Upland and Riparian Surface Soil Processes in an Urban Creek with Native and Non-Native Vegetation after Fire

Alicia M. Kinoshita,Natalie Mladenov,Marta Miletić,Rey Becerra

doi:10.3390/fire5020032

Abstract

Wildfires can pose environmental challenges in urban watersheds by altering the physical and chemical properties of soil. Further, invasive plant species in urban riparian systems may exacerbate changes in geomorphological and soil processes after fires. This research focuses on the 2018 Del Cerro fire, which burned upland and riparian areas surrounding Alvarado Creek, a tributary to the San Diego River in California. The study site has dense and highly flammable non-native vegetation cover (primarily Arundo donax) localized in the stream banks and has primarily native vegetation on the hillslopes. We estimated the post-fire organic matter and particle distributions for six time points during water years 2019 and 2020 at two soil depths, 0–15 cm and 15–30 cm, in upland and riparian areas. We observed some of the largest decreases in organic matter and particle-size distribution after the first post-fire rainfall event and a general return to initial conditions over time. Seasonal soil patterns were related to rainfall and variability in vegetation distribution. The riparian soils had higher variability in organic matter content and particle-size distributions, which was attributed to the presence of Arundo donax. The particle-size distributions were different between upland and riparian soils, where the riparian soils were more poorly graded. Overall, the greatest change occurred in the medium sands, while the fine sands appeared to be impacted the longest, which is a result of decreased vegetation that stabilized the soils. This research provides a better understanding of upland and riparian soil processes in an urban and Mediterranean system that was disturbed by non-native vegetation and fire.

Highlights

IntroductionHuman encroachment into fire-prone areas has increased the potential for ignition, as well as risks and damages to human communities, and costs to society [2]
We investigated the mass retained from the sieve analysis in general; the percent of mass retained in the upland soils was consistent across the clay and fine and medium sand (
After the fire, the organic matter in the uplands was higher in the upp soils than in deeper soils compared to the riparian area, where no significant differenc were observed (Figure 2)

Summary

Introduction

Human encroachment into fire-prone areas has increased the potential for ignition, as well as risks and damages to human communities, and costs to society [2]. The number of fires under 5 km in the urban riparian environment has increased in southern California since 2002 [3]. Landscape conversion due to the expansion of the human population and establishments in southern California (United States) has fragmented chaparral ecosystems and intensified the Wildland–Urban Interface (WUI), leading to the potential for fire ignition [4,5]. Urbanization contributes to larger dry weather baseflow due to impermeable land cover (“urban drool”), which alters native riparian vegetation density and community structure [6]. The high density of non-native vegetation biomass and plant infestations can alter riparian hydrological and geomorphological regimes [11–13] and fire patterns [7,8,11]

Methods

Results

Discussion

Conclusion