19The moisture content of live fuels is an important determinant of forest flammability. Current 20 approaches for modelling live fuel moisture content typically focus on the use of drought 21 indices. However, these have mixed success partly because of species-specific differences in 22 drought responses. Here we seek to understand the physiological mechanisms driving 23 changes in live fuel moisture content, and to investigate the potential for incorporating plant 24 physiological traits into live fuel moisture models. We measured the dynamics of leaf 25 moisture content, access to water resources (through stable isotope analyses) and 26 physiological traits (including leaf water potential, stomatal conductance, and cellular 27 osmotic and elastic adjustments) across a fire season in a Mediterranean mixed forest in 28 Catalonia, NE Spain. We found that differences in both seasonal variation and minimum 29 values of live fuel moisture content were a function of access to water resources and plant 30 physiological traits. Specifically, those species with the lowest minimum moisture content 31 and largest seasonal variation in moisture (Cistus albidus: 49-137 % and Rosmarinus 32 officinalis: 47-144 %) were most reliant on shallow soil water and had the lowest values of 33 predawn leaf water potential. Species with the smallest variation in live fuel moisture content 34 (Pinus nigra: 96-116 % and Quercus ilex: 56-91 %) exhibited isohydric behaviour (little 35 variation in midday leaf water potential, and relatively tight regulation of stomata in response 36 to soil drying). Of the traits measured, predawn leaf water potential provided the strongest 37 predictor of live fuel moisture content (R 2 = 0.63, AIC = 249), outperforming two commonly 38 used drought indices (both with R 2 = 0.49, AIC = 258). This is the first study to explicitly link 39 fuel moisture with plant physiology and our findings demonstrate the potential and 40 importance of incorporating ecophysiological plant traits to investigating seasonal changes in 41 fuel moisture and, more broadly, forest flammability.42 43 65LFMC. An eco-physiological approach to investigating LFMC dynamics may overcome 66 these limitations and lead to improved modelling (Jolly and Johnson, 2018). 67There are at least three different but mutually inter-related physiological processes 68 that might explain why leaf moisture varies differentially across species. One of them is 69 access to water resources, a function of rooting patterns and water availability in the soil 70 profile that, in turn, appears to be related to the post-fire regeneration strategy. For example, 71 species that can resprout following high intensity fire typically have greater allocation to 72 roots and deeper root systems than species lacking this capacity (Bell et al., 1996; Verdu, 73 2000). As a consequence, resprouting species often maintain greater water supply to foliage 74 during dry periods than obligate seeding species (which are fire-killed but regenerate via 75 seed) (Saura-Mas and...

Resprouting is an ancestral trait in angiosperms that confers resilience after perturbations. As climate change increases stress, resprouting vigor is declining in many forest regions, but the underlying mechanism is poorly understood. Resprouting in woody plants is thought to be primarily limited by the availability of non-structural carbohydrate reserves (NSC), but hydraulic limitations could also be important. We conducted a multifactorial experiment with two levels of light (ambient, 2-3% of ambient) and three levels of water stress (0, 50 and 80 percent losses of hydraulic conductivity, PLC) on two Mediterranean oaks (Quercus ilex and Q. faginea) under a This article is protected by copyright. All rights reserved. rain-out shelter (n = 360). The proportion of resprouting individuals after canopy clipping declined markedly as PLC increased for both species. NSC concentrations affected the response of Q. ilex, the species with higher leaf construction costs, and its effect depended on the PLC. The growth of resprouting individuals was largely dependent on photosynthetic rates for both species, while stored NSC availability and hydraulic limitations played minor and non-significant roles, respectively. Contrary to conventional wisdom, our results indicate that resprouting in oaks may be primarily driven by complex interactions between hydraulics and carbon sources, whereas stored NSC play a significant but secondary role.

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Background: Understanding the mechanisms that explain the spatial distribution of conifers across biogeographical gradients is important for anticipating potential range shifts owing to global change. Classical explanations have involved trade-offs between shade and drought tolerances, but more recent studies observed that trade-offs between fire and drought tolerances could also be important. Aims: Here we propose that a contributing mechanism to explain how conifer species are distributed across productivity gradients-with marked variation in the incidence of fire-involves a trade-off between allocation to bark, which serves to protect against fire, or to embolism resistance, which serves to protect against drought. Methods: We compiled information from different datasets and performed regression analyses. Results: We observed a trade-off between bark thickness and embolism resistance in conifer species such that species show either large investments of carbon to the bark or have thinner barks but xylem resistant to embolism; we did not observe conifer species concomitantly showing high fire tolerance and embolism resistance. Conclusions: This study serves as a starting point for a novel framework on how fire and drought adaptations affect conifer biogeography. Additional studies will be necessary to discover the generality of our findings by including other species of conifers, e.g. those in the Southern Hemisphere.

Stomatal closure is one of the earliest responses to water stress but residual water losses may continue through the cuticle and incomplete stomatal closure. Residual conductance (gres) plays a large role in determining time to mortality but we currently do not understand how do drought and shade interact to alter gres because the underlying drivers are largely unknown. Furthermore, gres may play an important role in models of water use, but the exact form in which gres should be incorporated into modeling schemes is currently being discussed. Here we report the results of a study where two different oak species were experimentally subjected to highly contrasting levels of drought (resulting in 0, 50 and 80% losses of hydraulic conductivity) and radiation (photosynthetic photon flux density at 1,500 μmol m–2 s–1 or 35–45 μmol m–2 s–1). We observed that the effects of radiation and drought were interactive and species-specific and gres correlated positively with concentrations of leaf non-structural carbohydrates and negatively with leaf nitrogen. We observed that different forms of measuring gres, based on either nocturnal conductance under high atmospheric water demand or on the water mass loss of detached leaves, exerted only a small influence on a model of stomatal conductance and also on a coupled leaf gas exchange model. Our results indicate that, while understanding the drivers of gres and the effects of different stressors may be important to better understand mortality, small differences in gres across treatments and measurements exert only a minor impact on stomatal models in two closely related species.