Green Roofs - Living Architecture Monitor - Fall 2019 - 6

PLANT PROFILE

ENHANCING BIODIVERSITY ON GREEN ROOFS
THROUGH PLANT SELECTION
DR. BRADLEY ROWE, MICHIGAN STATE UNIVERSITY, EAST LANSING

Most would agree that biodiversity is important for a sustainable
world. On a much smaller scale it also applies to green roof and walls.
Biodiversity can be enhanced by providing environmental conditions
that will support a diversity of plant species and then in turn these
plant communities may provide habitat for wildlife.

Images courtesy B. Rowe

O

LIVING ARCHITECTURE MONITOR / FALL 2019 / 6

ne of the main factors that influences plant diversity is substrate
moisture which is directly related to
substrate depth. The importance of
depth was evident in a plant community
study conducted on the MSU Molecular
Plant Sciences Building (Vandegrift et al
(2019), Ecological Engineering 138:264273, https://dx.doi.org/10.1016/j.
ecoleng.2019.07.032). Seventeen species of
native perennials and grasses were planted at
two depths, 10 cm and 20 cm, irrigated for
two years until established, and evaluated
over a period of eight years. Only seven of
the original species survived the entire time
and Allium cernuum was by far the dominant
species at both depths. Sporobolus heterolepis
and Koeleria macrantha were also present
in the 20 cm zone, but died off in the 10
cm zone. Substrate moisture was likely
the main cause as many species did not
survive a moderate drought that occurred
during 2016. The study emphasizes that
many herbaceous perennials may require
supplemental irrigation during drought
periods and it points out the importance
of long-term studies to accurately evaluate
green roof plant communities.
Although the lack of substrate moisture
contributed to plant death, a decrease in
plant diversity is not uncommon. A similar
decrease in diversity was observed on an
irrigated roof over six years in Sheffield, UK
(Dunnett et al (2008) Urban Ecosystems
11:373-384, https://doi.org/10.1007/s11252-

007-0042-7). Numerous other studies have
found similar decreases in diversity over
time with the main cause usually attributed
to drought. In contrast, a study conducted
on an intensive roof in Germany found that
diversity increased over time before reaching
an equilibrium (Catalano et al (2016) Landscape and Urban Planning 149:11-19, https://
doi.org/10.1016/j.landurbplan.2016.01.003).
The main differences between this roof
and the others were the deeper substrate
(26 cm) and the fact that ruderal colonizers
were not weeded. Colonizing species could
become established, whereas on the shallower
extensive roofs they were either removed or
died because of the shallower depth. If species
diversity is desired then removal of colonizing species should be avoided. In addition,
designing a roof with various depths will
increase plant diversity as different species
find their niche where they can compete
the best.
The plant community present will also
influence a roof's ability to provide wildlife
habitat for microorganisms, insects, birds,
and other animals. For example, edge sensitive avian species such as Branta canadensis
(Canada goose) and Charadrius vociferous
(killdeer) require vast open areas with short
vegetation characteristic of a sedum roof,
whereas other species prefer more vertical
structure often found on intensive roofs
where they can perch in trees or shrubs. In
a study we conducted involving 12 green
roofs in Michigan and Illinois, we identified
LIVINGARCHITECTUREMONITOR.COM
http://www.LIVINGARCHITECTUREMONITOR.COM
Green Roofs - Living Architecture Monitor - Fall 2019

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Fall 2019
