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

A novel dataset of 5 years of canopy disturbances across Barro Colorado Island, Panama, reveals abiotic drivers of spatial variation in disturbances.

The Science
Using 5 years of drone images over Barro Colorado Island, Panama, we identified new canopy disturbances resulting from tree mortality and damage. We show that disturbance rates vary locally depending on soils, topography, and forest age. Disturbances were most strongly associated with certain soil types, and were also higher in older forest, steeper slopes, and local depressions. Additionally, we found that disturbance rates are important for variation in forest height across the landscape.

The Impact
Tree mortality is a major control over tropical forest carbon stocks globally but the strength of associations between abiotic drivers and tree mortality within forested landscapes is poorly understood. Previous studies have shown that mortality rates are important for variation in standing biomass regionally and globally; we show that the same is true on a landscape scale for mature tropical forest and identify abiotic variables that control this variation.

Summary
We used repeat drone photogrammetry across 1500 ha of forest in Central Panama during 2015-2020 to quantify spatial variation in canopy disturbance rates and its predictors. We identified 11,153 canopy disturbances greater than 25 m² in area, including treefalls, large branchfalls, and standing dead trees, affecting 1.9% of area per year. Soil type, forest age, and topography explained up to 46-67% of disturbance rate variation at spatial grains of 58-64 ha. Further, disturbance rates predicted the proportion of low canopy area across the landscape, and mean canopy height in old growth forests. Thus abiotic factors drive variation in disturbance rates and thereby forest structure at landscape scales.

Features
This study was featured on a EurekAlert press release from AAAS which can be found at https://www.eurekalert.org/news-releases/942548.

Contact
KC Cushman, Postdoctoral Fellow, Smithsonian Tropical Research Institute, cushman.kc@gmail.com
Helene C. Muller-Landau, Staff Scientist, Smithsonian Tropical Research Institute, MullerH@si.edu

Funding
KCC 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; KCC was also supported by the Smithsonian Institution Fellowship Program; MD was supported by the Carbon Mitigation Initiative at Princeton University; data collection was supported by a Smithsonian Institution Competitive Grants Program for Science award to HCM.

Publications
K.C. Cushman, Detto, M., Garcia, M., and H.C. Muller-Landau, “Soils and topography control natural disturbance rates and thereby forest structure in a lowland tropical landscape” Ecology Letters (2022). https://doi.org/10.1111/ele.13978.

Using microwave remote sensing to observe vegetation water content

The Science
Hot droughts are becoming more common because of climate change, but we still do not know how forests respond to water stress conditions. Field measurements are too sparse and most satellite measurements cannot detect early signs of water stress in forests. Microwave measurements from space can be used to estimate vegetation water content and detect water stress across all forests on Earth. Vegetation water content is useful because plants lose internal water when they are stressed or when they die.

The Impact
A team of researchers with expertise from field measurements, remote sensing, and numerical models identified opportunities and challenges for using volumetric water content from remote sensing to detect water stress. For example, the time between measurements matters: changes from year to year are useful for seeing changes in forest structure, whereas changes across weeks relate to how much water plants are able to store. The team identified the need for a geostationary spaceborne observational system to measure water content. This system would provide important data on water fluxes from day to day, and could help to identify the earliest signs of water stress in forests.

Summary
This research review describes how extensive and frequent estimates of volumetric water content from microwave remote sensing could improve our ability to detect signs of water stress and anticipate critical conditions for fire and mortality in forests across the world. It could also allow for inference of belowground soil moisture and root water uptake conditions across large scales, which is challenging otherwise. Researchers also identified the need to establish relationships between volumetric water content and ecosystem-scale water potential to be able to detect signs of stress across different forest systems, and to be able to effectively link remote sensing measurements with terrestrial biosphere models. In addition, it is critical to improve methods to distinguish variations in water content due to changes in surface water (dew and rainfall interception) and changes in water stored inside plants.

This review also points to the need for field campaigns that will help establish the volume-potential relationships at ecosystem scale, which are critical to define thresholds for wilting, mortality, and fire risks in different forests. Finally, the monitoring of forest water stress could greatly benefit from geostationary measurements of volumetric water content, which would provide information at sub-daily scale, which could be more directly related to field measurements and improve the quantification of water stress.

Contact
Alexandra Konings, Stanford University, Department of Earth System Science, konings@stanford.edu

Funding
This work is a result from discussions initiated at the Sensing Forest Water Dynamics from Space: Towards Predicting the Earth System Response to Droughts workshop, which supported by the W.M. Keck Institute for Space Studies. This research was partially supported by the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. This research was also funded by NSF, and NASA Terrestrial Ecology. The research carried out at the Jet Propulsion Laboratory, California Institute of Technology, was under a contract with the National Aeronautics and Space Administration. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. DOE.

