Atmospheric CO₂ concentrations are still rising due to land-use change and fossil fuel burning, and have unambiguously influenced Earth’s climate system and terrestrial ecosystems. Plant responses to rising atmospheric CO₂ concentrations may have induced an increase in biomass and thus, increased the carbon sink in forests worldwide. Rising CO₂ directly stimulates photosynthesis (the so-called CO₂-fertilization effect) and tends to reduce stomatal conductance, leading to enhanced water-use efficiency, which may provide an important buffering effect for plants during adverse climate conditions. For these reasons, current global climate simulations consistently predict that tropical forests will continue to sequester more carbon in aboveground biomass. However, several lines of evidence point towards a decreasing carbon sink strength of the Amazon rainforest in the coming decades, potentially driven by nutrient limitation, droughts or other factors. Mechanistically modelling the effects of rising CO₂ in the Amazon rainforest are hindered by a lack of direct observations from ecosystem scale CO₂ experiments. To address these critical issues, we are currently building a free-air CO₂ enrichment (FACE) experiment in an old-growth, highly diverse, tropical forest in the Brazilian Amazon and we here present our main hypotheses that underpin the AmazonFACE experiment.  We focus on possible effects of rising CO₂ on carbon uptake and allocation, phosphorus cycling, water-use and plant-herbivore interactions, and discuss relevant ecophysiological processes, which need to be implemented in dynamic vegetation models to estimate future changes of the Amazon carbon sink. We also report on baseline measurements at the AmazonFACE site and show the progress on model development with regard to phosphorus uptake strategies.

How to cite: Anja, R. and David, L. and the AmazonFACE Team: AmazonFACE: A Free Air CO2 Enrichment Experiment in the Amazon rainforest is ready to launch, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11338, https://doi.org/10.5194/egusphere-egu23-11338, 2023.

Tropical rainforests in the Amazon play a vital role in absorbing atmospheric CO2, yet there remains a significant degree of uncertainty surrounding the process, magnitude, and potential future changes. In this study, we used outputs of Earth System Models (ESMs) to explore the impacts of dry-season precipitation changes on the terrestrial carbon cycle, with a specific focus on Gross Primary Production (GPP), which has yet to be thoroughly examined. The seasonal amplitude of GPP over the Amazon rainforests from the CMIP6 ensemble displayed significant variations among models resulting from dry-season decline, and these are much higher amplitude compared to reference data from 1981-2000. The regions with less precipitation are particularly vulnerable in the models, and this dry-season decline is anticipated to significantly intensify by the end of the 21st century due to the drying associated with global warming.

How to cite: Taguchi, T. and Ichii, K.: Analyzing the sensitivity of gross primary production to the water stress in the Amazon rainforest using CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6496, https://doi.org/10.5194/egusphere-egu23-6496, 2023.

Amazonia contains the most extensive tropical forests on Earth, but the role of the region as a carbon appears to be declining. Increasing deforestation, fire and climate change-associated increases in drought, threaten to push forests past a tipping point. However, forests are complex, exhibiting drought responses indicative of both resilience (photosynthetic “greening”) and vulnerability (browning and tree mortality) that are difficult to explain by climate variation alone. Still needed is a framework for understanding and predicting how different regions will respond to different kinds of future drought. Here, we combine remotely-sensed photosynthetic vegetation indices (enhanced vegetation index, EVI, corrected for sun-sensor geometry; and solar-induced chlorophyll fluorescence, SIF) with ground-based tree demography to test recent ecological hypotheses about forest drought resilience and vulnerability for different forest ecotypes across the basin, defined by their water-table depth, soil fertility and vegetation characteristics. In high-fertility southern Amazonia, drought response was importantly structured by water-table depth, with resilient greening in shallow-water-table-forests (where greater water availability heightened responsiveness to excess sunlight) contrasting with vulnerability (“browning” and excess tree mortality) over deeper water tables. Notably, shallow-water-table-forest resilience weakened as droughts lengthened. By contrast, low-fertility northern Amazonia, with slower-growing but drought-hardy trees (or tall trees, with deep-rooted water access), supported more drought-resilient forests independent of water-table depth. This work reveals a new biogeography of forest drought response that provides a framework for conservation decisions and improved prediction of heterogeneous forest responses to future climate changes, but warns that longer/more frequent droughts undermine these multiple ecohydrological strategies of Amazon forest resilience.

How to cite: Chen, S., Stark, S., Nobre, A., Cuartas, L., Amore, D., Restrepo-Coupe, N., Smith, M., Chitra-Tarak, R., Ko, H., Nelson, B., and Saleska, S.: Regional ecotypes structure biogeography of Amazon forest drought resilience and vulnerability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11005, https://doi.org/10.5194/egusphere-egu23-11005, 2023.

