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

Soil phosphorus (P) negatively moderated the pantropical litterfall resistance to cyclones: A 100mg P/kg increase in soil P corresponds to a ~35% decrease in resistance.

The Science
Tropical cyclone regimes are shifting with climate change. In 2020, tropical cyclones affected 36 million people and caused $56 billion in damages globally. Changing tropical cyclone regimes may lead to long-lasting effects on tropical forests under climate change. This pantropical meta-analysis investigated the importance of total soil P in mediating forest litterfall (flux of dead plant material from the canopy to the forest floor) resistance (ability to withstand change) and resilience (capacity to return to pre-cyclone condition) to 22 tropical cyclones. Results showed that soil P negatively moderated the pantropical litterfall resistance to cyclones. As soil P increased by 100 mg P/kg, resistance decreased by 32% to 38%.

The Impact
Our results suggest that soil P will partially determine the pantropical forest litterfall resistance and resilience in the face of intensifying cyclone disturbance. This study is the first to document the pantropical role of phosphorus as a factor mediating tropical forest responses to cyclones. Litterfall mass and nutrient pulses caused by cyclones both respond and contribute to resource heterogeneity that can affect species regeneration, growth, and competitive interactions. Additional research can test how plant functional groups and species across pantropical forest ecosystems differ in their resistance and resilience to cyclones to represent cyclone disturbance responses in predictive modeling.

Summary
While the influence of tropical cyclone frequency and intensity on the structure and function of tropical forests have been widely studied, much less attention has been given to the role of resource availability on the functional stability of tropical forests across the globe in the face of cyclone disturbance. Single-site studies in Australia and Hawaii suggest that litterfall on low-phosphorus (P) soils is more resistant and less resilient to cyclones. We conducted a meta-analysis to investigate the pantropical importance of total soil P in mediating forest litterfall resistance and resilience to 22 tropical cyclones. We evaluated cyclone-induced and post-cyclone litterfall mass (g/m 2 /day), and P and nitrogen (N) fluxes (mg/m 2 /day) and concentrations (mg/g), all indicators of ecosystem function and essential for nutrient cycling.

Across 73 case studies in Australia, Guadeloupe, Hawaii, Mexico, Puerto Rico, and Taiwan, total litterfall mass flux increased from ~2.5 ± 0.3 to 22.5 ± 3 g/m 2 /day due to cyclones, with large variation among studies. Litterfall P and N fluxes post-cyclone represented ~5% and 10% of the average annual fluxes, respectively. Post-cyclone leaf litterfall N and P concentrations were 21.6 ± 1.2% and 58.6 ± 2.3% higher than pre-cyclone means. Mixed-effects models determined that soil P negatively moderated the pantropical litterfall resistance to cyclones, with a 100 mg P/kg increase in soil P corresponding to a 32% to 38% decrease in resistance. Based on 33% of the resistance case studies, total litterfall mass flux reached pre-disturbance levels within one-year post-disturbance. Across pantropical forests observed to date, our results indicate that litterfall resistance and resilience in the face of intensifying cyclones will be partially determined by total soil P. This work will support benchmarking of ELM-FATES predictions against pantropical ground data.

Contact: Barbara Bomfim, Postdoctoral researcher NGEE-Tropics, Lawrence Berkeley National Laboratory, bbomfim@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. ORNL is managed by UT-Battelle, LLC, for the DOE. This research utilized data from Luquillo Long-Term Ecological Research (LTER) Program which is currently supported by NSF to the Institute for Tropical Ecosystem Studies, University of Puerto Rico, and to the International Institute of Tropical Forestry USDA Forest Service.

Publication
Bomfim, B., Walker, A.P., McDowell, W.H., Zimmerman, J.K., Feng, Y., Kueppers, L.M., Linking soil phosphorus with forest litterfall resistance and resilience to cyclone disturbance: a pantropical meta‐analysis. Global Change Biology gcb.16223 (2022) https://doi.org/10.1111/gcb.16223

Are Earth system models representing forest regeneration well enough?

The Science
Forests will only persist where future trees are able to reproduce, disperse, germinate, and grow into mature trees (i.e. “recruit”). These critical regeneration processes are generally not well represented in the models that ecologists use to predict future forests. We critically reviewed how regeneration processes are represented within models that strive to predict forest demography in Earth system models. We found a need to improve parameter values and algorithms for reproductive allocation, dispersal, environmental filtering in the seedling layer, and tree regeneration strategies adapted to wind, fire, and anthropogenic disturbance regimes. These improvements are needed so that models can capture forest responses to global change.

The Impact
Vegetation demographic models represent forest dynamics in the Earth system. They provide the opportunity to more fully integrate ecological understanding into predictions of future climate and ecosystem states. This paper identifies critical areas where models are not prepared to Forest regeneration is a series of environmentally sensitive processes that culminates in tree recruitment. Environmental variables act as filters (depicted as varying sieve mesh sizes) that impose constraints on each process capture future forest responses to global change variables such as changing precipitation and disturbance regimes. This review helps model developers identify what kinds of improvements are needed. It also helps field ecologists understand what types of data are best suited to supporting the improvement of models. Improving these models will advance our ability to predict the role that forests will play in sequestering and storing carbon, providing habitat for biodiversity, and provisioning critical natural resources for people.

