The Science
The role of phosphorus availability has not been considered in any of the CMIP5 models. This study shows that phosphorus availability could greatly reduce the projected CO2-induced carbon sink in Amazon rainforests. This study suggests that the Amazon rainforest response to increasing atmospheric CO2 depends upon the ability of trees to up-regulate phosphorus acquisition in response to increased carbon availability.

The Impact
Currently CMIP5 models predict that the Amazon rainforest will continue to act as carbon sinks in the future due to the CO2 fertilization effect. However the role of phosphorus availability (which is impoverished across the Amazon Basin yet controls forest functioning) has not been considered within CMIP5 simulations. This study suggests that the CMIP5 predicted carbon sink would likely be much less due to phosphorus limitation, suggesting Amazon rainforests may be less resilient to climate change than previously assumed.

Summary
An ensemble of 14 terrestrial ecosystem models was used to simulate the planned free-air CO2 enrichment experiment AmazonFACE. Model simulations showed that phosphorus availability reduced the projected CO2-induced carbon sink by about 50% compared to estimates from models assuming no phosphorus limitation. Large variations in ecosystem responses to elevated CO2 among Phosphorus-enabled models (ranging from 5 to 140 g Carbon m-2 yr-2 in biomass carbon response) are mainly due to contrasting representations of plant phosphorus use and acquisition strategies among models. This study highlights the importance of phosphorus acquisition and use, including alternative strategies, in Amazon rainforest responses to increasing atmospheric CO2 concentration.

 

 

 

Contacts (BER PM): Dan Stover and Sally McFarlane, Daniel.Stover@science.doe.gov (301-903-0289) and sally.mcfarlane@science.doe.gov (301-903-0943)

PI Contact: Jeffrey Q. Chambers, Lawrence Berkeley National Lab, jchambers@lbl.gov

Funding
DE-AC02-05CH11231 as part of their Next Generation Ecosystem Experiment-Tropics (NGEE-Tropics) and Exascale Energy Earth System Model (E3SM) programs.

Publication
Fleischer K, A. Rammig, M.G. De Kauwe, A. P. Walker, T.F. Domingues, L. Fuchslueger, S. Garcia, D. Goll, A. Grandis, M. Jiang, V.E. Haverd, F. Hofhansl, J. Holm, B. Kruijt, F. Leung, B. Medlyn, L.M. Mercado, R.J. Norby, B.C. Pak, B. Quesada, C. von Randow, K. Schaap, O. Valverde-Barrantes, Y. Wang, X. Yang, S. Zaehle, Q. Zhu, D. Lapola. Amazon forest responses to CO2 fertilization dependent on plant phosphorus acquisition, Nature Geoscience, https://doi.org/10.1038/s41561-019-0404-9, 2019.

The Science
In the Amazon basin, an estimated 25–50% of precipitation is recycled back to the atmosphere through forest transpiration, with important implications for the interactions between the biosphere and atmosphere. In this study, we present in situ field observations of environmental (direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)) and physiological (sap velocity (Vs), stomatal conductance (gs), and leaf water potential (ΨL)) variables and their correlations with leaf temperature (Tleaf) during the 2015–2016 El Niño-Southern Oscillation (ENSO). In order to observe the interactions between physiological variables and fast changing environmental conditions, we collected high temporal frequency data (15–60 min) in two primary rainforest sites located in the Eastern (Santarém) and in the Central (Manaus) Amazon. Since the 2015–2016 ENSO event was the warmest period in the Amazon forest over the past 13 years, we expected peak Tleaf to increase and subsequently hysteretic behavior of water use vs. Tleaf to become more pronounced. In this study, we explored the mechanisms that regulate tree transpiration and the diurnal hysteresis patterns between physiological and environmental variables to contrast different tree species responses to the extreme 2015 dry season (ENSO) and a normal 2017 dry season (“control scenario”).

The Impact
Despite expectations of a significant delay due to the large vertical distance between the observations of sap velocity (Vs) and leaf temperature (Tleaf), the two variables tightly track each other, during the day and night. On some days during the 2015 dry season, the differences between Tleaf and Tair were close to 8°C for some species (average difference between Tleaf and Tair for all species: 1.65 ± 1.07°C). For the first time in the Amazon forest, the quantitative differences and the hysteresis pattern between Tleaf and Tair were demonstrated and compared during the 2015 (ENSO) and 2017 (“control scenario”) dry seasons. The relationship between Tleaf and Tair was significantly different between these two periods and, in general, Tleaf was higher than Tair during the middle morning to early afternoon. The use of the variable Tleaf together with Tair is extremely important to ecophysiological observations due to the differences in terms of magnitude and temporal patterns. Also, Tleaf is an important variable to estimate the true water vapor pressure gradient between the substomatal cavity and the boundary layer of the air near the leaf surface (ΔVPD).