The study of forest fragmentation, the break-up of forests into smaller patches, has become increasingly important due to increases in human-induced forest clearance, with 12 million hectares of forest being lost per year and 32% of this loss being tropical. There is substantial evidence showing that edge effects can alter the ecology structure and vertical profile of remaining forests, even hundreds of meters from the forest edge.  However, implementing empirical experiments (for example in the framework of the Biological Dynamics of Forest Fragments Project (BDFFP)) to understand the effects of fragmentation on forest structural traits can be logistically and scientifically challenging and limited to smaller areas.  The use of forest models may help overcome these limitations, as they are able to quickly reproduce long-term data, as well as simulate a broad range of geographical conditions. This study aimed to reproduce the vertical distribution of plants in Amazonian forests affected by fragmentation using the forest model FORMIND. To achieve this, we optimized parameters driving plant demography and mortality, as well as their response to edge effects. FORMIND is an individual and process-based gap model suited for species rich vegetation communities, with the option of a fragmentation module. We modified processes and parameters in FORMIND to mimic the dynamics observed in the BDFFP experiment. Forest structural traits extracted from the FORMIND model output were compared with those obtained from terrestrial laser scans of the BDFFP fragments. The resulting simulations demonstrated that, after 40 years of edge effects, the optimized model was able to reproduce similar results to those observed using the terrestrial LiDAR system. Total plant area index (PAI), and PAI at varying height intervals (PAI 0-10m, PAI 10-20m, PAI 20-30m), showed consistent responses from edge effects, thus resulting in an adequate vertical plant distribution. Results demonstrate that, forest models such as FORMIND have strong potential to study the mechanisms and the impact of environmental changes on forests. Models can also expand the possibilities of in-situ studies, which are limited in time and space, when calibrated carefully with suitable in-situ data, here delivered by terrestrial LiDAR.

How to cite: Downie, E., Fischer, R., Knapp, N., Ghizoni Santos, E., Fassnacht, F., Camargo, J. L., Andrade, A., and Maeda, E.: Creating virtual forests to understand fragmentation in tropical ecosystems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11315, https://doi.org/10.5194/egusphere-egu23-11315, 2023.

Losses in above- and belowground biodiversity are linked to changes in land use practices and immediately affect processes in the upper soil horizons. Theoretically, these superficial changes are reversible unless a tipping point on an ecosystem level was crossed. This safety net of functional redundancy facilitates the return of vital soil functions and processes to the initial state. Little is known about how deep these changes have reached into the subsoil over time, because early warning indicators of a tipping point being about to be crossed are still sparse. This is especially important for the tropics, as soils are typically intensively weathered and nutrient depleted, therefore, plants relying largely on nutrients from below. Net nitrous oxide (N₂O) emission rates from the soil surface have proved to be valid proxies for detecting the crossing of tipping points in soil biogeochemistry. Here, we advanced this approach by looking deeply into the soil and reveal that potential greenhouse gas (GHG) production and consumption are useful proxies to estimate how deep the loss of aboveground biodiversity has already impacted soil microbial processes. We performed incubation experiments on forest and pasture soils stemming from shallow as well as 1 m deep profiles from the Peruvian Amazon Basin to determine the production and consumption potential of the GHGs at different water holding capacities. We expected pasture soils to have lost direct carbon(C)-related connections to deeper soil horizons. In forests, roots with different and greater depths also connect deeper soil layers. Therefore, in greater depth, carbon dioxide (CO₂) production declined faster under pastures than under forests because C limitations are reached sooner. In the surface, grasses are well known for their input of C and increased CO₂ production. Since old pastures are limited in nitrogen (N), almost no N₂O should be produced, possibly increasing the potential to take up N₂O. However, denitrification is a heterotrophic process also dependent on available C, therefore, the potentials of N₂O production and consumption in deeper forest soils were larger with increasing water holding capacity. These findings could indicate that losses in aboveground biodiversity due to forest conversion can have profound impacts on soil microbial processes, extending also to deeper soil layers while altering the connection of the subsoil to the top.

How to cite: Kilian Salas, S., Andrino, A., Díaz García, E., Boy, D., Horn, M. A., Boy, J., Guggenberger, G., and Jungkunst, H. F.: Forest conversion cuts off biogeochemical connections of the subsoil to the top, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17108, https://doi.org/10.5194/egusphere-egu23-17108, 2023.

