Fine-scale heterogeneity effects on green roof biodiversity
There is increasing interest in planning green roofs that will not only supply abiotic functions (insulation, runoff water management, etc.) but also enhance their ecological function such as providing increased biodiversity. Standard green roofs, which are typically extensive green roofs (light weight and low maintenance), have often been planned with a limited species composition (usually Sedum species) due to harsh rooftop conditions. In two experiments using local plant species in non-irrigated green roof experimental modules, we examined ways in which plant species diversity on green roofs may be enhanced. In one experiment, we crossed the presence of annuals with the presence and dispersion pattern (uniform or clumped) of Sedum sediforme. In a second factorial experiment, we tested the effects of substrate heterogeneity on an annual plant community. The substrate, comprised of 70% perlite, 10% tuff, 10% peat and 10% compost, was either spread homogeneously across plots or heterogeneously where organic (peat and compost) and/or mineral (perlite and tuff) components were concentrated in specific parts of the plot (i.e. subplots).
In the first year of the Sedum-annual interaction experiment, growth of Sedum was slowed and Sedum did not flower in the presence of annuals. In the second year of this experiment, Sedum presence had a negative effect on number of flowers and seed production per plot for several annual species. In the substrate-heterogeneity experiment, diversity of seed production at the end of the first growing season was higher under conditions of mineral heterogeneity (with or without the organic heterogeneity) while the seed production diversity in the organic heterogeneity plots was lower than in the homogeneous plots. Foliage height diversity was higher in tuff subplots than in perlite subplots, and lower in low-organic content subplots than in high organic subplots. These results provide initial data towards enhancing the diversity of green roofs via combining annuals and Sedum as well as designing heterogeneous substrates.