The Science
Floods account for a significant and increasing number of reported natural hazards globally. As extreme precipitation is projected to increase in a warmer climate, there is an urgent need to improve understanding and modeling of floods to improve flood prediction and inform infrastructure planning. Analyses of flood characteristics have focused on using streamflow data, but flood inundation area has more direct societal and ecological implications.

A team led by scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory calibrated and evaluated a newly developed floodplain inundation model in the Energy Exascale Earth System Model (E3SM). Global simulations of flood inundation area for 1953-2004 revealed significant changes in flood generation mechanisms in some basins around the world. In the Amazon basin, for example, increasing concentration of extreme rainfall events within the wet season has increased its contribution to floods in the recent decades by synchronizing the occurrence of extreme rainfall more often with saturated soil in the wet season.

The Impact
Despite the complex and highly dynamic nature of flood processes, this study demonstrated the ability of a physically-based inundation model in E3SM for realistic simulation of floodplain inundation. Global simulations of flood inundation provided insights on the mechanisms for floods and their trends in major basins around the world.

Summary
In this study, scientists applied a newly developed, physically-based inundation model coupled with a river routing model (Model for Scale Adaptive River Transport, MOSART) within the Energy Exascale Earth System Model (E3SM) framework to investigate flood inundation dynamics. After calibration using observed streamflow and satellite-derived flood extent, the model was used to simulate global flood inundation from 1953 to 2004. The mean date and seasonality of annual maximum flood, defined based on flood extent, exhibit significant regional differences across 16 major basins.

Generally, soil moisture and monthly maximum daily rainfall are the dominant drivers of floods in tropical basins while monthly maximum daily snowmelt is the dominant driver in high latitude basins. From 1953-1982 to 1975-2004, significant changes in flood generation mechanisms are found in some basins such as Amazon, Lena, Yenisey, and Kolyma. Analysis of the rainfall seasonality and water balance at grid scale reveals during the later period, the occurrence of extreme rainfall has concentrated more in the wet season in the Amazon, which increases the co-occurrence of extreme rainfall and wet soil to produce flooding.  Fewer extreme rainfall events and increasing soil moisture reduced the contribution of monthly maximum rainfall and increased the role of monthly maximum snowmelt in floods in the Lena and Yenisey basins, respectively. Lastly, increased soil moisture and frequency of large monthly maximum snowmelt reduced the contribution of the latter to floods in the Kolyma basin. This study demonstrates the usefulness of the floodplain inundation model in E3SM for understanding floods and predicting their future changes.

Contacts (BER PMs): Sally McFarlane, Earth System Modeling Program Area, Sally.McFarlane@science.doe.gov
Dan Stover, Terrestrial Ecosystem Science Program, Dan.Stover@science.doe.gov

PI Contact: L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov

Funding
The U.S. Department of Energy Office of Science, Biological and Environmental Research supported this research as part of the Earth System Modeling Program Area through the Energy Exascale Earth System Model (E3SM) project. LRL was also supported by the Terrestrial Ecosystem Science Program through the Next Generation Ecosystem Experiment (NGEE) Tropics project.

Publication
Mao, Y., T. Zhou, L.R. Leung, T.K. Tesfa, H.-Y. Li, K. Wang, Z. Tan, and A. Getirana. 2019. “Flood Inundation Generation Mechanisms and Their Changes from 1953 to 2004 in Global Major River Basins.” J. Geophys. Res., doi:10.1029/2019JD031381.

Related Links
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JD031381

The Science
The effects of rising atmospheric CO2 on tropical forests have been the focus of a large body of research, and the question of whether intact tropical forests will act as a large CO2 sink remains contested. Evidence supporting a sink, using pan-tropical inventory analyses, has suggested that the Earth’s intact tropical forests sequester roughly half of the global net terrestrial sink. However, more recent studies show these biomass sinks to be overestimated, or no increased forest productivity with rising CO2. An additional research need is to directly compare predictions between two terrestrial modeling approaches that are at the forefront of earth system modeling, which differ in vegetation structure competitive processes (i.e. ‘big-leaf’ models and Vegetation Demographic Models (VDMs)).

