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Description
The role of non-native riparian vegetation in urban fire regimes within Mediterranean riverine systems (Med-sys) is not well documented. A combination of satellite indices and in field surveys were used to investigate the impact of non-native), riparian, or upland vegetation on the spatial and temporal variability of fire severity and canopy loss was compared across eleven fires in southern California. Satellite-based metrics, differenced Normalized Burn Severity (dNBR) and differenced Normalized Difference Vegetation Index (dNDVI), were approximated for native and non-native vegetation for 0, 1, and 3 years after fire. Generally, the riparian class burned more severely and experienced a greater loss of green canopy than the upland land class. The presence of invasive vegetation affected the distribution of burn severity and canopy loss. On average, across both upland and riparian regions, fires with native vegetation cover burned at a higher severity and resulted in larger immediate loss of canopy than fires with non-native vegetation. The lower burn severity observed in the fires with non-native vegetation suggests a rapid regrowth of non-native grasses immediately after fire, resulting in a smaller measured canopy loss relative to native vegetation. A case study of the Del Cerro fire (San Diego, California) was used to compare observations of native and non-native vegetation and fluvial morphology in the upland and riparian regions. Field observations noted rapidly re-sprouting invasive grass species such as Arundo donax (Giant Reed). While, satellite-based metrics underestimated the initial green canopy loss after urban fires as did volumetric data derived from three-dimensional terrestrial laser scanning data (TLS). Fire damaged trees and chaparral and decomposition of burned branches contributed of vegetation volume throughout the first six months following the fire, exceeding the combined density of resprouting vegetation. The presence of Arundo donax significantly increased channel instability in the riparian region and encouraged the deposition of sediments in the floodplain (accretion). This geomorphological mechanism can promote riparian desiccation and fire risk. Ultimately, this work builds considerably upon our current knowledge of wildfire and recovery processes and informs understanding of the post fire mechanisms and anthropogenic feedbacks unique to urban Med-sys.
