Diverse biosphere influence on carbon and heat in mixed urban Mediterranean landscape revealed by high resolution thermal and optical remote sensing

A fundamental challenge in verifying urban CO2 emissions reductions is estimating the biological influence that can confound emission source attribution across heterogeneous and diverse landscapes. Recent work using atmospheric radiocarbon revealed a substantial seasonal influence of the managed urban biosphere on regional carbon budgets in the Los Angeles megacity, but lacked spatially explicit attribution of the diverse biological influences needed for flux quantification and decision making. New optical and thermal sensors from airborne and satellite platforms simultaneously resolve fine-scale urban land cover, land use (irrigation), phenology and water stress effects required to accurately quantify biological influences on CO2 exchange in complex urban environments. We use remote sensing constraints with an urban land surface model to quantify spatial and seasonal gross primary production (GPP) variability across the Southern California Air Basin (SoCAB) at 30 m resolution. High spatial resolution optical and thermal remote sensing imagery provides attribution of landscape influences related to vegetation type, fragmentation and irrigation across urban and non-urban gradients. Results showed that montane non-urban shrubs and trees dominate the regional carbon budget of SoCAB (80% of GPP). The urban carbon budget is strongly influenced by land use and landscape fragmentation. Irrigated vegetation accounts for only 21% of urban vegetation but 31% of annual GPP, driven by turf grass, and is twice as productive as non-irrigated vegetation during hot and dry Mediterranean summer months. Fragmented vegetation accounts for 25% of urban vegetation but 50% of annual GPP, with cooler interior vegetation enhancing tree and grass GPP in spring and summer, and irrigation mitigating stress-driven GPP loss in edge vegetation. Our satellite driven, very high resolution modeling framework highlights the important influence of land use and fragmentation on urban carbon budgets, and provides a path forward for quantifying urban biospheric influences globally.