NGEE–Tropics

Next-Generation Ecosystem Experiments

Tree Leaves Act as Bottlenecks to Water Flow in the Transpiration Stream

Disproportionate resistance to water flow in leaves helps trees maintain their vascular function, but the effect declines as trees grow larger.

Researchers used cranes to access tree crowns for measurements pressure gradients within trees, which were used to assess the constriction of water flow within leaves. 

The Science

Trees move large quantities of water from soil to the atmosphere in the transpiration stream, forming a major component in Earth’s water cycle. Water in the transpiration stream is pulled through trees under tension (i.e., negative pressure) in specialized conduits. When tension is excessive, vascular disfunction occurs. Compared to conduits in roots and stems, leaf conduits constrict water flow. This constriction reduces tension upstream, protecting stem and leaves from vascular disfunction. Variation among trees in this constriction is not well resolved. This study assessed variation in the extent to which tree leaves constrict water flow in the transpiration stream.

The Impact

Results showed that, on average, about half the total resistance to water flow within trees is located within leaves. This indicates that tree leaves are generally highly constrictive to water flow in the transpiration stream. Larger trees tended to have a lower proportion of total resistance to water flow within their leaves. This suggests that leaves in larger trees are less effective at protecting stems and roots from vascular disfunction associated with high water tension. These results help to understand and predict transpiration rates and tree mortality during droughts and heat waves.

Summary

Researchers compiled new and previously published measurements into a multibiome dataset to assess variation among trees in the fraction of whole-tree hydraulic resistance that is in leaves (fRleaf). The measurements relied on pressure differences between leaves, stems, and roots in transpiring trees. Among 80 samples, fRleaf averaged 0.51, was consistent among biomes, and declined with tree height. The results show that leaves play an important role in controlling the flow rate and tension of water in the transpiration stream. Because higher fRleaf protects stems from vascular disfunction and fRleaf declines with tree height, taller trees may be at greater risk of vascular disfunction during droughts. This effect may contribute to the disproportionate drought mortality that has been observed among tall trees.

Contact

Brett Wolfe
Louisiana State University Agricultural Center
wolfe1@lsu.edu 

Funding

-Next Generation Ecosystem Experiments‐Tropics funded by the US Department of Energy, Office of Science, Office of Biological and Environmental Research 

-United States Department of Energy contract to Brookhaven National Laboratory

-Smithsonian Tropical Research Institute post-doctoral fellowship 

-National Institute of Food and Agriculture, US Department of Agriculture, McIntire Stennis Project

-Innovation and Technology Fund (funding support to State Key Laboratories in Hong Kong of Agrobiotechnology) of the HKSAR, China

-US National Science Foundation 

 -USDA NIFA Hatch Project through the Maine Agricultural and Forest Experiment Station. 

Publications

Wolfe, B. T. et al. Leaves as bottlenecks: The contribution of tree leaves to hydraulic resistance within the soil−plant−atmosphere continuum. Plant, Cell & Environment 46, 736–746 (2023). [DOI: 10.1111/pce.14524

A path forward to understanding and mitigating drought impacts on trees

A review of the challenges and inter-disciplinary solutions to understanding tree drought physiology and management approaches to mitigate impacts.

Figure:  Tree mortality in the Sierra Nevada Mountains of California from 2014-2017.  Credit: US Forest Service.

 

The Science

Droughts of increasing severity and frequency are a primary cause of forest mortality. Yet, fundamental knowledge gaps regarding the complex physiology of trees limit the development of more effective management strategies to mitigate drought effects on forests. Here, a group of experts from around the world highlight basic research needs to better understand tree drought physiology and how new technologies and interdisciplinary approaches can be used to address them. Their discussion first identifies key research questions regarding how trees respond to drought, and then they identify opportunities to enhance or develop new strategies for managing drought effects on forests. They conclude with a discussion of the need for co-producing research with land managers.

 

The Impact

This paper addresses the accelerating challenge of drought-induced tree mortality, which has large, negative environmental, societal, and economic impacts. This information provides a roadmap to research and management that can maximize forest health and the subsequent provision of resources such as timber, clean water, animal habitat, and biodiversity.  A large discrepancy exists between the magnitude of societal and economic costs of tree mortality relative to the funding support for research and management.  Engaging local communities in understanding and mitigating drought impacts on forests is a critical step towards successful adaptation.  

 

Summary 

The paper emphasizes the urgency of bridging research gaps in tree drought physiology to inform better forest conservation and management practices under climate change. It calls for increased investment in interdisciplinary studies, new technologies, and collaborations to improve forest resilience.  The authors first review critical challenges in our understanding and application of tree genetics, hormones, wood properties, legacy impacts, and soil microbes to drought-mitigation strategies.  They then review key large-scale management strategies that show promise for sustaining forests under future droughts, including genetic selection, assisted migration, stress priming, microbiome manipulations, and advanced monitoring and prediction.  They conclude with an emphasis on the large value of cross-disciplinary efforts between scientists, land managers, and local communities to effectively develop and implement protection strategies.

