Linking Rainfall Variability and Carbon Cycling in a Green Roof Ecosystem

Abstract:

Whereas green roof hydrology is well-studied, these systems present a novel opportunity to examine plant-mediated linkages between rainfall and carbon cycling. For example, green roofs experience dramatic fluctuations in soil moisture because they have limited soil water holding capacity and 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 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. Concurrent measurements of net ecosystem CO₂ exchange (NEE) and ecosystem respiration (R_e) 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 by a reductions in both reduced midday CO₂ uptake associated with stomatal closure and nighttime net CO₂ efflux as CAM-driven assimilation offset respiratory losses. Not surprisingly, increased T and decreased θ negatively influenced GEE while R_e increased in response to increased T and θ. During a period of unusually hot, dry conditions the responses of GEE and R_e were reflected in a decline in daytime NEE. However, this decline in NEE was not associated with a similar reduction in nighttime R_e suggesting that these conditions were insufficient to induce CAM photosynthesis. Future ecohydrological investigations of green roofs may provide new insights into how rainfall variability interacts with plant traits, community diversity, and edaphic factors to shape ecosystem function.