Model results over Barro Colorado Island (BCI), Panama from an integrated model highlights important influence of topography and groundwater table on aboveground biomass of trees.

The Science
Topographic heterogeneity and lateral subsurface flow at the hillslope scale of ≤1 km may have outsized impacts on tropical forest through their impacts on water available to plants. However, vegetation dynamics and finer-scale hydrologic processes are not concurrently represented in Earth system models. This study integrated the Energy Exascale Earth System Model (E3SM) land model (ELM) that includes the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), with a three-dimensional hydrology model (ParFlow) to understand how hillslope-scale hydrologic processes modulate aboveground biomass (AGB) along water availability gradients at Barro Colorado Island (BCI), Panama. The new coupled model is a useful tool for understanding the diverse impact of local heterogeneity on vegetation dynamics and plant-water interactions.

The Impact
Applying ELM-ParFlow-FATES at BCI, this research shows that water table depth (WTD) can play a large role in governing AGB when drought-induced tree mortality is triggered by hydraulic failure (i.e., inability of a plant to move water from roots to leaves). Furthermore, spatial variations of the simulated AGB and WTD can be well explained by topographic attributes, including surface elevation, slope, and convexity. Contrary to the simulations, the observed AGB in the well-drained 50 ha forest census plot within BCI cannot be well explained by topographic attributes or observed soil water, suggesting other factors such as nutrient status, heterogeneity in soil property and/or plant traits may have a larger influence on the observed AGB. This study points to opportunities for improving understanding of hydrological and ecological processes using the newly developed coupled model combined with observations.

Summary
The team developed a coupled model that integrates a three-dimensional hydrology model into the Energy Exascale Earth System Model (E3SM). E3SM includes the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) for vegetation dynamics. The model was used to explicitly resolve hillslope topography and water flow underneath the land surface to understand how local-scale hydrologic processes modulate vegetation along water availability gradients. The team applied the model at BCI to simulate the AGB variability. They found AGB is higher in the wet areas compared to dry areas in the domain-wide simulations and AGB decreases nonlinearly with increasing WTD when WTD is less than 10 m. The degree of AGB variability differs depending on how hydraulic failure induced mortality is represented. Unlike for the modeled AGB, however, the team were not able to find similar relationships between topography and WTD with the observed AGB. To support the findings in this study calls for more data collection, e.g., soil moisture, WTD, AGB, and plant traits (e.g., wood density) across the hydrologic gradient.

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., Koven, C. D., Bisht, G., Detto, M., Cheng, Y., McDowell, N., Muller-Landau, H., Wright, S. J., and Chambers, J. Q.: Modeling the topographic influence on aboveground biomass using a coupled model of hillslope hydrology and ecosystem dynamics, Geosci. Model Dev., 15, 7879–7901, https://doi.org/10.5194/gmd-15-7879-2022, 2022.

Forest resilience has declined throughout much of the tropical and temperate regions globally since 2020, with increases in resilience in boreal regions.

The Science
Forest resilience to changing climate is suspected to be changing in many regions around the world, based on many independent forms of data.  However, no global study has examined changes in forest resilience over time, thus we are unaware of both the trends and spatial demographics of any changes in forest resilience.  This study examined the global patterns of forest resilience, indexed as the dTAC of NDVI (e.g. canopy greenness) towards or away from a tipping point after which disturbance is highly likely.  This study found declining forest resilience in tropical and temperate biomes, and an increase in forest resilience in the boreal biome.  There was no influence of forest management on the trends, indicating that changes in resilience were driven by regional-scale changes in water availability and temperature.

The Impact
Recent observations of increasing tree mortality from a variety of disturbances have raised concern over the global resilience of forests to changing climate.  Before this study, we did not know the global distribution of forest resilience to disturbance, the change in forest resilience due to climate drivers, nor did we have the ability to predict these disturbances.  This study suggests that a large fraction of the tropical and temperate zones will experience increasing disturbance in the near term, with a large impact on the terrestrial carbon sink.

