The Science
Trees adjust their leaf area based on their traits and environmental conditions, which has enormous impacts on global carbon fluxes. We use first principles to predict leaf area adjustment in response to global change.

The Impact
We provide and test methods for improving carbon cycle predictions through advancing model predictions of leaf area. Tree‐level carbon allocation to leaves should be derived from first principles using mechanistic plant hydraulic processes in vegetation models.

Summary
Forest leaf area has enormous impacts on the carbon cycle because it mediates both forest productivity and resilience to climate extremes. Trees are capable of adjusting to changes in environment, yet many vegetation models use fixed carbon allocation schemes independent of environment, which introduces uncertainty in predictions. We develop an optimization‐based model in which tree carbon allocation to leaves is an emergent property of environment and plant traits. A combination of meta‐analysis, observational datasets, and model predictions find strong evidence that optimal hydraulic–carbon coupling explains observed patterns in leaf allocation.

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

PI Contact: Anna T. Trugman, School of Biological Sciences, University of Utah, a.trugman@utah.edu

Funding
USDA National Institute of Food and Agriculture, Grant/Award Number: 2018‐ 67012‐28020 and 2018‐67019‐27850; National Science Foundation, Grant/Award Number: 1714972 and 1802880; German Research Foundation, Grant/Award Number: RU 1657/2‐1, SCHM 2736/2‐1 and YA 274/1‐1 ; David and Lucille Packard Foundation; University of Utah Global Change and Sustainability Center; Next‐ Generation Ecosystem Experiments‐Tropics; Biological and Environmental Research; German Federal Ministry of Education and Research; Helmholtz Association.

Publications
A. Trugman, “Climate and plant trait strategies determine tree carbon allocation to leaves and mediate future forest productivity.” Global Change Biology 25, 3395 (2019). 10.1111/gcb.14680.

The Science
Plant hydraulics has evolved dramatically in the last century, leading to knowledge of critical value to agriculture, forest management, and global modeling.  Here we review these developments and highlight some of the critical new directions for plant hydraulics in the 21st century.

The Impact
Plant hydraulics is the backbone of terrestrial ecology because of the critical role of water transport in regulating carbon uptake.  We highlight the impacts plant hydraulics have had on fields ranging from agricultural management to the terrestrial carbon and water cycles, and propose the path forward to new insights of value to both fundamental and applied sciences.

Summary
Plant hydraulic regulation of water uptake provides the backbone of the plant carbon cycle and ecology because of its direct control over, and tight coordination with, canopy photosynthesis. Advances in measurements and modeling over the last few decades have enabled far-reaching influence of hydraulic discoveries, including impacting how we view and simulate the global water and carbon cycles and manage crop systems (Figure 1). Perhaps most importantly in this era of a warming atmosphere and more variable droughts, is the critical role our understanding of plant hydraulics is having on our ability to predict and mitigate chronically-increasing stressors (e.g. temperature, vapor pressure deficit) on plant function and survival.  The 21st century offers a very exciting time for advancement of plant hydraulics understanding, approaches, and applications.  Future directions range in scale from understanding the molecular regulation and feedbacks with maximum conductance and embolism avoidance, to improved understanding of water potential regulation at landscape to global scales.  Inherently, developments in understanding will be associated with continued methodological improvements at micro- to macro-scales, and with applications of refined hydraulic models to allow strong, process-based inferences. Perhaps the most important directions that plant hydraulics science can go is in applications to the prediction and management of both wild and crop systems under rising temperature and vapor pressure deficit and drought frequency, which threatens food production and the global carbon cycle alike.

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

PI Contact: Nate McDowell, PNNL, nate.mcdowell@pnnl.gov

Funding
DOE-NGEE Tropics

Publications
McDowell NG, T Brodribb, A Nardini (2019). Plant hydraulics in the 21st century.  New Phytologist doi.org/10.1111/nph.16151.

The Science
Drought is forecasted to impact most terrestrial ecosystems in an increasing manner over the 21st century, with negative consequences on productivity, carbon storage, and yield.  We review the state-of-the-art methods for conducting drought manipulation experiments designed to understand the impacts of drought upon woody-plant dominated ecosystems.

The Impact
This manuscript provides a cutting-edge template for those scientists wishing to conduct drought manipulation studies.  We highlight the challenges, assumptions, and solutions to both scientific and logistical challenges associated with drought studies.