Sap velocity displayed species-specific diurnal hysteresis patterns that were strongly linked to gs and ΔVPD and reflected by changes in Tleaf. In the morning, gs was linearly related to Tleaf and sap velocity displayed a sigmoidal relationship with Tleaf. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. Hysteresis indices (Tleaf : Tair and Tleaf : ΨL) were much more pronounced during the ENSO event than during a typical dry season and varied between species, which reflects species-specific capacitance and tree hydraulic traits. The clockwise hysteresis in Vs-Tleaf and Vs-ΔVPD was evident with morning periods showing higher temperature sensitivities than afternoon and night periods and in this study is referred to as the “gs effect”. In Manaus, the scatter plot of Vs-direct solar radiation revealed a counterclockwise hysteresis pattern, on the same day as the Vs-ΔVPD clockwise hysteresis. For the same direct solar radiation values, higher Vs values in the afternoon were observed relative to the morning period, and in this study this pattern is referred to as the “VPD effect.”

Summary
Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015–2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (Vs), stomatal conductance (gs) and leaf water potential (ΨL) of multiple canopy trees, and their correlations with leaf temperature (Tleaf) and environmental conditions [direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of gs (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in Tleaf. In the morning, Tleaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. In Manaus, hysteresis indices of the variables Tleaf-Tair and ΨL-Tleaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season (“control scenario”). In some days during the 2015 ENSO event, Tleaf approached 40°C for all studied species and the differences between Tleaf and Tair reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, Tleaf was higher than Tair during the middle morning to early afternoon, and lower than Tair during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in ΨL during the afternoon period giving an observed counterclockwise hysteresis pattern between ΨL and Tleaf.

 

 

 

 

 

 

 

 

 

Contacts (BER PM): Daniel Stover SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Bruno O. Gimenez, Smithsonian Tropical Research Institute (STRI), bruno.oliva.gimenez@gmail.com
Kolby J. Jardine, Lawrence Berkeley National Laboratory (LBNL), Climate and Ecosystem Sciences Division, kjjardine@lbl.gov

Funding
This material is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research through contract No. DE-AC02-05CH11231 to LBNL, as part of DOE’s Terrestrial Ecosystem Science Program. Additional funding for this research was provided by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

Publications
Gimenez B., Jardine K., Higuchi N., Negrón-Juárez R., Sampaio-Filho I., Cobello L., Fontes C., Dawson T., Varadharajan C., Christianson D., Spanner G., Araújo A., Warren J., Newman B., Holm J., Koven C., McDowell N., Chambers J., (2019) Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015-2016 El Niño Event in the Amazon Forest Front. Plant Sci. 10:830. https://doi.org/10.3389/fpls.2019.00830

Related Links
https://www.frontiersin.org/articles/10.3389/fpls.2019.00830/full

The Science
During El Niño events, atmospheric teleconnections with sea surface temperature (SST) anomalies in the equatorial Pacific cause higher temperatures and reduced rainfall in the Amazon, leading to increased CO2 emissions. While some of the temperature increase results directly from the SST-atmosphere teleconnection, drier soil resulting from reduced rainfall can also contribute to higher temperatures and resulting CO2 flux anomalies. Researchers from the University of California, Irvine and the Oak Ridge National Laboratory modified the Energy Exascale Earth System Model (E3SM) to decouple the direct effects of SST anomalies from the resulting soil moisture anomalies, in order to determine the relative importance of each of these drivers.

The Impact
Soil moisture variability was found to amplify and extend the effects of SST forcing on eastern Amazon temperature and carbon fluxes in E3SM. During the wet season, the direct, circulation-driven effect of El NIño-Southern Oscillation (ENSO) SST anomalies dominated temperature and carbon cycle variability throughout the Amazon. During the following dry season, after ENSO SST anomalies had dissipated, soil moisture variability became the dominant driver in the east, explaining 67–82% of the temperature difference between El Niño and La Niña years, and 85–91% percent of the difference in carbon fluxes. These results highlight the need to consider the interdependence between temperature and hydrology when attributing the relative contribution of these factors to interannual variability of the terrestrial carbon cycle. Specifically, when offline models are forced with observations or reanalysis, the contribution of temperature may be overestimated when its own variability is modulated by hydrology via land-atmosphere coupling.

