Modeling the Impacts of Non-native Vegetation, Fire, and Management in an Urban Riverine Environment

Abstract:

Research is needed in urban Mediterranean systems where fire, geomorphic, and hydrologic regimes can be altered by non-native vegetation, human ignition sources, and urban landscape and riparian management. Opportunistic and invasive plants can dominate riparian vegetation patterns, outcompeting native plants and resulting in an increase in non-native vegetation biomass. Management efforts to restore waterways to native vegetation conditions may not only provide preventative measures to mitigate fire fuel loads, but also feedback conditions after fire. The goal of this research is to model the hydrologic and geomorphic impact of invasive riparian vegetation. Alvarado Creek, a tributary of the San Diego River in California, provides a unique opportunity to study multiple disturbances such as non-native vegetation, fire, and management efforts. Field data includes cross sections, vegetation surveys, grain size distributions, infiltration, turbidity, and stream flow measurements. Following a fire in 2018, which was fueled mainly by Washingtonia spp. (Mexican Fan Palms) and Arundo donax (Giant Reed), observations showed that A. donax rapidly resprouted. The highly invasive species altered channel stability in the riparian region and encouraged the deposition of sediments in the floodplain, which we hypothesize will encourage more invasive vegetation growth and consequently increased fire risk, flooding, and sediment transport. Study sites represent the following scenarios: 1) unburned reach with native vegetation; 2) non-native vegetation and burned conditions; and 3) burned landscape with vegetation restoration. FLO-2D, a physically-based and two-dimensional model, is ideal for complex and heterogeneous systems such as urban interfaces. We will parameterize and calibrate FLO-2D to simulate high resolution flooding patterns and trends in sediment transport for scenarios 1 and 2. The third scenario will be used to evaluate the response of a burned area that is restored with native vegetation after the fire. This work builds upon our current knowledge of wildfire and recovery processes, which can be utilized by local resource managers and engineers to inform and optimize management decisions in urban riverine environments prone to invasive-initiated riparian fires.