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Abstract and Figures

Most plant studies of green roof taxa have only been conducted for a duration of 1 or 2 years. The problem with this scenario is that it can result in premature conclusions and misleading recommendations because green roofs are dynamic systems. Plants that initially survive may eventually experience reduced coverage or disappear completely due to competition, variability in climate, and other factors. Setting up long-term studies similar to the National Science Foundation (NSF) Long Term Ecological Research (LTER) model would provide the opportunity to follow changes to green roof habitats over time and also examine impacts and ecosystem service outputs on similarly designed roofs across geographic locations. Without consciously considering the effects and changes over time mistakes are not only made, but also repeated. We review several important longitudinal studies and discuss factors that impact long-term plant communities such as substrate composition and fertility, substrate depth, substrate moisture, microclimates, roof slope, orientation, and irradiance levels; as well as initial plant choices, functional diversity and complexity, and maintenance practices. In addition, we discuss the potential of applying the LTER model to green roofs and close with future research needs and questions.
Content may be subject to copyright.
Richard K. Sutton
Editor
Green Roof Ecosystems
1 3
ISSN 0070-8356 ISSN 2196-971X (electronic)
Ecological Studies
ISBN 978-3-319-14982-0 ISBN 978-3-319-14983-7 (eBook)
DOI 10.1007/978-3-319-14983-7
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Richard K. Sutton
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311
Chapter 13
Long-term Rooftop Plant Communities
Bradley Rowe
B. Rowe ()
Department of Horticulture, Michigan State University, 1066 Bogue Street,
A212 Plant and Soil Sciences Bldg, 48824 East Lansing, MI, USA
e-mail: rowed@msu.edu
Abstract Most plant studies of green roof taxa have only been conducted for a
duration of 1 or 2 years. The problem with this scenario is that it can result in prema-
ture conclusions and misleading recommendations because green roofs are dynamic
systems. Plants that initially survive may eventually experience reduced coverage
or disappear completely due to competition, variability in climate, and other fac-
tors. Setting up long-term studies similar to the National Science Foundation (NSF)
Long Term Ecological Research (LTER) model would provide the opportunity to
follow changes to green roof habitats over time and also examine impacts and eco-
system service outputs on similarly designed roofs across geographic locations.
Without consciously considering the effects and changes over time mistakes are
not only made, but also repeated. We review several important longitudinal stud-
ies and discuss factors that impact long-term plant communities such as substrate
composition and fertility, substrate depth, substrate moisture, microclimates, roof
slope, orientation, and irradiance levels; as well as initial plant choices, functional
diversity and complexity, and maintenance practices. In addition, we discuss the
potential of applying the LTER model to green roofs and close with future research
needs and questions.
Keywords Long-term ecological research · Plant performance · Plant selection ·
Plant succession · Substrate composition · Substrate depth · Substrate moisture
13.1 Introduction
The long-term plant communities that exist on green roofs can have a major im-
pact on the ecological services provided (Oberndorfer et al. 2007; Rowe and Getter
2010). If green roofs are to deliver these benefits over time, as well as to meet
© Springer International Publishing Switzerland, 2015
R. Sutton (ed.), Green Roof Ecosystems, Ecological Studies 223,
DOI 10.1007/978-3-319-14983-7_13
312 B. Rowe
long-term client expectations, then plant selection and long-term plant performance
are extremely important. To date, most green roof research has been conducted to
measure performance of engineering traits such as stormwater retention and heat
flux through roofing membranes or has focused on whether a particular species
will survive. Unfortunately, there has been much less research involving green roof
ecological principles (Cook-Patton and Bauerle 2012). These facts raise questions
regarding how time will influence changes in plant communities, how these chang-
es influence the ecosystem services that a roof provides, and what can be done to
address these issues.
13.2 Review of Several Important Longitudinal Studies
One problem with the green roof literature dealing with plant evaluations is the lack
of long-term studies published in peer-reviewed journals. Here we review several
of the longest green roof studies of record that have been published in English.
