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Johnson, Michelle and Galbraith, David and Gloor, Manuel and De Deaurwaerder, Hannes and Guimberteau, Mattieu and Rammig, Anja and Thonicke, Kristin and Verbeeck, Hans and von Randow, Celso and Monteagudo Mendoza, Abel and Phillips, Oliver L. and Brienen, Roel and Feldpausch, Ted R. and Lopez Gonzales, Gabriela and Fauset, Sophie and Quesada, Carlos and Christofferson, Bradley and Ciais, Phillippe and Sampaio, Gilvan and Kruijt, Bart and Meir, Patrick and Moorcroft, Paul and Zhang, Ke and Alvarez-Davila, Esteban and Alves de Oliveira, Atila and Amaral, Ieda and Andrade, Ana and Aragao, Luiz and Araujo-Murakami, Alejandro and Arets, Eric and Arroyo, Luzmila and Aymard, Gerardo and Baraloto, Christopher and Barroso, Jocely and Bonal, Damien and Boot, Rene and Camargo, Jose and Chave, Jerome and Cogollo, Alvaro and Cornejo, Fernando and Lola da Costa, Antonio and Di Fiore, Anthony and Ferreira, Leandro and Higuchi, Niro and Honorio, Euridice and Killeen, Timothy and Laurance, Susan and Laurance, William and Licona, Juan and Lovejoy, Thomas and Malhi, Yadvinder and Marimon, Bia and Marimon Junior, Ben Hur and Matos, Darley and Mendoza, Casimiro and Neill, David and Pardo, Guido and Pena-Claros, Marielos and Pitman, Nigel and Poorter, Lourens and Prieto, Adriana and Ramirez-Angula, Hirma and Roopsind, Anand and Rudas, Augustin and Salomao, Rafael and Silveira, Marcos and Stropp, Juliana and ter Steege, Hans and Terborgh, John and Thomas, Raquel and Toledo, Marisol and Torres-Lezama, Armando and van der Heijden, Geertje M.F. and Vasquez, Rodolfo and Vieira, Imez and Vilanova, Emilio and Vos, Vincent and Baker, Timothy (2016) Variation in stem mortality rates determines patterns of above-ground biomass in Amazonian forests: implications for dynamic global vegetation models. Global Change Biology . ISSN 1365-2486

Abstract

Understanding the processes that determine above-ground biomass (AGB) in Amazonian forests is important for predicting the sensitivity of these ecosystems to environmental change and for designing and evaluating dynamic global vegetation models (DGVMs). AGB is determined by inputs from woody productivity [woody net primary productivity (NPP)] and the rate at which carbon is lost through tree mortality. Here, we test whether two direct metrics of tree mortality (the absolute rate of woody biomass loss and the rate of stem mortality) and/or woody NPP, control variation in AGB among 167 plots in intact forest across Amazonia. We then compare these relationships and the observed variation in AGB and woody NPP with the predictions of four DGVMs. The observations show that stem mortality rates, rather than absolute rates of woody biomass loss, are the most important predictor of AGB, which is consistent with the importance of stand size structure for determining spatial variation in AGB. The relationship between stem mortality rates and AGB varies among different regions of Amazonia, indicating that variation in wood density and height/diameter relationships also influences AGB. In contrast to previous findings, we find that woody NPP is not correlated with stem mortality rates and is weakly positively correlated with AGB. Across the four models, basin-wide average AGB is similar to the mean of the observations. However, the models consistently overestimate woody NPP and poorly represent the spatial patterns of both AGB and woody NPP estimated using plot data. In marked contrast to the observations, DGVMs typically show strong positive relationships between woody NPP and AGB. Resolving these differences will require incorporating forest size structure, mechanistic models of stem mortality and variation in functional composition in DGVMs.

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