The effects of soil depth, competition, and facilitation on plant growth on a green roof

Background/Question/Methods

Green roof architecture is becoming more prominent in North America. Green roofs are associated with a number of benefits including: the reduction of storm water runoff, decreased building temperatures, decreased air pollution and increased urban green space. Due to the weight restrictions of many buildings, shallow-substrate "extensive" green roofs (substrate < 20cm) are often the first choice for many consumers. However, this type of green roof can only support a limited range of species, reducing the potential benefits derived from the green roof. This study analyzed various approaches to improve plant survival and increase species diversity on an extensive green roof. Three different techniques were used: heterogenous soil depth, interspecies facilitation and the use of moss to enhance vascular plant survival.

Results/Conclusions

This study found that multiple soil depths could create niches allowing species with different growth forms and water requirements to co-exist. Three potential facilitators of a vascular target plant were tested.  Of the three, the moss had a net positive effect on growth of the target plant (suggesting facilitation), the lichen had no net effect, and the bunch-grass had a net negative effect (suggesting interspecific competition). Interestingly, even though the moss assisted the growth of neighboring species in one experiment this was not evident in the second experiment. This indicates that more research is necessary and that moss may only be able to facilitate some vascular plant species.