Studies where initial plantings were recorded which provide baseline information
include the Paul-Lincke-Ufer project, Berlin (20 years) (Köhler 2006; Köhler and
Poll 2010); Communication Arts Building, Michigan State University (9 years)
(Getter et al. 2009a); Horticulture Teaching and Research Center, Michigan State
University (7 years) (Durhman et al. 2007; Rowe et al. 2012); a commercial build-
ing in Sheffield, U.K. (6 years) (Dunnett and Nolan 2004; Dunnett et al. (2008); and
Seaton Hall, Kansas State University (5 years) (Skabelund et al. 2014). The study
on the Church of Jesus Christ of Latter-day Saints Conference Center in Salt Lake
City (Dewey et al. 2004) was only conducted for 2 years, but the roof still exists
and provides an excellent opportunity to go back and survey the roof after 14 years.
The Ufa-Fabrik Cultural Center, Berlin (13 years) (Köhler 2006) and the Thuring
and Dunnett (2014) paper that examined numerous old green roofs in Germany are
included even though the original plantings are unknown.
13.2.1 Paul-Lincke-Ufer Project, Berlin (20-years)
In the longest plant evaluation study of record, Köhler (2006) evaluated long-term
vegetation succession on the Paul-Lincke-Ufer (PLU) project located in Berlin.
The study area consisted of ten sub-roofs each with a substrate depth of 10 cm
(4 in) with varying orientation and slope. The green roofs were installed in 1985 as
pre-vegetated mats seeded with ten species : wild chives ( Allium schoenoprasum),
cheatgrass ( Bromus tectorum), orchardgrass ( Dactylis glomerata), sheep fescue
( Festuca ovina), red fescue ( Festuca rubra), junegrass ( Koeleria macrantha), pe-
313
13 Long-term Rooftop Plant Communities
rennial ryegrass ( Lolium perenne), Canada bluegrass ( Poa compressa), Kentucky
bluegrass ( Poa pratensis), and yellow stonecrop ( Sedum acre). Data were recorded
almost every year from 1985 until 2005 and measurements included the number of
plants, coverage for each species, plant heights, and percentage of “standing dead”
(living plants with dead leaves and stems). Over the 20-year period, 110 species
were observed, but only about 10–15 were present in large numbers. The average
number of plant species present at any given time was 15. Of the original ten spe-
cies only five were present every year ( A. schoenoprasum, B. tectorum, F. ovina, P.
compressa, and S. acre). Dactylis glomerata no longer existed after the first year,
and K. macrantha, P. pratensis, L. perenne, and F. rubra disappeared after 3, 5, 7,
and 8 years, respectively. By 2005, A. schoenoprasum became by far the dominant
species covering 56 % of the area followed by F. ovina, P. compressa, and B. tec-
torum. Initially, numerous weeds sprouted from the seed bank present in the grow-
ing substrate. However, these disappeared after a few years as the roofs were not
irrigated. Wet summers tended to encourage spontaneous species and enrich plant
diversity due primarily to colonization. Some colonizing species such as bulbous
bluegrass ( Poa bulbosa) persisted, likely because it forms a bulb, which allows it
to survive during dry periods. Also, the lichen, Cladonia coniocrea, colonized and
persisted because it can withstand dry periods.
Köhler attributed weather related factors such as temperature and rainfall to be
more important than roof size, slope, or age in regards to species richness. After the
initial decrease in plant diversity, roof age had limited impact on species richness
(defined as the number of different species present in a given habitat). It varied from
year to year due to weather conditions, but had more or less reached equilibrium.
Some species such as loose silky-bent ( Apera spica-venti) were more apparent dur-
ing wet summers compared to dry ones.