The Impact
With a doubling of CO2, three of the four models used here predicted an appreciable biomass sink (0.77 to 1.24 Mg ha-1 yr-1). ELMv1-ECA, the only model that includes phosphorus constraints, predicted the lowest biomass sink relative to initial biomass stocks (+21%), lower than the other biogeochemical (BGC) model, CLM5 (+48%). Model projections differed primarily through variations in nutrient constraints, then carbon allocation, initial biomass, and density- dependent mortality. The VDM’s performance (ELM-FATES and ED2) was similar or better than the BGC models run in carbon-only mode, suggesting that nutrient competition in VDMs will improve predictions. We demonstrate that VDMs are comparable to non-demographic (i.e.,‘big-leaf’) models, but also include finer-scale demography and competition that can be evaluated against field observations.

These VDMs were designed to operate at the ESM scale, but have never been fully tested for this application. This study takes a step at showing the potential advantages of coupling FATES to an ESM’s land surface model by comparing ED2 with ELM-FATES, which displayed a more realistically constrained absolute biomass sink, net primary productivity (NPP) flux, mortality rates, and leaf area index (LAI) compared to ED2. We conclude that VDMs are applicable and fruitful at ESM scale (e.g., similarities or improved performance compared to unconstrained C-only big-leaf models), but more testing needs to be done over regional to global scales.

Summary
For this study we simulated an old-growth tropical forest using six versions of four terrestrial models differing in scale of vegetation structure, and representation of BGC cycling, all driven with CO2 forcing from the preindustrial period to 2100. The models were benchmarked against tree inventory and eddy covariance data from a Brazilian site for present-day predictions. A quasi-factorial design was used to test structured vegetation dynamics and plant competition for nutrients as two critical processes to tropical forest dynamics and CO2 fertilization responses (i.e., direct “CO2 effect” or β-response). In addition to results listed above, our results revealed that all models used here predicted positive vegetation growth that outpaced mortality for an intact Brazilian forest, leading to continual increases in present-day biomass accumulation. Interestingly, the field data indicated that a quarter of canopy trees didn’t grow over the 15-year period, and biomass change was near-neutral. The results found here and model testing will benefit both DOE’s E3SM Project and the NGEE-Tropics Project.

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 William J. Riley, Lawrence Berkeley National Lab, jchambers@lbl.gov, wjriley@lbl.gov

Funding
DE-AC02-05CH11231 as part of their Next Generation Ecosystem Experiment-Tropics (NGEE-Tropics) and the Energy Exascale Earth System (E3SM) Program, as well as Laboratory Directed Research and Development (LDRD) funding from Berkeley Lab, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publication
Holm, J. A., Knox, R. G., Zhu, Q., Fisher, R. A., Koven, C. D., Nogueira Lima, A. J., Riley, W. J., Longo, M., Negron-Juarez, R. I., Araujo, A. C. de., Kueppers, L. M., Moorcroft, P. R., Higuchi, N., Chambers, J. Q. ( 2020). The Central Amazon biomass sink under current and future atmospheric CO2: Predictions from big-leaf and demographic vegetation models. Journal of Geophysical Research: Biogeosciences, 125, e2019JG005500. https://doi.org/10.1029/2019JG005500.

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

Related Links
https://www.nature.com/articles/s41558-019-0630-6

The Science
Trees adjust their leaf area based on their traits and environmental conditions, which has enormous impacts on global carbon fluxes. We use first principles to predict leaf area adjustment in response to global change.

The Impact
We provide and test methods for improving carbon cycle predictions through advancing model predictions of leaf area. Tree‐level carbon allocation to leaves should be derived from first principles using mechanistic plant hydraulic processes in vegetation models.

Summary
Forest leaf area has enormous impacts on the carbon cycle because it mediates both forest productivity and resilience to climate extremes. Trees are capable of adjusting to changes in environment, yet many vegetation models use fixed carbon allocation schemes independent of environment, which introduces uncertainty in predictions. We develop an optimization‐based model in which tree carbon allocation to leaves is an emergent property of environment and plant traits. A combination of meta‐analysis, observational datasets, and model predictions find strong evidence that optimal hydraulic–carbon coupling explains observed patterns in leaf allocation.

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

PI Contact: Anna T. Trugman, School of Biological Sciences, University of Utah, a.trugman@utah.edu

Funding
USDA National Institute of Food and Agriculture, Grant/Award Number: 2018‐ 67012‐28020 and 2018‐67019‐27850; National Science Foundation, Grant/Award Number: 1714972 and 1802880; German Research Foundation, Grant/Award Number: RU 1657/2‐1, SCHM 2736/2‐1 and YA 274/1‐1 ; David and Lucille Packard Foundation; University of Utah Global Change and Sustainability Center; Next‐ Generation Ecosystem Experiments‐Tropics; Biological and Environmental Research; German Federal Ministry of Education and Research; Helmholtz Association.

