The Problem with Manhattan's Green Roofs

Green Roofs.

They're gorgeous, and they can actively help to combat global warming.  They're a good thing.

In 2007, NYC's Mayor Bloomberg launched PlaNYC.  The goal was to green up the city and make it more environmentally sustainable.  It has been extraordinarily successful:

"In just four years we’ve built hundreds of acres of new parkland while improving our existing parks. We’ve created or preserved more than 64,000 units of housing. We’ve built whole new neighborhoods with access to transit. We’ve provided New Yorkers with more transportation options. We’ve enacted the most ambitious laws of any city in the country to make existing buildings more energy-efficient.  And we’ve reduced our greenhouse gas emissions 13% below 2005 levels. Over 97% of the 127 initiatives in PlaNYC were launched within one-year of its release and almost two-thirds of its 2009 milestones were achieved or mostly achieved." (source)

There is one slight problem, however...

You see, the green roofs that have been set up in Manhattan aren't exactly doing the job that was expected of them.

So what's the problem? 

Well, it turns out that the majority of plants that dot the roofs are of the sedum variety.  Sedum is a stonecrop plant.  A succulent.  This should be good, of course, because they're low-water plants.

Unfortunately, sedum comes with a downside, as explained in Scientific American magazine:

"Sedum does not absorb water as efficiently as other plant species, according to Scott MacIvor, a PhD student in biology at York University in Toronto who studies bee and wasp habitats on green roofs there, and co-wrote the city’s new guidelines for biodiverse green roofs. At certain times of the year, he says sedum actually absorbs heat instead of reflecting it. “The problem is that sedum plants aren’t really performing on green roofs,” he notes. “They’re just there.” One of the plant’s biggest failings, it turns out, is that it does not encourage biodiversity of plant species on the roof. According to MacIvor’s research, green roofs provide the most benefit when they are planted with a diverse group of species that are adapted to local conditions."

Oops.

That's ok, though...  it can be fixed.  You know how I know that?  Because...

Krista McGuire is here to save the day!

Well, ok.  Maybe she doesn't wear a cape.  That's ok, though... I'm still happy to place her in my personal Superhero Hall of Fame.  See, she's an assistant professor of biology at Barnard University, and she knows oodles about fungal communities.

Why am I so excited about things like mushrooms?

Well, because fungus says a lot about an environment.  You can get a general idea about an ecological system simply by studying its fungal communities.  Naturally, she got her hands dirty and examined the fungus growth on the green roofs of New York.

And what did she find?

"Her study, published in PLoS ONE last April, found that green roofs have distinct fungal communities that help plants to thrive in harsh, polluted environments and filter heavy metals. On average, 109 different types of fungi were present on each roof including Pseudallescheria fimeti, a fungus that grows in polluted soils and human-dominated environments. Rooftop soil also contained fungi from the genusPeyronellaea, which live in the tissues of plants to help them take in nutrients." (Scientific American)

So what does that mean? 

In a nutshell, it means that we can use the information she gained to plant appropriate native species on the roof tops.  Appropriate species, after all, will do a much better job of combating pollution and urban heat. The specific types of fungal communities can be used as a guide to inform us which plants are best suited to each roof.

Awesome!!!