Sun-induced fluorescence (SIF), which serves as a proxy of plant and ecosystem net carbon assimilation (or gross primary production, GPP, at the ecosystem level), has been observed to exhibit sustainability under optimal conditions. The recent evidence demonstrated a break of this relation under stress conditions, and a change in the fluorescence role for excessive energy dissipation.

In this study, our aim was to identify the physiological alterations responsible for the transition. To achieve this, we utilize both natural and induced stress conditions to investigate the changing role of fluorescence. Initially, we examine the impact of sustained drought on the correlation between plant leaf-level gas exchange and Pulse Amplitude Modulation (PAM) parameters, in conjunction with plant Sun-Induced Fluorescence (SIF) and other spectral indices. In the subsequent phase, we use chemical inhibitors to assess the reaction of both susceptible and tolerant plants.

The findings indicated that Sun-Induced Fluorescence (SIF) primarily interacts with the Non-Photochemical Quenching (NPQ) pathway. During the initial phases of rehydration, the SIF signal decreases correspondingly with the decline in photosynthetic activity. Subsequently, as NPQ levels reach saturation, the intensity of the SIF signal begins to rise. The use of photosystem inhibitors reinforces our observations from the drought experiment. The sensitive accession displays a rapid surge in the fluorescence signal, coinciding with a complete cessation of carbon assimilation. Conversely, in the tolerant accession, a simultaneous decrease in the fluorescence signal occurs alongside a partial decline in the rate of carbon assimilation.

The results underscore the dual roles of plant fluorescence within the plant. Distinguishing between these two phases can assist in monitoring both plant and ecosystem responses under stress conditions.

How to cite: Cochavi, A., Sadeh, Y., and Shemer, O. E.: Locating the shift in the plant fluorescence role under stress conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14620, https://doi.org/10.5194/egusphere-egu24-14620, 2024.

Peatlands are pivotal carbon sinks, storing approximately one-third of the terrestrial carbon. Nevertheless, peatland vegetation is vulnerable to environmental stressors, particularly heatwaves. Delving into the diurnal course of photosynthesis and its relationship with solar-induced fluorescence (SIF) unravels valuable insights into peatland vegetation's physiological responses to environmental stress.

The study examines the correlation between SIF and gross primary production (GPP), examining the photosynthetic vigour of peatland vegetation across heatwave and non-heatwave periods in the Rzecin peatland, Poland, from June to October 2019. CO₂ fluxes were measured by manual chambers, and GPP was calculated from consecutive net ecosystem exchange and ecosystem respiration measurements. Each campaign's GPP data was modelled with a Michaelis-Menten rectangular hyperbola model. SIF in the O₂-A band was retrieved by the improved Fraunhofer Line Depth (iFLD) method from the hyperspectral data measured by the FloX system. The SIF-GPP relationship was further examined based on dates and periods (before noon, noon, and afternoon), with the heatwave and non-heatwave scenarios.

Results showed the differences in correlations dependent on date, period, and heatwave scenarios. Generally, correlations increased under cloudy or partially cloudy conditions, where low light intensity and temperature alleviated plant stress, increasing photosynthetic efficiency. The SIF and GPP exhibit positive correlations in the morning and afternoon, but the relationship is broken during midday, underscoring the impact of factors such as intense light and high temperature on the peatland vegetation physiology.

Diurnal dynamics reveal a robust linear relationship between GPP and SIF O₂-A across non-heatwave days, losing coherence during heatwaves. A notable midday depression during heatwaves, characterized by a dip in SIF-GPP correlation at noon, points to changes in energy distribution in photosynthetic apparatus. The findings stress the significance of considering diurnal variations of SIF and GPP under heatwave conditions when assessing photosynthesis-climate interactions.

The findings showed that SIF is a good indicator of changes in plant physiology during midday depression caused by the high intensity of solar radiation and high temperature, mainly during the heatwave periods.

The Research was founded by National Science Centre, Poland: 2020/39/O/ST10/00775.