Summary
We demonstrated that in E3SM, soil moisture anomalies resulting from SST variability extended and strengthened the temperature and CO2 flux anomalies associated with ENSO. This indicates the need to consider the interdependent relationship between temperature and the hydrologic cycle  when attributing mechanisms to ENSO-driven variability in the tropical terrestrial carbon cycle.

 

Contacts (BER PMs): Dr. Renu Joseph (Primary), SC-23, Renu.Joseph@science.doe.gov (301-903-9237); Dr. Dorothy Koch, SC-23, Dorothy.Koch@science.doe.gov (301-903-0105); Dr. Daniel Stover, SC-23, Daniel.Stover@science.doe.gov (301-903-0289)

Author Contact: Paul A. Levine, University of California, Irvine (paul.levine@uci.edu, 949-824-9030) and Dr. James T. Randerson, University of California, Irvine (jranders@uci.edu, 949-824-9030)

PI Contact: Dr. Forrest M. Hoffman, Climate Change Science Institute (CCSI), Oak Ridge National Laboratory, forrest@climatemodeling.org (865-576-7680)

Funding
This research was funded, in part, by the Reducing Uncertainties in Biogeochemical Interactions through Synthesis and Computation Scientific Focus Area (RUBISCO SFA), which is sponsored by the Regional and Global Model Assessment (RGMA) Program in the Climate and Environmental Sciences Division (CESD) of the Office of Biological and Environmental Research (BER) in the US Department of Energy (DOE) Office of Science. This research used resources from Project m2467 of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility (DE-AC02-05CH11231), and from Project cli106bgc of the Oak Ridge Leadership Computing Facility, which is a DOE Office of Science User Facility (DE-AC05-00OR22725). P.A.L. received funding support from NASA Headquarters under the NASA Earth and Space Science Fellowship Program (NNX16AO38H). J.T.R. and Y.C. received additional funding support from the DOE Office of Science Earth System Modeling Program (DE-SC0006791) and NASA’s SMAP and CMS programs. M.S.P. received funding support from the DOE Early Career Program (DE-SC0012152). M.X. and F.M.H. received additional funding support from the Energy Exascale Earth System Model (E3SM) Project and the Next Generation Ecosystem Experiments–Tropics (NGEE-Tropics) Project, sponsored by BER in the DOE Office of Science.

Publications
Levine, P. A., J. T. Randerson, Y. Chen, M. S. Pritchard, M. Xu, and F. M. Hoffman (2019), Soil moisture variability intensifies and prolongs eastern Amazon temperature and carbon cycle response to El Niño-Southern Oscillation, J. Clim., 32(4):1273–1292, doi:10.1175/JCLI-D-18-0150.1.

Related Links
Reducing Uncertainties in Biogeochemical Interactions through Synthesis and Computation (RUBISCO) Scientific Focus Area. https://www.bgc-feedbacks.org/
Energy Exascale Earth System Model (E3SM). https://e3sm.org/
Next Generation Ecosystem Experiments–Tropics (NGEE-Tropics). https://ngee-tropics.lbl.gov/

Below versus above Ground Plant Sources of Abscisic Acid (ABA) at the Heart of Tropical Forest Response to Warming

Abscisic Acid mediates tropical forest response to warming

The Science
While assumed to be transported from roots to leaves where it controls transpiration and net photosynthesis by modifying stomatal conductance, we provide evidence that the powerful phytohormone abscisic acid (ABA) is produced mainly in leaves linked to photosynthesis (Scenario 2).

The Impact
We highlight the fact that ABA lies at the heart of tropical forest response to environmentally extremes by serving as a key endogenous factor that integrates oxidative stress and defense mechanisms, hydraulics, and carbon and energy metabolism with environmental variables. We discuss potential variations in ABA production and stomatal sensitivity among different plant functional types including isohydric/anisohydric and pioneer/climax tree species. We describe experiments that would demonstrate the possibility of a direct energetic and carbon link between leaf ABA biosynthesis and photosynthesis, and discuss the potential for a positive feedback between leaf warming and enhanced ABA production together with reduced stomatal conductance and transpiration. Finally, we propose a new modeling framework to capture these interactions.

