Plant species diversity improves ecosystem multifunctionality on a green roof

In this post, Jeremy Lundholm writes about his recent paper “Green roof plant species diversity improves ecosystem multifunctionality”.

We can improve the benefits provided by green roofs by planting more species.

Green roofs offer an attractive and useful solution to wasted space on building rooftops. Green roof coverage is growing in major cities around the world and more ecologists are paying attention to this habitat. Because they are intimately connected with the building they sit on, people who construct green roofs tend to see them first as engineered structures. Considerations like drainage and weight loading are the primary criteria in designing green roofs in order to keep the building interior dry. Researchers have also documented many different benefits of installing a green roof on a building. These “ecosystem services” are especially important in cities where these services have been compromised by habitat destruction at ground level. A green roof makes energy savings for the building by reducing the amount of energy passing through the roof, leading to lower heat gain in summer and less heat escaping in winter. Green roof plants grow in artificial soils and these help to retain water on the roof, reducing runoff into sewer systems. Green roofs also provide aesthetic relief, pollution control by trapping particulates, and several other benefits. Ecologists, horticulturalists and landscape architects are busy documenting the range of benefits provided by this unique kind of “green infrastructure”, and find that plant choice can make a big difference in the overall level of ecosystem services a green roof provides.

Figure 1b — Figure 1. Species-rich green roofs in Iceland (top); Australia (bottom). (Photos: J. Lundholm).

I study communities of plants, and I wanted to see whether combining different plant species on a green roof could lead to greater benefits. Since green roofs provide so many different ecosystem services, it seemed unlikely that any one plant species would be good at maximizing any one of those benefits, so I wanted to explore how multiple ecosystem services could be improved simultaneously by planting a diverse mixture of plants. This work builds on several experiments initiated by my MSc students at Saint Mary’s University in Halifax, eastern Canada (Lundholm et al. 2010, MacIvor & Lundholm 2011). In this study, I analyzed data collected over four years of a rooftop experiment. I compared monocultures of 13 different species with mixtures of three to 13 species. The perennial plant species used were selected from different growth forms: succulents (drought-tolerant species with fleshly leaves and stems favoured by the green roof industry), grasses, tall forbs (wildflowers) and creeping forbs (herbaceous ground covers) and creeping shrubs.

Figure 2. Plant diversity experiment at Saint Mary's University, Halifax, Canada. (Photo: J.S. MacIvor). — Figure 2. Plant diversity experiment at Saint Mary’s University, Halifax, Canada. (Photo: J.S. MacIvor).

When more species were planted, more water runoff was captured by the green roof, but the other services were not maximized by high plant diversity. When multiple benefits were considered at the same time, diversity showed a stronger effect, increasing multifunctionality. This study suggests that plant diversity can affect the services provided by an ecosystem even when that ecosystem is highly artificial and part of the built environment. New frontiers for research include determining whether green roofs rich in plant species are better habitats for other species, compared to species-poor vegetation (Williams et al. 2014) and how soil organisms respond to this novel environment (McGuire et al. 2013).

Literature cited:

Lundholm, J., MacIvor J.S., MacDougall, Z., & Ranalli, M. (2010) Plant species and functional group combinations affect green roof ecosystem functions. PLoS ONE, 5, e9677. doi:10.1371/journal.pone.0009677

MacIvor, J. S., & Lundholm, J. (2011) Performance evaluation of native plants suited to extensive green roof conditions in a maritime climate. Ecological Engineering, 37, 407-417.

McGuire, K.L, Payne, S.G., Palmer, M.I., Gillikin, C.M., Keefe, D., Kim, S.J., Gedallovich, S.M., Discenza, J., Rangamannar, R., Koshner, J.A., Massmann, A.L, Orazi, G., Essene, A., Leff, J.W., & Fierer, N. (2013) Digging the New York City skyline: soil fungal communities in green roofs and city parks. PLoS ONE, 8(3): e58020 doi:10.1371/journal.pone.0058020

Williams, N.S.G, Lundholm, J.T. & MacIvor, J.S. (2014) Can green roofs improve biodiversity conservation in cities? Journal of Applied Ecology 51, 1643-1649.