News Archive - NGEE

Collection of leaf and canopy level benchmark datasets for FATES testbeds

The Science

This paper showed an increasingly stronger impact on terrestrial gross primary production (GPP) by extreme droughts than by mild and moderate droughts over the twenty-first century. Specifically, the percentage contribution by extreme droughts to the total GPP reduction associated with all droughts was projected to increase from ~28% during 1850–1999 to ~50% during 2075–2099.

The Impact

Even though higher CO2 concentrations in future decades can increase GPP, low soil water availability and disturbances associated with droughts could reduce the benefits of such CO2 fertilization. This study conducted the first global analysis to quantify potential impacts of drought on future GPP, which could guide future modeling and field experiments.

Summary

Terrestrial gross primary production (GPP) is the basis of vegetation growth and food production globally and plays a critical role in regulating atmospheric CO2 through its impact on ecosystem carbon balance. Here we analysed outputs of 13 Earth system models to show an increasingly stronger impact on GPP by extreme droughts than by mild and moderate droughts over the twenty-first century. The droughts were defined on the basis of root-weighted plant accessible water. Due to a dramatic increase in the frequency of extreme droughts, the magnitude of globally averaged reductions in GPP associated with extreme droughts was projected to be nearly tripled by the last quarter of this century (2075–2099) relative to that of the historical period (1850–1999) under both high and intermediate greenhouse gas (GHG) emission scenarios. By contrast, the magnitude of GPP reductions associated with mild and moderate droughts was not projected to increase substantially. These drought impacts were widely distributed with particularly high risks for the Amazon, Southern Africa, Mediterranean Basin, Australia and Southwestern United States. Our analysis indicates a high risk of extreme droughts to the global carbon cycle with atmospheric warming; however, this risk can be potentially mitigated by positive anomalies of GPP associated with favorable environmental conditions.

Contacts (BER PM)

Daniel Stover
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289)

(PI Contact)

Chonggang Xu
Los Alamos National Laboratory, Los Alamos, NM
Email: cxu@lanl.gov; Phone: 505-665-9773

Funding
This work was funded by the Next Generation Ecosystem Experiment–Tropics project and the Survival/Mortality project sponsored by the DOE Office of Science, Office of Biological and Environmental Research, the Laboratory Directed Research and Development program of the Los Alamos National Laboratory and the University of California’s Laboratory Fees Research Program (grant no. LFR-18-542511).

Publications

Xu, C., McDowell, N.G., Fisher, R.A. , L. Wei, S. Sevanto, E. Weng, R. Middleton. Increasing impacts of extreme droughts on vegetation productivity under climate change. Nature Climate Chang. 9, 948–953 (2019) doi:10.1038/s41558-019-0630-6

Below versus above Ground Plant Sources of Abscisic Acid

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.

Age-dependent leaf physiology and consequences for crown-scale carbon

Photosynthetic Capacity of Branches Increases During the Dry Season in a Central Amazon Forest

First direct evidence from individual trees that new leaf growth and development cause overall forest green-up

The Science

Amazon forest ecosystems are observed by satellites to green-up and by towers to increase in photosynthetic uptake during the dry season, but the mechanisms for this at the tree and leaf scale have been much debated. Here we tested how leaf age-dependent physiology and leaf demography combine to affect photosynthetic capacity of a central Amazon forest during the dry season in a field-based study independent of remote sensing or eddy covariance methods. We found the first direct field evidence that branch-scale photosynthetic capacity increases during the dry season, with a magnitude consistent with increases in ecosystem-scale photosynthetic capacity derived from flux towers.

The Impact

This new study is the first to directly show the mechanistic basis for the much-debated Amazon forest dry season green up phenomenon. It highlights the role of endogenous phenological rhythms – not just seasonal variation in climate drivers – as a key mechanism regulating the seasonality of photosynthesis. This is important because in most earth system models, the seasonality of tropical evergreen ecosystems is driven by climatic seasonality, not biological phenology, and many of these models do not yet correctly simulate this pattern. This study thus strongly supports the incorporation of leaf phenology into earth system models as a means to represent our best understanding of the key processes regulating photosynthesis.

Summary

We conducted demographic surveys of leaf age composition, and measured age-dependence of leaf physiology in broadleaf canopy trees of abundant species at a central eastern Amazon site. Using a novel leaf-to-branch scaling approach, we used this data to independently test the much-debated hypothesis—arising from satellite and tower-based observations—that leaf phenology could explain the forest-scale pattern of dry season photosynthesis. We found that photosynthetic capacity, as indicated by parameters of biochemical limitations on photosynthesis (V_cmax, J_max, and TPU), was higher in recently matured leaves than either young or old leaves, and stomatal conductance was higher for recently matured leaves than old leaves. Most tree branches had several different leaf age categories simultaneously present, and the number of recently mature leaves on branches of our focal trees increased as the dry season progressed (before October 15 versus after October 15), as old leaves were exchanged for young leaves that then matured. Together, these findings suggest that aggregated whole-branch V_cmax increases during the dry season, with a magnitude consistent with increases in ecosystem-scale photosynthetic capacity observed from flux towers.