13.2.2 Ufa-Fabrik Cultural Center, Berlin (13-years)
The Ufa-Fabrik Cultural Center located in suburban Berlin was installed in 1986,
but the first data were not collected until 1992 and then data collection continued
until 2005 (Köhler 2006). The roof was planted with seed of wildflower meadow
species collected from the Alps, but it is not known exactly what species were origi-
nally sown. This roof was irrigated the first 11 years and exhibited higher species
richness during this time period. When irrigation was discontinued in 1997, herba-
ceous plant species started to decline and Sedum species began to dominate. In fact,
in 1998, the common green roof plants, S. acre and Caucasian stonecrop ( Phedimus
spurius) appeared for the first time. It is difficult to make major conclusions regard-
ing this roof since the exact original species are unknown and there wasn’t any data
collected until 8 years after installation.
314 B. Rowe
13.2.3 Communication Arts Building, Michigan State University,
East Lansing (9-years)
A study is being conducted on the third-story rooftop of the Communications Arts
and Sciences Building on the campus of Michigan State University to quantify the
effect of solar radiation (full sun vs. full shade) on several U.S. native and non-na-
tive species (Getter et al. 2009a) (Fig. 13.1). Plugs of six native and three non-native
species were planted in May 2005 on substrates of two different depths [8.0 cm
(3.1 in) and 12.0 cm (4.7 in)] both in sun and shade. Species tested included wild
nodding onion ( Allium cernuum), heath sedge ( Carex flacca), cascade stonecrop
( Sedum divergens), narrow-petaled stonecrop ( Sedum stenopetalum), largeflower
fameflower ( Talinum calycinum) (currently known as Phemeranthus calycinus by
taxonomists), and sunbright (Talinum parviflorum)(currently known as Phemeran-
thus parviflorus), as well as three non-natives ( Sedum acre (biting stonecrop), Se-
dum album (white stonecrop), and Sedum urvillei (stonecrop). Plots were irrigated
during the first year of establishment, but relied on natural rainfall thereafter.
At the end of the first growing season, C. flacca was one of the most abundant
species for both substrate depths in the shade. However, in subsequent years, it
decreased in abundance during the driest portions of the summer that likely im-
pacted overall regeneration. By the end of the 4 years, this species exhibited zero
or near-zero absolute cover (AC). Absolute cover is defined as the total number of
contacts recorded for each species divided by the number of data collection points.
Fig. 13.1  Replicated research plots located in the shade on the Communication Arts and Sciences
Building at Michigan State University. (Photo DB Rowe)
315
13 Long-term Rooftop Plant Communities
In contrast, at the end of the second growing season, S. acre had established itself
as the most abundant species for both substrate depths in the shade and exceeded
an AC of 0.6 by the third growing season. For both substrate depths in the shade,
A. cernuum was the next most abundant species by the fourth growing season, fol-
lowed by S. album and T. calycinum. In the sun, by the second growing season
both substrate depths were dominated by S. album, followed by T. calycinum and
S. acre. At 12 cm (4.75 in), A. cernuum closely followed as the fourth most abun-
dant, but this species was not nearly as abundant as it was in the shade at the same
depth. By week 174 (23 Sept 2008), most species exhibited different AC within a
depth between sun and shade. However, when all species were combined, overall
AC did not differ between sun and shade within a depth. This indicated that while
species make-up was changing among solar radiation levels, that overall coverage
was not significantly different between sun and shade. For all substrate depths and
solar levels, the most abundant species were S. acre, A. cernuum, S. album, and T.
calycinum. With the exception of T. calycinum, native species were less abundant
than non-native species. The native Talinum species ( T. calycinum and T. parviflo-
rum) were outside of their hardiness zone, but are prolific seeders. However, they
need bare soil in order to germinate. By the end of 9 years, the only species that still
existed were A. cernuum, S. acre, and S. album and represents a significant decrease
in species richness over time. Plots were weeded the first 4 years, but have received
no maintenance over the past 5 years.
