Using computer models and field data, researchers found that tropical forests recover from major hurricanes, as long as they are infrequent.

The Science
Scientists use computer programs called ecosystem models to predict and study the future of tropical forests. However, existing ecosystem models do not account for damage caused by hurricanes. This is a problem as hurricanes are becoming stronger because of climate change. A team of scientists modified one ecosystem model so it could simulate hurricane damage on tropical forests. They also used data from a forest in Puerto Rico to test and improve the ecosystem model predictions. Once the model results look good, they tested how long it takes for tropical forests to recover from hurricane damage.

The Impact
The ecosystem model agreed well with the observed forest damage from hurricane Hugo, and how fast forests recovered from the hurricane. The research team tested how long it would take for damaged forests to look similar to forests that never suffered any hurricane damage. To their surprise, damaged forests could accumulate more carbon than undamaged forests. This happened because hurricanes killed many small trees, so large trees grew even larger. These results indicate that infrequent hurricanes may have little impact on forests. With this model, researchers can explore other effects on forests resulting from changes in hurricane frequency and strength.

Summary
To develop the ecosystem model, the research team accounted for three observations. First, trees die much more when hurricane winds exceed 90 miles an hour. Second, hurricanes cause more damage on forests that have only a few large trees. Third, palms are more resistant to hurricane damage than trees. The team used data from the Luquillo Experimental Forest (Puerto Rico) to validate the model. The model correctly simulated the widespread loss of trees following hurricane Hugo. The model also represented well the forest growth and changes in tree and palm abundances over the following 30 years.

The team used the validated model to study the long-term impacts of hurricane disturbances. The team conducted three simulations: one without any hurricane damage, one with severe damage similar to Hugo, and one with moderate damage similar to Maria. The model predicted large losses of biomass immediately following the hurricane disturbances. However, after 80 years since the hurricane, the damaged forests recovered. Surprisingly, forests damaged by hurricane Maria showed 5% more biomass than undamaged forests. This result occurred because the hurricane killed small trees, which reduced the competition for light and water and allowed surviving trees to grow larger.

Contact: Marcos Longo, Lawrence Berkeley National Laboratory, mlongo@lbl.gov
Rafael L. Bras, Georgia Institute of Technology, rbras6@gatech.edu
Jiaying Zhang, Georgia Institute of Technology, jiaying.zhang@gatech.edu

Funding
This research was supported by the National Science Foundation; the K. Harrison Brown Family Chair; the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research; the NASA Postdoctoral Program, administered by the Universities Space Research Association under contract with NASA; and the U.S. Department of Agriculture, Forest Service, and the USDA Forest Service International Institute of Tropical Forestry works in collaboration with the University of Puerto Rico. The research carried out at the Jet Propulsion Laboratory, California Institute of Technology, was under a contract with the National Aeronautics and Space Administration.

Publications
Zhang, J., R. L. Bras, M. Longo, T. Heartsill Scalley (2022), The impact of hurricane disturbances on a tropical forest: Implementing a palm plant functional type and hurricane disturbance module in ed2-hudi v1.0. Geosci. Model Dev., 15 (13), 5107–5126. https://dx.doi.org/10.5194/gmd-15-5107-2022 

A novel crown damage module in the Functionally Assembled Terrestrial Ecosystem Simulator provides new capabilities for testing hypotheses related to disturbance and recovery in tropical forests.

The Science
Forest trees are exposed to a variety of disturbances such as windstorms and lightning. These disturbances can result in significant damage to their crowns, the part of a tree made up of branches and leaves. Little is known about how tree crown damage influences the growth and survival rates of trees, or interactions among different tree species. Now, however, a team of scientists have introduced a way to represent crown damage in a vegetation model. This new capability allows scientists to test how tree crown damage impacts forest dynamics and the carbon cycle.

The Impact
Forests cycle large amounts of water, energy and carbon with the atmosphere and play an important role in regulating the Earth’s climate. However, forest disturbances that cause crown damage are predicted to become more severe and more frequent in the future. Understanding how forests will respond to these disturbances is critical for understanding the long term role of forests in the biosphere.

