NGEE-Tropics modeling lead and staff scientist at Lawrence Berkeley National Laboratory (LBNL), Charlie Koven, helped play a key role in the Intergovernmental Panel on Climate Change (IPCC)’s recent 2021 Climate Change Report. This report is the United Nations (UN) IPCC’s sixth installment in a series of climate change reports designed as summaries for policymakers, with the previous iteration last published in 2014. You can find the full report here. Charlie Koven was one of four LBNL scientists to contribute to the report’s efforts.

Figure 1. Charlie Koven alongside the cover image of the IPCC Climate Change 2021 Report from the UN IPCC website, which can be found at https://www.un.org/en/climatechange/reports.

As the modeling lead for the NGEE-Tropics project, Charlie directs the development of the Functionally Assembled Terrestrial Ecosystem Simulator (FATES). FATES, as an ecosystem model, will ultimately enhance the U.S. Department of Energy’s Energy Exascale Earth System Model (E3SM), allowing for a more complete and complex representation of Earth’s ecosystems. A notable highlight of the 2021 IPCC report are simulations that show that the proportion of CO2 emissions taken up by land and ocean carbon sinks is smaller in scenarios with higher cumulative CO2 emissions (Figure 2). Figure 2 is based on  Earth system models like E3SM that NGEE-Tropics is building FATES to work within, with the goal of reducing uncertainty on this critical process needed for climate projections.

Figure 2. Figure 7 from the Summary for Policymakers in the 2021 IPCC report, which shows that the proportion of CO2 emissions taken up by land and ocean carbon sinks is smaller in scenarios with higher cumulative CO2 emissions.

A major theme surrounding the publication of the 2021 iteration of the IPCC Climate Change report is the pace at which we must tackle the climate crisis. There is a renewed sense of urgency sparked by the report. With climate science as sophisticated as it’s ever been, the 2021 report shows the future of climate change and global warming with a high degree of clarity. Various carbon emission and cycling scenarios are simulated in the report, with NGEE-Tropics’ own Charlie Koven playing an important role. In the report, a primary focus is the 2015 Paris Agreement’s goal of limiting warming to well below 2 degrees Celsius (C), with a target of 1.5 degrees C. There is one thing these scenarios have in common: swift and powerful action is required to avoid the escalating negative effects of climate change. To learn more about the 2021 IPCC report, and hear the perspective of Charlie and other LBNL scientists, check out the Berkeley Lab press release on its publication.

Lara Kueppers, another NGEE-Tropics science lead and LBNL scientist, has been helping spread the word on the 2021 IPCC Climate Change report. To provide a local perspective on the global climate change report, Lara Kueppers interviewed with the California Bay Area’s KQED to communicate some of the key messages contained within the 2021 IPCC report. The full interview can be found here, on the KQED website.

Figure 3. Lara Kueppers alongside the California Bay Area’s KQED logo from www.kqed.org.

Contact: Daniel Dores, ddores@lbl.gov, Berkeley Lab Program Operations Analyst. Charlie Koven, cdkoven@lbl.gov, Berkeley Lab Staff Scientist.

Predicting seasonal fluxes of water and energy across the Amazon

The Science
Forests regulate the water cycle, and store and absorb carbon dioxide from the atmosphere. However, their success to drive evaporative cooling and to do photosynthesis depends on how efficiently forests access and use light and water during the year. We used measurements from flux towers installed in Amazonia, and compared observations with predictions from four computer-based forest simulators (models). Models are useful for predicting how forests will cope with climate change, but need to be tested first. We found that models think the Amazon dry-up too often and too quickly, and that they reflect more light when compared to observations.

The Impact
Our knowledge about forest ecology, climate, structure, etc., is used by models to describe different ecosystems.  Moreover, models are representations of how forests work — how much trees transpire and grow, leaves reflect light, soil stores water, etc.  Models are tested by comparing their output to current observations as they will be used to forecast the future of tropical forests (e.g. we can infer tree growth if rainfall is reduced). This work describes measurements (e.g. temperature, tropical forest evaporation and transpiration) at different sites across Amazonia and points to processes that need to be improved on the tested models.

