Drought-induced diurnal and seasonal variability in fuel moisture content and leaf traits in two eucalyptus species: implications for wildfire behaviour

Abstract

Wildfire behaviour is influenced by the physiological state of vegetation, particularly live fuel moisture content (LFMC), which affects fire spread and ignition. LFMC represents the ratio of water mass to dry mass in plant tissues. To understand LFMC dynamics, it is important to investigate interrelated traits such as leaf water per area (LWA), dry matter content (DMC) and specific leaf area (SLA). LWA quantifies water content per unit leaf area while SLA and DMC capture different aspects of leaf physiology. Understanding the diurnal and seasonal variability of LFMC alongside LWA, DMC and SLA is essential for predicting wildfire dynamics. We examined the diurnal and seasonal variations in these traits under progressive drought in two Australian species: Eucalyptus torquata and Corymbia maculata. Sampling day and time influenced trait dynamics. LFMC showed clear diurnal patterns with lower values at midday than in morning or late afternoon, consistent with previous reports. We found that drought altered the diurnal behaviour of related leaf traits particularly LWA, DMC and SLA, and that these responses differed within species. In E. torquata, LFMC in drought-treated plants showed a steeper diurnal drop from day 29 onward with LFMC falling below 60% by day 73. LWA declined from morning to midday. DMC decreased before partially recovering at late afternoon and SLA increased slightly. In C. maculata, LFMC showed a more moderated drought decline. LWA declined, DMC declined slightly and SLA showed minor decreases. These results reveal diurnal and drought-driven shifts in water-related traits, while trait covariation showed that drought-driven LFMC decline was most strongly linked to reductions in LWA, indicating that water loss was the more dynamic component of LFMC decline during progressive drought. Our findings show that drought affects both the timing and magnitude of LFMC and leaf trait responses in a species-specific manner. The steeper LFMC and LWA declines in E. torquata suggest that drought may widen the daily period of elevated fire risk in more drought-sensitive species, whereas the more moderate response in C. maculata suggests a narrower risk window based on LFMC. Although differences may appear subtle, they are ecologically relevant as modest decreases can lower LFMC below critical ignition thresholds. These diurnal fluctuations also show that remote sensing approaches should consider related traits especially DMC and LWA because they may capture dry matter and water loss changes that can co-vary across species. Incorporating diurnal variation in LFMC and related traits into monitoring frameworks and remote sensing validation may strengthen vegetation flammability assessments in fire-prone ecosystems.