Seasonal and Interannual Dynamics of Drought Response in Urban Trees and Grasses using Landsat and AVIRIS Time Series

Abstract:

Drought impacts can manifest in vegetation across many physiological responses and time scales. In metropolitan areas, vegetation provides valuable shading and cooling during hot and dry conditions, but these benefits can be reduced during drought. Although many studies have evaluated interannual drought responses in vegetation, seasonal responses have rarely been studied at the same time, especially in cities that experience seasonal drought (e.g., in Mediterranean climates). Here, we used Landsat and Airborne Visible Infrared Imaging Spectrometer (AVIRIS) imagery to evaluate seasonal and interannual drought response during 2010-2019 in urban vegetation in Santa Barbara, California. We compared responses in functionally different vegetation: broadleaf, needleleaf, Eucalyptus, and evergreen oak trees, and unirrigated annual grass and irrigated turfgrass. To address different potential physiological responses to drought, we tracked changes in greenness (NDVI) and canopy temperature relative to paved surfaces (difference in land surface temperature, ΔLST) using Landsat and canopy water content (equivalent water thickness, EWT) using AVIRIS. NDVI was lower and ΔLST was closer to zero during drought years but they were seasonally correlated for only some vegetation types (e.g., broadleaf tree Pearson’s R = -0.76 but Eucalyptus Pearson’s R = 0.25). Changes in EWT revealed seasonal adjustments by vegetation that were not as readily apparent in the NDVI time series. We used the Standardized Precipitation Evapotranspiration Index (SPEI) to correlate vegetation response in Landsat variables (NDVI and ΔLST) to drought time scales during different seasons. NDVI and ΔLST were most strongly correlated with SPEI during summer for most vegetation types, except for annual grass NDVI (winter). Annual grass was generally correlated with SPEI at spans ≤12 months, whereas trees and turfgrass were commonly correlated with SPEI at spans >12 months in addition to seasonal time spans.