UC Santa Barbara

The Montecito debris flows of 9 January 2018 deposited sediment along many portions of the Santa Barbara coast, including Carpinteria Salt Marsh Reserve. Since disturbances have the potential to impact the ecosystem services and functions that wetlands provide, an understanding of how the ecosystem at Carpinteria Salt Marsh Reserve responded to this disturbance is important to its ongoing management. However, a lack of opportunity to collect field data around the time of disturbance makes this task difficult to complete by field methods. To address this gap, Sentinel-2 imagery from four dates (November 2017, January 2018, November 2018, November 2020) was used to calculate landcover fractions, normalized difference vegetation index (NDVI), and modified anthocyanin reflectance index (mARI) and used in tandem with random forest classification to produce maps of landcover before, during, and after the debris flow. Post-classification change detection was then performed on the classified maps to track changes in landcover through time. Results from the random forest classification showed that NDVI and green vegetation fractions were the most important variables in classifying landcover, though this varied on a date-by-date basis; error matrices indicated that the model had high accuracy with values of 0.994, 0.920, 0.956. and 0.963 for the respective dates. Change detection shows a pattern of returning to pre-debris flow vegetation extent in the marsh. While total vegetated area experienced little change (0.12% increase), there was a change in the extent of vegetation type with high marsh vegetation shifting to mid marsh vegetation in regions near where increases in bare soil landcover occurred. These results show that disturbance due to debris flow can cause vegetation changes which may affect ecosystem function, such as decrease primary productivity and marsh resilience to further disturbance, that will need to be taken into consideration when managing depositional event prone wetlands.