospheric effects (especially cloud contamination) whereas the second one is less sensitive to estimation errors (due to the greater number of spectral points in the fitting window). The retrieval scheme is not much different from the one from Caltech (in 743-758 nm) but relies on different cloud fraction inputs. We will present the principles of the retrieval scheme, the format of the output TROPOSIF product, and the results of the comparison of our SIF estimates against the Caltech ones as well as SIF data derived from OCO-2 which show a very good consistency between all SIF products.

How to cite: Bacour, C., Guanter, L., Schneider, A., Aben, I., Maignan, F., Grignon, L., El Hajj, M., and Retscher, C.: A new SIF (solar‐induced chlorophyll fluorescence) product derived from TROPOMI onboard Sentinel-5 Precursor, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8420, https://doi.org/10.5194/egusphere-egu21-8420, 2021.

Better constraining the ecosystem gross photosynthetic CO₂ uptake (GPP) is necessary to reduce the uncertainties on continental vegetation response to climate change. As GPP cannot be directly measured at the ecosystem scale, different proxies of vegetation CO₂ uptake have emerged. These proxies are essential for land surface modelers to estimate GPP at large scale. Carbonyl sulfide (COS) shows many similarities with CO₂, following the same diffusional pathways through the leaves. However, unlike CO₂ that is also emitted by plants during respiration, COS is essentially only taken up by leaves and not re-emitted back to the atmosphere. Therefore, COS is a promising proxy of photosynthetic activity. In previous studies, fixed values of the leaf relative uptake (LRU) ratio of COS to CO₂ fluxes normalized by their respective concentration have typically been used to infer GPP for the different biomes. However, it has been shown that LRU ratio changes with varying Photosynthetically Active Radiation (PAR), which limits its accuracy to constrain photosynthetic activity. Therefore, we redefined the COS-based GPP estimation approach to better capture GPP response to changing environmental conditions, by implementing a mechanistic model of COS exchange by continental vegetation in the ORCHIDEE land surface model. We compared the modelled fluxes against field measurements at two sites and studied the model behavior and environmental drivers. Then, we ran global simulations and computed the annual COS vegetation uptake that was found in the middle range values of previous reported budgets (-490 to -1335 Gg S yr^-1), with -756 Gg S yr^-1. The simulated fluxes were transported, and COS concentrations were evaluated against measurements from the NOAA atmospheric stations. Our results show that the mechanistic approach is more appropriate when studying photosynthetic activity at high temporal resolution, but similar results in concentrations are obtained between the mechanistic and LRU approaches at the global scale. Accurate evaluation of the continental vegetation COS uptake is necessary as it is the main COS sink. However, COS can also be absorbed or emitted by soils, a flux that complicates the use of eddy covariance COS flux measurements or atmospheric COS measurements to derive information on GPP estimates. Therefore, the soil COS exchange should also be represented in land surface models. We implemented two soil COS exchange models in ORCHIDEE, a mechanistic model (based on Ogée et al. 2016) and a second model based on an empirical relationship with soil respiration (following Berry et al., 2013). We evaluated the two models at several sites against field measurements. We also performed global simulations to evaluate the spatial distribution of soil COS fluxes and their seasonal variations. Finally, we estimated the contributions of the combined impact of soil COS exchange and leaf COS uptake (both from the ORCHIDEE model) to the global COS budget and on the COS atmospheric concentration latitudinal gradient.

How to cite: Abadie, C., Maignan, F., Remaud, M., Kooijmans, L. M. J., Kohonen, K.-M., Commane, R., Wehr, R., Campbell, J. E., Belviso, S., Montzka, S. A., Raoult, N., Seibt, U., Shiga, Y., Vuichard, N., Whelan, M., and Peylin, P.: Implementation of vegetation and soil carbonyl sulfide exchanges in the ORCHIDEE land surface model to better constrain ecosystem gross primary production, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8335, https://doi.org/10.5194/egusphere-egu21-8335, 2021.

How to cite: Koren, G., Adnew, G. A., Vilà-Guerau de Arellano, J., van der Molen, M. K., Kruijt, B., Röckmann, T., and Peters, W.: Exploring the diurnal cycle of Δ17O in CO2 at the ecosystem level, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13322, https://doi.org/10.5194/egusphere-egu21-13322, 2021.

In experimental ecosystem ecology, plot sizes are most often too small to apply eddy flux techniques and estimation of ecosystem gas exchange rates relies on various chamber measurement technologies. Furthermore, drained areas often results in increased growth of trees which complicates application of eddy flux measurements on small plots.

We combined ECO₂FluX ecosystem-level automatic chambers (prenart.dk) with an LI-8100/LI-8150 multiplexer systems (licor.com) in a range of Danish and Norwegian ecosystems experiments spanning agriculture, grassland/heathlands and peatland ecosystems. The automatic closed, none-steady state chambers each cover an area of 3,117 cm² (63 cm diameter), are 80 cm tall (volume: 250L), and are capable of switching automatically between transparent and darkened mode, enabling separation of light-sensitive and light-indifferent processes in the ecosystems covered. For CO₂ fluxes, net exchange (NEE) was estimated as the flux in transparent mode, ecosystem respiration (R_E) in darkened mode, while Gross Ecosystem Productivity (GPP) was estimated as NEE – R_E.

Chambers were set up to measure gas concentrations every second using enclosure times of 4-5 minutes, first in light mode and 10-30 minutes later in dark mode, with 3-48 repetitions per day. The longest time series spans 5 years of measurements and contain >60,000 point measurements.

In this presentation, we will present an analysis of the ability of the light-dark chamber data to infer ecosystem-level rates of gross primary productivity, respiration, net CO₂ exchange, and evapotranspiration. In the two Norwegian peatland sites, flux measurements may be compared directly with eddy flux measurements. We also compare the rates of the direct estimates of GPP from the light-dark chamber measurements to estimates inferred from using the light (NEE) measurements only followed by applying methodologies normally used for eddy flux measurements. This comparison may help constrain potential biases in both the closed chamber and eddy flux techniques. Finally, we investigate the ability of using such closed chambers to estimate ecosystem evapotranspiration rates at the plot scale. Such application may be useful for estimating the effects on evapotranspiration in field-scale experiments manipulating the ecosystem water balance either directly or indirectly.

How to cite: Larsen, K. S., Pullens, J. W. M., Avila, L., Bruun, S., Chen, J., Christiansen, J. R., Ibrom, A., Larsen, P., Lærke, P. E., Jørgensen, P., and Tariq, A.: Inferring ecosystem-level rates of gross primary productivity, respiration, and evapotranspiration with automatic light-dark measurement chambers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12464, https://doi.org/10.5194/egusphere-egu21-12464, 2021.