Estimates of energy partitioning, evapotranspiration, and net ecosystem exchange of CO2 for an urban lawn and a tallgrass prairie in the Denver metropolitan area under contrasting conditions

Lawns as a landcover change substantially alter evapotranspiration, CO₂, and energy exchanges and are of rising importance considering their spatial extent. We contrast eddy covariance (EC) flux measurements collected in the Denver, Colorado, USA metropolitan area in 2011 and 2012 over a lawn and a xeric tallgrass prairie. Close linkages between seasonal vegetation development, energy fluxes, and net ecosystem exchange (NEE) of CO₂ were found. Irrigation of the lawn modified energy and CO₂ fluxes and greatly contributed to differences observed between sites. Due to greater water inputs (precipitation + irrigation) at the lawn in this semi-arid climate, energy partitioning at the lawn was dominated by latent heat (LE) flux. As a result, evapotranspiration (ET) of the lawn was more than double that of tallgrass prairie (2011: 639(±17) mm vs. 302(±9) mm; 2012: 584(±15) mm vs. 265(±7) mm). NEE for the lawn was characterized by a longer growing season, higher daily net uptake of CO₂, and growing season NEE that was also more than twice that of the prairie (2011: −173(±23) g C m⁻² vs. -81(±10) g C m⁻²; 2012: −73(±22) g C m⁻² vs. -21(±8) g C m⁻²). During the drought year (2012), temperature and water stress greatly influenced the direction and magnitude of CO₂ flux at both sites. The results suggest that lawns in Denver can function as carbon sinks and conditionally contribute to the mitigation of carbon emissions - directly by CO₂ uptake and indirectly through effects of evaporative cooling on microclimate and energy use.