Summary
We used data collected in four eddy flux towers across the Amazon to quantify the seasonal cycle of sensible heat flux, evapotranspiration, emission of thermal infrared radiation, and the optical properties of the forest canopies.  We simulated the seasonal cycle of the same quantities using four terrestrial biosphere models that are often used to predict the future of the Amazon (namely IBIS, ED2, JULES, and CLM3.5). We compared the model predictions with the tower measurements and identified that most models predict a strong seasonality of the Bowen ratio (ratio between sensible and latent heat flux), and overall low water use efficiency. Consequently, models predicted that Amazon forests experience more frequent water stress than what has been observed. Likewise, the models predicted that the forest canopies would reflect more light than observed.

We identified possible explanations for such differences.  First, models do not represent when leaves shed or replace leaves, which may bias the canopy reflectance. Likewise, models seem to exaggerate the canopy interception of rainfall, which reduces the soil available water. Finally, the incorrect estimates of water stress also led to discrepancies between predicted and observed outgoing longwave radiation. Our findings can be used as references for future model development.

Figure. A 65-m eddy covariance flux tower in the Amazon Forest (Tapajos, Brazil) where we measure meteorology and exchanges of water, energy, and carbon between the forest and the atmosphere, and can help us to understand how tropical forests (Jaru and Tapajos shown here) will respond to climate change.  Image courtesy of Natalia Restrepo-Coupe.

Contact (BER): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)
Science Contact: Natalia Restrepo-Coupe, Saleska Lab, Ecology and Evolutionary Biology, University of Arizona, nataliacoupe@gmail.com

Funding
This research was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. This research was also funded by the Gordon and Betty Moore Foundation “Simulations from the Interactions Between Climate, Forests, and Land Use in the Amazon Basin: Modeling and Mitigating Large-Scale Savannization” project, and the National Aeronautics and Space Administration (NASA) through the NASA LBA-DMIP project.

Publications
Restrepo-Coupe, et al., “Understanding water and energy fluxes in the Amazonia: Lessons from an observation-model intercomparison” Global Change Biology 27, 1802–1819 (2021) [DOI: 10.1111/gcb.15555]

Scientists revealed the temporal variation in canopy disturbances using a unique dataset of five-year monthly drone-acquired imagery

Figure. Canopy disturbance visualized on canopy surface models and orthomosaics calculated from photogrammetric analyses of drone imagery. (a,b) Elevation models for a portion of the study area on two successive dates, 28 August 2019 (a) and 23 September 2019 (b). (c) Difference in elevation between the two dates, with black area indicating large decrease in canopy elevation. (d,e) RGB orthomosaics of the same dates.

The Science
A mechanistic understanding of how tropical tree mortality responds to climate variation is urgently needed to predict how tropical forest carbon pools will respond to anthropogenic global change. Researchers used five years of approximately monthly drone-acquired imagery for 50 ha of tropical forest on Barro Colorado Island, Panama, to quantify temporal variation and climate correlates of treefalls, branchfalls, or collapse of standing dead trees. They found that canopy disturbance rates are highly temporally variable, and are well-predicted by extreme rainfall events. Treefalls accounted for 77 % of the total area and 60 % of the total number of canopy disturbances in treefalls and branchfalls combined.

The Impact
Moist tropical forests account for 40% of the global biomass carbon stocks, and uncertainty regarding the future of these stocks is a major contributor to uncertainty in the future global carbon cycle. An improved understanding of the processes of forest disturbance is critical to constrain estimates of current and future carbon cycling in tropical forests under climate change. Results demonstrate the utility of repeat drone-acquired data for quantifying forest canopy disturbance rates at fine temporal and spatial resolutions, thereby enabling robust tests of how temporal variation in disturbance relates to climate drivers.

Summary
The size distribution of canopy disturbances was best fit by a Weibull function, and was close to a power function for sizes above 25 m2. Canopy disturbance rates varied strongly over time and were higher in the wet season, even though windspeeds were lower in the wet season. The strongest correlate of temporal variation in canopy disturbance rates was the frequency of 1-hour rainfall events above the 99.4th percentile (here 35.7 mm hour-1, r = 0.67). Scientists hypothesize that extreme high rainfall is associated with both saturated soils, increasing risk of uprooting, and with gusts having high horizontal and vertical windspeeds that increase stresses on tree crowns.

Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)
Science Contact: Raquel Fernandes de Araujo, STRI, araujo.raquelf@gmail.com

Funding
Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research; Smithsonian Institution Competitive Grants Program for Science; Smithsonian Tropical Research Institute fellowship program.

