Green Roofs - Living Architecture Monitor Winter 2020 - 9

in a more vigorous and healthy plant ecosystem. For example,
a groundcover plant species with a shallow root system such as
Sedum album will complement another plant with different functional traits such as Liatris aspera (Rough blazing star) that has an
upright form and a deeper root system. Each species is exploiting
different above ground space and soil depths so there is limited
competition. Sedum album actually benefits the Liatris by shading
the soil to help preserve soil moisture. Likewise, the much taller
Liatris can provide filtered shade for the sedum. A wide diversity
in functional traits allows the plant community as a whole to maximize its utilization of available resources, whether that resource is
water availability, nutrients in the soil, or sunlight. Plant diversity
will also decrease the chances of disease and insect attacks destroying an entire roof that was a monoculture.
As we already know, there is no doubt that green roofs retain
stormwater and save energy, but does biodiversity improve these
benefits and increase profits? In most cases, specific benefits can be
maximized through plant selection which should translate to economic benefits. Numerous studies have shown that vegetation type
influences stormwater retention. Plant photosynthetic metabolism
(C3, C4, and CAM) influences transpiration rates and plant types

In theory, plant species with
greater biomass and higher
transpiration rates should
provide a greater cooling effect
and this has been shown to be
true in most cases. However,
some other studies have shown
that during the summer, deeper
substrates absorb more heat
during the day and then continue to radiate that heat into
the building during the night.
Regardless, plant choices can
influence energy consumption
and the urban heat island effect.
The green roof plant community will also influence a
roof's ability to provide wildlife
habitat for microorganisms,
insects, birds, and other animals.
Plants that flower at different

Green roof on the MSU Molecular Sciences Building showing a diversity in flowering times. The Liatris aspera (Rough
blazingstar) is in full bloom, whereas the Echinacea purpurea
(purple coneflower) is nearing the end of flowering and the
Symphyotrichum laeve (smooth aster) and Symphyotrichum
oolentangiense (azure aster) have yet to bloom.

with greater biomass such as forbs and grasses that possess greater
shoot and root biomass are generally more effective in reducing
water runoff. Rainwater is intercepted by foliage and may evaporate
before reaching the substrate surface and plants with greater
biomass will require deeper substrates that provide additional water
storage space. Providing conditions that allow a greater number of
plant species to be grown will increase biodiversity.
Similarly, intuition would also tell us that deeper green roofs
planted with herbaceous perennials and grasses would be better
insulators and thus save more energy than shallow sedum roofs.
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characteristics of a sedum roof,
whereas other species prefer a
more vertical structure often
found on intensive roofs where
they can perch in trees or
shrubs. In fact, as I glance out
the window while writing this
article, I have been distracted
by two cardinals (Cardinalis
cardinalis), a bluejay (Cyanocitta
cristata), and another bird
species I can't identify that are
visiting the green roof over
my garage and screened porch.
Proof that green roofs do attract
wildlife.
Installing green roofs
provides an opportunity to add
biodiversity to our communities,
especially in urban areas. Varying substrate depths will provide

Photo courtesy B. Rowe

times of the year can provide
food over an extended time
period. Diversity in plant
vertical structure is also a very
important factor. For example,
edge sensitive avian species
such as Canada goose (Branta
canadensis) and killdeer (Charadrius vociferous) require vast
open areas with short vegetation

favorable microenvironments
for a greater number of plant
species with variations in season
of flowering, plant height, and
spread. Different species find
their niche where they can compete the best. This structural
diversity in plant species will
then provide the microhabitats
for various flora and fauna to

LIVING ARCHITECTURE MONITOR / WINTER 2020 / 9

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Green Roofs - Living Architecture Monitor Winter 2020

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