Global urbanization effects on vegetation activity and temperature

VERÓN, S.; PROPATO, T. ; M.F. GARBULSKY; CASADEI, P.; SEMMARTIN, M.

New Orleans, EE.UU.

Congreso; 103rd Annual Meeting of the Ecological Society of America; 2018

Ecological Society of America

Urbanization, one of the most accelerated land use changes worldwide, drastically affects ecosystems. Although cities account for a minor fraction of total land surface, they sustain half of total human population and consume most of the energy and food provided by not urban ecosystems. In turn, the rate of consumption is relatively independent from local resource availability and seasonality. Therefore, cities are considered an environmental homogenizing force. Our understanding of urban impact at large spatial and temporal scales mediated by climate, ecological, and socio-economic drivers is still limited.We studied the impact of urbanization on the average and variability of vegetation activity and temperature worldwide. For 143 large cities worldwide (>750,000 inhabitants), we studied urban/not urban pairs. Urban and not urban sampled areas (maximum of 1 km2) were located at 10-30 km from each other, with an altitude difference of less than 50 m. Not urban members were natural vegetation when possible or agricultural lands where natural vegetation was completely modified. We used moderate resolution satellite data (2001-2016) to asses both vegetation activity (normalized difference vegetation index, NDVI) and temperature at soil surface. Environmental context was derived from mean annual precipitation and temperature (40-year series of meteorological station data, Worldclim). Results/Conclusions On average, urbanization reduced NDVI by 40% (p≤ 0.001) and increased day temperature by 4°C (p≤ 0.01). However, urbanization strongly interacted with environmental context. NDVI reduction was more important in most humid areas, whereas in extremely arid areas urban NDVI was higher than that of their natural/agricultural counterparts. Temperature increments of urban areas were more pronounced in temperate and cold areas. On the contrary, urbanization reduced mean temperature in extremely warm areas. Regarding the spatial and temporal variability, urbanization reduced the spatial, inter-annual and seasonal variability of NDVI by 50%, 30% and 40% respectively. Urbanization had no effect on temperature spatial and seasonal variability, but decreased its inter-annual variability by 40%.This study shows that urban impact worldwide is strong and context-dependent. The spatial and temporal homogenizing impact of urbanization is more consistent on vegetation activity than on temperature. Therefore, global and regional models of urban impact must consider the environmental context in which cities are imbedded.