Summary
Warming surface temperatures and increasing frequency and duration of widespread droughts threaten the health of natural forests and agricultural crops. High temperatures (HT) and intense droughts can lead to the excessive plant water loss and the accumulation of reactive oxygen species (ROS) resulting in extensive physical and oxidative damage to sensitive plant components including photosynthetic membranes. ROS signaling is tightly integrated with signaling mechanisms of the potent phytohormone abscisic acid (ABA), which stimulates stomatal closure leading to a reduction in transpiration and net photosynthesis, alters hydraulic conductivities, and activates defense gene expression including antioxidant systems. While generally assumed to be produced in roots and transported to shoots following drought stress, recent evidence suggests that a large fraction of plant ABA is produced in leaves via the isoprenoid pathway. Thus, through stomatal regulation and stress signaling which alters water and carbon fluxes, we highlight the fact that ABA lies at the heart of the Carbon-Water-ROS Nexus of plant response to HT and drought stress. We discuss the current state of knowledge of ABA biosynthesis, transport, and degradation and the role of ABA and other isoprenoids in the oxidative stress response. We discuss potential variations in ABA production and stomatal sensitivity among different plant functional types including isohydric/anisohydric and pioneer/climax tree species. We describe experiments that would demonstrate the possibility of a direct energetic and carbon link between leaf ABA biosynthesis and photosynthesis, and discuss the potential for a positive feedback between leaf warming and enhanced ABA production together with reduced stomatal conductance and transpiration. Finally, we propose a new modeling framework to capture these interactions. We conclude by discussing the importance of ABA in diverse tropical ecosystems through increases in the thermotolerance of photosynthesis to drought and heat stress, and the global importance of these mechanisms to carbon and water cycling under climate change scenarios.

Contacts (BER PM): Dan Stover, SC-23.1

PI Contact: Kolby J. Jardine, Lawrence Berkeley National Laboratory (LBNL), Climate and Ecosystem Sciences Division, kjjardine@lbl.gov

Funding
This material is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research through contract No. DE-AC02-05CH11231 to LBNL, as part of DOE’s Terrestrial Ecosystem Science Program. Additional funding for this research was provided by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

Publications
Sampaio Filho I, Jardine K, Oliveira R,  Gimenez B, Cobello L, Piva L, Candido L, Higuchi N, Chambers J (2018) Below versus above ground plant sources of abscisic acid (ABA) at the heart of tropical forest response to warming, International journal of molecular sciences, 19(7), 2023. http://dx.doi.org/10.3390/ijms19072023.

Climate Sensitive Size-dependent Survival in Tropical Trees

Survival of trees in the tropics can be grouped into four types and the abundance of each type is related to climatic factors.

The Science
Discovered size-dependent survival of tropical tree species is classifiable into four major categories of plants, is not related to common plant traits and whose abundance is related to climatic factors.

The Impact
Our work indicates the size-dependent models of forest dynamics can represent tropical forests survival dynamics with four groups of species. This can have a major impact on how we model forests at the global scale and improve our predictions of carbon in tropical forests which is an outstanding area of uncertainty thus far.

Summary
We found species were classifiable into four “survival modes” that explain life-history variation that shapes carbon-cycling and the relative abundances within forests. Frequently collected functional traits, such as wood density, leaf mass per area, and seed mass, were not generally predictive of species’ survival modes. Mean annual temperature and cumulative water deficit predicted the proportion of biomass of survival modes, indicating important links between evolutionary strategies, climate, and carbon cycling. The application of survival modes in demographic simulations predicted biomass change across forest sites. Our results reveal globally identifiable size-dependent survival strategies that differ across diverse systems in a consistent way. The abundance of survival modes and interaction with climate ultimately determine forest structure, carbon storage in biomass, and future forest trajectories.

Contacts (BER PM):
Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)
Dorothy Koch, SC-23.1, Dorothy.Koch@science.doe.gov (301-903-0105)

PI Contact: Nate McDowell, Pacific Northwest National Laboratory, nate.mcdowell@pnnl.gov

Funding
The development of this project benefited from ForestGEO workshops in 2015, 2016 and 2017 (NSF DEB-1046113 to S.J. Davies). Contributions by C. Xu, J. Chambers, S. Davies and N. McDowell were supported by the Next-Generation Ecosystem Experiments (NGEE-Tropics) project, funded by the U.S. Department of Energy, Office of Biological and Environmental Research. D. Johnson was supported by Los Alamos National Laboratory (Director’s Post-doctoral Fellowship).

Publications
DJ Johnson et al. (47 coauthors), “Climate sensitive size-dependent survival in tropical trees.” Nature Ecology and Evolution (2018). (in press)]