Some challenges for forest fire risk predictions in the21st Century

V.R.d.D. acknowledges funding from the National Natural Science Foundation in China(U20A20179, 31850410483), the talent proposals in Sichuan Province (2020JDRC0065), from SouthwestUniversity of Science and Technology (18ZX7131), and the MICINN (RTI2018-094691-B-C31). R.H.N.was supported with funding from the New South Wales Department of Planning, Industry andEnvironment, via the NSW Bushfire Risk Management Research Hub

Physiological drought responses improve predictions of live fuel moisture dynamics in a Mediterranean forest.

The moisture content of live fuels is an important determinant of forest flammability. Current approaches for modelling live fuel moisture content typically focus on the use of drought indices. However, these have mixed success partly because of species-specific differences in drought responses. Here we seek to understand the physiological mechanisms driving changes in live fuel moisture content, and to investigate the potential for incorporating plant physiological traits into live fuel moisture models. We measured the dynamics of leaf moisture content, access to water resources (through stable isotope analyses) and physiological traits (including leaf water potential, stomatal conductance, and cellular osmotic and elastic adjustments) across a fire season in a Mediterranean mixed forest in Catalonia, NE Spain. We found that differences in both seasonal variation and minimum values of live fuel moisture content were a function of access to water resources and plant physiological traits. Specifically, those species with the lowest minimum moisture content and largest seasonal variation in moisture (Cistus albidus: 49–137% and Rosmarinus officinalis: 47–144%) were most reliant on shallow soil water and had the lowest values of predawn leaf water potential. Species with the smallest variation in live fuel moisture content (Pinus nigra: 96–116% and Quercus ilex: 56–91%) exhibited isohydric behaviour (little variation in midday leaf water potential, and relatively tight regulation of stomata in response to soil drying). Of the traits measured, predawn leaf water potential provided the strongest predictor of live fuel moisture content (R2 = 0.63, AIC = 249), outperforming two commonly used drought indices (both with R2 = 0.49, AIC = 258). This is the first study to explicitly link fuel moisture with plant physiology and our findings demonstrate the potential and importance of incorporating ecophysiological plant traits to investigating seasonal changes in fuel moisture and, more broadly, forest flammability.This study was made possible thanks to the collaboration of and the staff from the Natural Park of Poblet, P Sopeña, and the technical staff from MedForLab. This study was funded by the Spanish Government (RYC-2012-10970, AGL2015-69151-R). R. H. Nolan was supported with funding from the New South Wales Office of Environment and Heritage, via the Bushfire Risk Management Research Hub. We benefitted from critical comments from J Voltas, JM Moreno and L Serrano and instrument loans from R Savín

A trade-off between embolism resistance and bark thickness in conifers: Are drought and fire adaptations antagonistic?

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, for example those in the Southern Hemisphere.This work was supported by the Ministerio de Economía y Competitividad [RYC-2012-10970, AGL2015-69151-R, AGL2015-68274-C3-]