 

DOE PM Contacts

  • Daniel Stover, Environmental System Science Program, daniel.stover@science.doe.gov

Research Contacts

 

Funding

Nate McDowell was supported by the Next Generation Ecosystem Experiment-Tropics, a multi-institutional project supported by the Department of Energy, Office of Science, Biological and Environmental Research as part of the Environmental System Science Program. This synthesis was an international effort supported by a number of different agencies around the world.

Publication

Groover, A., Holbrook, N.M., Polle, A., Sala, A., Medlyn, B., Brodersen, C., Pittermann, J., Gersony, J., Sokołowska, K., Bogar, L. and McDowell, N., 2024. Tree drought physiology: critical research questions and strategies for mitigating climate change effects on forests. New Phytologist.

Related Links

https://ngee-tropics.lbl.gov/

 

Building a Global Network to Monitor and Understand Tree Mortality

A global survey of forest monitoring plots points the way forward for improved monitoring of tree mortality and support of robust collaborations. 

A global tree mortality monitoring system is crucial for climate mitigation. Data integration and representative collaboration can improve assessments and policy decisions. (Figure from Esquivel-Muelbert, et al. 2025)

The Science

Environmental change is driving greater mortality of trees worldwide, but no comprehensive assessment of global tree mortality exists. To address this gap, a large international and multidisciplinary team integrated data from long-term forest monitoring plots around the world, leveraging satellite data for a truly global picture based on almost a half-million plots across 89 countries and five continents. The authors are from 132 different institutions around the world and propose integrating ground-based data with satellite technology to create a global monitoring system to better understand tree death and develop strategies to protect forests and combat climate change. They emphasize that a truly global monitoring effort should promote geographically broad collaborations that support temporally and methodological consistency and alleviate issues of limited spatial and temporal gaps in forest monitoring records.

 

The Impact

This research addresses the growing problem of tree mortality worldwide, which affects forests and human societies. This is the first effort to integrate forest inventory networks from around the world, including scientists and citizens by combining ground surveys with satellite images to track tree deaths more accurately. This information helps elucidate the long-term impacts of tree mortality on forest ecosystems, their biodiversity, and the ecosystem services they provide. Such a network also helps scientists predict future tree mortality and its impact on Earth system processes to support better-informed management of global forests. 

 

Summary 

This research highlights the urgent need for a comprehensive global system to monitor tree mortality, as increasing rates of tree death have significant implications for forest ecosystems and their services. Rates of tree mortality are rising but there is a lack of consistent global data to monitor and understand these trends. By synthesizing data from 466,865 forest monitoring plots across 89 countries, a large and international team of authors argue that there are significant gaps in ground-based monitoring, particularly in regions like Russia, West Africa, and Central America. They propose integrating ground-based forest inventories with remote sensing and modeling to fill these gaps. This integration requires developing technical solutions for data comparability, enhancing data collection in understudied areas and fostering robust collaborations by engaging local scientists. More data with standardized metrics for tree mortality and regular ground surveys complemented by use of new technologies will support robust predictions of tree mortality.

 

DOE PM Contacts

  • Daniel Stover, Environmental System Science Program, daniel.stover@science.doe.gov

Research Contacts

  • Vanessa L. Bailey, vanessa.bailey@pnnl.gov, Pacific Northwest National Laboratory, COMPASS-FME Principal Investigator
  • Nate McDowell, nate.mcdowell@pnnl.gov, Pacific Northwest National Laboratory
  • Ben Bond-Lamberty, BondLamberty@pnnl.gov, Pacific Northwest National Laboratory

 

Funding

A portion of this research was supported by COMPASS-FME, a multi-institutional project supported by the Department of Energy, Office of Science, Biological and Environmental Research as part of the Environmental System Science Program. This synthesis was an international effort supported by a number of different agencies around the world.

 

Publication

International Tree Mortality Network, Cornelius Senf, Adriane Esquivel‐Muelbert, Thomas AM Pugh, William RL Anderegg, Kristina J. Anderson‐Teixeira, Gabriel Arellano et al. “Towards a global understanding of tree mortality.” New Phytologist (2025). https://doi.org/10.1111/nph.20407

Related Links

COMPASS-FME Project Page

International Tree Mortality Network Webpage

 

Vertical Canopy Gradients of Respiration Drive Plant Carbon Budgets and Leaf Area Index

 

Field-based Canopy Gradients in Leaf Respiration Improve Modeled Canopy Structure Scientists reduced bias in simulated forest dynamics by updating model assumptions to align with field observations. 

