Fine-scale remote sensing unveils drivers and mechanisms of drought-induced mortality

The Science
We utilized Landsat Enhanced Vegetation Index (EVI), an indicator of vegetation function and resilience, to investigate the controls over forest mortality in the dry-tropical region of Costa Rica.  The EVI based estimates of mortality correlated well with in-situ, ground-based inventories, leading to robust prediction capacity.  We observed significant fine-scale heterogeneity in forest mortality that was related primarily to the degree of cumulative water deficit during the drought, leaf deciduousness, and topography.

The Impact
This work is important for improving our fundamental understanding, observational capacity, and predictive certainty for dry-tropical forest vegetation dynamics.  Understanding the controls over mortality is greatly enhanced by this remote sensing approach in which fine-scale patterns can be detected and related to key abiotic drivers.  Improving our ability to map forest mortality at the global scale is one of the grand challenges in the forest mortality community, and our approach greatly improves the utility of Landsat measurements for this purpose.  Scientists can now use this dataset or future datasets based on this approach to benchmark model performance.

Summary
Remote sensing provides a powerful approach to quantify changes in vegetation on the Earth’s surface.  We used Landsat 30x30m resolution imagery to quantify changes in vegetation biomass due to mortality during a severe drought in 2015 in the dry-tropical forests of Costa Rica.  After strong validation with in-situ ground inventories of tree mortality, we applied the approach to examine the local drivers of mortality.  The degree of drought played a strong role in localized mortality.  Ecosystems with a higher fraction of evergreen tree species experienced greater mortality than those with a higher abundance of deciduous species.  Topographic position also played a significant role, with sun-exposed and steep slopes having the highest mortality.

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

Funding
We thank funding from Cornell University CALS to X. X., National Science Foundation CAREER grant DEB-1053237 to J.S.P. and U.S. Department of Energy, Office of Science, Terrestrial Ecosystem Science Program, Award DE-SC0014363 for funding the field plots. We thank Roger Blanco and Maria Marta Chavarria for logistical help in the field. N.G.M. was supported by the U.S. Department of Energy’s Next Generation Ecosystem Experiment-Tropics project. A.S-A is supported by National Science and Engineering Research Council of Canada (NSERC) –Discovery Grant Program. 

Publications
Wu, D., Vargas, G.G., Powers, J.S., McDowell, N.G., Becknell, J.M., Pérez‐Aviles, D., Medvigy, D., Liu, Y., Katul, G.G., Calvo‐Alvarado, J.C. and Calvo‐Obando, A., 2021. Reduced ecosystem resilience quantifies fine‐scale heterogeneity in tropical forest mortality responses to drought. Global Change Biology. https://doi.org/10.1111/gcb.16046

Observations of precipitation isotopes are used to determine the spatial and temporal performance of modeled precipitation recycling in the tropics

The Science
The variability in precipitation recycling ratios has important implications for the availability of water to plants as well as tracking the movement of water through the water cycle. Field-based measurements of precipitation recycling over large areas are lacking due to the challenges associated with distributing the necessary equipment. As such, scientists have relied more on modeled precipitation recycling estimates over large scales, while precipitation isotopes have been used as a proxy at the local scale. By aggregating these isotopic observations through space and time, we were able to provide the first global-scale assessment of modeled precipitation recycling estimates using observations over the humid tropics region.

The Impact
The quantity of precipitation recycling represents the amount of precipitation made available to a given area by plants. Heavily forested areas like the Amazon rainforest are known to derive anywhere from one third to one half of its water from precipitation recycling. As such, changes in this quantity are important to understand, as they may provide a direct indication of the health and sustainability of plant ecosystems. Our approach can be used to check how models are doing so modifications to the models can be made to improve performance. Such efforts will be critical to understand how plants and the water cycle will be impacted by climate change at local to regional scales.

Summary
The amount of precipitable water derived locally through transpiration from plants is known as precipitation recycling. By transpiring water that recently fell as rain, plants are effectively recycling water back to the atmosphere so it can fall as rain again. A multi-international team of scientists covering both modelers and experimentalists developed a new approach that uses observed isotopes in precipitation record (a known proxy of precipitation recycling) to constrain estimates from models, which had largely been used to evaluate these quantities over larger scales. Two types of models were assessed in this study – the mass balance and particle tracking models – the latter of which were only made possible very recently through advancements in computational power that were required to perform such simulations. This research highlights which of these models tend to perform better over different times of year based on comparisons to the isotopic observations. In addition, the models were assessed over different regions of the tropics that were broken down by climate zone to see how these might be playing a role in performance based on the physics of the model. Our new approach can be used to improve future modeled estimates of precipitation recycling so that we may better understand its variability and potential impact on plants in response to climate change.

