Exploring the role of plant hydraulics in canopy fuel moisture content: insights from an experimental drought study on Pinus halepensis Mill. and Quercus ilex L.

Abstract

Key Message: Understanding the impact of extreme drought on the canopy fuel moisture content (CFMC) is crucial to anticipate the effects of climate change on wildfires. Our study demonstrates that foliage mortality, caused by leaf embolism, can substantially diminish CFMC during drought on Pinus halepensis Mill. and Quercus ilex L. It emphasizes the importance of considering plant hydraulics to improve wildfire predictions. Context: Canopy fuel moisture content (CFMC), which represents the water-to-dry mass ratio in leaves and fine twigs within the canopy, is a major factor of fire danger across ecosystems worldwide. CFMC results from the fuel moisture content of living foliage (live fuel moisture content, LFMC) and dead foliage (dead fuel moisture content, DFMC) weighted by the proportion of foliage mortality in the canopy (αDead). Understanding how LFMC, αDead, and ultimately CFMC are affected during extreme drought is essential for effective wildfire planning. Aims: We aimed to understand how plant hydraulics affect CFMC for different levels of soil water deficit, examining its influence on both LFMC and αDead. Methods: We conducted a drought experiment on seedlings of two Mediterranean species: Aleppo pine (Pinus halepensis Mill.) and Holm oak (Quercus ilex L.). Throughout the drought experiment and after rewatering, we monitored CFMC, LFMC, and αDead along with other ecophysiological variables. Results: LFMC exhibited a significant decrease during drought, and as leaf water potentials reached low levels, αDead increased in both species, thereby reducing CFMC. Distinct water use strategies resulted in species-specific variations in dehydration dynamics. Conclusion: Our findings demonstrate that as drought conditions intensify, foliage mortality might become a critical physiological factor driving the decline in CFMC.