Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng Inter-annual thermoregulation of extensive green roofs in warm and cool seasons: Plant selection matters Nicholas Sookhan a, ⁎ , Liat Margolis b , J. Scott MacIvor a a Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada b John H. Daniels Faculty of Architecture, Landscape, and Design, University of Toronto, 1 Spadina Crescent, Toronto, Ontario, Canada ARTICLE INFO Keywords: Sedum Multi-year study Urban cooling Green infrastructure Climate variability Thermal benefits ABSTRACT The thermoregulation of buildings and cities by green roofs is a primary driver of their integration into urban environments. In warm seasons, green roofs cool buildings (thereby reduce interior air conditioning costs), and cities (impervious surfaces contribute to urban heat islands and vegetation mitigates contributions by conven- tional roof surfaces). In cool seasons, green roofs insulate buildings by reducing heat flux through the roof surface. Here we investigate thermoregulation services provided by extensive green roofs in warm and cool seasons from temperature data points recorded at 5-minute intervals over a four-year period, and from modules containing either Sedum or perennial grasses and herbaceous flowers, mineral- or organic-based substrate, 10 cm or 15 cm substrate depth, and supplemental irrigation or none. We demonstrate that Sedum outperformed a mixture of perennial grasses and herbaceous flowers over the total inter-annual survey period. The meadow mixture was more dependent on supplemental irrigation than Sedum, but more susceptible to inter-annual cli- mate variability. Our findings point to the durability of Sedum as a plant for extensive green roof cooling, as well as the importance of plant selection and identifying traits that match not just microclimatic conditions in summer, but also in winter. 1. Introduction Green roofs are designed plant communities in specialized substrate atop conventional roof surfaces (Oberndorfer et al., 2007). Green roofs provide many social, economic and environmental benefits, for which they are now encouraged or mandated in cities around the world due to their role in mitigating the environmental impacts of urbanization (Li et al., 2004; Tzoulas et al., 2007; Fujibe, 2009; Benvenuti, 2014). Green roofs contribute to the thermoregulation of buildings and of cities, which is one of the primary drivers of their proliferation in different climates around the world in an effort to mitigate impacts of a changing climate (MacIvor and Lundholm, 2011). In warm seasons, conventional roof surfaces which are opaque and dark absorb heat which is re-emitted to the indoor and outdoor en- vironment. This results in additional temperature regulation costs for indoor environments, and a hotter and more extreme outdoor urban environment (Del Barrio, 1998; Rinner and Hussain, 2011; Wang et al., 2016). Through a combination of shading, solar reflectance, insulation, and evapotranspiration, green roof plants and substrate enhance building and urban thermoregulation. In cool seasons, green roofs contribute to the reduction of heat flux through roof surfaces of buildings, thereby insulating and reducing potential energy costs for heating indoor spaces. These benefits are enhanced when plant biomass is left over winter, which accumulates snow, leading to greater in- sulation values (Lundholm et al., 2014b). Thermoregulation in warm and cool seasons has a demonstrable impact on green roof cooling and heating load, respectively (Jim and Peng, 2012a; Zhao and Srebric, 2012). For example, Liu and Minor (2005) found in an experimental study in Toronto, that green roofs reduced heat flux from the roof substrate to the indoor environment by up to 90% during summer, and indoor heat loss by up to 30% during the winter compared to conventional bare roof surfaces. While at the mesoscale where cityscapes can be comprised of up to 25% rooftop area (Akbari and Levinson, 2008), the wide scale implementation of green roofs could lead to a reduction in the urban heat island effect (Santamouris, 2014). For instance, a recent simulation study conducted by Sharma et al. (2016) estimated that the widespread application of green roofs through the city of Chicago could reduce peak daily urban heat island by 0.84–3.41 °C during a summer heat wave. Rooftop microclimates are extreme growing environments for plants, and are subject to high wind speeds, temperatures and solar irradiance. As a result, when choosing plants, practitioners tend to https://doi.org/10.1016/j.ecoleng.2018.08.016 Received 19 April 2018; Received in revised form 4 August 2018; Accepted 14 August 2018 ⁎ Corresponding author. E-mail address: nicholas.sookhan@mail.utoronto.ca (N. Sookhan). Ecological Engineering 123 (2018) 10–18 0925-8574/ © 2018 Elsevier B.V. All rights reserved. T