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

Abstract

Ø Water 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. Ø In 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. Ø The 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. Ø At 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. Ø A 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. Ø We 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.

Drought acclimation of Quercus ilex leaves improves tolerance to moderate drought but not resistance to severe water stress

“…We find a difference of more than 1 MPa between K leaf P 50 and the stem xylem P 50 reported in the literature for Q. ilex , with K leaf P 50 being more in the range of stem P 12 (Lobo et al, 2018; Sergent et al, 2020). This supports the vulnerability segmentation hypothesis according to which leaves may act as a ‘safety‐valve’ protecting the branches from xylem embolism during severe water stress (Scoffoni & Sack, 2017; Tyree & Ewers, 1991; Zhu et al, 2016), and is in accordance with observations of widespread leaf browning and shedding in Puéchabon during the severe 2016 and 2017 droughts while branch mortality remained very limited (Martin‐StPaul et al, 2020).…”

“…2) drought‐induced embolism in the leaf xylem would vary with the minimum Ψ leaf experienced the previous summer, and would either lower K leaf,max and K leaf P 50 in the rainfall exclusion treatments (Cochard et al, 2013), or alternatively would increase K leaf P 50 due to cavitation fatigue (Hacke et al, 2001); (hyp. 3) partial leaf drying and shedding observed in Puéchabon in 2017 as a consequence of the extreme water stress (Martin‐StPaul et al, 2020) would have selected the most hydraulically resistant leaves in this site.…”

Prediction of regional wildfire activity in the probabilistic Bayesian framework of Firelihood

“…Live fuel moisture content dynamics depends on the processes of the water (soil water uptake, plant water storage, and transpiration) and the carbon (photosynthesis, respiration, carbon allocation, and canopy phenology) cycles (Jolly and Johnson 2018). Key water processes during an extreme drought and heat wave can be modeled according to plant hydraulics, depending on plant traits (e.g., Martin‐StPaul et al 2020). As far as the carbon cycle is concerned, the evolution of fuel moisture linked to the production of new shoots could be taken into account thanks to tree phenological models (e.g., Vitasse et al 2011).…”

SurEau: a mechanistic model of plant water relations under extreme drought

“…This also offers promising opportunities for a wide range of applications. These include comparison with remote sensing products for moisture content (Fan et al 2018;Marino et al 2020) or promising prediction of moisture content of living fuels for fire risk assessment (Martin-StPaul et al 2020), which is a critical factor in fire behavior (e.g., Ruffault et al 2018;Pimont et al 2019). SurEau also departs by the fact it includes an explicit description of cuticular losses connected to the symplasm of the different plant organs described.…”