Image courtesy of Jessica Needham, LBNL.

Steeper canopy gradients of leaf maintenance respiration (dashed line) based on field measurements changes the point in which leaves are in negative carbon balance which impacts allocation of carbon to leaf biomass.

The Science

Around half of all carbon that plants take up through photosynthesis is released back to the atmosphere by plant respiration. Despite its importance as a major component of the carbon cycle, most Earth system models represent plant respiration very simply. Scientists from the Next Generation Ecosystem Experiments Tropics project updated assumptions in the DOE’s earth system model about how respiration changes from sunlit leaves at the top of the canopy to shaded leaves deep in the forest understory based on measurements from a field site in Panama. Global simulations with the updated model were a better match to observations of forest structure.

The Impact

Leaf area index, a measure of the amount of leaf area in the forest canopy per unit ground area, is an important forest attribute, driving exchanges of carbon, water and energy between the land and atmosphere through impacts on photosynthesis and evapotranspiration. By updating model assumptions based on field observations, scientists were able to improve modelled leaf area index. This will further our understanding of the role of forests in the Earth system over the coming decades. 

Summary

When field scientists from the NGEE-Tropics project found that vertical gradients in leaf respiration differed from model assumptions, scientists from the modelling team responded by updating the ELM-FATES model to align with observations. When respiration has a steeper canopy gradient and is lower in the understory, leaves deeper in the canopy remain in positive carbon balance, with more carbon taken up by photosynthesis than is used in leaf construction and respiration. As a result, plants grow more leaves, which increases simulated rates of photosynthesis and evapotranspiration. The updated parameterization of ELM-FATES led to an increased number of understory plants, and higher leaf area index, both of which improved alignment of simulations with ground and satellite observations. 

Contact
Jessica Needham
Lawrence Berkeley National Laboratory
jfneedham@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 and as part of the Energy Exascale Earth System Model (E3SM) project, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research Earth Systems Model Development Program area of Earth and Environmental System Modeling. LBNL is managed and operated by the Regents of the University of California under prime contract number DEAC02-05CH11231. R.F. also acknowledges support of the EU Horizon2020 under grant agreement 101003536 (ESM 2025).

Publications

For each publication, link to the publication’s record in DOE Pages in OSTI (https://www.osti.gov/pages/) in the title of the article. This is now required for all research funded by DOE. The researcher submitting the highlight will need to first submit the accepted manuscript of the journal article(s) to the DOE Office of Scientific and Technical Information (OSTI) via the DOE Energy Link System (E-Link).  If the researcher is a financial assistance awardee, they can submit a completed DOE Announcement Notice (AN) 241.3 by going to: https://www.osti.gov/elink-2413. In the case of lab researchers, they will need to consult with their lab’s STI Manager for more information about submission. A listing of STI Managers can be found here: https://www.osti.gov/stip/stimanagers#stimanagers.  

Needham, J. F., et al, “Vertical Canopy Gradients of Respiration Drive Plant Carbon Budgets and Leaf Area Index.” New Phytologist, (2025). [DOI: 10.1111/NPH.20423]

Lamour, J., et al. “The Effect of the Vertical Gradients of Photosynthetic Parameters on the CO2 Assimilation and Transpiration of a Panamanian Tropical Forest.” New Phytologist 238 (6), 2345–62 (2023). https://doi.org/10.1111/nph.18901.

Future climate doubles the risk of hydraulic failure in a wet tropical forest

Simulations of future climate in tropical forests show a risk of increased mortality due to plant
water stress, unmitigated by rising CO 2 levels.

Robbins, Z., Chambers, J., Chitra‐Tarak, R., Christoffersen, B., Dickman, L. T., Fisher, R., … & Xu, C. (2024). Percentage of days with hydraulic failure at (a) > 60% and (b) > 80% loss of conductivity projected by the vegetation model, FATES-HYDRO, at Barro Colorado Island, Panama, under contemporary climate conditions), two future climate scenarios,and two CO2 levels.

The Science

There will be a future risk of increased tropical forest mortality under climate change due to increasing plant water stress due to warming temperatures. Rising atmospheric carbon levels do not mitigate mortality risk but increase plant productivity. We further find that plant traits are crucial to determining this mortality risk.

The Impact

Tropical forests have significant impacts on global water and carbon cycles. Adapting to global climate change will require an understanding of the change to these crucial resources. We show that increasing temperature and drying may increase tropical forest mortality. This is due to increased plant water stress. Increased atmospheric carbon levels were thought to potentially mitigate this loss, but our model shows it does not at these levels of warming. These increasing mortality levels could significantly reduce tropical carbon storage. Global climate change will increase the annual productivity of these forests but remove more water from these forests.

