The response of urban greenness to precipitation at the continental scale

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Dion Kucera, Kristin Hamilton and Darrel Jenerette, Botany and Plant Sciences, University of California, Riverside, Riverside, CA, Diane Pataki and Elizaveta Litvak, School of Biological Sciences, University of Utah, Salt Lake City, UT

Background/Question/Methods
Urban plant greenness is influenced by both irrigation and rain, however, the role of urban irrigation on plant greenness is unknown, making the urban Precipitation-NDVI relationship uncertain. Delineating the relationship between urban greenness and precipitation provides insight into the impacts of climate change on urban areas, in particular, of drought. Here, we ask: how does the urban Precipitation-NDVI relationship change at the continental scale? To answer this question, we chose 25 cities based on population sizes of 200,000 or more, and their location within an aridity gradient across the United States. We utilized Landsat 8 Level-2 NDVI from March-August 2013-2020, and precipitation, humidity, and temperature from MesoWest, averaged monthly per city. We also compared urban vegetation-precipitation relationships within cities with neighborhood characteristics of income, impervious cover, and tree canopy cover. We first assessed the Precipitation-NDVI slope, then tested climate and social factors which may influence the slope.
Results/Conclusions
15 of our 25 sample cities exhibited a significant Precipitation-NDVI relationship while the effect of precipitation was highly variable. We found, first, that the Precipitation-NDVI relationship was strong on a monthly time scale, regardless of mean annual precipitation, i.e., cities in the heavily irrigated Southwest had significant relationships as did the mesic eastern cities. Climate played the most significant role in the Precipitation-NDVI slope. In a multiple regression of the significant slopes, mean maximum temperature and humidity jointly had an r2 of 0.72. Where precipitation was correlated with greenness, in hot, humid cities, the effect of precipitation was limited, whereas in cooler cities, regardless of humidity, the effect of precipitation was greater. These results highlight the importance of both social and climatic factors in determining the Precipitation-NDVI slope in irrigated cities. Urban plants provide multiple ecosystem services, including the reduction of urban heat exposure and consequent morbidity and mortality of urban residents. This research provides a quantitative understanding of how urban plants respond to precipitation at the continental scale, allowing ecologists to better distinguish the roles of precipitation and irrigation in shaping urban greenness, its responses to drought, and other environmental stresses along a climatic gradient.