13.2.4 Horticulture Teaching and Research Center, Michigan
State University, East Lansing (7-years)
This study followed the succession of 25 succulents grown at three substrate depths
over the course of 7 years (Durhman et al. 2007; Rowe et al. 2012). Absolute cover
was determined using a point-frame transect every two weeks during the first three
growing seasons and monthly during years four through seven to measure com-
munity composition and change (Fig. 13.2). At the 7.5 cm (3 in) depth, 22 species
were present at the end of the first growing season, but these numbers were reduced
to 13, 8, and 7 after 2, 3, and 5 years, respectively. Similar results occurred at the
shallower depths except that the number of species was reduced at a faster pace. For
the most part, the species present did not change after 4 years, but the relative abun-
dance for each species continued to change. At 5.0 cm (2 in) and 7.5 cm (3 in), both
Caucasian stonecrop ( Phedimus spurius) and Chinese mountain sedum ( Sedum
middendorffianum) continued to expand through year 7 at the expense of the other
remaining species. At 2.5 cm (1 in), S. acre and S. album were the dominant species.
Results show that the length of the study can have a dramatic effect on conclu-
sions and plant recommendations. The initial paper published from the first two sea-
sons of data from this study (Durhman et al. 2007) recommended P. spurius, S. acre,
S. album, S. middendorffianum, Jenny’s stonecrop ( S. reflexum), pale stonecrop ( S.
sediforme), and P. spurius for extensive green roofs ranging from 2.5 cm (1 in) to
316 B. Rowe
7.5 cm (3 in) in depth. Additional recommendations for subsidiary species that were
present at specific substrate depths, but may not exhibit an ability to cover large
areas included Burnatti sedum ( S. dasyphyllum ‘Burnatii’), lilacmound sedum ( S.
dasyphyllum ‘Lilac Mound’), diffuse sedum ( S. diffusum), Spanish sedum ( S. his-
panicum), and orange stonecrop ( S. kamtschaticum syn. Phedimus kamtschaticus).
As can be seen from the results following 7 years, recommendations were mislead-
ing as S. sediforme, S. dasyphyllum ‘Burnatii’, S. dasyphyllum ‘Lilac Mound’, S.
diffusum, and S. hispanicum no longer existed at any depth.
13.2.5 A Commercial Building in Sheffield, U.K. (6 years)
Dunnett and Nolan (2004) and Dunnett et al. (2008) conducted a study on top of a
three story commercial building in Sheffield, U.K., over a period of 6 years from
2001–2006. The objectives of their study were to evaluate potential plant taxa for
use on green roofs that experience a maritime U.K. climate and to test how substrate
depth [10 cm (4 in) and 20 cm (8 in)] influenced plant establishment and survival, as
well as visual and aesthetic criteria. Species included white thrift seapink ( Armeria
Fig. 13.2  Use of a point
frame to measure absolute
cover on the Horticulture
Teaching and Research Cen-
ter green roof at Michigan
State University. (Photo DB
Rowe)
317
13 Long-term Rooftop Plant Communities
maritima ‘Alba’), lesser calamint ( Calamintha nepeta), maiden pink ( Dianthus
deltoides) dwarf blue fescue ( Festuca ovina glauca), bearskin fescue ( Festuca sco-
paria), Lindheimer’s beeblossom ( Gaura lindheimeri), white creeping babysbreath
( Gypsophila repens ‘Alba’), Border Ballet red hot poker ( Kniphofia X‘Border Bal-
let’), sea-lavender ( Limonium platyphyllum), Fassen’s catnip ( Nepeta Xfassenii),
Herrenhuasen oregano ( Origanum laevigatum ‘Herrenhausen’), roseroot ( Rhodiola
rosea), yellow stonecrop ( Sedum acre), lambsear ( Stachys byzantina), and dwarf
spiked speedwell ( Veronica spicata ‘Nana’).
Species tested were all native to dry and nutrient-stressed habitats, but differed
widely in their heights, flowering times, life spans, growth forms, and locations
where they are considered native. In addition to annually measuring plant height,
spread, flowering performance and percent vegetation cover for the 15 planted spe-
cies, they also recorded the numbers and percent cover for colonizing species.