Publications
Araujo, R. F., Grubinger, S., Celes, C. H. S., Negrón-Juárez, R. I., Garcia, M., Dandois, J. P., and Muller-Landau, H. C. (2021), Strong temporal variation in treefall and branchfall rates in a tropical forest is explained by rainfall: results from five years of monthly drone data for a 50-ha plot, Biogeosciences Discuss. [DOI: 10.5194/bg-2021-102]

Modeling and measurements show plant water loss responds more to dry soil than dry air at the Barro Colorado Island during the 2015-2016 El Niño drought event.

The Science
Water stress from dry soil and vapor pressure deficit (VPD) can both limit plant transpiration and hence plant response to drought. However, separating the response of plant functioning to these two interactive stresses is challenging. Using statistical models fitted to two types of field observation data and results from a land surface model with an added capability to simulate water movement in the soil and water transport within the plant at a tropical forest site in Panama, this study found that dry soil is more important than VPD in limiting plant water loss at the site during the El Niño drought of 2015-2016.

The Impact
Carbon sequestered by tropical forests during normal and wet years can be released during drought years due to tree mortality and reduced ecosystem productivity. Recent drought-related plant mortality has been attributed to increasing VPD associated with climate change. This research disentangled the relative impact of VPD and soil water stress on canopy conductance that controls plant transpiration at a tropical forest site in Panama. The results highlighted the need for new data collection as well as new model development to improve understanding of tropical forest responses to drought.

Summary
In this research, field data and numerical modeling were used to isolate the impact of dry soil and vapor pressure deficit (VPD) on evapotranspiration (ET) and gross primary productivity (GPP) at a tropical forest site in Barro Colorado Island (BCI), Panama, focusing on their response to the drought induced by the El Niño event of 2015-2016.  Numerical simulations were performed using a plant hydrodynamic scheme (HYDRO) and a heuristic approach that ignores stomatal sensitivity to leaf water potential in DOE’s Energy Exascale Earth System Model (E3SM) Land Model (ELM).  The sensitivity of canopy conductance to (VPD) obtained from eddy-covariance fluxes and measured sap flux shows that, at both ecosystem and plant scale, soil water stress is more important in limiting canopy conductance than VPD at BCI during the El Niño event.  The model simulations confirmed the importance of water stress limitation on canopy conductance, but overestimated the VPD impact compared to that estimated from the observations. During the dry season at BCI, seasonal ET, especially soil evaporation at VPD > 0.42 kPa, simulated using HYDRO and ELM, was too strong and will require alternative parameterizations.

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

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

Funding
This research was supported by the U.S. Department of Energy Office of Biological and Environmental Research as part of the Terrestrial Ecosystem Science program through the Next Generation Ecosystem Experiment (NGEE) Tropics project.

Publications
Fang, Y., Leung, L. R., Wolfe, B. T, Detto, M., Knox, R. G., McDowell, N. G., et al. (2021). Disentangling the effects of vapor pressure deficit and soil water availability on canopy conductance in a seasonal tropical forest during the 2015 El Niño drought. Journal of Geophysical Research: Atmospheres, 126, e2021JD035004. https://doi.org/10.1029/2021JD035004

The Science
Los Alamos scientists lead the development of the first inverse model of trees’ rooting depths that is well-tested and integrated with plant physiology, in a rainforest of Barro Colorado Island, Panama. Deep-rooted species had water transport systems that were likely to fail under potential dehydration. Under a variety of droughts, however, deep-rooted species were less dehydrated and survived better than shallow-rooted species especially among evergreen trees. This emphasizes the need to integrate drought exposure in evaluating trees’ drought tolerance.

The Impact
Rooting depths are a critical unknown for modeling forest response to droughts, which are projected to intensify. Due to challenges in measuring rooting or water-sourcing depths, researchers have relied on above-ground traits to assess trees’ likelihood of drought-induced mortality. The models developed here will allow wider integration of rooting depths and drought exposure in drought resilience studies.

Summary
For the long-term, 50 ha forest dynamics observatory in Barro Colorado Island, Panama, whole soil column moisture profiles were reconstructed over 1990-2015. An inverse model of rooting depths was developed (29 species) using 5-yearly tree growth re-censuses, leaf hydraulic vulnerability curves, VPD and soil moisture. The model was validated against existing data of potential water-sourcing depths based on isotopes. Associations of rooting depths with properties of the water transport system were evaluated, as well as, with tree mortality rates over 35 years.

