Global photosynthesis modeling is stymied by competing hypotheses on scaling of plant traits

Uncertainty in how maximum photosynthetic rates scale across the Earth leads to substantial uncertainty predictions of terrestrial carbon uptake

The Science
A major source of uncertainty in modeling of global photosynthesis, and associated carbon cycle dynamics, is the calculation of maximum photosynthetic carboxylation rate, which is one of two plant traits that closely determines photosynthetic rate. Various methods are used in terrestrial biosphere models to calculate these traits; each representing a different theory about how these traits scale but the resultant errors have not yet been quantified.

The Impact
This research highlights the need for robust estimates of global photosynthesis and a better understanding of how maximum photosynthetic rates scale across the Earth’s surface.

Summary
The impact on global patterns of photosynthesis of four trait-scaling hypotheses (plant functional type, nutrient limitation, environmental filtering, and plant plasticity) was investigated by an international team of researchers. Led by a DOE researcher at Oak Ridge National Laboratory, the study finds that global photosynthesis estimates from the different trait-scaling hypotheses ranged between 108 and 128 PgC yr^-1, representing around 65% of the uncertainty range found in photosynthesis model intercomparison exercises. The uncertainty propagated through to a 27% variation in net biome productivity, the net amount of carbon removed from the atmosphere by land ecosystems. All hypotheses produced global photosynthesis estimates that were highly correlated with proxies of global photosynthesis. Nevertheless, nutrient limitation appeared to be marginally the best method to simulate the scaling of maximum photosynthetic rates. The comparison of model photosynthesis with ‘observed’ photosynthesis was stymied by the fact that no robust methods exist to measure photosynthesis at the global scale. For this reason, researchers used three proxies of global photosynthesis to compare with the model estimates. Interestingly, photosynthesis in agricultural regions of Earth were much higher in the satellite-based photosynthesis proxies that measure solar induced fluorescence of the photosynthetic machinery in a leaf. Higher photosynthesis in these regions when measured from space suggest that models and other photosynthesis proxies may be missing an important component of global photosynthesis in these managed ecosystems.

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

PI Contact: Anthony P. Walker, Oak Ridge National Laboratory, walkerap@ornl.gov

Funding
DOE Office of Science BER, Next Generation Ecosystem Experiments – Tropics

Publications
Walker, A. P. et al. The impact of alternative trait-scaling hypotheses for the maximum photosynthetic carboxylation rate (Vcmax) on global gross primary production. New Phytol Early View (2017). doi:10.1111/nph.14623

Related Links
Next Generation Ecosystem Experiments – Tropics https://ngee-tropics.lbl.gov/

A metadata reporting framework (FRAMES) for synthesis of ecohydrological observations has been developed as part of the NGEE-Tropics Data Synthesis and Management Framework.

See Metadata Templates: https://ngt-data.lbl.gov/dois/NGT0041/
See R code to work with templates: https://ngt-data.lbl.gov/dois/NGT0042/

The Science
FRAMES is a set of excel and on-line templates that standardize reporting of diverse ecohydrological data and the necessary metadata required for data synthesis to study earth systems.

The Impact
Detailed metadata, information that describes when, where, and how data is generated, is required for interpreting, comparing, validating, and synthesizing ecohydrological observations collected with diverse methods in different ecosystems. FRAMES bridges the gap between complex data information models that are needed to organize detailed metadata and specific ecohydrological data reporting protocols that lack enough detail for earth system science research.

Summary
FRAMES is a set of excel and on-line templates that standardize reporting of diverse ecohydrological data and metadata for data synthesis required for earth system science research. We developed FRAMES iteratively with data providers and consumers who are developing a predictive understanding of carbon cycling in the tropics. Key features include: (1) Best data science practices; (2) Modular design that allows for addition of new measurement types; (3) Data entry formats that enable efficient reporting; (4) Multiscale hierarchy that links observations across spatiotemporal scales; and (5) Collection of metadata for integrating data with earth system models.

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 Contacts: Danielle S. Christianson, Lawrence Berkeley National Laboratory, dschristianson@lbl.gov
Charuleka Varadharajan, Lawrence Berkeley National Laboratory, cvaradharajan@lbl.gov

Funding
Research supported by Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics), funded by U. S. Department of Energy, Office of Science.

Publications
D.S. Christianson, C. Varadharajan, B. Christoffersen, M. Detto, B. Faybishenko, B.O. Gimenez, V. Hendrix, K. J. Jardine, R. Negron-Juarez, G.Z. Pastorello, T.L. Powell, M. Sandesh, J.M. Warren, B.T. Wolfe, J.Q. Chambers, L.M. Kueppers, N.G. McDowell, D.A. Agarwal, “A metadata reporting framework (FRAMES) for synthesis of ecohydrological observations.” Ecological Informatics (2017). https://doi.org/10.1016/j.ecoinf.2017.06.002.

Related Links
https://doi.org/10.1016/j.ecoinf.2017.06.002

Dr. Richard Norby of Oak Ridge National Laboratory was recently named a Fellow in the American Geophysical Union (AGU).

From Climate Change Science Institute, Oak Ridge National Laboratory

Dr. Richard Norby, UT Battelle Corporate Research Fellow in the Environmental Sciences Division, Climate Change Science Institute at Oak Ridge National Laboratory and Joint Professor in the Bredesen Center for Interdisciplinary Research and Graduate Education, has been named a Fellow in the American Geophysical Union (AGU).

