The Science
The amount of water that tropical trees lost from their stems during drought conditions, when trees lack access to soil water, was correlated with their bark water vapor conductance, i.e., the leakiness of bark to water vapor. This suggests that water loss through bark may be an important and overlooked mechanism that influences stem dehydration and drought performance.

The Impact
Predicting drought performance among trees is a major challenge. These results suggest that incorporating the rate of water loss from bark can help to predict the rate at which trees dehydrate and die during droughts.

Summary
Saplings of several tree species in Panama were measured for stem water content during well-watered conditions and drought conditions in forest understories and in a shadehouse experiment to assess stem water deficit during drought. Saplings of the same species were collected and measured for bark water vapor conductance. In both datasets, bark water vapor conductance was correlated with stem water deficit among species that lacked assess to soil water.

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

PI Contact: Brett Wolfe, School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA, Wolfe1@lsu.edu

Funding
This project was supported by the Next Generation Ecosystem Experiments-Tropics and by the US Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publications
Wolfe BT. 2020 Bark water vapour conductance is associated with drought performance in tropical trees. Biol. Lett. 16: 20200263. http://dx.doi.org/10.1098/rsbl.2020.0263

The Science
Terrestrial ecosystem dynamics, including carbon stocks and biosphere-atmosphere fluxes of carbon dioxide (CO2), water (H2O), and volatile organic compounds (VOCs) are dramatically changing in response to climate factors such as trends in surface warming and a higher frequency and intensity of large-scale droughts and associated insect infestation epidemics. Understanding the mechanisms that drive forest responses to climate change is vital for predicting how the structure and function of natural and managed ecosystems will react to environmental change including alterations in carbon and H2O cycling, and the ecosystem services and products provided. Therefore, understanding the underlying biochemical, physiological, and ecological processes including plant growth and development, abiotic and biotic stress responses, mortality, and ecological succession and forest recovery are critical for accurately predicting the future of forest structure and function.

The Impact
We highlight cell wall-derived emissions of methanol and acetic acid, along with associated changes in cell wall structure and function as a common thread among the processes that underlie ecosystem responses to climate change. Thus, interdisciplinary studies linking cell wall biochemistry and metabolism with plant physiology and biosphere-atmosphere gas exchange will lead to better predictive understanding of the mechanisms through which cell wall esters facilitate forest response to climate extremes. Of particular interest are high latitude forests responding to rapid warming through expansion of deciduous broadleaf trees and commensurate declines in evergreen conifer trees. These distinct plant functional types vary in their leaf phenological cycles and cell wall composition, with deciduous trees undergoing seasonal leaf emergence and senescence while conifer trees retain their needles over the winter months. This may impact the timing, spatial distribution, and magnitude of biosphere-atmosphere fluxes of VOCs, CO2, and H2O in such changing forests in the future.

Summary
While little is known about the functions of cell wall ester modifications in trees, evidence from model plant systems like Arabidopsis thaliana suggests that they may be highly dynamic, playing central roles in cell growth, tissue development and function, participating in sensing and signaling pathways involved in cell wall remodeling in response to stress, and integrate into primary C1-3 metabolism. Although the reservoirs and fluxes of carbon through acetylated and methylated cell wall polysaccharides are potentially large, they remain poorly characterized. Methanol and acetic acid products of cell wall de-esterification may integrate whole plant metabolism by being transported over large distances within the plant via the transpiration stream and extracellular air space and enter central metabolism in distant tissues, or be emitted into the atmosphere as gases. Moreover, cell wall-derived methanol and acetic acid may be intimately involved in signaling and immune responses as an essential component of environmental plant monitoring systems; changes in cell wall esters during stress induce signaling via damage-associated molecular patterns, which in turn activate immunity responses. This link is mediated by the role of cell wall esters in central metabolism and plant sensing and signaling pathways involved in cell wall structural remodeling and “green immunity”.

Contact: Kolby Jardine, Lawrence Berkeley National Lab, kjjardine@lbl.gov

Funding
This material is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research (BER), Biological System Science Division (BSSD), Early Career Research Program (FY18 DOE National Laboratory Announcement Number: LAB 17-1761), Topic: Plant Systems for the Production of Biofuels and Bioproducts, under Award number FP00007421 to Lawrence Berkeley National Laboratory.

Publications
R.A. Dewhirst, J.C. Mortimer, K.J. Jardine, (2020) Do cell wall esters facilitate forest response to climate?, Trends in Plant Science.

The Science
As an interplay of chronic drivers and transient disturbances are introduced to forests, dynamic changes occur in the reproduction, growth, and mortality of trees. Combining analyses from multiple studies, researchers led by Pacific Northwest National Laboratory investigated the demographic drivers of forest dynamics, the disturbances that drive them, and the pervasiveness and impact of these effects worldwide.

The Impact
Ongoing changes in both environmental drivers and disturbance regimes appear to be consistently increasing mortality and forcing forests towards shorter and younger stands. This reduces potential carbon storage and impacts the biodiversity and, ultimately, the surrounding climate. The pervasive shifts in forest vegetation dynamics are already occurring and are likely to accelerate under future global changes, with consequences for climate forcing.

Summary
In a collaborative study, more than twenty researchers investigated the influence of today’s climate conditions on the dynamics and stature of forests. Combining data and observations from more than 160 previous studies worldwide, the research team found that tree stands are losing their potential for obtaining height and enduring lifespans. Researchers identified consistent mortality trends resulting from specific chronically changing drivers—rising temperature, increasing CO2 levels, and transient disturbances including wildfire, drought, and land-use change. These factors have thrown out of balance three important characteristics of a diverse and thriving forest: (1) recruitment, which is the addition of new seedlings to a community; (2) growth, the net increase in biomass or carbon; and (3) mortality, the loss of a plant’s ability to reproduce. As a result, forest vegetation and canopies decline and overall recovery is lessened, resulting in ecosystems dominated by novel species that have replaced the original community. Since the resulting forest loss comes from both natural and land-use change drivers, all of which are predicted to increase in magnitude in the future, it is highly likely that tree mortality rates will continue to increase while recruitment and growth will respond to changing drivers in a spatially and temporally variable manner. The net impact will be a reduction in forest coverage and biomass, with mixed effects on biodiversity. This study forms the basis for investigations regarding the patterns and processes underlying the shifts in forest dynamics, all of which can be tested using emerging terrestrial and satellite-based observation networks.

Contacts (BER PMs): Dan Stover, Program Manager, Terrestrial Ecosystems Science, Dan.Stover@science.doe.gov

PI Contact: Nate McDowell, Pacific Northwest National Laboratory, Nate.McDowell@pnnl.gov

Funding
This research was supported by the Office of Science of the U.S. Department of Energy (DOE) as part of the Terrestrial Ecosystem Science (TES) Program and the Next Generation Ecosystem Experiment-Tropics (NGEE-Tropics) project. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830.

Publication
McDowell, NG, et al. 2020. “Pervasive shifts in forest dynamics in a changing world.” Science 368(6494):eaaz9463. https://doi.org/10.1126/science.aaz9463

Related Links
https://science.sciencemag.org/content/368/6494/eaaz9463

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