Contacts (BER PM)

Daniel Stover
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289)

(PI Contact)

Lead author contact information
Loren Albert
Brown University
loren_albert@brown.edu

Institutional contact

Scott Saleska
University of Arizona
saleska@email.arizona.edu

Funding

This project received U.S. DOE support through GoAmazon, award DE-SC0008383. It was also supported by the US National Science Foundation (NSF) (award OISE-0730305 to S. Saleska), the Philecology Foundation through University of Arizona Biosphere 2 and a Marshall Foundation of Arizona dissertation fellowship to L.P.Albert. J. Wu was supported in part by the Next-Generation Ecosystem Experiment (NGEE-Tropics) project of DOE’s Office of Biological and Environmental Research.

Publications

Albert, L.P., J. Wu, N Prohaska, P.B. de Camargo, T.E. Huxman, E.S. Tribuzy, V.Y. Ivanov, R.S. Oliveira, S. Garcia, M.N. Smith, RC Oliviera, Jr., N. Restrepo-Coupe, R. da Silva, S.C. Stark, G.A. Martins, D.V. Penha, S.R. Saleska. (2018) Age-dependent leaf physiology and consequences for crown-scale carbon uptake during the dry season in an Amazon evergreen forest. New Phytologist. DOI: 10.1111/nph.15056

Climate sensitive size-dependent survival in tropical trees

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)]

New DOE Earth System Model Released

The U.S. Department of Energy’s Energy Exascale Earth System Model (E3SM) project, supported by the Office of Science in the Biological and Environmental Research Office, released its new high-resolution earth modeling system to the broader scientific community on April 23, 2018. E3SM will have weather-scale resolution and use advanced computers to simulate aspects of Earth’s variability and anticipate decadal changes that will critically impact the U.S. energy sector in coming years. NGEE-Tropics’ next-generation dynamic vegetation model, FATES (the Functionally-Assembled Terrestrial Ecosystem Simulator), was fully integrated into E3SM and its land model in 2017, and continues to be an important part of E3SM’s model development and science campaigns.

Warm ENSO phase drives coordination between leaf and fruit phenology

Resource acquisition and reproductive strategies of tropical forest in response to the El Niño–Southern Oscillation

The Science
It has been suggested that tree phenology may be regulated by climatic oscillations. Here, we present a 30 year tropical forest dataset that suggests leaf and fruit production is coordinated with ENSO cycles, with greater leaf fall observed prior to El Niño followed by greater seed production.

Coordination between leafing and fruiting at multiple temporal scales. a Wavelet coherence between leaf fall and seed fall shows strong coordination at both seasonal and ENSO cycles (red regions). b Phase-angle histogram for leaf fall and seed fall for the seasonal cycle and c for periods of 2–7 years, corresponding to the ENSO cycle. Phase angles were calculated for areas inside the cone of influence and for coherence >0.5 in a. Negative angles (blue area) indicate that leaf fall leads seed fall

The Impact
The response of tropical forest to ENSO events and in general to drought and other environmental stress are still under exploration. Here, we show a relatively strong response of tropical phenology (fruiting and leafing) to a warming phase of ENSO. This discovery can help to understand the mechanisms of response or adaptation of plants to climate variability and pave the road to their implementation into Earth Ecosystem Models.

Summary
For the first time an interaction between phenophases of tropical plants (leafing and fruiting) is shown to be driven by large scale periodic climate variations. This interaction mirrors the dynamics between dry and wet season, suggesting adaptive strategies to optimize reproduction and resource acquisition in response to environmental stress.

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

(PI Contact)
Matteo Detto
Associate Researcher, Dept. of Ecology and Evolutionary Biology Princeton University and Smithsonian Tropical Research Institute
mdetto@princeton.edu

Funding
The Environmental Sciences Program of the Smithsonian Institution funded the data collection. Matteo Detto was partially supported by NGEE-Tropics. Raul Rios, Brian Harvey, and Steven Paton collected the BCI climate data.

Publications
Detto, M., Wright, S. J., Calderón, O. & Muller-Landau, H. C. Resource acquisition and reproductive strategies of tropical forest in response to the El Niño-Southern Oscillation. Nature Communications, 9, 1–8 (2018).