The high life - going green on the roofs of London
21 July 2008
Rare species of bees, spiders and ladybirds have been found high in London's famous skyline. City ecologists, Chloe Molineux and Alan Gange, see the future of London could be a skyline carpeted in green roofs.
If you've ever peered down from a plane at the vast expanse of grey, dull rooftops, or walked along a busy street and glanced up at the high-rises, you might agree that cities severely lack greenery.
Cities are rapidly losing natural habitats and city ecologists are running out of space at ground level. Many new developments are built on derelict or brownfield sites. But, these are often rich in rare plant and animal species, such as the black redstart, a bird that likes to nest in rubble.
Roofs are good habitats for many rare plants and insects.
Instead, ecologists are looking to the skies as a last refuge for rare life. By using wasted space high above us on rooftops, we can recreate lost habitats and help make cities greener at the same time.
The idea of green roofs - rooftops that have plants growing on them - is well established in continental Europe, but relatively new to the UK. Originally, architects designed them to insulate the building below, keeping it cool in summer and warm in winter, reducing energy costs.
But they have other advantages. They reduce the risk of sewers flooding during heavy rain by trapping excess water. The plants on a living roof absorb carbon dioxide and trap pollution. For councils wanting to reduce their carbon footprints, green roofs provide a range of economical and ecological benefits.
Rare plants and insects
Previous work at Royal Holloway University of London by PhD student Gyongyver Ka'das has shown that, unexpectedly, these roofs are good habitats for many rare plants and insects. This makes them ideal substitutes for lost brownfield sites. Rare species include a brown banded bumble bee, a shrill carder bee, rare spiders: Arctosa leopardus, Pirata latitans, Pardosa agrestis, Erigone aletris and a rare ladybird Hippodamia variegate. We are now investigating how we can make these habitats even more attractive to wildlife.
Pellets made from sewage sludge make a good bed for plants
So, how do you create the ideal habitat for rare species at 30 stories? We lug demolition waste, usually crushed brick or concrete, up onto the roofs. The idea behind this is to imitate wasteland sites on the ground. The advantages of using these materials as a base, or substrate, is that they are much cheaper than soil and will not get blown away by wind.
The huge weight pressures on roofs means these substrates must be very shallow, often just 3-5cm deep. This creates a problem: the green-roof plants are unable to survive in dry, hot summers because of the limited water supply, and the lack of nutrients often leaves the plants looking extremely stressed. We've been working with the the organisation Livingroofs.org and our PhD partner the company Carbon8 Technologies to investigate the performance of lighter substrates. These materials may retain more water and nutrients and promote plant growth, making them potentially a better long-term sustainable solution.
The estimated area of roof space that could have a green roof could be 28 times the size of Richmond Park.
Several recycled waste materials such as sewage sludge, paper ash and quarry fines (a waste by-product of limestone extraction) could turn out to be good replacements. A few of these materials are very fine powders so to make them useful we convert them to pellets - the quarry fines end up looking like chocolate-covered raisins. To test their potential, we're about to set up a new green roof in Kentish Town, north London. We'll divide this roof into a number of plots containing different substrates and measure for plant growth and soil microbial activity. Our aim is to hit two birds with one stone: an improved environment for plants; and a new use for waste materials that would otherwise go to landfill.
So far, laboratory trials indicate that the alkalinity of the recycled pellets could be a key factor affecting plant growth. We are thinking of rejecting one aggregate already, as the pH of water percolating through it was over 11! The aggregates differ in their water-holding capacity. The key will be finding one that will hold a lot of water and allow plants easy access to it when times are hard.
Encouraging back wildlife
Green roofs are excellent for increasing biodiversity, providing habitats for many insects and birds
We are also looking at biological ways to improve the existing traditional crushed brick green roofs. Four years ago, we established an experimental green roof on top of the gift shop at London Zoo. We drew on previous research at Royal Holloway, where we added bacteria and fungi to microbially-deficient soils in sports turf. Results are good.
Initial findings at the new site indicate that the crushed brick is almost entirely lacking in microbes. But, when we added microbes, they remained and established a community. Already, we have seen evidence that plants are healthier in microbe-rich areas and more able to withstand drought.
In London, the estimated area of roof space that could have a green roof, with little or no structural modification to the building below could be as much as 28 times the size of Richmond Park. That's about 175 premiership football pitches! Our vision for the future is that cities such as London are carpeted in green roofs, recycling vast quantities of waste material and encouraging back local wildlife.
The next time you are on the approach to Heathrow or Gatwick, grab a window seat and play spot the green roof.
Chloe Molineux is a NERC CASE PhD studentship at Royal Holloway, University of London. Her supervisor, Professor Alan Gange, is interested in the links between the soil microbial community and the plant community above ground.
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