UC Santa Barbara

Expanding invasive herbaceous vegetation (non-native grasses and forbs or herbs) is replacing portions of native shrublands in San Diego County, California, USA through a grass-fire cycle, which contributes to an increased risk of wildfire ignition and spread as well as a changing fire regime. Despite the association between herb abundance and wildfire risk, remote sensing and image processing approaches for quantification of fractional herb cover in shrublands are not well established, nor is the association between herb fraction and proportion of ignitions. In this research, I comparatively assess the accuracy of herb cover estimation and mapping based on three spectral unmixing models applied to Landsat-derived spectral reflectance and spectral vegetation index data from multiple 2020 dates. Based on the model and methods that most accurately and reliably represent herb cover, I then reconstruct the spatial-temporal distribution of herb growth using Landsat images from 1988, 1997, and 2011 and assess the extent to which herb cover has expanded and replaced woody vegetation cover (1988 to 2020). Finally, I combine the herb cover maps with historical ignition points to evaluate the spatial association between herb fractions and locations where fires initially ignited and spread (1992 to 2020). When compared to generated reference data, results demonstrate the parsimonious spectral unmixing approach applied to a fall date estimates herb cover at the 10% accuracy level and more accurately than the more sophisticated unmixing models. Absolute change estimates derived from the earliest and most recent herb cover maps show approximately 25% of the study area exhibited an increase in herb cover > 20%, and roughly 5% experienced a decrease in herb < -20%, with the greatest concentration of change occurring in wildland-urban interface areas. Factors most strongly associated with substantial increase in herb cover include fire return interval, drought, proximity to development, and elevation. The results of evaluating historical ignitions in herbs show the largest proportion of ignitions occurred in areas with > 20% herbaceous fractional cover. Results from this study will enable improved detection of sensitive habitats by satellite for wildfire-prone communities and help identify target areas for mitigating and combating the grass-fire cycle.