Abstract: Green roofs have several measurable benefits in urban areas. Green roofs reduce storm runoff, provide habitat for wildlife, improve energy use in buildings, mitigate urban heat island effects, and provide aesthetic value. Green roofs also indirectly help reduce CO2 emissions via decreased energy consumption. Whether or not extensive green roof ecosystems are net sources or sinks of CO2 and how they compare to similar and adjacent natural ecosystems is not fully resolved. We begin to address this knowledge gap by assessing how annual variation in temperature and precipitation affect above and below ground biomass accumulation, soil organic matter, and soil CO2 efflux in a newly established extensive green roof and an adjacent prairie site.
The prairie and green roof had similar precipitation, but volumetric water content was significantly lower on the green roof due to the porous nature of the green roof substrate. Mean midday soil temperature on the green roof was 20 +/- 0.5 C which was significantly higher than the prairie site at 17.31 +/- 0.4 (p < 0.001). Above and below ground biomass were significantly higher at the prairies site and, as expected, soil respiration was significantly greater for the prairie site when compared to the green roof (F = 686.747, df = 1, 7.98, p < 0.001). Soil flux at the prairie site ranged from 4.03 +/- 0.18 μmol CO2 m-2 s-1 in June to a low of 0.27 +/- 0.04 μmol CO2 m-2 s-1 in December. In contrast, the highest measured CO2 efflux on the green roof was 1.46 +/- 0.12 μmol CO2 m-2 s-1 on one day in July, and the lowest was 0.05 +/- 0.01 in December. The response of respiration to temperature was fitted with an exponential model and we estimated the respiration quotient (Q10) for the prairie and the green roof.
Overall these results indicate that the green roof is more sensitive to the variation in temperature and precipitation. Both the depth (< 15cm) and the composition of the green roof substrate create a soil environment that leads to greater variance in soil moisture and temperature than the prairie. The green roof substrate lacks buffering against temperature and moisture fluctuations, which suggests that green roof ecosystems will be more sensitive to temperature extremes under climate change than native ecosystems.