Summary

We used a dynamic vegetation model with plant hydrodynamics  (FATES-HYDRO) to simulate the stand-level responses to future climate changes in a wet tropical forest in Barro Colorado Island, Panama. We calibrated the model by selecting plant trait assemblages that performed well against NGEE tropics observations of plant hydrodynamics. These assemblages were run with temperature and precipitation changes for two greenhouse gas emission scenarios  (2086–2100: SSP2-45, SSP5-85) and two CO2 levels (contemporary, anticipated). Simulations show an increase of 5.7% to 10.1–11.3% under future climate scenarios due to increasingly negative leaf water potentials. Gross primary productivity increased 27–53% under future climate but decreased (-21%–8.6% ) without rising CO2. Minimum annual leaf water potential ( a measure of plant stress) under contemporary simulations decreased  under both future scenarios with anticipated CO2  and under contemporary CO2 scenarios (indicating greater plant stress). Trait assemblage members which experienced hydraulic failure had substantially more negative minimum season leaf water potential  (−1.376 MPa) than those who did not (−0.815 MPa). Simulation analysis shows plant traits played a more significant role in the risk of hydraulic failure (98%) than climate scenarios or models.

Contact:

Zachary Robbins
Los Alamos National Lab, Postdoctoral researcher
zjrobbins@lanl.gov

Chonggang Xu
Los Alamos National Lab, Staff Scientist
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.

Publications

Robbins, Z. et al. Future climate doubles the risk of hydraulic failure in a wet tropical forest. New Phytologist (2024) doi:10.1111/nph.19956.

Why models underestimate West African tropical forest primary productivity

Fractional absorbed photosynthetic radiation, and photosynthetic traits are responsible for widespread underestimation of West African forests productivity. Photo Credit: Huanyuan Zhang-Zheng. One of the study site: Ankasa forest reserve

The Science

Tropical forests are some of the most important ecosystems on Earth, contributing a massive portion of the planet’s biomass and carbon absorption. They act as vital carbon sinks, absorbing CO₂ from the atmosphere and playing a crucial role in regulating the global climate. Accurate estimation of their productivity is vital for understanding how effectively they sequester carbon and how they might respond to changing environmental conditions. Historically, models have struggled to estimate tropical forest productivity accurately, particularly in West Africa.

The Impact

The study explores why current global models underestimate the Gross Primary Productivity (GPP)—the rate at which forests convert carbon dioxide into biomass through photosynthesis—of West African tropical forests. In short, the team led by Huanyuan discovered that these forests are more productive than previously thought, often outperforming even the Amazon in terms of carbon assimilation. Yet, existing models fail to capture this, underscoring the need for improved data inputs and modelling approaches.

Summary

This region’s GPP is often underestimated, with field measurements revealing far higher productivity than global models suggest. The reasons for this mismatch lie in the models’ inputs, particularly in terms of light use efficiency (LUE), the fraction of absorbed photosynthetically active radiation (fAPAR), and how local climate conditions are represented. The study focused on three sites in Ghana, representing the major forest types in West Africa: the Ankasa rainforest (ANK), the semi-deciduous Bobiri forest (BOB), and the dry Kogyae forest (KOG). Each of these sites revealed significant discrepancies between biometric GPP—measured directly in the field—and the GPP estimates from widely used global models like MODIS, FLUXCOM, and dynamic global vegetation models (DGVMs). On average, the models underestimated GPP by 56.3%, with the most significant discrepancies occurring at the semi-deciduous BOB site. Interestingly, this gap between field measurements and models narrowed when local data were incorporated into the models. For example, when a simple photosynthesis model (P-model, a new generation model led by Prof Colin Prentice) was adjusted to use field-measured inputs like photosynthetic traits and absorbed sunlight, its GPP estimates came closer to the real values, though small discrepancies remained in the wetter rainforest site at Ankasa. 

Contact

Huanyuan Zhang-Zheng
University of Oxford
huanyuan.zhang@ouce.ox.ac.uk

Funding

The study 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. The author team also received funding from UK Natural Environment Research Council.

Publications

Zhang-Zheng, H., Deng, X., Aguirre-Gutiérrez, J. et al. Why models underestimate West African tropical forest primary productivity. Nat Commun 15, 9574 (2024). https://doi.org/10.1038/s41467-024-53949-0

Zhang-Zheng, H., Adu-Bredu, S., Duah-Gyamfi, A. et al. Contrasting carbon cycle along tropical forest aridity gradients in West Africa and Amazonia. Nat Commun 15, 3158 (2024). https://doi.org/10.1038/s41467-024-47202-x

Related Links

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Los Angeles Times: https://www.latimes.com/world-nation/story/2024-11-25/moo-deng-baby-pygmy-hippo-thailand-social-media-celebrity