Summary
A multi-institutional team of scientists introduce a crown damage module into the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), a submodel of the DOE’s Energy Exascale Earth System Model (E3SM). Using this new functionality, scientists were able to test how crown damage alters forest dynamics relative to equivalent increases in tree mortality. Simulated growth and survival rates were benchmarked against data from Barro Colorado Island in Panama. Results revealed how the largest impact of crown damage on aboveground biomass and carbon residence time are due to increases in mortality associated with crown damage. However, simulated crown damage caused changes to forest canopy organization and competitive dynamics between plant functional types. Representing crown damage in vegetation models is important to capture the legacy effects of disturbance and the ways that disturbances that overlap in space or time may interact to increase forest mortality.

Contact: Jessica Needham, Research Scientist, 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. CK also acknowledges support from the DOE Early Career Research Program. LBNL is managed and operated by the Regents of the University of California under prime contract number DE-AC02-05CH11231. BCI dendrometer data collection was supported by the HSBC Climate Partnership and Smithsonian ForestGEO.

Publication
Needham, J.F., Arellano, G., Davies, S.J., Fisher, R.A., Hammer, V., Knox, R., Mitre, D., Muller-Landau, H.C., Zuleta, D., Koven, C.D., “Tree crown damage and its effects on forest carbon cycling in a tropical forest”, Global Change Biology (2022) DOI:10.1111/gcb.16318

Jennifer Holm, research scientist at Berkeley Lab’s Climate and Ecosystem Science Division, is a collaborator with the NGEE-Tropics project and serves as the project liaison to E3SM’s Functionally Assembled Terrestrial Ecosystem Simulator (FATES). She was recently a guest feature on Aliyah Kovner’s A Day in the Half-Life podcast. Her appearance on the podcast was dedicated to explaining and demystifying the concept of climate models. For years, models have been employed by climate scientists to anticipate Earth’s long term weather patterns and conditions. Despite the long history of the technology and the importance of its function, many people are still uninformed of the details and significance of climate modeling. Luckily, scientists like Holm have been taking great strides to introduce the public to the ins and outs of these vital processes. While featured on the podcast, Holm detailed the goals of climate science and computing, the kinds of projections you can expect from the modeling processes, the impact the prediction technology has had, the history of climate modeling and the different eras the subject has undergone, and more. At the end, Holm states that the future of climate research will be dependent on the “human element” and the interactions between humans and the environment. We offer a big thank you to Holm for taking steps to bridge the information gap and reaching out to educate audiences. Listen to the full podcast here to learn more about what Jennifer has to say about climate change.

Headshot of Jennifer Holm

Berkeley Lab’s Earth and Environmental Sciences Area (EESA)’s Staff Scientist Robinson Negron-Juarez is going to receive an honorary degree from the National University of the Peruvian Amazon in July. Negron-Juarez has been a part of EESA since 2013, and has been making significant contributions to the Next Generation Ecosystem Experiments – Tropics project. He also created the Wildfire Research Element at EESA.  His research aims to understand the interactions between terrestrial ecosystems and climate conditions. Due to his work on understanding windthrow variability in the Central Amazons, his research on the vulnerability of forests due to severe storm conditions, and his use of Landsat imagery to detect treefall gaps, he has been recognized by the National University of the Peruvian Amazon for his research efforts. When Negron-Juarez isn’t continuing his contributions to the NGEE-Tropics project through his research on deep convection and extreme rainfall events on forestry dynamics, he’s supporting and encouraging indigenous scientists. We applaud his dedication to uplifting his fellow scientists, asking questions few think to ask, and his tireless efforts to discover the truth. Read more about Robinson’s research and this award from the National University of the Peruvian Amazon here.

Image courtesy of EESA’s News and Events site https://eesa.lbl.gov/robinson-negron-juarez-to-receive-honorary-degree/

Soil moisture thresholds of sap velocity.

The Science
Transpiration is the process of water moves through a plant from soil to atmosphere. In humid tropical rainforest region, soil water is recharged during the wet season to support dry season transpiration. Therefore, transpiration is often considered to be light- but not water limited in humid tropical rainforests. However, it’s not clear whether the tropical rainforests with abundant water will become water limited under extreme climate conditions. We used field data to examine dynamics of transpiration, soil water, and meteorological variables during the record-breaking 2015-16 El Niño drought in Central Amazon. We found a shift from light- to water-limitation of sap velocity, and identified a soil moisture threshold of water limitation in Central Amazon.

The Impact
This study suggests a progressively critical role of soil moisture under a drier future. This could happen in Central Amazon and other places that we previously thought have plenty of water. The soil moisture threshold provides a crucial benchmark to test and improve model simulations of future land-atmosphere feedbacks in Amazon under climate change, which are currently inadequate.