Summary
Past forest drought experiments employed a variety of study designs related to treatment level, replication, plot and infrastructure characteristics, and measurement approaches. Important considerations for establishing new drought experiments include: selecting appropriate treatment levels to reach ecological thresholds; balancing cost, logistical complexity, and effectiveness in infrastructure design; and preventing unintended water subsidies. Response variables in drought experiments were organized into three broad tiers reflecting increasing complexity and resource intensiveness, with the first tier representing a recommended core set of common measurements.  Differences in site conditions combined with unique research questions of experimentalists necessitate careful adaptation of guidelines for drought experiments to balance local objectives with coordination among experiments. We advocate adoption of a common framework for coordinating forest drought experiment design to enhance cross‐site comparability and advance fundamental knowledge about the response and sensitivity of diverse forest ecosystems to precipitation extremes.

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

PI Contact: Nate McDowell, PNNL, nate.mcdowell@pnnl.gov

Funding: DOE-NGEE Tropics

Publications
Asbjornsen, H., J. Campbell, K. Jennings, C. McIntire, M. Vadeboncoeur, P.H. Templer, R. Phillips, T.L. Bauerle, F. Bowles, M. Dietze, S. Frey, P. Groffman, R. Guerrieri, P.J. Hanson, E. Kelsey, A. K. Knapp, N.G. McDowell, P. Meir, K.A. Novick, S.V. Ollinger, W.T. Pockman, P.G. Schaberg, S.D. Wullschleger, M.D. Smith, L. Rustad.  Guidelines and considerations for designing precipitation manipulation experiments in shrubland, woodland, and forest ecosystems.  Methods in Ecology and Evolution 9(12) p 2310-2325 (2018). https://doi.org/10.1111/2041-210X.13094.

The Science
Much research has examined the sensitivity of tropical terrestrial ecosystems to various environmental drivers. The predictability of tropical vegetation greenness based on sea surface temperatures (SSTs), however, has not been well explored. This study employed fine spatial resolution remotely-sensed Enhanced Vegetation Index (EVI) and SST indices from tropical ocean basins to investigate the predictability of tropical vegetation greenness in response to SSTs and established empirical models with optimal parameters for hindcast predictions. Three evaluation metrics were used to assess the model performance, i.e., correlations between historical observed and predicted values, percentage of correctly predicted signs of EVI anomalies, and percentage of correct signs for extreme EVI anomalies. Our findings reveal that the pan-tropical EVI was tightly connected to the SSTs over tropical ocean basins. The strongest impacts of SSTs on EVI were identified mainly over the arid or semi-arid tropical regions.

The Impact
Our work provides a basis for the prediction of changes in greenness of tropical terrestrial ecosystems at seasonal to intra-seasonal scales. Moreover, the statistics-based observational relationships have the potential to facilitate the benchmarking of Earth System Models regarding their ability to capture the responses of tropical vegetation growth to long-term signals of oceanic forcings.

Summary
Three tropical regions, namely northeastern Brazil, eastern tropical Africa, and northern Australia, that are located in arid or semi-arid climate zones and covered mainly by sparse vegetation including open shrub, were found to be associated with evident influences of SSTs on vegetation growth and consequently high ecological predictability on seasonal to intra-seasonal time scales. Over the tropical rainforests, however, the weakest oceanic influences and thus lowest predictability were identified. The developed statistical models partially predicted the EVI dynamics based on selected SST indices over the pan tropics with limitations owing to the impacts from factors other than climate and model simplicity. As a future direction, more sophisticated statistical models will be tested. We will also evaluate the reliability of the statistics-based model for extracting key oceanic impacts on tropical terrestrial ecosystem production using dynamic experiments with an Earth system model, for example those from the High Resolution Model Intercomparison Project driven by observed SSTs. We will include vegetation data that is more physiologically related to plant photosynthesis, including the observation-based gross primary productivity and solar-induced fluorescence. Furthermore, dimension reduction techniques will be applied to quantify the leading modes of SSTs and precipitation and their relationships with vegetation greenness, thus to reduce dimensionality.

Contacts (BER PM): Renu Joseph and Daniel Stover, renu.joseph@science.doe.gov (301-903-9237) and Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Jiafu Mao: Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, maoj@ornl.gov (865-576-7815)

Funding
Yan, J. Mao, X. Shi, F.M. Hoffman, N. Mcdowell, M. Xu, L. Gu and D.M. Ricciuto are supported by DOE Office of Science, Biological and Environmental Research, including support from the following programs: Terrestrial Ecosystem Science Program (NGEE-Tropics project); Regional and Global Climate Modeling Program (ORNL RUBISCO SFA).

Publications
Yan, B., J. Mao*, X. Shi, F. M. Hoffman, M. Notaro, T. Zhou, N. Mcdowell, R.E. Dickinson, M. Xu, L. Gu, and D.M. Ricciuto, Predictability of tropical vegetation greenness using sea surface temperatures. Environ. Res. Commun. 1 (2019) 031003. doi: 10.1088/2515-7620/ab178a.

The Science
Climate – specifically annual precipitation – modulates the speed with which water is transported through tropical trees.

