ORCHIDEE_CAN_NHA

Abstract

Extreme drought events in Amazon forests are expected to become more frequent and more intense with climate change, threatening ecosystem function and carbon balance. Yet large uncertainties exist on the resilience of this ecosystem to drought. A better quantification of tree hydraulics and mortality processes is needed to anticipate future drought effects on Amazon forests. Most state-of-the-art dynamic global vegetation models are relatively poor in their mechanistic description of these complex processes. Here, we implement a mechanistic plant hydraulic module within the ORCHIDEE-CAN-NHA r7236 land surface model to simulate the percentage loss of conductance (PLC) and changes in water storage among organs via a representation of the water potentials and vertical water flows along the continuum from soil to roots, stems and leaves. The model was evaluated against observed seasonal variability in stand-scale sap flow, soil moisture and productivity under both control and drought setups at the Caxiuanã throughfall exclusion field experiment in eastern Amazonia between 2001 and 2008. A relationship between PLC and tree mortality is built in the model from two empirical parameters, the cumulated drought exposure duration that triggers mortality, and the mortality fraction in each day exceeding the exposure. Our model captures the large biomass drop in the year 2005 observed four years after throughfall reduction, and produces comparable annual tree mortality rates with observation over the study period. Our hydraulic architecture module provides promising avenues for future research in assimilating experimental data to parameterize mortality due to drought-induced xylem dysfunction. We also highlight that species-based (isohydric or anisohydric) hydraulic traits should be further tested to generalize the model performance in predicting the drought risks.

Details of model improvement

The improvement of the new version mainly focus on the simulation of dynamic water potential profile, hydraulic conductance, water storage dynamics regulated by capacitance, and drought-induced tree mortality in aspects of tree density and biomass. Transpiration supply is updated in ./src_sechiba/sechiba_hydrol_arch.f90. Dynamic water potential profile and percentage loss of stem hydraulic conductance (PLC) are also solved in this script. Subroutine hydrol_arch is modified. Then PLC is transferred to./src_stomate/stomate_mark_kill.f90, where the number of continuous days with PLC above 50% is calculated as drought exposure. Then the amount of trees that would be killed is derived. The update of tree density and transfer of live biomass to litter are solved in ./src_stomate/stomate_kill.f90.

Code access

• See the version on the webinterface here : ​https://forge.ipsl.fr/orchidee/browser/branches/publications/ORCHIDEE_CAN_NHA
• Extract it on a terminal as follow, type anonymous as password:

  svn co --username anonymous svn://forge.ipsl.fr/orchidee/branches/publications/ORCHIDEE_CAN_NHA  ORCHIDEE
  

Metadata

DOI	[under request]
Creator	Yitong Yao
Affiliation	LSCE
Title	ORCHIDEE-CAN-NHA
Publisher	Institut Pierre Simon Laplace (IPSL)
PublicationYear	2021
ResourceType	Software
Rights	This software is distributed under the CeCILL license
rightsURI	​http://www.cecill.info/
Subject	Plant hydraulic architecture, land surface model, terrestrial water cycle
DataManager	Karim Ramage (IPSL)
DataCurator	Josefine Ghattas (IPSL)
ContactPerson	Yitong Yao (LSCE)
FundingReference	ANR CLAND Convergence Institute; Make Our Planet Great Again Scholarship