The Impact of Drought on Native Southern California Vegetation: Remote Sensing Analysis Using MODIS-Derived Time Series

Abstract

Ecosystems in Mediterranean climates are adapted to seasonal drought. Multiannual drought, however, may significantly affect Mediterranean ecosystems and, further, may affect their constituent communities in different ways with differences in responses emerging during severe drought and over the course of long-term climate change. This study investigates the response to recent prolonged drought of two important Mediterranean-climate vegetation types: chaparral and coastal sage scrub. Their greenness was monitored from 2000 to 2017 using the Normalized Difference Vegetation Index (NDVI) derived from the Moderate Resolution Imaging Spectroradiometer. A bootstrapped multiple regression of NDVI anomalies against Palmer Drought Severity Index was used to assess the response of vegetation to drought. The results suggest that drought had a greater effect on chaparral-dominated ecosystems than coastal sage over the entire region. Shallow-rooted coastal sage scrub displayed higher interannual variability in NDVI than chaparral but had a lower correlation with Palmer Drought Severity Index. This suggests that this vegetation community is generally less responsive to drought on longer timescales than chaparral, which may be more sensitive to soil water depletion in a prolonged drought. Soil texture plays a critical role in affecting the community sensitivity to drought. For chaparral communities, elevation is negatively correlated with drought sensitivity, suggesting the role of temperature-related water stress on their long-term response to drought. Future predicted climate change in the form of increased temperature and drought hazards in Southern California will influence the native ecosystems in nonuniform ways. Long-term trends in soil moisture depletion and temperature may be particularly important in chaparral-dominated areas.