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.

Vegetation Responses to 2012–2016 Drought in Northern and Southern California

“…Another remote sensing-based study by Brodrick and Asner (2017) reported a close relationship between sustained canopy water loss and mortality in conifers over the Sierra Nevada mountain range in California. However, the vegetation responses to drought represented by NDVI may vary with vegetation types (e.g., Fensholt et al, 2012;Gamon et al, 1995;Okin et al, 2018;Reed et al, 1994;Vicente-Serrano et al, 2013). However, the vegetation responses to drought represented by NDVI may vary with vegetation types (e.g., Fensholt et al, 2012;Gamon et al, 1995;Okin et al, 2018;Reed et al, 1994;Vicente-Serrano et al, 2013).…”

“…Satellite-derived Normalized Difference Vegetation Index (NDVI) data have long been used to monitor the impacts of drought on vegetation greenness, which is sensitive to soil moisture conditions and precipitation (Gu et al, 2007(Gu et al, , 2008Karnieli et al, 2010;Kogan, 1995;Pettorelli et al, 2005;Reed et al, 1994;Williams et al, 2013). However, the vegetation responses to drought represented by NDVI may vary with vegetation types (e.g., Fensholt et al, 2012;Gamon et al, 1995;Okin et al, 2018;Reed et al, 1994;Vicente-Serrano et al, 2013). Geographically differentiated average climate can also exert influence on the vegetation responses to drought.…”

Quantifying Drought Sensitivity of Mediterranean Climate Vegetation to Recent Warming: A Case Study in Southern California

“…Jacobsen and Pratt [116] found that the Southern California shrublands are relatively resilient to short-term high-intensity droughts, but displayed significant dieback and mortality during the recent prolonged drought (2012-2016); they concluded that the highly cavitation-resistant shrubs are the ones most affected by long-term high-intensity drought. This is supported by [50], who argued that the deep-rooting system of chaparrals buffers them from summer drought but makes them particularly vulnerable to multi-year drought, where the soil water stored in the deep layers is reduced by persistent evaporation. Our results further reveal that the chaparral communities seem to be less resilient to climate disturbance than the other vegetation types.…”

“…This approach better reflects the regional vegetation distribution than the nearest-neighbor interpolation, and is less affected by data noise and misclassifications [80]. Additionally, a rigorous data quality control was implemented by the California Department of Fish and Wildlife in generating the SWAP database and, thus, it is the "best available" land cover data for California at present [50]. Vegetation cover typical of chaparral is comprised of sclerophyllous, evergreen-leaved shrubs with hard stems and deep roots with a variety of species.…”

“…A recent study by [50] revealed that the favorable soil water conditions of the deep-rooted chaparral makes them less sensitive to short-term drought events, but particularly vulnerable to prolonged droughts, as they are not adapted to soil water depletion, such as that which occurred in the 2012-2016 California drought. By contrast, the shallow-rooted coastal sage scrub (CSS) displayed a low sensitivity to the recent multi-year drought, indicating a better adaptation to persistent dry conditions [50]. The differential behavior of the two shrublands during drought is a result of long-term environmental adaption.…”