The Science
Normally moist tropical forests suffer from the effects of droughts.  In a model study we found that degraded forests (forests that had serious impacts from logging and or burning) absorbed less carbon dioxide from the atmosphere, evaporated less water and got hotter relative to intact forests under mild to moderate drought conditions.  Under severe drought conditions, all forests had similar responses in terms of water loss and warming.

The Impact
Tropical forests contribute substantial amounts of water vapor to the atmosphere that falls as rain downwind.  Today more tropical forests are degraded than intact and this may be changing the amount of atmospheric water vapor.  Future exploration of these results may help to explain why tropical forest regions are suffering longer and more severe droughts.

Summary
We integrated small-footprint airborne lidar data with forest inventory plots across precipitation and degradation gradients in the Amazon. We provided the forest structural information to the Ecosystem Demography Model (ED-2.2) to investigate how degradation-driven forest structure affects sensible heat, evapotranspiration and gross primary productivity.  Tropical forest degradation effects were the strongest in seasonal forests. Increased water stress in degraded forests resulted in up to 35% reduction in evapotranspiration and gross primary productivity, and up to 43% increase in sensible heat flux. Relative to intact forests, degradation effects diminished during extreme droughts, when water stress dominated the response in all forests.  Our results indicate a much broader influence of land cover and land use change in energy, water, and carbon cycles that is not limited to deforested areas and highlight the relevance models that consider forest structure such as FATES in predicting biophysical and biogeochemical cycles.

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

PI Contact: Michael Keller, USDA Forest Service, International Institute of Tropical Forestry, Rio Piedras, Puerto Rico, mkeller.co2@gmail.com

Funding
This research was funded NASA (16-IDS16-0049), the NASA Postdoctoral Program, administered by Universities Space Research Association under contract with NASA and the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research as part of the Next­Generation Ecosystem Experiments–Tropics.

Publications
Longo, et al. “Impacts of Degradation on Water, Energy, and Carbon Cycling of the Amazon Tropical Forests. ”J. Geophys. Res.-B., e2020JG005677 (2020), doi:10.1029/2020JG005677.

The Science
Tropical forests are a critical ecosystem in governing terrestrial feedbacks to global change, and representing the complex ecological dynamics that determine these processes represents a crucial problem in Earth system models.  We have developed the FATES model to explore and represent these dynamics, and are testing the model at tropical forest field sites to explore how the representation of plant traits and ecosystem parameters govern forest structure and function.

The Impact
This article represents a first benchmarking and parameter sensitivity of the full-complexity FATES model using multiple dimensions of plant trait variation alongside other ecosystem parameters.  We find that the representation of competition fundamentally alters tropical forest function, and that parameters that control the dynamics of competition, such as disturbance rate and intensity, thus control ecosystem structure and function.

Summary
Tropical forests are a critical and dynamic ecosystem, but the ecological complexity of these regions are not represented in existing Earth system models.  We have developed the FATES model for use in E3SM to address this.  Here we test FATES within the E3SM Land Model (ELM) to explore how plant trait variation, and competition between different plant functional types at a tropical forest site, governs model predictions of the function and structure of the forest.  Using a set of 12 plant traits whose variability we have observations of at that site, we use ensembles of model runs to explore both the trait variation, as well as how structural differences in the representation of competition, determine model outcomes, and compare these to observations at the site.  Key findings are that adding larger numbers of competing plant types increases the productivity of the forest, and thus pointing to a need to better represent tradeoffs that prevent any one type from dominating an ecosystems; and that the balance between early successional and late successional functional types is highly sensitive to the representation of disturbance intensity, disturbance extent, and the degree of determinism in light competition by the trees, thus pointing to the need to focus on these processes in testing and benchmarking the model.

Contacts (BER PM): Dan Stover and Renu Joseph, Daniel.Stover@science.doe.gov (301-903-0289) and Renu.Joseph@science.doe.gov (301-903-9237)

PI Contact: Charlie Koven, Staff Scientist, Lawrence Berkeley National Lab, cdkoven@lbl.gov, 510.486.6724

Funding
NGEE-Tropics, DOE Early Career Research Program

Publications
Koven, C. D., Knox, R. G., Fisher, R. A., Chambers, J. Q., Christoffersen, B. O., Davies, S. J., Detto, M., Dietze, M. C., Faybishenko, B., Holm, J., Huang, M., Kovenock, M., Kueppers, L. M., Lemieux, G., Massoud, E., McDowell, N. G., Muller-Landau, H. C., Needham, J. F., Norby, R. J., Powell, T., Rogers, A., Serbin, S. P., Shuman, J. K., Swann, A. L. S., Varadharajan, C., Walker, A. P., Wright, S. J., and Xu, C.: Benchmarking and parameter sensitivity of physiological and vegetation dynamics using the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) at Barro Colorado Island, Panama, Biogeosciences, 17, 3017–3044, https://doi.org/10.5194/bg-17-3017-2020, 2020.