The greatest survival, diversity, size, and flowering performance of planted spe-
cies occurred at a substrate depth of 20 cm (8 in) relative to 4 in (10 cm) and the
herbaceous species developed 85 and 58 % coverage at the end of two growing sea-
sons at depths of 20 cm (8 in) and 10 cm (4 in), respectively. By the end of 5 years,
all species survived at both depths, however, 14 of 15 species maintained at least
50 % of their original numbers at 20 cm (8 in) whereas, only eight did so at 10 cm
(4 in). Bare ground and moss cover was greatest at 10 cm (4 in) as was diversity
of colonizing species, presumably due to the presence of open space for invading
seeds to germinate.
Species-richness (mean number of taxa per subplot) decreased over time at both
substrate depths, but the rate of decline was greater at 10 cm (4 in). As mentioned,
some species performed better at the 10 cm (4 in) or 20 cm (8 in) depth. The low-
growing species such as sedum that are typical of shallow extensive roofs were not
as competitive at 20 cm (8 in). Likewise, the perennial plants that normally possess
greater biomass could not survive as well at 10 cm (4 in). Even when drought toler-
ant plant species are selected, the limiting factor for plant survival is often substrate
moisture, which is often a function of substrate depth (Chaps. 4, 5). The authors
emphasized the importance of long-term monitoring of green roofs because of the
changes that occurred in plant communities from the first to the sixth year of their
experiment.
13.2.6 Seaton Hall, Kansas State University, Manhattan,
KS (5-years)
The first green roof project installed at Kansas State University was planted on
Seaton Hall in May 2009 (Skabelund et al. 2014). The main goal of the project was
to see if a semi-intensive green roof consisting of native grasses and forbs grow-
ing in a substrate profile ranging from 10 cm (4 in) to 18 cm (7.1 in) was feasible
in this relatively dry climate with minimal maintenance and irrigation. The roof
is south facing and receives reflected light off windows and limestone especially
318 B. Rowe
during spring and fall. The 28.3 m2 (305 ft2) roof was planted with plugs of five
species of grasses, ten forbs, and one forb-like shrub. Grasses included side-oats
grama ( Bouteloua curtipendula), blue grama ( Bouteloua gracilis), little bluestem
( Schizachyrium scoparium), prairie dropseed ( Sporobolus heterolepis), and Indian-
grass ( Sorghastrum nutans). Forbs consisted of smooth aster ( Aster laevis), purple
poppy-mallow ( Callirhoe involucrata), purple prairieclover ( Dalea pupurea), tall
gayfeather ( Liatris aspera), dotted gayfeather ( Liatris punctata), prairie coneflow-
er ( Ratibida columnifera), gray-headed prairie coneflower ( Ratibida pinnata), wild
blue sage ( Salvia azurea), rigid goldenrod ( Solidago rigida), and common spider-
wort ( Tradescantia ohiensis). The forb-like shrub was New Jersey tea ( Ceanothus
americanus). The study is still ongoing and has yet to be published other than in a
proceedings from a meeting (Skabelund et al. 2014).
Along with plant survival and dynamics, a range of climatic variables was moni-
tored. A subset of selected grasses was evaluated for height, basal diameter, and
number of flowering stalks at the end of each growing season between 2009 and
2013. In 2009 and 2010, supplemental irrigation was provided on an as-needed ba-
sis and growing conditions were favorable, resulting in nearly 100 % plant survival.
Most grasses exhibited flowering stalks and increased basal diameter. The west side
of the green roof was not irrigated in 2011, the entire roof was irrigated in 2012,
and then supplemental irrigation ceased during mid-August 2012. Between 2010
and 2011 the original plantings decreased from 130 to 98 for individual grasses and
from 98 to 39 for forbs. At the end of 2012 grasses exhibiting visibly-green above
ground biomass remained at 98 while forbs increased to 54. By November 2013
original grasses numbered 68 and forbs 21. After the first year many new native
grasses and forbs established themselves from germinating seeds produced by the
original plantings. This was particularly pronounced in 2010 and 2012. Notably,
plants of B. gracilis were taller in deeper substrates, with 12–18 cm (4.75–7.1 in)
depths producing plants approximately 10.5 cm (4.1 in) taller than 7.5–9 cm (3.0–
3.5 in) depths. Between 2009 and 2012, 15–18 cm (5.9–7.1 in) substrate depths
produced B. gracilis 10.8 cm (4.2 in) taller than those at 10 cm (4 in) depths.
