Vegetation Type Conversion in Los Peñasquitos Lagoon, California: An Examination of the Role of Watershed Urbanization

The objective of this study was to examine the role that watershed urbanization has played in changes to the vegetation types within Los Peñasquitos Lagoon, San Diego, California. Aerial photographs taken between 1928 and 1999 were used to examine changes in vegetation types. The aerial photographs were scanned into digital format and incorporated into a geographic information system (GIS). A combination of image classification techniques was used to differentiate the vegetation types. Land use/cover of the Carmel Valley watershed of the Los Peñasquitos Lagoon was mapped for the same dates as the aerial photographs. A temporal geographic analysis was conducted on the conversion of areal extent of lagoon vegetation types compared to areal extent of urban development in the watershed. Soil salinity and dry season (June–September) stream discharge were measured. The results show that approximately 80.3% of the vegetation in the study area has changed types between 1928 and 1999. Increases in the areal extent of urban development within the watershed show a strong positive relationship compared to the areal extent of brackish marsh and riparian vegetation, and a strong negative relationship to the areal extent of salt panne and mudflats. There was no significant relationship between urban development and salt marsh vegetation. Dry season stream discharge has increased by an order of magnitude. The increase in stream discharge supports the hypothesis that increased freshwater has lowered soil salinity, allowing for invasion by glycophytic species. Hydrology and soil salinity appear to be significant factors for maintaining the distribution of the lagoon vegetation types and the biotic communities that rely upon them.