The Science
Satellite images before and after hurricane María show a major shift in color from green to reddish, indicating widespread impact on forests. Most intense forest disturbances were found on steeper slopes, high elevations, wind-facing direction, close to the hurricane track. Different types of trees respond differently to hurricanes.

The Impact
Previously, scientists needed to step into the field and measure the forest damage after hurricanes. It is a difficult job and also takes a long time to get only a small plot of data. This research mapped the forest disturbance on the landscape scale, so we can quickly get access to the damage level on the whole island of Puerto Rico. To better understand the disturbance level, we also explored a number of factors that affect the spatial variation of the disturbance intensity.

Summary
Widely recognized as one of the worst natural disasters in Puerto Rico’s history, hurricane María made landfall on September 20, 2017 in southeast Puerto Rico as a high-end category 4 hurricane on the Saffir-Simpson scale causing widespread destruction, fatalities and forest disturbance. This study focused on hurricane María’s effect on Puerto Rico’s forests as well as the effect of landform and forest characteristics on observed disturbance patterns. Our analyses showed that forest structure, and characteristics such as forest age and forest type affected patterns of forest disturbance. Among forest types, highest disturbance values were found in sierra palm, transitional, and tall cloud forests; seasonal evergreen forests with coconut palm; and mangrove forests. For landforms, greatest disturbance metrics were found at high elevations, steeper slopes, and windward surfaces. As expected, high levels of disturbance were also found close to the hurricane track, with disturbance less severe as hurricane María moved inland. This study demonstrated an informative regional approach, combining remote sensing with statistical analyses to investigate factors that result in variability in hurricane effects on forest ecosystems.

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

PI Contact: Jeffrey Chambers, Lawrence Berkeley National Laboratory,  jchambers@lbl.gov

Funding
This research was supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE), funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under contract number DE-AC02-05CH11231.

Publications
Y. Feng, R.I. Negrón-Juárez, J.Q. Chambers, “Remote sensing and statistical analysis of the effects of hurricane María on the forests of Puerto Rico.” Remote Sensing of Environment 247 (2020), https://doi.org/10.1016/j.rse.2020.111940

Related Links
Hurricane María impact visualization:
https://ylfeng.users.earthengine.app/view/forestdisturbancemapinpr
https://ylfeng.users.earthengine.app/view/forestdisturbanceafterhurricanemaria

The Science
The Future of Fire Consortium (FFC), composed of ecologists from around the globe with expertise ranging from paleoecology to atmospheric science, identified critical research frontiers in six areas of fire ecology and three emergent themes for future fire ecology research including: (1) the need to study fire across temporal and spatial scales, (2) the need to assess the mechanisms underlying a variety of feedbacks in the fire system, and (3) the need to improve representation of fire in a range of modeling contexts.

The Impact
As fire regimes and our relationships with fire continue to change, prioritizing these research areas and emergent themes will facilitate understanding of the ecological causes and consequences of future fires and fire management. 

Summary
In this review, critical research frontiers in six areas of fire ecology were identified: (1) expanding concepts of fire regimes, (2) understanding changing fire regimes, (3, 4) examining fire effects on aboveground and belowground ecology, (5) increasing fuels characterization in determining fire behavior, and (6) improving representation of fire processes in a variety of modeling contexts. Within these areas, three emergent themes for future fire ecology research including: (1) the need to study fire across temporal and spatial scales, (2) the need to assess the mechanisms underlying a variety of feedbacks in the fire system, and (3) the need to improve representation of fire in a range of modeling contexts. This review offers guidance to further our effort to understand both the fundamental role of fire in ecological systems and the human role in shaping fire activity. As fire regimes and our relationships with fire continue to change, prioritizing these research areas and emergent themes will facilitate understanding of the ecological causes and consequences of future fires and fire management.

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

PI Contact: Kendra McLauchlan, Kansas State University, mclauch@ksu.edu

Funding
This manuscript is a product of discussions at the Future of Fire workshop held in November 2017 in Boulder, Colorado, USA. Support for this workshop was provided by NSF‐DEB‐1743681 to K.K.McLauchlan and A.J.Tepley. J.K. Shuman was supported by Next-Generation Ecosystem Experiments (NGEE Tropics), a project supported by the Office of Biological and Environmental Research in the Department of Energy, Office of Science.

Publications
McLauchlan, K. K., P. E. Higuera, J. Miesel, B. M. Rogers, J. Schweitzer, J. K. Shuman, A. Tepley, J. M. Varner, T. T. Veblen, et al., 2020, “Fire as a fundamental ecological process: research advances and frontiers.” Journal of Ecology 00:1-23 doi: 10.1111/1365-2745.13403

Related Links
Article URL: https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.13403

The Science
Researchers used simulations to explore how size- and age-dependent mortality of trees will affect changes in forest carbon storage in response to rising CO2. They found that faster growth as a result of increased CO2 caused increases in forest biomass that were twice as large when mortality was age-dependent compared with size-dependent.

