And the fungal communities are quite different from what's found in the city's parks.
Demand for green roofs might plummet if they became known as "fungal roofs." But that is what they are, at least in New York – and contrary to what it may sound like, it's not a bad thing.
The world just became a little more aware of the hidden-but-teeming biomass of green roofs thanks to the intrepid work of researchers from Barnard College, Columbia University, Fordham and the University of Colorado. Recently, these guys found themselves wondering if the gardens in the sky might support different kinds of life than the stuff at dog-pee level. It's a realm into which few scientific minds have tread. While green roofs as heat-island dampeners and rainwater-runoff plugs have been widely discussed, the extent to which they serve as urban "biodiversity reservoirs" (in the researchers' words) is something of a mystery.
So in the summer of 2011, the team set out to test the soil composition of 10 green roofs stationed at recreation centers throughout the five boroughs:
Using soil corers, they hunted for fungi, because fungal communities play a key role in a roof garden's health and longevity. For comparison's sake, they also took samples from five city parks near some of the roofs, including Central Park and the High Line. A little magic from "[i]nductively coupled plasma atomic emission spectroscopy" at Alabama's Auburn University Soil Testing Laboratory, as well as a dollop of phospholipid fatty-acid extraction and Illumina-dye sequencing, and they had their results, which were published this month in the journal PLOS ONE.
So what were the conclusions? For one, these sun-kissed carpets of gray goldenrod and smooth blue aster are absolutely crawling with fungi. The researchers logged an average of 109 types of fungi per roof, such as Glomus, Acaulospora, Rhizophagus and Funneliformis, suggesting that green roofs can indeed contribute to urban biodiversity. As they explained:
[W]e found that green roofs supported a diverse fungal community, with numerous taxa belonging to fungal groups capable of surviving in disturbed and polluted habitats. Across roofs, there was significant biogeographical clustering of fungal communities, indicating that community assembly of roof microbes across the greater New York City area is locally variable. Green roof fungal communities were compositionally distinct from city parks and only 54% of the green roof taxa were also found in the park soils.
In other words, the roofs are home to fungi not typically given to squelching around in normal parkland. They also seem to be better for growing stuff you might, you know, put in your mouth: While the soil in New York's parks showed a greater biomass of microbes, it also tested higher for heavy metals, a scourge of urban gardens that can be unhealthy if consumed in larger quantities.
Here's a comparison the researchers put together illustrating how the roofs stacked up against the parks, in terms of the abundance of fungal phyla:
Needless to say, this is hardly the first news of green roofs supporting life. The elevated gardens are routinely patrolled by insects and in some cases much larger fauna. In Australia, for instance, the Adelaide Zoo maintains several grassy roofs that are designed as homes for urban plants and wildlife, like reptiles, insects and bats.
And an immense green roof in the U.K., mounted on a wastewater treatment facility near Brighton, attracts seagulls and crows that pluck at its quaking grass in search of food. To fight those hungry birds, the roof's overseers have released even more animals over the roof – ferocious goshawks, a golden eagle and even a great horned owl.
Top microscopic photo of an arbuscular mycorrhizal fungus courtesy of the USDA's Agricultural Research Service.