Publications
A. Trugman, “Climate and plant trait strategies determine tree carbon allocation to leaves and mediate future forest productivity.” Global Change Biology 25, 3395 (2019). 10.1111/gcb.14680.

The Science
Plant hydraulics has evolved dramatically in the last century, leading to knowledge of critical value to agriculture, forest management, and global modeling.  Here we review these developments and highlight some of the critical new directions for plant hydraulics in the 21st century.

The Impact
Plant hydraulics is the backbone of terrestrial ecology because of the critical role of water transport in regulating carbon uptake.  We highlight the impacts plant hydraulics have had on fields ranging from agricultural management to the terrestrial carbon and water cycles, and propose the path forward to new insights of value to both fundamental and applied sciences.

Summary
Plant hydraulic regulation of water uptake provides the backbone of the plant carbon cycle and ecology because of its direct control over, and tight coordination with, canopy photosynthesis. Advances in measurements and modeling over the last few decades have enabled far-reaching influence of hydraulic discoveries, including impacting how we view and simulate the global water and carbon cycles and manage crop systems (Figure 1). Perhaps most importantly in this era of a warming atmosphere and more variable droughts, is the critical role our understanding of plant hydraulics is having on our ability to predict and mitigate chronically-increasing stressors (e.g. temperature, vapor pressure deficit) on plant function and survival.  The 21st century offers a very exciting time for advancement of plant hydraulics understanding, approaches, and applications.  Future directions range in scale from understanding the molecular regulation and feedbacks with maximum conductance and embolism avoidance, to improved understanding of water potential regulation at landscape to global scales.  Inherently, developments in understanding will be associated with continued methodological improvements at micro- to macro-scales, and with applications of refined hydraulic models to allow strong, process-based inferences. Perhaps the most important directions that plant hydraulics science can go is in applications to the prediction and management of both wild and crop systems under rising temperature and vapor pressure deficit and drought frequency, which threatens food production and the global carbon cycle alike.

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

PI Contact: Nate McDowell, PNNL, nate.mcdowell@pnnl.gov

Funding
DOE-NGEE Tropics

Publications
McDowell NG, T Brodribb, A Nardini (2019). Plant hydraulics in the 21st century.  New Phytologist doi.org/10.1111/nph.16151.

The Science
Drought is forecasted to impact most terrestrial ecosystems in an increasing manner over the 21st century, with negative consequences on productivity, carbon storage, and yield.  We review the state-of-the-art methods for conducting drought manipulation experiments designed to understand the impacts of drought upon woody-plant dominated ecosystems.

The Impact
This manuscript provides a cutting-edge template for those scientists wishing to conduct drought manipulation studies.  We highlight the challenges, assumptions, and solutions to both scientific and logistical challenges associated with drought studies.

Summary
Past forest drought experiments employed a variety of study designs related to treatment level, replication, plot and infrastructure characteristics, and measurement approaches. Important considerations for establishing new drought experiments include: selecting appropriate treatment levels to reach ecological thresholds; balancing cost, logistical complexity, and effectiveness in infrastructure design; and preventing unintended water subsidies. Response variables in drought experiments were organized into three broad tiers reflecting increasing complexity and resource intensiveness, with the first tier representing a recommended core set of common measurements.  Differences in site conditions combined with unique research questions of experimentalists necessitate careful adaptation of guidelines for drought experiments to balance local objectives with coordination among experiments. We advocate adoption of a common framework for coordinating forest drought experiment design to enhance cross‐site comparability and advance fundamental knowledge about the response and sensitivity of diverse forest ecosystems to precipitation extremes.

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

PI Contact: Nate McDowell, PNNL, nate.mcdowell@pnnl.gov

Funding: DOE-NGEE Tropics

Publications
Asbjornsen, H., J. Campbell, K. Jennings, C. McIntire, M. Vadeboncoeur, P.H. Templer, R. Phillips, T.L. Bauerle, F. Bowles, M. Dietze, S. Frey, P. Groffman, R. Guerrieri, P.J. Hanson, E. Kelsey, A. K. Knapp, N.G. McDowell, P. Meir, K.A. Novick, S.V. Ollinger, W.T. Pockman, P.G. Schaberg, S.D. Wullschleger, M.D. Smith, L. Rustad.  Guidelines and considerations for designing precipitation manipulation experiments in shrubland, woodland, and forest ecosystems.  Methods in Ecology and Evolution 9(12) p 2310-2325 (2018). https://doi.org/10.1111/2041-210X.13094.