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      Pervasive decreases in living vegetation carbon turnover time across forest climate zones

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          Significance

          With a limited understanding of spatiotemporal trends of carbon turnover time and its drivers, we are unable to quantify future changes in the forest carbon sink strength. By comparing long-term forest plot data and Earth system model (ESM) projections, we found a pervasive increase in carbon loss from tree mortality, likely driving declines in living aboveground vegetation carbon turnover time across forest climate zones. The climate correlations between temperature or precipitation and temporal trends of living vegetation carbon turnover time differed between forest plots and ESMs. Our results indicate that a mechanistic representation of tree mortality in ESMs and its sensitivity to climate is a crucial uncertainty in predicting the future forest carbon sink.

          Abstract

          Forests play a major role in the global carbon cycle. Previous studies on the capacity of forests to sequester atmospheric CO 2 have mostly focused on carbon uptake, but the roles of carbon turnover time and its spatiotemporal changes remain poorly understood. Here, we used long-term inventory data (1955 to 2018) from 695 mature forest plots to quantify temporal trends in living vegetation carbon turnover time across tropical, temperate, and cold climate zones, and compared plot data to 8 Earth system models (ESMs). Long-term plots consistently showed decreases in living vegetation carbon turnover time, likely driven by increased tree mortality across all major climate zones. Changes in living vegetation carbon turnover time were negatively correlated with CO 2 enrichment in both forest plot data and ESM simulations. However, plot-based correlations between living vegetation carbon turnover time and climate drivers such as precipitation and temperature diverged from those of ESM simulations. Our analyses suggest that forest carbon sinks are likely to be constrained by a decrease in living vegetation carbon turnover time, and accurate projections of forest carbon sink dynamics will require an improved representation of tree mortality processes and their sensitivity to climate in ESMs.

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          Most cited references29

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          Global response of terrestrial ecosystem structure and function to CO2and climate change: results from six dynamic global vegetation models

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            A multi-species synthesis of physiological mechanisms in drought-induced tree mortality

            Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function.
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              A drought-induced pervasive increase in tree mortality across Canada's boreal forests

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                Author and article information

                Journal
                Proc Natl Acad Sci U S A
                Proc. Natl. Acad. Sci. U.S.A
                pnas
                pnas
                PNAS
                Proceedings of the National Academy of Sciences of the United States of America
                National Academy of Sciences
                0027-8424
                1091-6490
                3 December 2019
                18 November 2019
                18 November 2019
                : 116
                : 49
                : 24662-24667
                Affiliations
                [1] aSchool of Biological Sciences, University of Utah , Salt Lake City, UT 84112;
                [2] bSchool of Natural Resources and the Environment, University of Arizona , Tucson, AZ 85721;
                [3] cDepartment of Geography, University of California, Santa Barbara , CA 93106;
                [4] dThe Morton Arboretum , Lisle, IL 60532;
                [5] eDepartment of Ecology and Evolutionary Biology, University of California, Los Angeles , CA 90095;
                [6] fConsejo Superior de Investigaciones Científicas, Global Ecology Unit (Center for Ecological Research and Forestry Applications–Consejo Superior de Investigaciones Científicas–Universitat Autònoma de Barcelona) , 08193 Bellaterra (Catalonia), Spain;
                [7] gCenter for Ecological Research and Forestry Applications , 08193 Cerdanyola del Vallès (Catalonia), Spain;
                [8] hDepartment of Biological Sciences, University of Quebec at Montreal , Montréal, QC H3C 3J7, Canada;
                [9] iState Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest Agriculture and Forestry University , Yangling, 712100 Shaanxi, China;
                [10] jDepartment of Environmental Studies, University of California, Santa Cruz , CA 95064;
                [11] kThe Swiss Federal Institute for Forest Snow and Landscape Research (WSL) 8903 Birmensdorf, Switzerland;
                [12] lUMR RECOVER, University of Aix-Marseille , Institut National de Recherche en Sciences et Technologies pour l’Environnement et l’Agriculture, 13182 Aix-en-Provence, France;
                [13] mSlovenian Forestry Institute , 1000 Ljubljana, Slovenia;
                [14] nInstitute of Terrestrial Ecosystems, Eidgenössische Technische Hochschule Zürich , 8092 Zürich, Switzerland
                Author notes
                1To whom correspondence may be addressed. Email: ky9hc@ 123456virginia.edu .

                Edited by Christopher B. Field, Stanford University, Stanford, CA, and approved October 15, 2019 (received for review December 15, 2018)

                Author contributions: K.Y. and W.R.L.A. designed research; K.Y. performed research; K.Y., W.K.S., A.T.T., R.C., S.P.H, J.S, C.P, K.Z., J.P., M.C., T.L., A.G., M.S., M.F., and W.R.L.A. contributed new data and analytic tools; K.L.Y. analyzed data with input from W.K.S.; and K.Y., W.K.S., A.T.T., and W.R.L.A. wrote the paper.

                Author information
                http://orcid.org/0000-0003-4223-5169
                http://orcid.org/0000-0002-5785-6489
                http://orcid.org/0000-0002-7903-9711
                http://orcid.org/0000-0003-1587-3317
                http://orcid.org/0000-0002-7215-0150
                http://orcid.org/0000-0002-1910-9589
                http://orcid.org/0000-0001-6551-3331
                Article
                201821387
                10.1073/pnas.1821387116
                6900527
                31740604
                ad418088-4be3-4fba-9fa7-690b21e8f368
                Copyright © 2019 the Author(s). Published by PNAS.

                This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

                History
                Page count
                Pages: 6
                Funding
                Funded by: NSF | BIO | Division of Environmental Biology (DEB) 100000155
                Award ID: 1714972
                Award Recipient : William R.L. Anderegg
                Categories
                Biological Sciences
                Ecology

                carbon cycle,carbon turnover,forest carbon stocks,forest productivity,tree mortality

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