NGEE-Tropics participated in a workshop jointly organized by the Nanyang Technological University (Singapore) and the Smithsonian Institute. The Future of Tropical Forests in Asia: Experimental and Modeling Approaches workshop took place November 14–19, 2016, at NTU’s Asian School of the Environment.  Over thirty international researchers came together to explore new approaches and potential collaboration across different fields, such as hydrology, demography, and modeling of tropical forests in Asia. The workshop focused on discussing empirical and modeling needs to improve Earth system models for tropical forests.  The workshop was also complemented by field site visits in Singapore (Bukit Timah Nature Reserve and Upper Seletar Reservoir Park) and Malaysia (Lambir Hills National Park and Pasoh Forest Reserve).

NGEE-Tropics participants included Lead PI and Director Jeffrey Chambers (LBNL), Deputy Director Lara Kueppers (LBNL), Chief Scientist Stuart Davies (Smithsonian), and Science Leads Charles Koven (LBNL), Richard Norby (ORNL), and Alistair Rogers (BNL). Countries represented at the workshop included Australia, China, India, Japan, Malaysia, Philippines, Singapore, Thailand, United Kingdom, USA, and Vietnam.

Researchers identified key model, data, and process knowledge improvements needed to advance the representation of photosynthesis in next-generation climate models.

The Science 
A collaboration between modelers and plant physiologists compared the projected physiological responses of photosynthesis to key environmental drivers in seven terrestrial biosphere models (TBMs) that form the land components of major Earth system models. The study identified research activities needed to improve process representation of photosynthesis in TBMs.

The Impact
A widely held assumption is that the representation of photosynthesis in TBMs is settled science and that model uncertainty is driven largely by other processes downstream of carbon acquisition. This study demonstrates that model divergence in the physiological response of photosynthesis to key environmental drivers is high and likely a major source of model divergence. This finding is critical because the response of the terrestrial biosphere to global change is driven by these same physiological responses and their accurate representation should be an essential component of improved TBMs. This study lays out the steps needed to improve model representation of photosynthesis.

Summary
Accurate representation of photosynthesis in TBMs is essential for robust projections of global change. However, current representations vary markedly between TBMs, contributing uncertainty to projections of global carbon fluxes. In this study, researchers compared the representation of photosynthesis in seven TBMs by examining leaf and canopy-level responses of photosynthetic carbon dioxide (CO₂) assimilation to key environmental variables: light, temperature, CO₂concentration, vapor pressure deficit, and soil water content. They identified research areas where limited process knowledge prevents inclusion of physiological phenomena in current TBMs and research areas where data are urgently needed for model parameterization or evaluation. The study provides a roadmap for new science needed to improve the representation of photosynthesis in the next generation of terrestrial biosphere and Earth system models.

Contact: Alistair Rogers, Brookhaven National Laboratory, arogers@bnl.gov

Funding
The New Phytologist Trust provided support of the 9th New Phytologist Workshop: Improving Representation of Photosynthesis in Earth System Models. AR and SPS were supported by the Next-Generation Ecosystem Experiments (NGEE; NGEE-Arctic and NGEE-Tropics) projects funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research through contract number DE-SC00112704 to Brookhaven National Laboratory. DW acknowledges support from the Natural Sciences and Engineering Research Council, Canada Foundation for Innovation, and an Ontario Early Researcher Award. JSD received support from the National Science Foundation (DEB-0955771).

Publications
Rogers, A., B. E. Medlyn, J. S. Dukes, G. Bonan, et al. 2017. “A Roadmap for Improving Representation of Photosynthesis in Earth System Models,” New Phytologist 213, 22-42. DOI: 10.1111/nph.14283. (Reference link)

Rogers, A., B. E. Medlyn, and J. S. Dukes. 2014. “Improving Representation of Photosynthesis in Earth System Models,” New Phytologist 204, 12-14. DOI: 10.1111/nph.12972. (Reference link)

Models of phosphorus-limited tropical forests may be improved through empirical relationships between photosynthesis and nutrients

The Science
Gas exchange and nutrient content data were collected from upper canopy leaves of 144 trees at two forest sites in Panama, differing in species composition, rainfall, and soil fertility. Relationships between photosynthesis, foliar N and P, and wood density were evaluated against mechanistic and empirical models.

The Impact
This study provides a basis for improving models of photosynthesis based on phosphorus nutrition and thereby increase the capability of models to predict future conditions in P-limited tropical forests.