Contact
Kurt Solander, Los Alamos National Laboratory, ksolander@lanl.gov

Funding
This research was 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.

Publications
Cropper, S., Solander, K., Newman, B.D. et al. “Comparing deuterium excess to large-scale precipitation recycling models in the tropics.” npj Climate and Atmospheric Science 4, 60 (2021). https://doi.org/10.1038/s41612-021-00217-3

Surprising events of widespread tree mortality cause concern for future tree loss

The Science
Forest loss through tree mortality events appears to be occurring more frequently in response to rising temperature and more frequent and severe droughts. However, most of these events were unexpected by global and regional experts. This study reviewed the literature to identify such unexpected events. The authors used these examples to highlight the unpredictable nature of mortality events. They subsequently discussed solutions to this challenge, including the use of remote sensing as an early warning system, and improved modeling to better predict such events.

The Impact
Recent observations of elevated tree mortality following climate extremes, like heat and drought, raise concerns about climate change risks to global forest health. We currently lack both sufficient data and understanding to identify whether these observations represent a global trend toward increasing tree mortality. The impact of this paper comes in the form of a global warning of increasing mortality events that are currently unpredictable. Further, this paper identifies a path forward for improved detection and prediction of such mortality events. Ultimately, this paper will lead to improved motivation and awareness of the growing issue of forest loss globally.

Summary
Here we document events of sudden and unexpected elevated tree mortality following heat and drought events in ecosystems that previously were considered tolerant or not at risk of exposure. These events underscore the fact that climate change may affect forests with unexpected force in the future. We use the events as examples to highlight current difficulties and challenges for realistically predicting such tree mortality events and the uncertainties about future forest condition. Advances in remote sensing technology and greater availably of high-resolution data, from both field assessments and from satellites, are needed to improve both understanding and prediction of forest responses to future climate change.

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

Funding
Mireia Banqué, Víctor Granda, Francisco Lloret and Jordi Vayreda provided useful suggestions for an earlier version of Section 3.T.A.M.P. and A.E.-M. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant 758873,TreeMort).N.G.M. acknowledges support from the Department of Energy’s Next-Generation Ecosystem Experiments (NGEE)-Tropics and Coastal Observations, Mechanisms, and Predictions Across Systems and Scales (COMPASS) projects, and A.J.D. acknowledges support from the United States Geological Survey (USGS) Ecosystems Mission Area (EMA). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. This study draws on and contributes to the International Tree Mortality Network, an initiative of the International Union of Forest Research Organizations (IUFRO) Task Force on monitoring global tree mortality trends and patterns.

Publications
Hartmann H, Bastos A, Das AJ, Esquivel Muelbert A, Hammond WH, Martinez-Vilalta J, McDowell NG, Powers J, Pugh TAM, Ruthrof K, Allen CD. 2022. Climate change risks to global forest health – emergence of unexpected events of elevated tree mortality world-wide. Annual Reviews of Ecology and Environment. doi.org/10.1146/annurev-arplant-102820-012804

Shifting root water uptake to deeper soil depths under drought allows homeostatic regulation of aboveground physiology in wet-tropical forest trees

The Science
This study examined how tropical rainforest trees respond to artificially induced drought.  We used a canopy crane located in Queensland, Australia, to measure a variety of aboveground physiological traits such as photosynthesis and transpiration, among others.  We coupled these measurements to an optimization model to calculate shifts in rooting depths.  We discovered that trees maintained homeostasis in aboveground traits through increasing the soil depth in which they foraged for water.

The Impact
Drought is the major culprit of increasing rates of tropical tree mortality.  This has large implications for the carbon cycle because tree death reduces the potential carbon storage of forests, and drought is anticipated to become more frequent and severe globally.  These results point to a key trait that we must quantify to understand and predict future tree responses to drought, namely rooting depth.  These results further suggest that tropical forests may be more resilient to drought than previously anticipated.