How to cite: Abdelmajeed, A. Y. A., Antala, M., Albert-Saiz, M., Stróżecki, M., Rastogi, A., Poczta, P., Julitta, T., Burkart, A., Schuettemeyer, D., Chojnicki, B., and Juszczak, R.: Diurnal variations of solar-induced fluorescence and photosynthesis in a heterogeneous peatland ecosystem under heatwave and non-heatwave conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20509, https://doi.org/10.5194/egusphere-egu24-20509, 2024.

On a global scale, the scientific community is to date unable to close the CO2 budget, with implications for policymakers in regard to limiting global warming to internationally agreed levels. Recently, remote sensing of Solar-Induced Chlorophyll Fluorescence (SIF) has emerged as a promising proxy for Gross Primary Productivity (GPP). The upcoming Fluorescence Explorer (FLEX) mission by the European Space Agency (ESA) is anticipated to offer unprecedented spatio-temporal and spectral resolution of SIF data, raising high expectations towards assessing GPP on a global scale. However, the relationship between SIF and GPP is intricate, varying with environmental conditions due to the influence of a third process — Nonphotochemical Quenching (NPQ). NPQ critically affects this relationship by utilizing the same energy pool. Hence, NPQ is not unfortunately not directly measurable through remote sensing but previous studies have employed the Photochemical Reflectance Index (PRI) as a proxy for NPQ. Yet, a systematic assessment of the PRI-NPQ relationship is still lacking, as previous works were limited to case studies, confined to the studied ecosystem and the environmental conditions during the study period.

In this contribution, our primary aim is to contribute to enhancing SIF as a remotely sensed proxy for GPP. We present the initial findings of our investigation into the robustness of the PRI-NPQ correlation under diverse environmental conditions. To achieve this goal, we leverage a unique dataset that includes joint measurements of hyperspectral reflectance (Data: Fluorescence BoX, JB Hyperspectral Devices GmbH) and active chlorophyll fluorescence (Data: MONI-TORING-PAM Multi-Channel Chlorophyll Fluorometer, Walz) from seven European ecosystems spanning the years 2018 to 2022.

How to cite: Hänchen, L., Martini, D., Sakowska, K., Migliavacca, M., Pacheco-Labrador, J., Duveiller, G., Schwarz, M., Hammerle, A., Scholz, K., Galvagno, M., Julitta, T., Spielmann, F., Van Wittenberghe, S., and Wohlfahrt, G.: Establishing the Photochemical Reflectance Index (PRI) as a reliable proxy for Non-Photochemical Quenching (NPQ), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1901, https://doi.org/10.5194/egusphere-egu24-1901, 2024.

Carbonyl sulphide (COS) has been used in earlier studies as a proxy for determining photosynthesis. To this end, the concepts of leaf relative uptake (LRU, ratio of COS and CO₂ deposition velocities at leaf scale) and ecosystem relative uptake (ERU, ratio of COS and CO₂ deposition velocities at ecosystem scale) are often used. We have constructed a new canopy model that simulates LRU and ERU. This model consists of multiple layers, each having its own air temperature, COS, CO₂ and H₂O mixing ratio. Sunlit and shaded leaves are modelled separately. We coupled this model to the Chemistry-Land Surface Soil Slab (CLASS) model to simulate the atmospheric mixed layer and surface layer above the canopy. An inverse modelling framework is built around these models, allowing for an optimisation of model parameters. In our presentation we will mostly focus on using this framework to analyse the differences in leaf relative uptake in the model, that together influence the overall ERU. We find large differences in LRU between sunlit and shaded leaves, to a large extent caused by differences in stomatal conductance.

How to cite: Bosman, P. and Krol, M.: Disentangling carbonyl sulphide ecosystem relative uptake using (inverse) canopy modelling , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9059, https://doi.org/10.5194/egusphere-egu24-9059, 2024.

Carbonyl sulphide has potential as a tracer of gross primary productivity. However, its use at the ecosystem scale requires us to understand something about its atmospheric transport and the distribution of sources and sinks within the environment of interest. Despite the importance of understanding the controls on carbon uptake and release by Amazonian forests, very little is known about the carbonyl sulphide cycle of widespread terra firme ecosystems.