Summary
The objective of this study was to analyze and summarize data describing photosynthetic parameters and foliar nutrient concentrations from tropical forests in Panama to inform model representation of phosphorus limitation of tropical forest productivity. Gas exchange and nutrient content data were collected from upper canopy leaves of 144 trees from at least 65 species at two forest sites in Panama, differing in species composition, rainfall, and soil fertility. The relationships between photosynthetic parameters and nutrients were of similar strength for nitrogen and phosphorus and robust across diverse species and site conditions. The strongest relationship expressed maximum electron transport rate (J_max ) as a multivariate function of both nitrogen and phosphorus, and this relationship was improved with the inclusion of independent data on wood density. Models that estimate photosynthesis from foliar nitrogen content would be improved only modestly with the inclusion of additional data on foliar phosphorus, but doing so may increase the capability of models to predict future conditions in phosphorus-limited tropical forests, especially when combined with data on edaphic conditions and other environmental drivers.

Contact: Richard J. Norby, Oak Ridge National Laboratory, rjn@ornl.gov

Publications
J. Norby et al. “Informing models through empirical relationships between foliar phosphorus, nitrogen and photosynthesis across diverse woody species in Panama.” New Phytologist (2016). doi: 10.1111/nph.14319

Related Links
Data posted at http://dx.doi.org/10.15486/NGT/1255260

A trait-based plant hydraulics model for tropical forests, developed for use within size-structured models

The Science
We developed a trait-based plant hydraulics model for tropical forests. It successfully predicts how individual trees in a forest vary in water status based on their size, canopy position and hydraulic traits, which improved simulations of total ecosystem transpiration.

The Impact
A substantial amount of diversity in tropical forests can be represented by a reduced set of dimensions. This sub-model can be used in conjunction with other demographic ecosystem models to predict how forest composition evolves under a changing climate.

Summary
We developed a plant hydraulics model for tropical forests based on established plant physiological theory, in which all parameters of the constitutive equations are biologically-interpretable and measureable plant hydraulic traits (e.g., the turgor loss point, hydraulic capacitance, xylem hydraulic conductivity, water potential at 50% loss of conductivity for both xylem and stomata, and the leaf:sapwood area ratio). Next we synthesized how plant hydraulic traits coordinate and trade-off with each other among tropical forest species. We first show that a substantial amount of trait diversity can be represented in the model by a reduced set of trait dimensions. We then used the most informative empirical trait-trait relationships derived from this synthesis to parameterize the model for all trees in a forest stand. The model successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, and also improved simulations of total ecosystem transpiration. Collectively, these results demonstrate the importance of plant hydraulic traits in mediating forest transpiration and overall forest ecohydrology. When used in conjunction with other demographic ecosystem models, this modeling approach can be used to predict how forest composition evolves under a changing climate.

Contact: Brad Christoffersen, Los Alamos National Laboratory, bradley@lanl.gov, 505-665-9118

Funding
This research was supported in part by the European Union Seventh Framework Program under the project AMAZALERT, and by the Next-Generation Ecosystem Experiments (NGEE-Tropics) project, funded by the U.S. Department of Energy, Office of Biological and Environmental Research. Funding was also contributed by the Los Alamos National Laboratory LDRD.

Publications
Christoffersen, et al. “Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v.1-Hydro).” Geoscientific Model Development Discussions (2016). doi:10.5194/gmd-2016-128.

Related Links
doi:10.5194/gmd-9-4227-2016-supplement
doi:10.15486/NGT/1256473
doi:10.15486/NGT/1256474

Dr. Melanie Mayes was one of four Oak Ridge National Laboratory (ORNL) researchers to receive a 2016 US Department of Energy (DOE) Office of Science (SC) Early Career Research Program research grant. The program, now in its seventh year, is designed to bolster the nation’s scientific workforce by providing support to exceptional researchers during the crucial early career years, when many scientists do their most formative work.

Dr. Mayes’ proposal, “A Comprehensive Framework for Modeling Emissions from Tropical Soils and Wetlands,” selected for funding by the DOE Office of Biological and Environmental Research (BER), has as its goal development of a framework for modeling greenhouse gas (GHG) emissions in tropical regions that takes into account factors such as microbial traits and functions, soil characteristics, different microbial energy sources, and soil moisture. Tropical wetlands are important contributors to GHG emissions worldwide; however, tropical biomes are extremely complex, and many of the factors contributing to GHG emissions are poorly understood and thus poorly represented in models.

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