Summary
We used remotely sensed estimates of kNDVI (canopy greenness) at the global scale to quantify changes in NDVI from 2000-2020.  The response of dTAC was particularly strong over time, with divergent patterns between the tropics and temperate biomes, where there was a decline in resilience, to the boreal zone, where there was an increase in resilience. This paper revealed that ~23% of undisturbed forests globally have reached a tipping point by which disturbance is likely imminent without a rapid change in climate.  Because this represents a large amount of carbon uptake and storage globally, and particularly due to the large impact of tropical forest loss on the global carbon budget, these results are of particular concern.

Figure 1a. The change in temporal autocorrelation (dTAC) of the kernel normalized difference vegetation index (kNDVI) over 2000-2020.  Positive values indicate slowing down of physiological processes, or a decline in resilience, towards a threshold for a tipping point i.e. disturbance. Negative values indicate the opposite, or an increase in resilience. Tropical and temperate regions exhibit significant trends towards decreasing resilience, with notable regional variability, while boreal regions show increased resilience.  Figure 1b.  the data from 1a binned as a function of climatological temperature and precipitation.  The color scheme is as in 1a.  Significant changes in dTAC over time are indicated as solid dots.

Principal Investigator: Giovanni Forzieri, giovanni.forzieri@unifi.it
Program Manager: Dan Stover. U.S. Department of Energy, Biological and Environmental Research (SC-33), Environmental System Science, dan.stover@science.doe.gov

Funding
This study was funded by the Exploratory Project FOREST@RISK of the European Commission, Joint Research Centre. N.G.M. was supported by the Department of Energy’s project Next Generation Ecosystem Experiment-Tropics.

Publication
Forzieri, G., Dakos, V., McDowell, N.G., Ramdane, A. and Cescatti, A., 2022. Emerging signals of declining forest resilience under climate change. Nature, pp.1-6. https://doi.org/10.1038/s41586-022-04959-9

Differential Amazon tree responses to dry conditions and a shift to deeper soil water uptake.

The Science
Tree water use and soil water extraction patterns were monitored during a month-long dry period in a Central Amazon upland tropical forest. During the 2018 dry period, tree water use increased, remained the same, or decreased, depending on species. Water use was dependent on tree size and the amount of conducting sapwood in the trunk. While most roots were in the upper soil layers, some roots exceeded 2 m depth. As the upper soil dried out, more water was taken up from deeper depths.

The Impact
While the upper 2 m of soil can provide much of the water needed during a dry period, deeper sources of water will be required during drought. Tree hydraulic strategies vary, and those that access and shift to deep water sources may be better able to survive drought. Knowing how different tree species respond to drought and how soil water availability changes with drought is important for modeling the responses of tropical forests to projected changes in precipitation patterns.

Summary
With current observations and future projections of more intense and frequent droughts in the tropics, understanding the impacts that extensive dry periods may have on tree and ecosystem-level water use and photosynthesis has become increasingly important. This research investigated soil and tree water extraction dynamics in an old-growth upland forest in central Amazonia during the 2018 dry season. Tree water use was measured by sap flow sensors installed in eight dominant canopy trees, each a different species with a range in diameter, height, and wood density. Soil moisture probes were installed near six of those trees and measured water content and soil water extraction within the upper 1 m. To link depth-specific water extraction to patterns to root distribution, fine root biomass was measured through the soil profile to 235 cm. To scale plot-level tree water use, tree diameters were measured for all trees within a 5 m radius around each soil moisture probe.

The sensitivity of tree water use to reduced rainfall varied by tree, with some increasing and some decreasing water use during the dry period. Tree-level water use ranged from 11-190 liters per day. Stand level water use based on multiple plots encompassing sap flow and adjacent trees varied from ~1.7 to 3.3 mm per day, increasing with tree density. Soil water extraction was dependent on root biomass, which was dense at the surface (i.e., 45% in the upper 5 cm) and declined dramatically with depth. As the dry season progressed and the upper soil dried, soil water extraction shifted to deeper levels, and model projections suggest that much of the water used during the month-long dry-down could be extracted from the upper 2-3 m. Results indicate variation in rates of soil water extraction across the research area and temporally through the soil profile. These results provide key information on tree water use and soil water extraction as water availability changes and can be used in models that project tropical forest response to drought.

