Shrubs and their role in modifying green roof microclimate

The main hypothesis of this study is that different canopy types will have distinct effects on rooftop temperatures at substrate level, with plant spatial arrangement strongly influencing microclimate underneath the canopies.

Veronica Love started her PhD project, “Shrubs and their role in modifying green roof microclimate:, in the Department of Landscape in 2015. Supervised by Dr. Ross Cameron, the overall aim of the study is to evaluate the potential of woody shrubs to provide ecosystem services (ES) on a green roof, particularly in respect to temperature abatement and thermal insulation of the building. The project also proposes innovative green roof designs based on findings and tailored to the changing conditions of the year. Despite plentiful evidence showing advantages in using a wider range of plant types to improve building energy performance, there is relatively little research on the implementation of woody shrubs. Lower-growing and short-lived plants, such as Sedum sp., grasses, and perennial herbs, are much more common choices for green roof vegetation, but these are potentially less capable of bringing about a key ES – rooftop temperature mitigation to reduce cooling/heating energy costs – compared to shrubs. In fact, the shrub lifeform has a greater potential to improve building energy management and savings by inherently having higher rates of evapotranspiration, growth, shading capacity, coverage, longevity, and even by acting as a windbreak. Further investigation into the effects of this particular group of plants on rooftop temperatures, with a comparison of different combinations of species, canopy types and plant spatial distribution, is essential.

Preliminary results from an experiment carried out during winter (December 2015–March 2016), in which three evergreen species were arranged in two different spatial dispositions on a rooftop (densely-arranged vs scattered individuals), show that there is a significant relationship between canopy type (i.e., shape, density, branching), spatial distribution of plants, irradiance, and temperature and relative humidity at substrate level. These initial findings suggest that, through opportune selection, shrub canopy type and spacing could represent potentially important factors in increasing green roof thermal performance. During the summer months following this winter experiment, morphological and physiological measurements were carried out on six different species (3 evergreen and 3 deciduous) arranged in the same spacing categories.

summer-hadfield

Shrubs provide greater shading capacity, longevity, transpiration rates, phenological/physiological plasticity, and capacity to create a microclimate. (Rundel 1991, Bond et. al. 2007, Lundholm et.al. 2010)

Also to be investigated are the differences in temperature profiles between ‘monospecific’ and ‘multispecific’ shrub communities (i.e., different combinations of species). Plant water use will be measured to assess the cooling capacity of the range of species. Finally, semi-controlled experiments, in which plants will be exposed to simulated levels of irradiance, wind, air temperature and rain, will be performed to identify specific leaf and canopy traits that contribute to green roof microclimate.

Veronica was selected to attend the prestigious Valuing Nature Business Impact School, to present her research to a panel of business representatives. Read more…..

References and further reading:

Bond, B.J., Meinzer, F.C., Brooks, J.R., 2007. How trees influence the hydrological cycle in forest systems, 7-36. In: Wood, P.J., Hannah, D.M., Sadler, J.P. (Eds.) Hydroecology and Ecohydrology: Past, Present and Future. John Wiley and Sons, Ltd., Chichester, UK.

Lundholm, J.T., MacIvor, J.S., MacDougall, Z., Ranalli, M., 2010. Plant species and functional group combinations affect green roof ecosystem functions. PLoS One 5 (3), 1-11.

Rundel, P.W., 1991. Shrub life-forms, 345-370. In: Mooney, H.A., Winner, W.E., Pell, E.J., Chu, E. (Eds.), Response of Plants to Multiple Stresses. Academic Press, Inc., NY.

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