Summary
We discovered that wet-tropical trees can maintain homeostatic regulation of aboveground traits, such as photosynthesis and transpiration, through an experimentally imposed, multi-year drought.  The trees achieve this apparent non-response to drought through increasing the soil depth at which they obtain water for transpiration.  Drought induced declines in surface soil moisture content, but deeper soils maintained sufficient water to provide the tree’s transpirational requirements, leading to homeostasis in aboveground traits.

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

Funding
This work was funded by NGEE Tropics.

 Publications
Pivovaroff, et al. 2021. Stability of tropical forest tree carbon‐water relations in a rainfall exclusion treatment through shifts in effective water uptake depth. Global Change Biology, 27(24), pp.6454-6466. https://doi.org/10.1111/gcb.15869

New analyses of hydraulic architecture, species distributions, and mortality across the Isthmus of Panama

The Science                                

We compiled literature values for hydraulic traits that regulate water stress, species distributions, and species mortality rates for 27 species that live across the moisture gradient formed by the Isthmus of Panama.  The hydraulic traits investigated included parameters such as the safety a plant maintains from hydraulic failure during drought, and associated traits that regulate these safety margins.  Correlation and cluster analyses were conducted to investigate if any traits were correlated with species distributions or mortality rates.

The Impact
Tree mortality in tropical forests has been increasing in some regions, with the primary culprit thought to be drought.  Increasing tree mortality results in a decrease in the potential carbon sink of tropical forests, which has major implications for the global carbon budget.  This paper provided a novel test of the relationship between mortality, species distributions, and tree hydraulic architecture.  The results of this study provide new information on the regulation of plant mortality and distribution in tropical forests, and guide future modeling efforts intended to predict the future tropical carbon budget.

Summary
We discovered that hydraulic safety margins, that is, the risk of exceeding stress thresholds that lead to fatal dehydration, were not correlated with tree mortality rates measured during an El-Nino drought.  However, these traits were correlated with species distributions across the moisture gradient, suggesting that long-term acclimation to drought does manifest through avoidance of hydraulic failure.

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

Funding
This work was supported by the Next-Generation Ecosyste Experiments (NGEE Tropics) projects that was supported by the Office of Biological and Environmental Research in the Department of Energy, Office of Science. AR and SPS were supported by the United States Department of Energy contract to Brookhaven National Laboratory. CG was supported by the Swiss National Science Foundation SNF. BTW was supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, McIntire Stennis project under LAB94493. The BCI forest dynamics research project was made possible by National Science Foundation (NSF) grants to Stephen P. Hubbell, with support from the Forest Global Earth Observatory, the Smithsonian Tropical Research Institute, the John D. and Catherine T. MacArthur Foundation, the Mellon Foundation, the Small World Institute Fund, and numerous private individuals, and through the hard work of over 100 people from 10 countries over the past three decades. The plot project is part the Forest Global Earth Observatory (ForestGEO), a global network of large-scale demographic tree plots. The CTFS R Package was developed with the support of NSF to Stuart J. Davies through the NSF-IRCN program on the Dimensions of Biodiversity. 

Publications
Pivovaroff, A.L., Wolfe, B.T., McDowell, N., Christoffersen, B., Davies, S., Dickman, L.T., Grossiord, C., Leff, R.T., Rogers, A., Serbin, S.P. and Wright, S.J., 2021. Hydraulic architecture explains species moisture dependency but not mortality rates across a tropical rainfall gradient. Biotropica. https://doi.org/10.1111/btp.12964

A new global database of sap flow measurements for understanding and modeling plant water use.

The Science
Plant transpiration links vegetation physiology with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the dominant terrestrial evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/). SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants of 174 species.

The Impact
SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes.  Through provision of this global database, discovery of new controls over vegetation water use can be achieved.  This dataset also provides the first global benchmark of transpiration for model evaluation.  Ultimately this database will rapidly advance our ability to understand and predict vegetation function and climate and hydrological feedbacks.

Summary
To generate Sapfluxnet, we harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET has a broad bioclimatic coverage. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings.

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

Funding
No funding was acknowledged in this paper due to the large number of coauthors and associated funding sources.

Publications
Poyatos R, V. Granda, V. Flo, M. Adams, B. Adorján, et al. 2021. Global transpiration data from sap flow measurements: the SAPFLUXNET database.  Earth System Science Data. 13(6), pp.2607-2649. doi.org/10.5194/essd-13-2607-2021