Linking drought and fire: a biophysical approach to the moisture content of living forest fuel.

GOALS

Drought&Fire is part of an INRAE project that aims to acquire knowledge and develop models to propose, in the long term, new indicators of fire danger and vulnerability to drought that take into account the physiology of plants under water deficit conditions and its impact on the combustibility of stands and fire activity. Specifically, the objectives of Drought&Fire were as follows:

1. To assess the relevance of existing drought and fire danger indicators
2. To quantify the effect of vegetation moisture content on fire behaviour and activity
3. To better understand and model the physiological determinants and processes of woody plant moisture dynamics in response to drought

Expected outcomes and possible applications

The results of Drought&Fire justify pursuing the objective of producing new fire danger indicators that take into account the water content of plant foliage. The development of the SUREAU-ECOS plant desiccation model is ongoing and part of a research programme funded by the US Department of Defense and coordinated by the USDA Forest Service.

Measurements of functional traits and modelling of plant physiological processes are used to predict the differential response of species to drought in terms of water content, both in Mediterranean regions and in regions less subject to fire risk. The model paves the way for spatialization of the water content of plant cover, as a complement to remote sensing, or for projections of the effects of climate change on the dynamics of this water content.

Determinants and physiological processes of vegetation water content variations under drought conditions.

The SUREAU-ECOS model (Ruffault et al. 2022) combines water balance principles with hypotheses of plant hydraulic functioning. In this model, the plant is represented by water reservoirs: symplasmic (the living compartment) and apoplasmic (the vascular compartment). Water dynamics within and between the plant compartments depend on a series of hydraulic traits, which vary according to the species. The cavitation process, which causes mortality of affected organs and thus strongly affects their water content, is also represented.

SUREAU-ECOS was evaluated under natural conditions, on the holm oak stand of the Puéchabon forest (instrumented site, ICOS network). The model captures the summer dynamics of water content well, but a bias exists in some years on the level of water content. This led to a hypothesis of acclimatisation according to seasonal conditions.

Modelling analysis showed that moderate droughts lead to water content variations mainly related to leaf-level water content adjustments, whereas severe droughts lead to leaf mortality reducing canopy water content irreversibly on a season-wide basis. By performing a sensitivity analysis, we identified the most sensitive parameters involved in the dynamics of water content at the leaf and canopy scales.