How are carbon dioxide assimilation by photosynthesis and (shallow) cumulus clouds connected? What is the local interaction between rainforest evapotranspiration and cloud formation modulated by incoming regional air masses? These interrelated questions were the main drivers of the experimental campaign CloudRoots-Amazonia22 that took place at the ATTO/Campina supersites in the pristine Amazon rainforest during August 2022 (dry season). CloudRoots-Amazonia22 collected observational data to derive relationships between leaf level processes to canopy scales and connected them to the diurnal evolution of the clear to cloudy atmospheric boundary layer. At leaf level, first results indicate a diurnal asymmetry of the leaf conductance with maximum openings before midday. These observations are related to radiative energy fluxes to study the partitioning into sensible and latent heating and of plant-soil carbon dioxide exchnages. By coupling measurements of carbon and water stable isotopologues by fast laser instruments to turbulence measurements we aim to quantify flux variations related to the radiation fluctuations driven by clouds. These observations are integrated with 75 soundings of state variables and greenhouse gas profiles taken by flights below, through and above the cloud layers. We investigate what controls the transition from shallow to deep convection and the causality between the surface-cloud shear and the moisture transport at the interface between the different atmospheric layers. The observational analysis is completed with conceptual modelling and systematic large-eddy simulation experiments, which include dynamic vegetation models to advance our understanding of the diurnal energy, water and carbon cycles over the Amazon rainforest.

How to cite: Vila-Guerau de Arellano, J. and Hartogensis, O. and the CloudRoots and collaborators from Brasil and Germany: CloudRoots-Amazonia22: Integrating clouds with photosynthesis by crossing scales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1307, https://doi.org/10.5194/egusphere-egu23-1307, 2023.

The spatial representativeness of anemometric tower measurements and the underlying hypothesis of horizontal homogeneity of the atmospheric flows have long being questioned in the literature. Further, forest are rarely situated on uniform and flat terrain in which the horizontally homogeneity of turbulence holds. The orography around the Amazon Tall Tower Observatory (ATTO) site makes no exception. The measurement site is located on a narrow plateau SW-NE oriented, surrounded by lower hills. It is then natural to investigate the spatial variability of the turbulent field around the ATTO and the INSTANT towers to understand the role played by gentle topography. The Parallelized Large-Eddy Simulation Model (PALM) is used to simulate the atmospheric flow over the Amazon Forest and to investigate the effect of topography on the Atmospheric flow within and above the roughness sublayer. The real topography of the ATTO site is considered, while a horizontally homogeneous leaf area density (LAD) profile is assumed for the canopy over the whole area. Hence, the flow variability can be totally ascribed to spatial orography gradients. The influence of orography is assessed comparing the profiles of the turbulent kinetic energy components and fluxes evaluate over flat terrain and over the real topography at different position on the horizontal plane. The dependence of the orography influence on the wind direction is investigated considering two different wind directions.

How to cite: de Oliveira, R., Brondani, D., Mortarini, L., Giostra, U., Alves, E., Quesada, C. A., and Dias-Junior, C. Q.: Topography influence on turbulent profiles over the ATTO site, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14926, https://doi.org/10.5194/egusphere-egu23-14926, 2023.

For tropical forests, such as the Amazon Forest, the turbulence intensity at the forest-atmosphere interface is high since in this region there is strong convective activity during the day and the aerodynamic roughness of the forest canopy is high. Heat and other scalar properties are exchanged between the flow and the canopy. Understanding these exchange mechanisms is essential for a variety of applications in various fields of science. Furthermore, it is known that the Amazon region has a strong influence on the transport of heat and water vapor to regions located at higher latitudes and plays an important role in the carbon cycle. Measurements carried out in micrometeorological towers are crucial for the correct quantifications of the turbulent fluxes. However, the use of micrometeorological towers in the Amazon is recent. High frequency measurements (eg Eddy covariance systems) in the Amazon rainforest were usually performed at a single point, often above the forest canopy. The first analyses from the fast response data clearly showed the existence of what is now known as the roughness sublayer (RSL). In these works, it was speculated that the surface boundary layer, was higher up. Within Amazonian RSL, important discoveries have already been made, for example: (i) the Monin-Obuhkov similarity functions are not the most appropriate for estimating turbulent fluxes in the region immediately above the forest canopy. (ii) The Amazonian nocturnal boundary layer is often populated by submeso phenomena, which create episodes of intermittent turbulence and increase the complexity of exchange processes between the forest and the atmosphere during the night. (iii) Above the Amazonian RSL, it was possible to verify that there is no evidence of a classic inertial layer. Since July 2021, the ATTO (Amazon Tall Tower Observatory) tower has been performing continuous measurements, carried out by nineteen 3D-sonic installed from 5 m (inside the forest canopy) to 316 m (above the RSL). Therefore, in this work we will show the profiles of different turbulent fluxes measured since mid-2021 under different stability conditions and at different periods of the year (dry and rainy season). These new measurement profiles, with high vertical resolution, are unique and they will allow us to understand the turbulent exchange processes in regions of the Amazon planetary boundary layer that have not been previously explored.

How to cite: Dias-Junior, C. Q., Dias, N., Acevedo, O., Mortarini, L., Brondani, D., Oliveira, P., Araújo, A., Oliveira, L., Ferreira, R., Acosta, R., Takeshi, B., and Quesada, C. A.: Turbulent Fluxes Within and Above the Amazon Roughness Sublayer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10412, https://doi.org/10.5194/egusphere-egu23-10412, 2023.