Swedish University of Agricultural Sciences, Sweden
Current understanding of green (vegetated) roof ecosystems comes mainly from short-term studies from newly-constructed roofs, giving little insight into likely changes in over the expected 50+ year life span. To address this knowledge gap, we examined long-term variation in the plant community, substrate (engineered soil) characteristics, and several different indicators of ecosystem service provision, from a suite of green roofs in two northern cities: Malmö, Sweden, and Helsinki, Finland. Using a large number of similarly-constructed but different-aged green roofs (up to 25 years old), and repeated surveys over multiple years, we addressed the questions (i) How do the soil characteristics, plant communities, and a suite of associated ecosystem services change over time in the widely used thin-substrate sedum-moss green roofs (SMR)? and (ii) How do these temporal changes vary between the SMR vs. more plant-diverse, thicker-substrate “meadow” roofs? We found that green roof substrate characteristics changed predictably, with one notable change being a several-fold increase in N stocks, accompanied by a decrease in substrate 15N indicating a shift in nutrient source. In terms of vegetation community, SMR commonly showed a decrease over time in the coverage of the originally dominant (planted) succulent Sedums, accompanied by an increase in moss and lichen coverage, with only sparse volunteer forbs or grasses. Meadow roofs supported a substantially richer plant community than SMR. We did not find evidence of a change in vascular plant species richness over time for either roof type, in spite of shifting plant species assemblages over time. Environmental characteristics of the roof setting, in particular the degree of shading, also influenced the plant community development. With regards to ecosystem services, the shift to dominance by mosses as SMR aged, was accompanied by an increase in carbon and nitrogen storage, and an increase in substrate water-holding capacity; all of which could indicate enhanced provision of regulating ecosystem services. At the same time, it is known that a dominance by mosses can lead green roofs to be more vulnerable to erosion. An analysis of the changes in the public perception of aesthetics and stress-reducing capacity of the roofs is underway, and will lend a cultural dimension to the analysis. Overall, the changes observed suggest intriguing potential synergies and tradeoffs among different ecosystem services as green roofs age.