Can we make green roofs more biodiverse?

Desdemona Despair

By Leslie Nemo
13 April 2018

(CityLab) – Every time Kelly Ksiazek-Mikenas scrambled onto a new green roof, it was hard to tell exactly where she was. The city below was definitely Berlin or Neubrandenburg, but the expanse of scraggly greens ahead of her looked a lot like the green roofs in Chicago, her home.

The only difference was that the German green roofs were much older than anything found in the United States: three to nine times older. Which is why the Northwestern University Ph.D. student in plant biology spent her summer there a few years ago.

The ability of plants to absorb and evaporate storm water, reduce a building’s energy use, and clean up some air pollution makes green roofs effective as a sustainable-building technique. They also just look nice. Germany began tinkering with green-roof technology back in the late 1800s, when owners of some buildings tried fireproofing with gravel, sand, and sod.

In 1975, German construction businesses got together to document the nitty-gritty construction standards. Their 2002 manual detailed everything from the ideal roof slope to the best soil depth and waterproof barriers. By the time Americans started experimenting with green roofs, their German counterparts were already professionals.

America’s green roofs were modeled after Germany’s. In both countries, the standard design is a thin layer of lightweight, low-moisture, and low-nutrient dirt blanketed by sedum, a hardy genus of succulent. Landscapers can easily install a roof of this type and check in on it once or twice a year.

“We ended up just copying what the Germans did,” said Ksiazek-Mikenas. By “we,” she meant the organization that defines American construction standards, which used the German protocol as a template in the early 2000s, as the green-building movement was taking off in the U.S.

The German model was dependable and low-maintenance. Why start from scratch, Americans figured, when someone else had done the stressful experimentation and developed the final product? Besides, even in cities that offered substantial financial incentives for green roofs, you got nothing extra for keeping them lush. Developers could follow the German method, stick hardy plants in a roof, and walk away, rewarded for their environmentally friendly choice.

Ksiazek-Mikenas wanted to know if green roofs ever come to host a wide range of species. American roofs were too young for her to tell. “As an ecologist, I realized a decade is such a tiny period of time as far as a succession of a plant community goes,” she said.

So she examined the diversity of 16 German installations that were between one and 93 years old. She collaborated with Manfred Köhler, a long-time researcher in the German green roof scene, who hadmonitored about a third of the plots at least once a year for between 12 and 27 years. The pair also closely studied 13 other roofs of different ages for one season. That way, they could measure how individual green roofs evolve, and approximate how one might look after nearly a century.

The results, published earlier this year in Urban Naturalist, make a case for breaking with tradition and investing more resources in green roofs. [more]

Can We Make Green Roofs More Biodiverse?

ABSTRACT:  Cities can support biodiversity and provide the ecosystem services upon which life depends. Green roofs are increasingly common in cities and could be designed to increase biodiversity, but community assembly and succession patterns on green roofs are poorly documented. We used long-term vegetation surveys at 6 extensive green roofs and sampled a 1–93-year chronosequence at 13 extensive green roofs in northeast Germany to determine if plant and arthropod diversity increased over time in a deterministic pattern. We also explored abiotic factors that may contribute to community diversity on green roofs. We found that vegetation cover increased over time, but beyond the first 2 years, vegetation richness and diversity did not. There is no evidence for broadly applicable patterns of succession of plant communities on green roofs. Although the abundance, richness, and diversity of arthropods increased slightly over time, this trend was not statistically significant for ants, bees, beetles, or spiders. The size of the vegetated area of the roof, the conditions of the growing substrate, species richness and diversity of the vegetation, and the proportion of ground-level green space surrounding the roof at 0.5-km and 1.0-km radii were associated with increased arthropod abundance, richness, and diversity. We conclude that community diversity on green roofs is highly variable and dependent on several biotic and abiotic factors that are not consistent among extensive green roofs. Community successional patterns are not conserved; thus, each green roof may support a novel community and contribute to urban biodiversity.

If You Build It, Will They Come? Plant and Arthropod Diversity on Urban Green Roofs Over Time