Whereas green roof hydrology is well-studied, these systems present a novel opportunity to examine plant-mediated linkages between hydrologic processes and carbon cycling. For example, green roofs experience dramatic fluctuations in soil moisture because they have limited soil water holding capacity and experience high rates of evaporation. Stonecrop (Sedum spp.) is widely planted in green roofs and its traits reflect an overall strategy of water conservation. In addition to succulent leaves and a slow growth rate, several stonecrop species possess inducible CAM photosynthesis. We made continuous measurements of ecosystem CO2 and H2O exchange, soil temperature (T), and volumetric soil moisture (θ) using a chamber-based automated monitoring system installed on a 3-year old green roof located in Buffalo, New York during the middle portion of the 2015 growing season. Concurrent measurements of net ecosystem CO2 exchange (NEE) and ecosystem respiration (Re) allowed us to estimate gross ecosystem CO2 exchange (GEE). We predicted that CAM photosynthesis by stonecrop would be induced by high T and low θ and would manifest at the ecosystem scale as a reduction in midday CO2 uptake associated with stomatal closure as well as a reduction in nighttime net CO2 efflux as CAM-driven assimilation offset respiratory losses.
Soil moisture varied dramatically in response to periodic rainfall during the growing season. Not surprisingly, increasing temperatures and decreasing soil moisture negatively influenced GEE while Re increased in response to increased T and θ. Ecosystem respiration showed a hysteretic response to diel fluctuations in soil temperature and the magnitude of this hysteresis was strongly influenced by soil moisture availability. Moreover, across a wide range of soil moisture conditions the temperature optima of Re did not vary. During a period of unusually hot, dry conditions the responses of GEE and Re were reflected in a decline in daytime NEE and, consistent with the onset of CAM photosynthesis by Sedum, brief periods of nighttime CO2 assimilation. This unusual pattern of ecosystem metabolism reflects the anthropogenic nature of green roofs which are engineered to mitigate storm water runoff and ameliorate surface energy budgets in urban environments. Future ecohydrological investigations of green roofs may provide new insights into how hydrologic processes interact with plant traits, community diversity, and edaphic factors to shape ecosystem function and the supplementary goods and services which these ecosystems may provide.