Publications
Konings, A. G., S. S. Saatchi, C. Frankenberg, M. Keller, V. Leshyk, W. R. Anderegg, V. Humphrey, A. M. Matheny, A. Trugman, L. Sack et al., Detecting forest response to droughts with global observations of vegetation water content. Global Change Biology 27, 6005–6024 (2021). [DOI: 10.1111/gcb.15872]

Trade-offs in fine-root traits of common tree species in Puerto Rico’s tropical forests

The Science
This study measured root traits from five tropical tree species. We used a combination of traits related to soil phosphorus acquisition. We also measured these traits before and after two hurricanes in Puerto Rico. We found that architectural traits were oppositely adjusted to phosphatase activity and fungal colonization. These strategies were grouped by pioneer trees and non-pioneer trees. This study showed no change in root trait adjustments after the hurricanes, except for phosphatase activity.

The Impact
How plants adjust their root traits to better obtain nutrients is relevant for understanding their distribution. Understanding these adjustments under climatic disturbance can help us predict future scenarios. Although there are general gradients for some of these trait adjustments, most traits are not represented. Tropical plants have been less studied than temperate plants. This study highlights the negative relationships between architectural and physiological traits. Non-pioneer trees relied more on architectural traits than pioneer trees. Additionally, the unchanged adjustments of most root traits after the hurricanes shows the stability of the traits. Our results can help better understand root adjustments of some tropical trees under soils with low phosphorus availability.

Summary
Tropical trees might adjust their traits to better obtain soil phosphorus. For example, they can adjust their root length or root branching. They can also adjust their colonization by fungi or phosphatase activity. It is still not clear which combination of adjustments tropical trees might have to obtain soil phosphorus. Here, we measured seven root traits of five common trees in Puerto Rico. We described these trait adjustments and followed their changes after two hurricanes. We found that roots with high colonization of fungi and high phosphatase activity presented less branching. This strategy was mostly shown in pioneer trees. The opposite happened in non-pioneers. We also found that root traits adjustments did not change before and after the hurricanes, except for root phosphatase activity. Our results showed a combination of root trait adjustments for better obtain soil phosphorus in tropical trees and the stability on most of the root traits adjustment after hurricane disturbances.

Contact
Daniela Yaffar, Functional Forest Ecology, University of Hamburg, danielayaffar@uni-hamburg.de

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.

Publications
Yaffar D, Defrenne CE, Cabugao KG, Kivlin SN, Childs J, Carvajal N and Norby RJ. 2021. Trade-Offs in Phosphorus Acquisition Strategies of Five Common Tree Species in a Tropical Forest of Puerto Rico. Frontiers in Forests and Global Change 4:698191. [DOI: 10.3389/ffgc.2021.698191]

Related Links
Yaffar D, Cabugao KG, Norby RJ, Childs J. 2021. Fine-root traits from common tree species in Puerto Rico before and after Hurricane María (2017-2018). NGEE Tropics Data Collection. (dataset). [DOI: 10.15486/ngt/1778242]

Leaf respiration in the Tropics

The Science
Leaf respiration contributes to an estimated 50% of total autotrophic respiration, but with few observations in the tropics, the uncertainty around this component is high. In addition, little is known about how it varies across common tree species as a function of height within the forest and how light influences the respiratory process. In this study, we show that canopy position has an important influence on leaf respiratory rates and the degree of light suppression.

The Impact
This study also provides a better understanding of the carbon cycle, energy metabolism, and the connections with leaf functional traits in different environmental conditions. Our results highlight the importance of representing the light suppression of leaf respiration in dynamic vegetation models aimed at predicting the future of tropical forests under climate change by accounting for environmental and biological vertical gradients within forest canopies and connections with leaf functional traits.

Summary
Leaf respiration is a major contributor to autotrophic respiration but is poorly characterized in diverse tropical ecosystems. Light can inhibit this process, but little information is known about the light suppression of leaf respiration. Due to the great importance of the Amazon rainforest in the global climate context, this study quantified the rates of the day and dark respiration and investigate if the canopy position influences the variation of leaf respiration rates and light suppression. We studied 26 tree individuals of different species distributed in three different canopy positions: canopy, lower canopy, and understory. We found that rates of leaf respiration and light suppression followed an opposite pattern as function of canopy position. Canopy trees had significantly higher rates of R_dark and R_day than trees in the understory. However, the difference between R_dark and R_day (the light suppression of respiration) was greatest in the understory (68 ± 9%, 95% CI) decreasing in the lower canopy (49 ± 9%, 95% CI), and reaching the lowest values in the canopy (37 ± 10%, 95% CI). Our results highlight the importance of including representation of the light suppression of leaf respiration in terrestrial biosphere models and also of accounting for vertical gradients within forest canopies and connections with functional traits.

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
C. Souza, et al. “Canopy Position Influences the Degree of Light Suppression of Leaf Respiration in Abundant Tree Genera in the Amazon Forest”. Frontiers in Forests and Global Change 4, 13, (2021). DOI 10.3389/ffgc.2021.723539.

Related Links
https://www.frontiersin.org/articles/10.3389/ffgc.2021.723539/full