Since 1981, Dr. Norby’s career has focused on ecological research on the responses of organisms and ecosystems to elevated carbon dioxide (CO₂) and associated environmental variables. He fostered the transition of the discipline from short-term laboratory to multi-year, field relevant studies. This has allowed the science community to understand organism and ecosystem responses to elevated CO₂ to reflect important biogeochemical cycling interactions and long-term adjustments through time. Dr. Norby’s work transformed understanding of vegetation responses to elevated CO₂ through the addition of mechanisms responsible for constraining the overall response to enhancements in gross primary production including carbon allocation, mineral cycling, root-mycorrhizal-microbial interactions, and water limitation.

Currently, Dr. Norby is working with the international science community to establish a new  elevated CO₂ study in the wet tropical rain forests of Brazil that should provide critical insights on this globally important ecosystem. In addition, he is the task lead for Nutrient Constraints as part of the DOE-funded NGEE-Tropics project.

Dr. Norby has more than 185 peer-reviewed publications with 31 cited more than 100 times according to the Web of Science.

Dr. Norby is an editor of New Phytologist and was an associate editor of the Journal of Plant Ecology (2008-2016). He was a guest editor of a special issue and served on the editorial board of Ecological Applications (1998-2002). As a member of the board of the New Phytologist Trust, Dr. Norby has stimulated the establishment of workshops and symposia on wide-ranging subjects including biogeochemical cycling, climate change impacts, and global terrestrial modeling, to a broader plant sciences community.

The rank of AGU Fellow is given to individual AGU members who have made exceptional scientific contributions and gained prominence in their respective fields of Earth and space sciences.  Since the AGU Fellows program was established in 1962, no more than 0.1 percent of the total membership of AGU is recognized annually.  Sixty-one new Fellows will be honored at the 2017 Fall meeting for their leadership and scientific excellence in their respective fields.

Dr. Norby has a B.A. in chemistry from Carleton College and a Ph.D. in Forestry and Botany from the University of Wisconsin. He is a member of the Ecological Society of America, the Association of Tropical Biology and Conservation, and the American Geophysical Union, and in 1995 was elected Fellow of the American Association for the Advancement of Science and selected Fellow of the Ecological Society of America in 2016.

The American Geophysical Union is dedicated to advancing the Earth and space sciences for the benefit of humanity through its scholarly publications, conferences, and outreach programs. AGU is a not-for-profit, professional, scientific organization representing nearly 60,000 members in 139 countries.

https://eos.org/agu-news/2017-class-of-agu-fellows-announced​

This is the first paper to synthesize the results on mechanisms of mortality from all known drought manipulation studies, and found that hydraulic failure is a universal component of death while carbon starvation is frequent but not universal.

 

The Impact
This paper 1) tests a contentious hypothesis regarding hydraulic failure and carbon starvation, for the first time, at a global scale, 2) provides modelers a direct path to improving vegetation dynamics simulations.

Summary
About half of carbon dioxide emissions are absorbed by plants, but this service is threatened by increasing frequency of hot droughts. One of the largest uncertainties in land surface modeling is how vegetation will respond to greater exposure to life-threatening droughts. One of the most contentious theories in ecology today regards the mechanisms of responses e.g. how plants regulate hydraulic failure and carbon starvation (if they even occur at all) during drought. We reviewed all known drought studies that killed trees and found that hydraulic failure was a universal characteristic proceeding death, and co-occurring carbon starvation occurred in approximately 50% of studies. The most advanced land-surface models today simulate mortality via carbon starvation but not via hydraulic failure, thus a change in model development priority to simulate hydraulic failure is merited.

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

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

Funding
Funding was provided by DOE, Office of Science, NGEE-Tropics, via the Los Alamos and the Pacific Northwest National Lab’s LDRD program, an via NSF.

Publications
Adams et al. (61 co-authors). A multi-species synthesis of physiological mechanisms in drought-induced mortality. Nature Ecology and Evolution (In Press)

FATES (the Functionally-Assembled Terrestrial Ecosystem Simulator), a next-generation dynamic vegetation model, has been fully integrated into the E3SM ESM and its land model (ALM)

The Science
FATES, a dynamics vegetation model that predicts tree size distributions, disturbance dynamics, and plant trait competition, has been integrated into the E3SM Land Model, and released as an open source tool to the public.

The Impact
FATES will allow a richer representation of the potential ecosystem responses to weather, land-use, and atmospheric compositional changes, and how these ecosystem changes alter the dynamics of the Earth system. The coupled E3SM ESM will benefit from these changes to allow it to be applied to scientific questions about the role of ecosystem change in the context of larger global changes.

Summary
The Functionally Assembled Terrestrial Ecosystem Simulator (FATES) is a demographic vegetation model that includes dynamics that are not included in the current E3SM Land Model, such as individual tree growth, death, and competition for light; explicit representation of both natural and anthropogenic disturbance; and competitive dynamics between different plant functional types as a result of their differing plant traits. The FATES model has been designed for modularity so as to allow scientific isolation of component processes and clean scientific experimental design. Because FATES makes predictions about tree size distributions, disturbance dynamics, and physiological dynamics at the level of individual trees, it can be more robustly tested against field measurements and can therefore serve as an organizing model for DOE field activities, particularly in forested ecosystems, such as NGEE Tropics. Now that FATES has been fully integrated into the ACME Land Model, such activities are directly feeding into  science.

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: Charles Koven, Lawrence Berkeley National Lab, Phone: 510-486-6724, cdkoven@lbl.gov

Funding
Support for this activity is from the Department of Energy, Office of Science, Biological and Environmental Research, through the Climate and Environmental Sciences Division and the Terrestrial Ecosystem Sciences Division as part of the Next-Generation Ecosystem Experiments (NGEE-Tropics) Project.

Related Links
The link to the FATES-release github repository is: https://github.com/NGEET/fates-release