Green roofs alter the surface energy balance and can help in mitigating urban heat islands. However, the cooling
of green roofs due to evapotranspiration strongly depends on the climatic conditions, and vegetation type and
density. In the Southern Central Plains of the United States, extreme weather events, such as high winds, heat
waves and drought conditions pose challenges for successful implementation of green roofs, and likely alter
their standard performance. The NationalWeather Center ExperimentalGreen Roof, an interdisciplinary research
site established in 2010 in Norman, OK, aimed to investigate the ecological performance and surface energy balance
of green roof systems.
Starting in May 2010, 26 months of vegetation studies were conducted and the radiation balance, air temperature,
relative humidity, and buoyancy fluxes were monitored at two meteorological stations during April–October 2011.
The establishment of a vegetative community trended towards prairie plant dominance. High mortality of succulents
and lowgermination of grasses and herbaceous plants contributed to lowvegetative coverage. In this condition
succulent diversity declined. Bouteloua gracilis and Delosperma cooperi showed typological dominance in harsh climatic
conditions, while Sedumspecies experienced high mortality. The plant community diversified through volunteers
such as Euphorbia maculate and Portulaca maculate. Net radiation measured at a green-roof meteorological
station was higher than at a control station over the original, light-colored roofing material. These findings indicate
that the albedo of the green roofwas lower than the albedo of the original roofing material. The lowvegetative coverage
during the heat and drought conditions in 2011, which resulted in the dark substrate used in the green roof
containers being exposed, likely contributed to the low albedo values. Nevertheless, air temperatures and buoyancy
fluxes were often lower over the green roof indicating that higher evapotranspiration rates compensated for the
higher net radiation at the green roof.