The Impact
Understanding how forests will respond to rising CO2 is critical for predicting changes in the Earth’s climate. The results of this study highlight in particular the importance of understanding large tree mortality.

Summary
Little is known about how the probability of death changes as trees get older and larger. However, as rising CO2 is expected to cause trees to grow faster, it is important that we understand whether this will lead to trees growing larger, or whether they will continue to die at the same size, but in less time. This has important implications for the amount of carbon that is stored in forest ecosystems, and how long it is stored. Researchers used simulations to explore how different mechanisms of tree mortality could affect forest carbon storage. They found that increased growth from simulated increases in CO2 caused increases in biomass that were twice as large when mortality was age-dependent compared with size-dependent. Further, they found a much larger decrease in carbon storage time when mortality was size-dependent, as trees move through their life cycles more rapidly.

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

PI Contact: Charles Koven, Lawrence Berkeley National Laboratory, cdkoven@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. RF acknowledges the support of the National Center for Atmospheric Research, which is funded by the National Science Foundation.

Publications
Needham, J. F., Chambers, J., Fisher, R., Knox, R., Koven, C. D., “Forest response to simulated elevated CO2 under alternate hypotheses of size- and age-dependent mortality.” Global Change Biology, 2020.

The Science
The increase in global temperature directly affects the net primary productivity of the forest. High temperatures can influence the rates of chemical reactions in cells, such as photosynthesis, electron transport and isoprene emissions. In this study, we asked the following research questions:
1) Are reductions in photosynthesis at high leaf temperatures in tropical forests linked to a reduction in stomatal conductance (gs) rather than direct negative temperature effects on photosynthesis?
2) Do current isoprene emission models that link photosynthetic electron transport rates to isoprene emissions rates as a function of temperature hold true in tropical species?
3) What is the role of isoprene on thermal tolerance of photosynthesis at high temperatures?
We discovered that in a “thermophile” early successional species in the Amazon, photosynthetic electron transport rates increased linearly with temperature in concert with isoprene emissions, even as stomatal conductance and net photosynthetic carbon fixation declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported and that blocking isoprene production induced a temperature-dependent loss of photosynthetic capacity.  

The Impact
Tropical forests absorb large amounts of atmospheric CO2, but substantial decreases in tropical forest gross primary productivity have been repeatedly demonstrated in the Amazon basin during periodic widespread drought associated with high temperature. Therefore, the physiological mechanisms through which tropical forests respond to high temperature are critically important to understand. While extreme warming will decrease stomatal conductance and net photosynthesis in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of photosynthetic electron transport (ETR) and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results demonstrate that models which link isoprene emissions to the rate of ETR are ideal for tropical species and provide necessary “ground-truthing” for simulations of the large predicted increases in tropical isoprene emissions with climate warming.

Summary
Tropical forests absorb large amounts of atmospheric CO2 through photosynthesis, but high surface temperatures suppress this absorption while promoting isoprene emissions. While mechanistic isoprene emission models predict a tight coupling to ETR as a function of temperature, direct field observations of these phenomena are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here, we demonstrate that in the early successional species Vismia guianensis in the central Amazon, ETR rates increased with temperature in concert with isoprene emissions, even as gs and net photosynthetic carbon fixation (Pn) declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported (50°C). While Pn showed an optimum value of 32.6 ± 0.4°C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (qL) increased with leaf temperature with strong linear correlations for ETR (ƿ = 0.98) and qL (ƿ = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non-photochemical quenching (NPQ), were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressed Pn at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decrease gs and Pn in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of ETR and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results confirm that models which link isoprene emissions to the rate of ETR hold true in tropical species and provide necessary “ground-truthing” for simulations of the large predicted increases in tropical isoprene emissions with climate warming.

PI Contact: Kolby Jardine, Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, One Cyclotron Road, 84-155 Berkeley, CA, USA 94720, kjjardine@lbl.gov

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

Funding
This material is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research`s Terrestrial Ecosystem Science Program through contract No. DE-AC02-05CH11231 to Lawrence Berkeley National Laboratory, DE-AC05-00OR22725 to Oak Ridge National Laboratory, and DE-SC0012704 to Brookhaven National Laboratory. Additional funding for this research was provided by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Logistical and scientific support is acknowledged by the Forest Management laboratory (LMF), Climate and Environment (CLIAMB), and Large Scale Biosphere-Atmosphere (LBA) programs at the National Institute for Amazon Research (INPA).

Publications
Rodrigues T, Baker C, Walker A, McDowell N, Rogers A, Higuchi N, Chambers J, Jardine K (2020) Stimulation of isoprene emissions and electron transport rates as a key mechanism of thermal tolerance in the tropical species Vismia guianensis, Global Change Biology. https://doi.org/10.1111/gcb.15213

Related Links
Data DOIs: Jardine K; Rodrigues T (2019): Isoprene, Chlorophyll fluorescence, and leaf temperature data from Manaus, Brazil, 2017 – 2018. 1.0. NGEE Tropics Data Collection. (dataset). http://dx.doi.org/10.15486/ngt/1570407