Here we report on carbonyl sulphide exchange estimated from concentration measurements of carbonyl sulphide at various heights, atmospheric conditions and net carbon dioxide exchange on an 80 m tower, and from soil flux chambers at the Amazon Tall Tower Observatory. The landscape surrounding the measurement site, 150 km north-east of the Brazilian city of Manaus, is typical of the central Amazon consisting of plateaus and steep valleys. Growing on highly weathered and well-drained Ferralsols, these plateaus are covered by old-growth, terra firme forests reaching 30 to 35 m in height. The region experiences relatively stable temperatures throughout the year, but pronounced seasonality in rainfall with a minimum in August and maximum in March.

Atmospheric measurements suggest that the forest within the tower footprint is generally a net sink for carbonyl sulphide. However, fires likely represent a regionally significant source of carbonyl sulphide during the dry season. Net uptake of carbonyl sulphide is greater during the day than the night indicating a strong link to light control of stomatal opening. Estimating gross primary productivity from this uptake is complicated by transport dynamics and soil activity. The tall canopy and diurnal variations in atmospheric mixing, with overnight drawn down followed by entrainment of the upper atmosphere after dawn, means storage has a large influence on net exchange at sub-daily timescales. At longer timescales these exchanges appear to cancel out, simplifying the estimation of average uptake. Similarly, uptake of carbonyl sulphide by the soil represents a significant and variable proportion of the estimated net exchange that needs to be considered when estimating the contribution of photosynthesis.

How to cite: Jones, S. P., Acosta, R., Ferreira, R. R., Cely Toro, I. M., Quaresma Dias-Junior, C., Wolff, S., Martins, G., Kesselmeier, J., and Trumbore, S.: Carbonyl sulphide uptake by a terra firme forest in the central Amazon., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2379, https://doi.org/10.5194/egusphere-egu24-2379, 2024.

The isotopic composition of water vapor (e.g., δ¹⁸O, δ²H) can be a powerful tracer to disentangle water vapor transport, mixing, and phase-changes (e.g., evaporation and condensation) that govern processes of the atmospheric hydrological cycle. The development and improvement of commercial laser-based spectrometers has expanded in situ continuous observations of water vapor isotope composition in a variety of sites worldwide. Nevertheless, until recently, no observations exist from the Amazon basin, a region influenced by the largest tropical rain forest that recycles significant fraction of precipitation as evapotranspiration (ET), thereby influencing regional and global atmospheric water cycling. Continuous water vapor isotope observation combined with meteorological and flux tower measurements in the Amazon rainforest are of high importance to better understand how rainforest ET contributes to regional atmospheric moisture cycles.

We report initial observations of water vapor isotope compositions at the Amazon Tall Tower Observatory (ATTO) site, located in an intact upland forest in the central Amazon, during August-September (dry-season) in 2022. A commercial cavity-ring down (CRDS) analyzer (L2140-i model, Picarro, Inc., USA) continuously measured water vapor concentration and isotope composition at four tower heights (79, 38, 24, and 4 m above ground) in and above the canopy (canopy height ~30 m). We assessed δ¹⁸O and δ²H relationships (i.e., local meteoric water line, LMWL) of different water sources (e.g., water vapor, soil water, leaf water) and deuterium excess (D-excess; D-excess = δ²H − 8 × δ¹⁸O) to trace processes that contribute to atmospheric moisture variations inside and above the canopy. For assessing the contribution of local ET to the total atmospheric moisture (i.e., local moisture recycling), the Keeling plot and intersection point methods were applied to estimate the isotope signatures of ET and background vapor, respectively.  

The LMWL of water vapor at ATTO site was δ²H = 5.2 × δ¹⁸O − 12.6, with a considerably lower slope than the Global Meteoric Water Line (δ²H = 8 × δ¹⁸O + 10). This indicates that rainforest ET significantly influences local atmospheric moisture signals. D-excess in water vapor generally increased from the early morning towards the afternoon, and reached maximum values between 12 pm and 4 pm, indicating that local processes in evaporation and transpiration contribute to local atmospheric moisture signals during daytime. In addition, the diel D-excess variation showed the positive logarithmic relationship with VPD, which indicates that VPD is the key factor for regulating diel moisture isotope signals. Based on isotope mixing models, the estimated contribution fraction of rainforest ET to the total atmospheric moisture showed maximum values (c.a., 20 % to 60 %) in the afternoon, indicating the significant contribution of rainforest ET to regional atmospheric moisture. 

