Association of herbaceous fractional cover and wildfire ignition in Mediterranean-type ecosystems

Background Mediterranean-type ecosystems, like those in Southern California’s native shrublands, are being invaded and sometimes replaced by non-native herbaceous plant species, potentially increasing wildfire ignition risk and spread in a positive feedback loop called the grass–fire cycle. Aims To assess whether a grass–fire cycle is occurring in Southern California, we evaluated the association between historical wildfire ignitions and Landsat-derived, spectrally unmixed herbaceous fractional cover (HFC) maps. Methods We examined differences in the spatial distribution of wildfire ignitions relative to differences in HFC interval classes over 29 years (1992–2020). We also compared results for percentages of ignitions across land ownership, ignition source, and outcomes of the ignitions. Key results Analyses of annual data showed increasing percentages of ignitions in >40% HFC over time. When the entire 29-year period was evaluated, analyses showed the largest percentage of ignitions occurred in >20% HFC. Conclusions Study results provided support documenting a grass–fire cycle in Southern California. Annual trends reinforced that >40% HFC increased fire occurrence. Implications Results from the full 29-year period can be used to determine HFC-dependent ignition potential and will be useful for identifying target areas for combatting the grass–fire cycle.