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

PI Contact: Rutuja Chitra-Tarak, Los Alamos National Lab, rutuja@lanl.gov
Chonggang Xu, Los Alamos National Lab, cxu@lanl.gov

Funding
This research was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. Funding from the following sources also supported this research: U.S. National Science Foundation, U.S. Department of Agriculture and IRD France.

Publications
Chitra-Tarak, Rutuja et al. “Hydraulically-vulnerable trees survive on deep-water access during droughts in a tropical forest.” New Phytologist (In press) [https://doi.org/10.1111/nph.17464]

Chitra-Tarak, Rutuja et al. “Soil Water Potentials (1990-2018) from a calibrated ELM- FATES, and rooting depth analyses scripts, PA-BCI, Panama. 2.0.” NGEE Tropics Data Collection. (dataset). (2020) [http://dx.doi.org/10.15486/ngt/1696806]

The Science
In tropical forests available phosphorus can limit plant growth. Enzymes released by plant roots and soil microbes can increase phosphorus availability throughout the soil profile. Phosphatase enzymes convert phosphorus bound in organic molecules to an inorganic form that is available to plants. Roots of different tree species can have different effects on phosphatase activity. The amount of roots and their activities vary with depth in soil. Current models distribute roots through the soil column; new data on how root traits, soil characteristics, and phosphorus availability vary with soil depth will improve how models represent tree growth in tropical forests.

The Impact
This study pairs new data on soil and root phosphatase with fine-root and soil factors. The root and soil factors regulate enzyme activity in the soil profile. The results improve our understanding of root-soil interactions that influence phosphorus dynamics. These findings from a tropical forest in Puerto Rico generated predictive relationships that were robust across a wide range of soil conditions. The best equation predicted root phosphatase from specific root length and soil available phosphorus content. These relationships will enable more accurate models of phosphorus control on tropical forest productivity under changing environmental conditions. 

Summary
Our objective was to determine fine-root traits and soil measurements that influenced soil and root phosphatase activity in the soil profile. We measured soil and root phosphatase to 1 m and 30 cm in soil depth respectively, including corresponding soil conditions (phosphorus concentrations, soil texture, bulk density) and fine-root traits (specific root length and fine-root mass density). We found that soil phosphatase can be predicted by bulk density, organic phosphorus, and fine-root mass density and that variation in root phosphatase can be explained by available phosphorus and specific fine-root length. Thus, both fine-root traits and soil phosphorus measurements are needed to understand mechanisms, like phosphatase, that mediate phosphorus availability in tropical forests. These findings strengthen the link between phosphatase activity and existing root and soil phosphorus parameters in ecosystem models enabling a more accurate representing of the phosphorus cycle. Our data merge phosphatase activity —a root and microbial function important to phosphorus acquisition— with fine-root traits and soil data, informing our understanding of phosphorus acquisition throughout the soil profile and the potential feedbacks to tropical forest growth. 

Contact: BER Program Manager, Daniel Stover, U.S. Department of Energy Office of Science, Office of Biological and Environmental Research, Earth and Environmental Systems Sciences Division (SC-33.1), Environmental System Science daniel.stover@science.doe.gov

Brian Benscoter, U.S. Department of Energy Office of Science, Office of Biological and Environmental Research, Earth and Environmental Systems Sciences Division (SC-33.1), Environmental System Science, brian.benscoter@science.doe.gov

Principal Investigator: Richard J. Norby, Oak Ridge National Laboratory, currently, University of Tennessee, Knoxville rnorby@utk.edu

Kristine G. Cabugao, Lawrence Berkeley National Laboratory, KCabugao@lbl.gov

Funding
This research was supported as part of the Next Generation Ecosystem Experiments‐Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publications
G. Cabugao, et al., “Bringing function to structure: Root-soil interactions shaping phosphatase activity throughout a soil profile in Puerto Rico.” Ecology and Evolution 11, 1150-1164 (2021). [DOI: 10.1002/ece3.7036]

Norby, K. G. Cabugao, D. Yaffar.“Root-soil depth profile in Luquillo Experimental Forest, Puerto Rico, February, 2019”. NGEE Tropics Data Collection. (dataset). [DOI: 10.15486/ngt/1574087]