The Impact
This study demonstrates that local climate plays an important role in the sap flux response of humid tropical forests to evaporative demand. Moreover, we highlight that trees growing in wetter regions in the tropics may be subjected to a reduced sap flux velocity with the high evaporative demand predicted by most climate models.

Summary
Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remain elusive. With a progressively warming climate, atmospheric evaporative demand (i.e., vapor pressure deficit, VPD) will be increasingly important for plant functioning, becoming the major control of plant water use in the 21st century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime (mean annual precipitation, MAP; seasonal drought intensity, PDRY) and two functional traits related to foliar and wood economics spectra (leaf mass per area, LMA; wood specific gravity, WSG). We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely-used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.

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

PI Contact: Grossiord Charlotte(1) & Brad Christoffersen(2), (1) Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf 8903, Switzerland; (2) Department of Biology and School of Earth, Environmental, and Marine Sciences, University of Texas Rio Grande Valley, Edinburg, TX, USA; charlotte.grossiord@wsl.ch; bradley.christoffersen@utrgv.edu

Funding
This project was supported in part by the Next Generation Ecosystem Experiments Tropics, funded by the US Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Sciences Program, under Award Number DE-SC-0011806. CG was supported by the Swiss National Science Foundation SNF (5231.00639.001.01). BC was supported in part by the Laboratory Directed Research and Development Program Project 8872 of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. This work has benefited from an “Investissements d’Avenir” grant managed by Agence Nationale de la Recherche (CEBA, ref. ANR-10-LABX-25-01). Data recorded in French Guiana (FRG) were collected at the Guyaflux sites which belong to the SOERE F-ORE-T and is supported annually by Ecofor, Allenvi and the French national research infrastructure, ANAEE-F.

Publications
Grossiord, B. Christoffersen et al., “Precipitation mediates sap flux sensitivity to evaporative demand in the neotropics.” Oecologia (2019), doi: 10.1007/s00442-019-04513-x

Related Links
https://link.springer.com/article/10.1007/s00442-019-04513-x

The Science
Very few trees die. That results in very sparse information and great uncertainty about mortality. This work develops a technique to use spatial information to increase accuracy in the estimation of mortality.

The Impact
The general method described here, with modifications, could be applied to reduce uncertainty in the estimation of proportions related to any temporally or spatially structured phenomenon with two possible outcomes.

Summary
Many ecological applications, like the study of mortality rates, require the estimation of proportions and confidence intervals for them. The traditional way of doing this applies the binomial distribution, which describes the outcome of a series of Bernoulli trials. This distribution assumes that observations are independent and the probability of success is the same for all the individual observations. Both assumptions are obviously false in many cases. I show how to apply bootstrap and the Poisson binomial distribution (a generalization of the binomial distribution) to the estimation of proportions. Any information at the individual level would result in better (narrower) confidence intervals around the estimation of proportions. As a case study, I applied this method to the calculation of mortality rates in a forest plot of tropical trees in Lambir Hills National Park, Malaysia. I calculated central estimates and 95% confidence intervals for species‐level mortality rates for 1,007 tree species. I used a very simple model of spatial dependence in survival to estimate individual‐level risk of mortality. The results obtained by accounting for heterogeneity in individual‐level risk of mortality were comparable to those obtained with the binomial distribution in terms of central estimates, but the precision increased in virtually all cases, with an average reduction in the width of the confidence interval of ~20%. Spatial information allows the estimation of individual‐level probabilities of survival, and this increases the precision in the estimates of mortality rates. The general method described here, with modifications, could be applied to reduce uncertainty in the estimation of proportions related to any spatially structured phenomenon with two possible outcomes. More sophisticated approaches can yield better estimates of individual‐level mortality and thus narrower confidence intervals.

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

PI Contact: Gabriel Arellano, University of Michigan, gabriel.arellano.torres@gmail.com

Funding
This study was developed 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). The Lambir 52‐ha plot was established as a collaboration between the Forest Department of Sarawak, Malaysia, Harvard University (NSF awards DEB‐9107247 and DEB‐9629601) and Osaka City University (grants 06041094, 08NP0901 and 09NP0901). The research has been supported by the Asia program of the Arnold Arboretum (Harvard University), the Center for Tropical Forest Science (Smithsonian Tropical Research Institute), the NSF award DEB‐1545761 to Stuart J. Davies, and the Sarawak Forest Department.

Publications
Arellano, G. (2019), Calculation of narrower confidence intervals for tree mortality rates when we know nothing but the location of the death/survival events, Ecology and Evolution, 9, 9644-9653. [DOI: https://doi.org/10.1002/ece3.5495]

Related Links
https://onlinelibrary.wiley.com/doi/abs/10.1002/ece3.5495