Dry Live Fuels Increase the Likelihood of Lightning‐Caused Fires

Abstract

Live fuel moisture content (LFMC) is a key determinant of landscape ignition potential, but quantitative estimates of its effects on wildfire are lacking. We present a causal inference framework to isolate the effect of LFMC from other drivers like fuel type, fuel amount, and meteorology. We show that in California when LFMC is below a critical flammability threshold, the likelihood of fires is 1.8 times as high statewide (2.25% vs. 1.27%) and 2.5 times as high in shrubs, compared to when LFMC is greater than the threshold. This risk ratio is >2 times when LFMC is 10% less than the threshold. Between 2016 and 2021, the risk ratio was highest in 2020 (2.3 times), potentially contributing to the record‐breaking wildfire activity in 2020. Our estimates can inform several wildfire prediction and management applications, including wildfire suppression, prescribed burn planning, and public safety power shutoff implementation.

Autoregressive Forecasting of the Number of Forest Fires Using an Accumulated MODIS-Based Fuel Dryness Index

“…Furthermore, the generally lower thresholds for fire occurrence observed in wetter states and, conversely, higher FDI 99 found for drier states, support similar previous observations for FPI by [60], who documented the highest FPI thresholds for the drier bioclimatic regions in Southern Europe. This also agrees with previous observations of higher thresholds of live fuel moisture for fire occurrence in drier ecosystems than in wetter, more productive forests (e.g., [40,43,88]).…”

“…Beyond a potential influence on coarse dead fuel moisture or on deep soil layers, it is likely that the apparent advantage of the selection of a 50-day period might be related to live fuel moisture dynamics, which are known to influence fire occurrence and spread (e.g., [39,43,103]), although its mathematical contribution to fire modeling remains still as an open research question (e.g., [38,42,43]). In this sense, unlike short-term fire danger indices and dead fuel moisture codes that are driven mainly by short-term weather conditions, live fuel moisture depends not just on recent hydrometeorology [38].…”

“…In this sense, unlike short-term fire danger indices and dead fuel moisture codes that are driven mainly by short-term weather conditions, live fuel moisture depends not just on recent hydrometeorology [38]. Accumulated vegetation stress is also driven by dynamic and non-linear interactions between weather conditions, soil properties, and plant physiological processes (e.g., [40,42,43]). Plant responses to drought and dry mass changes associated with phenology are particularly critical ( [38,42,104]).…”

“…This study offers useful information for a hybrid index that, unlike studies that have focused on weather-driven fire danger indices only (e.g., [22,23,37]]) or only on remotely sensed estimates of live fuel moisture (e.g., [39,43,46]), combines both moisture components. This is, to our best knowledge, the first study analyzing several accumulation periods from 10 to 90 days for FPI, one of the few operational fire danger indices integrating both weather-driven dead fuel moisture and remotely sensed live fuel moisture estimates.…”

“…Nevertheless, the flammability of live fuels depends not only on weather variability, but also on plant and soil response to it, which is species-and landscape-specific (e.g., [38][39][40]). Although live fuel moisture has been shown to be crucial in predicting fire behavior and fire severity (e.g., [39][40][41][42]), its role is not explicitly accounted for by current fire danger models [43]. In this sense, in spite of an emerging body of literature that has demonstrated a correlation of remotely sensed estimates of live moisture (e.g., [44,45]) with fire occurrence and behavior (e.g., [46][47][48]), there are still uncertainties for incorporating such remotely sensed fuel moisture estimates into operational fire prediction modeling (e.g., [38,39,42]).…”