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bioRxiv · 10.1101/2020.06.03.127167

Modelling live fuel moisture content at leaf and canopy scale under extreme drought using SurEau-Ecos, a lumped plant hydraulic model

Martin-StPaul, N.·Ruffault, J.·Blackmann, C.·Cochard, H.·De Caceres, M.·Delzon, S.·Dupuy, J.-L.·Fargeon, H.·Lamarque, L.·Moreno, M.·Parsons, R.·Pimont, F.·Ourcival, J.-M.·Torres-Ruiz, J.·Limousin, J.-M.
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Abstract

O_LIWater content in living vegetation (or live fuel moisture content, LFMC), is increasingly recognized as a key factor linked to vegetation mortality and wildfire ignition and spread. Most often, empirical indices are used as surrogates for direct LFMC measurements. C_LIO_LIIn this paper, we explore the functional and ecophysiological drivers of LFMC during drought at the leaf and canopy scale using the SurEau-Ecos model, and a three years dataset of leaf and canopy scale measurements on a mature Quercus ilex forest, including an extreme drought. The model is based on forest hydrology and plant hydraulics and allows to simulate temporal variations of water potential and content at a daily time step. At leaf level, it simulates the relationship between water potential and water content by separating the apoplasm and the symplasm. Symplasm water content is modeled using the pressure volume curve theory, and apoplasm water content is modelled using the xylem vulnerability to cavitation. Fuel moisture content was upscaled to the canopy level by accounting for foliage mortality estimated from drought induced cavitation. C_LIO_LIThe model was parameterized either with site-measured traits or using a calibration procedure, and compared with water potential and LFMC measured at leaf level, and NDVI variation measured at canopy level and taken as a surrogate for foliage mortality. C_LIO_LIAt leaf level, LFMC prediction using measured hydraulic traits could be improved by considering year-to-year osmotic adjustments. At canopy level, foliage mortality due to drought induced cavitation was a key driver of LFMC decline during the most extreme drought. C_LIO_LIA sensitivity analysis showed that parameters driving soil water balance (leaf area index, soil water capacity, and regulation of transpiration) and parameters determining pressure volume curves are key traits driving LFMC dynamics at leaf level. At the canopy level, parameters that drives hydraulic failure were the most sensitive and included, both soil water balance parameters and hydraulic traits (the leaf vulnerability to cavitation) were the main drivers of LFMC decline during extreme drought. C_LIO_LIWe also showed that under normal historic weather conditions, most variation of LFMC are linked to reversible symplasm dehydration, however under future, hotter and dryer conditions, most variations are due to the decline canopy of LFMC driven by foliage mortality. C_LI

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Martin-StPaul, N., Ruffault, J., Blackmann, C., Cochard, H., De Caceres, M., Delzon, S., Dupuy, J.-L., Fargeon, H., Lamarque, L., Moreno, M., Parsons, R., Pimont, F., Ourcival, J.-M., Torres-Ruiz, J., Limousin, J.-M.. 2020-06-03. Modelling live fuel moisture content at leaf and canopy scale under extreme drought using SurEau-Ecos, a lumped plant hydraulic model. https://doi.org/10.1101/2020.06.03.127167

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