Summary
Earth system models must predict forest responses to global change in order to simulate future global climate, hydrology, and ecosystem dynamics. These models are increasingly adopting vegetation demographic approaches that explicitly represent tree growth, mortality and recruitment, enabling advances in the projection of forest vulnerability and resilience, as well as evaluation with field data. To date, simulation of regeneration processes has received far less attention than simulation of processes that affect growth and mortality in spite of its critical role maintaining forest structure, facilitating turnover in forest composition over space and time, enabling recovery from disturbance, and regulating climate-driven range shifts. Our critical review of regeneration process representations within current Earth system vegetation demographic models reveals the need to improve parameter values and algorithms for reproductive allocation, dispersal, seed survival and germination, environmental filtering in the seedling layer, and tree regeneration strategies adapted to wind, fire, and anthropogenic disturbance regimes. These improvements require synthesis of existing data, specific field data collection protocols, and novel model algorithms compatible with global scale simulations. Vegetation demographic models offer the opportunity to more fully integrate ecological understanding into Earth system prediction; regeneration processes need to be a critical part of the effort.

Contact: Adam Hanbury-Brown, University of California, Berkeley, ahanburybrown@gmail.com

Funding
Funding was provided via Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics), funded by the US Department of Energy, Office of Science, Office of Biological and Environmental Research. The lead author also received support from the National Science Foundation and the National Aeronautics and Space Administration during this research.

Publication
Hanbury-Brown AR, RE Ward , LM Kueppers (2022), Forest regeneration within Earth system models: current process representations and ways forward. New Phytologist. https://doi.org/10.1111/nph.18131

Researchers found that degraded forests (burned and logged) do less photosynthesis during the
dry season, but recover productivity in about 4 years.

The Science
Large areas of the Amazon forest are being degraded through fires and logging. Degraded forests may suffer more water stress than intact forests during the dry season, but it remains an open question. A team of researchers used multiple remote sensing data to test this possibility. They used laser sensors on an aircraft to see changes in forest structure caused by degradation. They also used a satellite sensor that can see how much photosynthesis plants are doing. They compared both data sets to check how long it takes for forests to recover structure and photosynthesis after disturbance.

The Impact
The team found that fires cause much more damage to forests than logging. They also noticed that recently burned forests did less photosynthesis than intact forests. To their surprise, in just 4 years after disturbance, burned and logged forests were already doing as much photosynthesis as intact forests. However, the structure of burned forests remained very different from intact forests even after 14 years. This is important because each forest characteristic may take a very different time to recover from degradation.

Summary
Human cause disturbances that degrade tropical forests. Forest degradation from selective logging and fires alter forest structure and function. These changes also impact the ability of forests to uptake carbon. This study used airborne laser scanning data over the Amazon to investigate how forest structure varies across burned and logged forests of different ages since disturbance. In addition, the team used solar induced chlorophyl fluorescence (SIF) data from the TROPOMI mission. SIF is a proxy for photosynthesis, and the TROPOMI data provide information on how photosynthesis varies across seasons and across degraded and intact forests.

The researchers found that forest fires suffered the largest changes in the vertical distribution of foliage and canopy height compared to logged and intact forests. Moreover, SIF in recently burned forests were significantly lower than in intact forests. In contrast, within 4 years after the disturbance, SIF values were higher in regenerating forests than in intact forests. This was surprising because regenerating forests still had lower leaf area. These findings highlight that degraded forests recover photosynthesis rates faster than they recover forest structure. The results also indicate that degraded forests can accumulate large amounts of carbon during recovery from disturbance.

Contacts: Ekena Rangel Pinagé, College of Forestry, Oregon State University, rangelpe@oregonstate.edu
Marcos Longo, Lawrence Berkeley National Laboratory, mlongo@lbl.gov

Funding
This research was funded by the Australian Government Research Training Program Scholarship, the USDA Forest Service Pacific Northwest Research Station and International Programs, and by the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. 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
Pinagé, E. R., D. M. Bell, M. Longo, S. Gao, M. Keller et al. “Forest structure and solar-induced
fluorescence across intact and degraded forests in the Amazon”. Remote Sensing of Environment
274, 112 998 (2022). [DOI: 10.1016/j.rse.2022.112998]

A synthetic review leads to a new hypothesis framework that sheds light on how and why plants are dying more under a warming climate.

