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.

Simulating environmentally sensitive tree recruitment shows promise for helping ecologists predict where trees will grow in the future.

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 represented in the models that ecologists use to predict future forests. The recently developed Tree Recruitment Scheme (TRS) was developed specifically to capture how changing environmental conditions will affect future trees’ ability to recruit. The TRS is shown to improve  predictions of tree recruitment rates in a tropical forest in Panama and captures how reduced soil moisture and light constrains tree recruitment.

The Impact
By improving predictions of tree recruitment using environmentally sensitive processes the TRS is well-positioned to improve predictions of future forest range boundaries, composition, and function. This is important for predicting the role that forests will play in sequestering and storing carbon, providing habitat for biodiversity, and provisioning critical natural resources for people. By representing the early stages of tree development the TRS will allow ecosystem modelers to simulate more complicated interactions between vegetation and changing disturbance regimes such as the affect of more severe fire on vegetation composition.

Summary
The TRS was developed and evaluated at Barro Colorado Island (BCI), Panama where ecologists have collected a significant amount of forest demography and meteorological data since the early 1980s. These data allowed researchers at Lawrence Berkeley National Lab to parameterize TRS algorithms that represent how soil moisture and light affect critical regeneration processes such as seedling emergence, seedling mortality, and seedling to sapling transition rates. By simulating recruitment under observed meteorological conditions researchers were able to compare TRS predictions of recruitment to census observations at BCI. Compared to prior models the TRS makes significant improvements in predicting which types of trees recruit and at what rate under the current climate. By also running the TRS under El Niño, wetter-than-observed, and drier-than-observed precipitation scenarios researchers found that the TRS predicts recruitment responses to varying soil moisture and light levels that are consistent with ecological expectations.

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.

Publications
A.R. Hanbury-Brown et al., “Simulating environmentally-sensitive tree recruitment in vegetation demographic models”. New Phytologist (2022). [DOI: 10.1111/nph.18059]

NGEE-Tropics scientist Barbara Bomfim was featured in an interview on the “Tree Talk” Podcast hosted by Katie Davies. The episode is titled “Tropical Forestry,” and discusses tropical forest science and soils. Barbara also gives insight into the impact of tropical forest research through the social and economic issues surrounding the Amazon forest. Barbara is a scientist on the NGEE-Tropics project, working as a postdoc on Research Focus Area 2 (RFA2) science questions. RFA2 is primarily focused on forest structure and composition along environmental gradients. You can find Barbara’s full interview by following this link: https://pod.co/treetalkpodcast/2-tropical-forestry.

Headshot of Barbara Bomfim, PhD — NGEE-Tropics Scientist and postdoc.

NGEE-T scientists studied trunk and crown vertical volume profiles to describe the volume contained up to any height in tropical trees.

The Science
As climate changes, it is crucial to monitor the health of tropical forests. Recent studies have reported tree-level damage (i.e., branch fall, trunk breakage, and the decay caused by wood decomposition in standing trees) as one of the most important conditions preceding deaths in tropical trees. However, field-based damage assessments are very limited, in part, due to the lack of whole-tree (trunk + branches) volume equations in tropical trees. Using terrestrial laser scanning, forest ecologists studied the vertical distribution of trunk and crown (i.e., branches) volumes to provide models to estimate the proportion of volume contained up to any height in tropical trees.

The Impact
Field-based assessments of tree damage are increasingly needed to better estimate biomass losses and drivers of tree mortality. This research provides a set of models that can be used to estimate volume losses in living trees when the living length of the trunk and the proportion of newly broken branches are available.

Summary
Tree volume models are critical for forest management and for obtaining accurate forest carbon estimates. In this paper, researchers present species- composite cumulative volume profile models that describe the volume contained up to a given height in the trunks and crowns of tropical trees. They used terrestrial laser scanning (TLS) and quantitative structure models to estimate the trunk and crown volume of 177 trees (49 species) in a lowland tropical forest in the Barro Colorado Island in Panamá. The researchers found that (1) the rate at which volume accumulated with height was much higher and variable in the whole tree (trunk + branches) than only in the trunk; (2) the variability in the rate of volume accumulation was three times higher in the trunk and nine times higher in the whole tree across individuals within species than between species; and (3) parameters describing the rate of volume accumulation significantly depended on the height of attachment of the lowest branch, but not on the tree size.

Contacts
Principal Investigator: Daniel Zuleta, Ph.D., Ecologist (Postdoctoral fellow), Forest Global Earth Observatory at the Smithsonian Tropical Research Institute, dfzuleta@gmail.com
Program Manager: Brian Benscoter, U.S. Department of Energy, Biological and Environmental Research (SC-33), Environmental System Science, brian.benscoter@science.doe.gov

Funding
This project was supported as part of the Next Generation Ecosystem Experiments–Tropics and was funded by the Office of Biological and Environmental Research (BER) within the U.S. Department of Energy’s (DOE) Office of Science. Data collection was supported by the Forest Global Earth Observatory (ForestGEO) of the Smithsonian Institution.

References
Zuleta, D., et al. “Vertical distribution of trunk and crown volume in tropical trees” Forest Ecology and Management 508 120056 (2022). DOI: 10.1016/j.foreco.2022.120056

Stem respiration and growth in the Tropics.

