Green roofs as urban habitat: Plant species colonization on green roofs in Michigan

Background/Question/Methods

Urbanization has led to habitat loss, some of which may be replaced by green roofs.  We evaluated colonization rates of plants on green roofs to test two hypotheses: 1) native and invasive plants colonize green roofs at equal rates and 2) colonization rates of green roofs are independent of roof area, roof age, number of plant collections, soil depth, and succulent density.  We studied 9 green roofs in the summer of 2011.  Soil depth and initial density measurements were taken in May, colonizing plants were collected through August, and native and invasive plant colonization rates were calculated for each roof.

Results/Conclusions

We found no difference between native and invasive species colonization rates (t=-0.65, P=0.52) or between native and invasive individual colonization rates (W=88.0, P=0.86).  Preliminary analysis of the factors influencing plant colonization suggest that roof age is the most important factor in predicting plant colonization on green roofs.  Native individual colonization rates, native species colonization rates, and invasive species colonization rates showed a positive linear relationship with roof age (R²=0.70, P<0.005; R²=0.40, P=0.04; R²=0.48, P=0.03 respectively).  Our results suggest that green roofs could act as conservation areas for native plant species to preserve habitat in urbanized areas. However, we suggest initial selective weeding of invasive species to facilitate native species establishment and persistence.