13.2.7 Church of Latter-Day Saints Convention Center,
Salt Lake City, Utah
An example of a short-term study is the Church of Jesus Christ of Latter-day Saints
Conference Center in Salt Lake City, Utah (Dewey et al. 2004) (Fig. 13.3). How-
ever, because original substrate conditions and some information on plantings were
recorded, the opportunity exists to monitor this roof into the future.
The objective of the original study was to observe the relative competitiveness
of native grass and wildflower species growing in a range of different radiation/
temperature environments. For research purposes, the roof was partitioned into
seven radiation zones: (1) maximum sunlight, maximum reflection/radiation, (2)
maximum sunlight, moderate refection/radiation, (3) maximum sunlight only, (4)
319
13 Long-term Rooftop Plant Communities
minimal shading, (6) moderate shading, and (7) maximum shading. Zone 5 was
eliminated from the study as it was considered similar to zone 4. The main compo-
nent of the substrate was heat-expanded shale and it was placed at a depth of 1 m
(3.3 ft). The roof was planted with plugs during the summer of 2000, overseeded in
April 2001 with some of the same species in addition to others. Weeds were pulled
as needed and the roof was irrigated twice a week.
During fall 2001, the roof was evaluated by counting the number of plants pres-
ent for each species in a given sample area. Twenty one species were identified
that should at least be considered for future grass/wildflower green roofs. How-
ever, Canada bluegrass ( Poa compressa) and white sage ( Artemisia ludoviciana)
were too aggressive when planted in this grass and wildflower mixture. In contrast,
the alpine bluegrass ( Poa alpina), big bluegrass ( Poa secunda), mutton bluegrass
( Poa fendleriana), blue bellflower ( Campanula rotundifolia), columbine ( Aquile-
gia spp.), purple meadowrue ( Thalictrum purpurea), and tickseed. ( Coreopsis spp.)
may not be competitive enough. Since there was no experimental design to the orig-
inal planting, no replication, and only an estimate of the number of plugs planted
in each zone, the study is only observational. Still, if monitored in to the future it
would provide valuable information as to the long term succession of a grass and
wildflower meadow on a green roof.
Fig. 13.3  Meadow consisting of native plants on the Church of Latter-day Saints Convention
Center in Salt Lake City, Utah. (Photo DB Rowe)
320 B. Rowe
13.2.8 Old Green Roofs in Germany
It would be a travesty to discuss long-term plant communities on green roofs with-
out acknowledging the long tradition of over 100-years of green roofs in Germany.
Unfortunately, much of the original information on these roofs was never recorded,
has been lost, or was anecdotal; studies were observational in nature without repli-
cation and thus not scientifically sound by today’s standards; were not published in
peer-reviewed journals; or are not easily accessible to the scientific world as they
were not written in English. However, in addition to the Paul-Lincke-Ufer project
and the Ufa-Fabrik Cultural Center in Berlin (Köhler 2006) described above, two
recent papers published in scientific journals have gone back and looked at some of
these older German roofs (Köhler and Poll 2010; Thuring and Dunnett 2014).
The purpose of the Köhler and Poll (2010) study was to compare vegetation and
substrate characteristics between the old Tar-Paper-Green roofs (TPG-roofs) that
were installed between 1880 and 1930 to the first Modern Extensive Green roofs
(MEG-roofs) that were established in the 1980’s. These roofs, subjects of previ-
ously published studies written in German from 1960, 1982, 1986, 1987, 1990, and
1995 were surveyed in 2008. While the Paul-Lincke-Ufer project (Köhler 2006)
discussed earlier focused on ecological succession, this study concentrated on
growing substrate, vegetative quality, and species richness.
