Drought response of urban trees and turfgrass using airborne imaging spectroscopy

Vegetation growth and physiological functioning, including carbon uptake and transpiration, can be strongly affected by drought. During water-limited conditions, urban vegetation may be limited in its ability to provide ecosystem services, such as shading and evaporative cooling. While there has been extensive research on drought response in natural vegetation and agricultural systems using remote sensing, drought impacts on urban vegetation have remained understudied. With >50% of people worldwide living in cities and the increased potential for future droughts, it is important to understand how urban vegetation will respond and to evaluate potential impacts. Here, we used repeat flights of NASA airborne imaging spectroscopy (AVIRIS and AVIRIS-NG) to quantify the responses of common urban tree species and turfgrass in downtown Santa Barbara, California, USA to the 2012–2016 California drought. The effects of the drought on vegetation were quantified using several spectral indicators: fractional cover from multiple endmember spectral mixture analysis (MESMA), vegetation indices, and continuum removed absorption features. We used existing tree crown object polygons that were classified to species and had associated estimates of pre-drought leaf area index and biomass. We also included polygons of turfgrass parks and recreational fields. All tree species and turfgrass had four or more spectral indicators with significantly lower mean values (p < 0.05) during the drought in 2014, and evidence of recovery was observed for some species in 2017, which was after the drought had ended. In general, needleleaf and native trees were more affected than broadleaf and non-native trees, respectively, but with considerable interspecific variability. Based on the spectral indicators, turfgrass cover senesced in the middle of the drought but recovered soon after the drought ended, whereas the majority of tree species had not fully recovered. Mean green vegetation cover declined in most tree canopies during the drought, primarily exposing paved surfaces and/or non-photosynthetic vegetation, depending on the site type and tree patch size. Drought impacts as expressed in spectral indicators were generally more severe in tree patches with high pre-drought leaf area index, suggesting that denser, leafier canopies were more difficult for trees to maintain during the drought. Most spectral indicators were correlated with one another (R > 0.4), suggesting co-occuring adjustments of greenness, water content, and plant pigments.