The Science
Increasing rates of woody-plant mortality, including trees and shrubs, presents a large scientific challenge because we do not yet understand what is causing the increasing loss of plants. This study reviewed the literature to identify the key mechanisms underlying warming-induced mortality, and presented a testable framework that yields insight into the drivers of plant death as well as how to better model these processes. Ultimately, mortality under drought, rising temperature, and rising carbon dioxide results from depletion of water and carbon stores, leading to irreversible dehydration and the inability to maintain metabolism. Warming exacerbates these storage declines, while elevated carbon dioxide has mixed impacts. The net result of the increasing rate and severity of warming and drought overwhelms the benefits of elevated carbon.

The Impact
Plant mortality is rising globally, leading to negative impacts on ecosystem services of value to society including economic, aesthetic, and ecological consequences. Plant mortality reduces carbon uptake and increases carbon loss, promoting a decline in terrestrial carbon storage. Despite these consequences, our ability to predict plant mortality is limited by a lack of knowledge of the underlying mechanisms, their response to climate, and their integration into models. Here, scientists reviewed the literature to generate a synthetic hypothesis framework that pinpoints the key mechanisms driving mortality under a changing environment. The result is a roadmap for future research, including the provision of a set of testable hypotheses that will rapidly increase our knowledge, and identification of key mechanisms that should be included in process models to enable more accurate representation of the impacts of climate change on plant survival.

Summary
Plant mortality is rising in concert with increasing droughts, warming, and carbon dioxide, but the mechanisms driving the increased mortality are poorly known. This knowledge gap leads to large challenges for prediction of the future of terrestrial ecosystems, including their role in water, carbon, and nutrient cycling. Here we integrated the literature on plant mortality and subsequently generated a synthetic and testable hypothesis framework describing the mechanisms underlying plant death in a warming and drying world. The stores of carbon and water are depleted under changing climate, with some amelioration due to rising carbon dioxide. The decline in these stores leads to failure to maintain hydration and metabolism, and can promote death outright or through failure to defend against attacking biotic agents. Acclimation can promote survival to an extent. Determining the net impacts of rising carbon dioxide versus drought and warming remain a major science challenge.

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

Funding
N. G. M. and C. X. were supported by the Department of Energy, Office of Science project Next Generation Ecosystem Experiment-Tropics (NGEE-Tropics). G. S. was supported by the NSFBII-Implementation (2021898). D. T. acknowledges support from the Australian Research Council (DP0879531, DP110105102, LP0989881, LP140100232). M. D. K acknowledges support from the Australian Research Council (ARC) Centre of Excellence for Climate Extremes (CE170100023), the ARC Discovery Grant (DP190101823) and the NSW Research Attraction and Acceleration Program. C. G. was supported by the Swiss National Science Foundation (PZ00P3_174068). M. M. and J.M.V were supported by the Spanish Ministry of Science and Innovation (MICINN, CGL2017‐89149‐C2‐1‐R). A.T.T. acknowledges funding from the NSF Grant 2003205, the USDA National Institute of Food and Agriculture, Agricultural and Food Research Initiative Competitive Programme Grant No. 2018-67012-31496 and the University of California Laboratory Fees Research Program Award No. LFR-20-652467. W.M.H. was supported by the NSF GRFP (1-746055). A.M.T. and H.D.A. were supported by the NSF Division of Integrative Organismal Systems, Integrative Ecological Physiology Program (IOS-1755345, IOS-1755346). H.D.A. also received support from the USDA National Institute of Food and Agriculture (NIFA), McIntire Stennis Project WNP00009 and Agriculture and Food Research Initiative award 2021-67013-33716. D.D.B. was supported by NSF (DEB-1550756, DEB-1824796, DEB-1925837), USGS SW Climate Adaptation Science Center (G18AC00320), USDA NIFA McIntire Stennis ARZT 1390130-M12-222, and a Murdoch University Distinguished Visiting Scholar award. D.S.M. was supported by NSF (IOS-1444571, IOS- 1547796). R.S.O. acknowledges funding from NERC-FAPESP 19/07773-1. W.R.L.A. was supported by the David and Lucille Packard Foundation, NSF grants 1714972, 1802880 and 2003017, and USDA NIFA AFRI grant no. 2018‐67019‐27850. R.S.O. acknowledges funding from NERC-FAPESP 19/07773-1. B.E.M. is supported by an Australian Research Council Laureate Fellowship (FL190100003. A.S. was supported by a Bullard Fellowship (Harvard University) and the University of Montana.

Publications
McDowell NG, Sapes G, Pivovaroff A, Adams H, Allen CD, Anderegg WRL, Arend M, Breshears DD, Brodribb T, Choat B, Cochard H, Cáceres MD, De Kauwe M, Grossiord C, Hammond WH, Hartmann H, Hoch G, Kahmen A, Klein T, Mackay DS, Mantova M, Martínez-Vilalta J, Medlyn BE, Mencuccini M, Nardini A, Oliveira RS, Sala A, Tissue DT, Torres-Ruiz JM, Trowbridge A, Trugman AT, Wiley E, Xu C. (2022) Mechanisms of woody plant mortality under rising drought, CO2, and vapor pressure deficit. Nature Reviews Earth and Environment. https://doi.org/10.1038/s43017-022-00272-1.