According to the specified criteria set by Köhler and Poll (2010), they concluded
that the performance of the MEG-roofs with engineered substrates composed of
heat expanded clay, etc. was higher than the older TPG-roofs that originally utilized
sandy soils. Even so, both roof types were still functional after many years and
exhibited an increase in pedogenesis, a trend toward higher organic carbon, and a
neutral pH. The old TPG-roofs were significantly richer in humus (mean organic
C content of 4 %) than the MEG-roofs. Initial mean organic carbon content on the
MEG-roofs was 2.5 % and then declined to 1.9 % due to microbial oxidation. Af-
ter the roof stabilized after about 10 years, their organic carbon content increased
steadily for the next 25 years up to the point that by 2008, the organic C content
of both roof types were not significantly different. Total porosity of the MEG-roof
substrates rose over a period of 10 years from 50 to 60 %. This change is likely due
to processes such as the continuous formation and decay of plant roots, microbial
activity, freezing and thawing.
Regarding plant species, 70 different species were recorded on the MEG-roofs,
compared to 45 on TPG-roofs. Of course, this difference in species richness could
be due to differences in substrate properties, as well as many other factors such as
initial plantings. The most successful species were generally grasses such as cheat-
grass ( Bromus tectorum), poverty brome ( Bromus sterilis), fescues Festuca spp.,
perennial ryegrass ( Lolium perenne), annual bluegrass ( Poa annua), and Canada
bluegrass ( Poa compressa) (most common).
The second study took place in southwestern Germany where Thuring and Dun-
nett (2014) surveyed vegetation and substrates on nine of the oldest extensive green
roofs in the Stuttgart area during 2010 and 2011. Roof ages at the time of the sur-
321
13 Long-term Rooftop Plant Communities
vey ranged from 20 to 33-years-old. Unfortunately, there was little information on
original substrate composition and depth or original species planted on these roofs
so the results serve as a snapshot in time of present conditions. They could only
speculate on how the substrates and plant communities changed over time. How-
ever, the roofs likely all adhered to early FLL standards and had a substrate depth
less than 20 cm (8 in), a pH between 6.5 and 8.0, and organic content below 4.1 lbs/
ft3 (65 g/L) when constructed (FLL 2008).
Results suggested a decrease in substrate depth, substrate pH, and plant biomass
over time while substrate organic content increased. This increase in organic matter
agrees with the Köhler and Poll (2010) study discussed above and with the findings
of Getter et al. (2007) who reported that organic matter nearly doubled from 2.33
to 4.25 % in just 5 years where the primary component of the substrate was heat-
expanded slate. Similarly, Getter et al. (2009b) reported that the amount of carbon
sequestered on shallow sedum based roofs increased with age and that 100 g C/
cm2 (57.8 oz/in3) were sequestered during the first 2 years after installation of 6 cm
(2.4 in) deep plots. The increase in organic carbon makes sense when one consid-
ers that the engineered substrates often used on extensive green roofs have limited
initial organic matter because they are designed to hold moisture by manipulating
particle size distributions (FLL 2008). Also, low substrate pH could result in an
accumulation of substrate organic matter because some microbes are adversely af-
fected by low pH, thus reducing decomposition (Berendse 1998).
Regarding plant cover, Thuring and Dunnett (2014) reported that succulents
dominated these roofs either by themselves or as a consistent groundcover under-
neath other herbaceous perennials or grasses. Over time species diversity decreased
which agrees with the work of Liesecke (1998) who reported that one or two suc-
culents, a single herb, and one or two moss species often dominated older, extensive
green roofs or two moss species.
13.3 Factors Impacting Long-Term Plant Communities
Numerous factors impact long-term plant communities on green roofs. Factors in-
clude substrate composition and fertility, substrate depth, substrate moisture, mi-
croclimates, roof slope, orientation, and irradiance levels; as well as initial plant
choices, functional diversity and complexity, and maintenance practices.