Principal Investigator: Jeffrey Warren, Oak Ridge National Laboratory, warrenjm@ornl.gov
Program Manager: Daniel Stover, U.S. Department of Energy, Biological and Environmental Research (SC-33), Environmental System Science, daniel.stover@science.doe.gov

Funding
This material is based upon work supported as part of the Environmental System Science program’s Next Generation Ecosystem Experiments–Tropics (NGEE–Tropics), which is funded by the U.S. Department of Energy’s (DOE) Office of Science Biological and Environmental Research (BER) Program through contract No. DE-AC02-05CH11231 to LBNL). Additional funding for this research was provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

References
Spanner, G.C., et al. “Dry Season Transpiration and Soil Water Dynamics in the Central Amazon.” Frontiers in Plant Science 13, 825097 (2022). DOI: 10.3389/fpls.2022.825097.

Five ways to re-envision fire science and stimulate discovery that help communities better navigate our fiery future.

The Science
Fires can be both useful to and supportive of human values, safe communities and ecosystems, and threatening to lives and livelihoods. Climate change, fire suppression, and living closer to the wildland-urban interface have helped create a global wildfire crisis. There is an urgent, ethical need to live more sustainably with fire. By re-envisioning fire science we can stimulate discovery that help communities better navigate our fiery future. We argue that only through overcoming institutional silos and accessing knowledge across diverse communities can we effectively undertake research that improves outcomes in our more fiery future.

The Impact
Fire has historically been studied from distinct disciplines, as an ecological process, a human hazard, or an engineering challenge. In isolation, connections among human and non-human aspects of fire are lost. We need to shift from observation and modeled representations of varying components of climate, people, vegetation, and fire to more integrative and predictive approaches that support pathways towards mitigating and adapting to our increasingly flammable world, including the utilization of fire for human safety and benefit.

Summary
Fire is fundamental part of ecosystems globally and has been used to manage landscapes for millennia. Humans change wildfire activity via climate change, fire suppression, land development, and population growth. Altered fire regimes impact health, infrastructure, and ecosystem services. A group of 87 fire experts from many disciplines outlined barriers and opportunities in the next generation of fire science. Understanding, mitigating and managing the impacts of fire require addressing key challenges to inform how we serve environmental and social justice by sustainably living and interacting with fire. Utilizing a coordinated and integrated proactive approach across fire science, social science and ecological research is needed. Knowledge from diverse communities is essential to inform progress to safer and more sustainable communities and ecosystems. Establishing infrastructure and reducing barriers to information will accelerate scientific discovery towards advances that promote fire-resilient communities. Fire experts agree that management, including utilization of fire, is essential to supporting safe communities and ecosystems. Inclusion and consideration of human dimensions and values (where we live and how we impact our world) are critical to forecasting and anticipating future fire. Supporting a holistic and collective approach is fundamental for science to inform policy and action in our more firey world.

Contact
Jacquelyn Shuman, National Center for Atmospheric Research, jkshuman@ucar.edu
Brian Benscoter (Program Manager), Department of Energy Office of Science, Biological and Environmental Research at Environmental System Science, Brian.Benscoter@science.doe.gov

Funding
This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the US National Science Foundation (NSF) under Cooperative Agreement No. 1852977. This manuscript is a product of discussions at the Wildfire in the Biosphere workshop held in May 2021 funded by the NSF through a contract to KnowInnovation. J.K. Shuman was supported as part of the Next Generation Ecosystem Experiments – Tropics, funded by the US Department of Energy, the Office of Science, the Office of Biological and Environmental Research, and by the NASA Arctic Boreal Vulnerability Experiment grant 80NSSC19M0107. R.T. Barnes was supported by the NSF grant DEB-1942068. P.E. Higuera was supported by the NSF grant DEB-1655121. J.K. Balch and E.N. Stavros were supported by CIRES, the University of Colorado Boulder.

Publications
J.K. Shuman, et al., “Reimagine fire science for the Anthropocene.”, PNAS Nexus (2022). pgac115, [https://doi.org/10.1093/pnasnexus/pgac115]

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