The Response of Whole-City Urban Vegetation to Severe Drought in Los Angeles, CA, USA

Urban plants provide extensive ecosystem services to urban residents; however, the provisioning of these services is contingent on how plants fare under stress. Between 2011-2016 California experienced the most severe drought in over 1,000 years. We ask: How does severe multiyear drought affect urban plant greenness? We use this megadrought as a case study to evaluate hypotheses that both income and distance from coast moderate the drought stress response via altering local temperature and vapor pressure deficit. This work will allow us to better understand and model how urban plants respond to meteorological drought in the context of societal and environmental dynamics.

To answer our question, we examine a 19-year time series of satellite-derived vegetation indices, from 2000-2018 in the greater Los Angeles region. The greater Los Angeles region has the largest human population in California, exhibiting a large disparity in socio-demographic and biophysical variables. The Los Angeles region also has highly variable winter rainfall which can be 50%-150% of the prior year, thus allowing a more thorough understanding of the relationship between vegetation and rainfall. We use vegetation greenness as the dependent variable as it is known to be sensitive to the plant water status.

We found that, first, prior winter precipitation had a moderate effect (avg. adj. r²= 0.13) on urban plant productivity in Los Angeles. Second, income had a stronger effect on urban plant productivity than did distance from coast. In support of one of our hypotheses, the temporal variability of urban plant productivity was unchanged during drought in high-income tracts but exhibited a statistically significant decrease (p<0.001) in low-income tracts, indicating greater sensitivity in low-income tracts to prolonged drought.

This research provides a quantitative understanding of how urban plants respond to drought in a semi-arid city. The microclimate created by distance from the coast and wealth moderate how urban plants respond to drought; this indicates resources should be focused on low-income areas of the city during drought. Future research should focus on the national scale, where income, precipitation, and temperature likely differentiate how different city's vegetation respond to varying amounts of rainfall.