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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-409
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-409
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 14 Oct 2019

Submitted as: research article | 14 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Benchmarking and Parameter Sensitivity of Physiological and Vegetation Dynamics using the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) at Barro Colorado Island, Panama

Charles D. Koven1, Ryan G. Knox1, Rosie A. Fisher2,3, Jeffrey Chambers1,4, Bradley O. Christoffersen5, Stuart J. Davies6, Matteo Detto7,8, Michael C. Dietze9, Boris Faybishenko1, Jennifer Holm1, Maoyi Huang10, Marlies Kovenock11, Lara M. Kueppers1,12, Gregory Lemieux1, Elias Massoud13, Nathan G. McDowell10, Helene C. Muller-Landau6,7, Jessica F. Needham1, Richard J. Norby14, Thomas Powell1, Alistair Rogers15, Shawn P. Serbin15, Jacquelyn K. Shuman2, Abigail L. S. Swann11,16, Charuleka Varadharajan1, Anthony P. Walker14, S. Joseph Wright7, and Chonggang Xu17 Charles D. Koven et al.
  • 1Climate and Ecosystem Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA, USA
  • 2Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO, USA
  • 3Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique, Toulouse, France
  • 4Department of Geography, University of California, Berkeley, CA, USA
  • 5Department of Biology, University of Texas, Rio Grande Valley, Edinburg, TX, USA
  • 6Forest Global Earth Observatory, Smithsonian Tropical Research Institute, Washington, D.C., USA
  • 7Smithsonian Tropical Research Institute, Apartado 0843–03092 Balboa, Republic of Panama
  • 8Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, USA
  • 9Department of Earth and Environment, Boston University, Boston, MA, USA
  • 10Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
  • 11Department of Biology, University of Washington, Seattle, WA, USA
  • 12Energy and Resources Group, University of California, Berkeley, USA
  • 13Jet Propulsion Laboratory, Pasadena, CA, USA
  • 14Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, USA
  • 15Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY, USA
  • 16Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
  • 17Earth and Environmental Sciences Division, Los ALamos National Laboratory, Los Alamos, NM, USA

Abstract. Plant functional traits determine vegetation responses to environmental variation, but variation in trait values is large, even within a single site. Likewise, uncertainty in how these traits map to Earth system feedbacks is large. We use a vegetation demographic model (VDM), the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), to explore parameter sensitivity of model predictions, and comparison to observations, at a tropical forest site: Barro Colorado Island in Panama. We define a single 12-dimensional distribution of plant trait variation, derived primarily from observations in Panama, and define plant functional types (PFTs) as random draws from this distribution. We compare several model ensembles, where individual ensemble members vary only in the plant traits that define PFTs, and separate ensembles differ from each other based on either model structural assumptions or non-trait, ecosystem-level parameters, which include: (a) the number of competing PFTs present in any simulation, and (b) parameters that govern disturbance and height-based light competition. While single-PFT simulations are roughy consistent with observations of productivity at BCI, increasing the number of competing PFTs strongly shifts model predictions towards higher productivity and biomass forests. Different ecosystem variables show greater sensitivity than others to the number of competing PFTs, with the predictions that are most dominated by large trees, such as biomass, being the most sensitive. Changing disturbance and height-sorting parameters, i.e. the rules of competitive trait filtering, shifts regimes of dominance or coexistence between early and late successional PFTs in the model. Increases to the extent or severity of disturbance, or to the degree of determinism in height-based light competition, all act to shift the community towards early-successional PFTs. In turn, these shifts in competitive outcomes alter predictions of ecosystem states and fluxes, with more early-successional dominated forests having lower biomass. It is thus crucial to differentiate between plant traits, which are under competitive pressure in VDMs, from those model parameters that are not, and to better understand the relationships between these two types of model parameters, to quantify sources of uncertainty in VDMs.

Charles D. Koven et al.
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600 Years of Forest Dynamics using FATES model at Barro Colorado Island, Panama C. Koven https://doi.org/10.5446/43627

Charles D. Koven et al.
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Short summary
Tropical forests play a crucial role in governing climate feedbacks, and are incredibly diverse ecosystems, yet most Earth system models do not take into account the diversity of plant traits in these forests and how this diversity may govern feedbacks. We present an approach to represent diverse competing plant types within Earth system models, test this approach at a tropical forest site, and explore how the representation of disturbance and competition govern traits of the forest community.
Tropical forests play a crucial role in governing climate feedbacks, and are incredibly diverse...
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