Summary
We measured sap velocity, soil water content, and meteorological variables in an old-growth upland forest in the Central Amazon throughout the 2015-16 drought. We found a rapid decline in sap velocity and in its temporal variability during the drought, accompanied by a marked decline in soil moisture and an increase in temperature and vapor pressure deficit. To understand water or light limitation, we examined the covariation of sap velocity with soil water content and net radiation using partial correlation analysis. We found sap velocity was largely limited by net radiation during normal dry seasons, but it shifted to be limited by soil water during drought. Next to identify the timing when this shift occurred, we used a moving window approach to conduct partial correlation analysis at every 10-day and examined how the coefficient changed during the whole period. We found the water stress started to occur in late August to early September in 2015. The soil moisture control continued throughout September, then became intermittent and disappeared after several rainfall events. During the strong water control period, the light control disappeared. We further identified the threshold of soil moisture at 0.33 m 3 /m 3 (around -150 kPa in soil matric potential).

Contact: Lin Meng, Lawrence Berkeley National Laboratory, Postdoc researcher, linmeng@lbl.gov

Funding
This work is supported by the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics), as part of DOE’s Terrestrial Ecosystem Science Program – Contract No. DE-AC02-05CH11231. We would like to thank the Large Scale Biosphere-Atmosphere Program (LBA), coordinated by the National Institute for Amazon Researches (INPA), for the use and availability of data, for logistical support and infrastructure during field activities.

Publications
Meng, Lin, et al. “Soil moisture thresholds explain a shift from light-limited to water-limited sap velocity in the Central Amazon during the 2015-16 El Niño drought.” Environmental Research Letters (2022), 17: 064023. https://doi.org/10.1088/1748-9326/ac6f6d

Large uncertainty exists in predictions of future transpiration due to complex interactions between plant physiology and the water and energy budgets.

The Science
Plant transpiration is the largest hydrologic flux of water globally, after precipitation, and therefore plays a large role in driving surface water availability. Transpiration responds to rising atmospheric CO₂ at stomatal to whole-plant to regional-scales, with feedbacks between scales. This study reviewed the biophysical mechanisms by which rising CO₂ impacts global scale plant transpiration. The authors then identify a path forward by which both empirical and modeling focused science approaches work together towards more mechanistic, evaluated prediction of transpiration under future conditions.

The Impact
Recent observations of rising plant transpiration at the global scale are consistent with increasing leaf area due to CO₂ fertilization, but at odds with the well-known stomatal closure response. This increasing water demand results from a complex set of interacting factors and mechanisms, many of which are non-linear. This paper ultimately provides a testable framework of hypotheses regarding how transpiration responds globally to rising atmospheric CO₂. The paper further serves as a call to arms for global empiricists and modelers to unify their efforts to better understanding and predicting transpiration under future conditions.

Summary
Here we review the myriad ways by which rising CO₂ can directly and indirectly influence plant transpiration at the global scale. Many compensating mechanisms and feedbacks make the prediction of transpiration challenging with rising CO₂. Global changes in plant transpiration in response to rising CO₂ will manifest through droughts, vapor pressure deficit, plant physiological processes including shifting leaf area and phenology, and through forest loss (disturbance). We place these mechanisms into a testable framework of hypotheses that outlines a path forward for both empiricists and modelers. The impacts of changing transpiration at large scales are significant for water provision and utilization demands.

Contacts: Sergio Vicente-Serrano, Instituto Pirenaico de Ecología, svicen@ipe.csic.es
Brian Benscoter, U.S. Department of Energy, Biological and Environmental Research (SC-33), Environmental System Science, brian.benscoter@science.doe.gov

Funding
This work was supported by the Spanish Ministry of Science and FEDER; CROSSDRO project financed by the AXIS – sectorial climate impacts and pathways for sustainable transformation, JPI-Climate co-funded call of the European commission; NGM and LRL were supported by the Department of Energy’s Next Generation Ecosystem Experiment-Tropics. DGM acknowledges support from the European Research Council. AK acknowledges support from the European Union Horizon 2020 program.

Publication
Vicente-Serrano S, Miralles D, McDowell N, Brodribb T Domínguez-Castro F, Leung R, Koppa A. The uncertain role of rising atmospheric CO₂ on global plant transpiration. Earth Science Reviews, p. 104055. https://doi.org/10.1016/j.earscirev.2022.104055