The Science
Current models predict that autotrophic respiration increases with growth rates and temperature. We found that when averaged over the annual timescale, there was a positive relationship between stem growth of trees and CO₂ emitted from the stem into the atmosphere as a part of growth respiration. However, over a single day, growth and respiration are suppressed during the warmer periods associated with high transpiration and water use.

The Impact
The mechanisms involved in this apparent suppression of respiration are a hot topic of research because it behaves in an opposite pattern to what we would expect considering only temperature. Mechanisms under investigation include Increased CO₂ transport in the transpiration stream and an actual decrease in cellular respiration rates linked to reduced stem water potentials during warmer daytime periods of high transpiration and inhibited growth.

Summary
Tropical forests cycle a large amount of CO₂ between the land and atmosphere, with a substantial portion of the return flux due to tree respiratory processes. However, in situ estimates of woody tissue respiratory fluxes and carbon use efficiencies (CUE_W) and their dependencies on physiological processes including stem wood production (P_w) and transpiration in tropical forests remain scarce. Here, we synthesize monthly P_w and daytime stem CO₂ efflux (E_S) measurements over one year from 80 trees with variable biomass accumulation rates in the central Amazon. On average, carbon flux to woody tissues, expressed in the same stem area normalized units as E_S, averaged 0.90 ± 1.2 µmol m^-2 s^-1 for P_w, and 0.55 ± 0.33 µmol m^-2 s^-1 for daytime E_S. A positive linear correlation was found between stem growth rates and stem CO₂ efflux, with respiratory carbon loss equivalent to 15 ± 3% of stem carbon accrual. CUE_W of stems was non-linearly correlated with growth and was as high as 77-87% for a fast-growing tree. Diurnal measurements of stem CO₂ efflux for three individuals showed a daytime reduction of E_S by 15-50% during periods of high sap flow and transpiration. The results demonstrate that high daytime E_S fluxes are associated with high CUE_W during fast tree growth, reaching higher values than previously observed in the Amazon Basin (e.g. maximum CUE_W up to 77-87%, versus 30-56%). The observations are consistent with the emerging view that diurnal dynamics of stem water status influences growth processes and associated respiratory metabolism.

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

Funding
This research is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE Tropics) as a part of work package 1.4 (Autotrophic respiration) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research’s Terrestrial Ecosystem Science Additional funding for this research was provided by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

Publications
Jardine K, Cobello L, Teixeira L, East M, Levine S, Gimenez B, Robles E, Spanner G, Koven C, Xu C, Warren J, Higuchi N, McDowell N, Pastorello G, Chambers J (2022). Stem respiration and growth in a central Amazon rainforest, Trees, 20:1-4. https://doi.org/10.1007/s00468-022-02265-5

An analysis of demographic rates shows that the biomass and turnover of forests depend on which tree demographic strategies are present.

The Science
Plants take up carbon from the atmosphere via photosynthesis and store it in their tissues. The growth and survival of trees determine how much, and for how long, carbon is stored by forests. A recent analysis of the growth and survival rates of thousands of tree species explored how the number of species in a forest plot is related to the range of tree growth and survival rates (demographic diversity), and how that influences carbon cycling dynamics. The study reveals that demographic diversity plateaus as the numbers of species increases. Further, the presence of species with particular demographic rates, rather than demographic diversity, govern carbon dynamics.

The Impact
Forests play a critical role in regulating the world’s climate by cycling large amounts of carbon, water and energy with the atmosphere. Yet forests are threatened by changes to climate and an increase in the frequency and intensity of disturbances which are both likely to alter the species composition of forests globally. It is therefore essential that we understand how the species composition of forests relate to demographic rates, and forest dynamics. This study highlighted the importance of high survival, large statured species for carbon storage.

Summary
The growth and survival of individual trees determine the physical structure of a forest with important consequences for forest function. The authors of this study calculated growth and survival rates of 1,961 tree species from temperate and tropical forests and explored how the range of demographic rates, and the presence or absence of distinct demographic strategies differ across forests, and how these differences in demography relate to the number of species in the forest, and carbon storage. The authors found wide variation in demographic rates across forest plots, which could not be explained by the number of species or climate variables alone. There is no evidence that a large range of demographic rates lead to higher carbon storage. Rather, the relative abundance of high-survival, large-statured species, predicts both biomass and carbon residence time. Linking the demographic composition of forests to resilience or vulnerability to climate change, will improve the precision and accuracy of predictions of future forest dynamics.

Contact: Jessica Needham; Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory; jfneedham@lbl.gov

Funding
This project began and was developed at ForestGEO workshops in 2016, 2017 and 2018 (NSF DEB-1046113 to S. J. Davies). M. McMahon was partially funded by the USA National Science Foundation (NSF 640261 to S. M. McMahon). 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. LBNL is managed and operated by the Regents of the University of California under prime contract number DE-AC02-05CH11231. For individual forest plot funding acknowledgements see the SI of this published manuscript.

Publications
Needham, J.F., et al. “Demographic composition, not demographic diversity, predicts biomass and turnover across temperate and tropical forests.” Glob Change Biol. (2022) https://doi.org/10.1111/gcb.16100