How to cite: Komiya, S., Jones, S. P., Moonen, R., Adnew, G. A., Botia, S., Asperen, H. V., Dias-Júnior, C. Q., Cely Toro, I. M., Kondo, F., and Trumbore, S.: The isotopic composition of atmospheric water vapor during dry season in a central Amazon rainforest: Insights into local moisture recycling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9637, https://doi.org/10.5194/egusphere-egu24-9637, 2024.

Climate change is projected to increase the frequency and intensity of droughts, inducing further water stress on vegetation. These water stress conditions impact both vegetation carbon and water exchanges that are regulated through stomatal diffusion. Land surface models (LSMs) have been developed to simulate the amount of carbon taken up by vegetation through photosynthesis (GPP), and the emission of water into the atmosphere through plant transpiration. However, LSMs struggle to accurately simulate vegetation response to drought, which contributes to the strong uncertainty on GPP and plant transpiration estimates and is critical for future projections. Moreover, LSMs represent different vegetation types by grouping plants with similar characteristics in terms of structure, behavior, and climatic conditions, therefore not accounting for possible differences in vegetation response to drought due to the diversity of environmental conditions within the same biome. In the ORCHIDEE LSM, we used in situ GPP and latent heat flux (LE) estimates at more than 40 sites from the FLUXNET Warm Winter 2020 network, which captures the recent drought years over Europe, to evaluate and refine the simulated vegetation response to soil water stress. We performed multisite data assimilation experiments based on the dominant vegetation type at these sites to optimize the parameters involved in vegetation response to soil water stress using the in situ GPP and LE estimates. We found that the optimized values of the coefficient that determines the speed of vegetation response to soil water stress can be defined as a function of the mean annual vapor pressure deficit (VPD). This new function enables to consider the environmental conditions on site through VPD in vegetation response to soil water stress, instead of having a response that only depends on the vegetation type. During a drought event, this soil water stress function induces vegetation growing under low VPD to close its stomata faster than vegetation acclimated to higher VPD conditions. Finally, regional simulations were performed to evaluate the impact of including this dependency on VPD in vegetation response to water stress over the recent European drought years.  

How to cite: Abadie, C., Maignan, F., Peylin, P., Vuichard, N., Raoult, N., and Bastrikov, V.: Revisiting vegetation carbon and water flux representation during drought events in the ORCHIDEE land surface model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5665, https://doi.org/10.5194/egusphere-egu24-5665, 2024.

Ecosystem-scale estimates of net photosynthesis may be derived from eddy covariance measurements of net ecosystem exchange through the application of flux-variance similarity theory. Net photosynthesis, which is defined as carboxylation minus photorespiration and leaf respiration, differs from gross primary production by the leaf respiration term, which has been implicated as a potential source of error for traditional flux partitioning approaches. Here, we focus on seasonal dynamics of net photosynthesis and leaf respiration by deriving relevant variables (e.g. magnitude of dark respiration, light-saturated rate of net photosynthesis, sensitivity of leaf respiration to light intensity) through rectangular hyperbolic fits of net photosynthesis to photosynthetically active radiation (PAR) throughout the growing season. We find that the magnitude of dark leaf respiration decreases throughout the growing season, while the sensitivity of leaf respiration to light intensity and light-saturated net photosynthesis remain relatively stable. The level of PAR required for carboxylation minus photorespiration to exceed leaf respiration increases over the course of the growing season. We examine how environmental variables, specifically air temperature and volumetric soil moisture, influence these aspects of net photosynthesis. Estimates of leaf-level water use efficiency, a key parameter in the flux-variance similarity theory approach, are evaluated through comparisons with co-located measurements of solar induced fluorescence and sap flux.

How to cite: Scanlon, T. and Tatham, E.: Growing season dynamics of net photosynthesis and leaf respiration for a mixed hardwood forest as inferred from flux-variance similarity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13648, https://doi.org/10.5194/egusphere-egu24-13648, 2024.