13.3.1 Substrate Composition and Fertility
Substrate composition influences plant communities primarily through moisture
retention and nutrient availability. Ideally they should be lightweight, permanent,
and able to sustain plant health without leaching nutrients that may pollute receiv-
ing water bodies. For these reasons substrates often incorporate aggregate materials
322 B. Rowe
such as heat expanded slate, shale, or clay as their main component (FLL 2008).
Water holding capacity can be altered by manipulating the particle size distribution
of the aggregates and by adding organic matter. Although organic matter will retain
moisture and provide nutrients, high levels are not recommended because it decom-
poses resulting in substrate shrinkage and can leach nutrients such as nitrogen (N)
and phosphorus (P) in the runoff (Rowe 2011). The same runoff problems can occur
when fertilizer is applied. A detailed discussion of nutrient cycling in green roof
ecosystems can be found in Chap. 5.
In a study that looked at the effects of substrate composition and fertility, Rowe
et al. (2006) found that sedum achieved 100 % cover regardless of the percent-
age of heat-expanded slate in the substrate, but that the herbaceous perennials and
grasses required greater percentages of organic matter or supplemental irrigation.
They also reported that a greater number of smooth aster ( Aster laevis), junegrass
( Koeleria macrantha), and showy goldenrod ( Solidago speciosa) survived when
they were not fertilized. Presumably, these plants could survive drought conditions
for a longer period of time since they had less biomass to maintain. In contrast, if the
purpose of the green roof is urban agriculture then fertility levels must be relatively
high to produce acceptable yields for fruits and vegetables (Whittinghill and Rowe
2012a; Whittinghill et al. 2013).
Most commercial green roofs are built within German FLL guidelines (FLL
2008) and are composed of manufactured plastic layers topped with engineered
growing substrates. These standards help to assure consistency of materials and
success of green roof projects. However, many are being built without these expen-
sive components, especially in Switzerland (Brenneisen 2006; Kiers 2013). Stephan
Brenneisen, from the University of Applied Sciences Wädenswil, has been a pro-
ponent for the construction of green roofs with the primary purpose of promoting
biodiversity. For example, some roofs utilize gravel or layers of straw or grasses
such as maidengrass ( Miscanthus sinensis) as the drainage layer, use native soils
blended with other components such as lava rock or gravel, and are planted with
native wildflowers. A commercial installer may be hesitant to go outside FLL sub-
strate specifications, but other systems do work (Chaps. 3, 6).
An excellent example of a green roof constructed with non-standard green roof-
ing materials is the Moos Lake water filtration plant in Wollishofen, Zürich, Swit-
zerland. Installed in 1914, the roof was built long before German FLL guidelines
were written and adopted (Fig. 13.4). The original drainage layer consisted of grav-
el topped with 12.5 cm (5 in) of sand and 15–20 cm (6–8 in) of local topsoil. After
100 years those layers are no longer distinguishable, but there are neither problems
with drainage nor any negative effects on the vegetation, and the original roofing
membrane is still in place. The 30,000 m2 (322,917 ft2) roof is home to 175 plant
species, several of which are now endangered or rare. The roof consists of nine spe-
cies of orchids and approximately 6000 specimens of green-winged orchid ( Orchis
morio) a species that is now extinct in the landscape surrounding Zürich. The roof
reflects species richness of the surrounding area from 100 years ago as well as
today. The original vegetation developed from the seed bank that was part of the
323
13 Long-term Rooftop Plant Communities
original topsoil. Today, plant composition consists of these original species as well
as any new species that colonized from the surrounding landscape.
13.3.2 Substrate Depth
Substrate depth has a major impact on plant survival and long-term plant commu-
nities. Depending on climate and the availability of supplemental irrigation, most
shallow extensive green roofs are limited to drought tolerant species such as suc-
culents. This is primarily due to a lack of moisture (Dunnett and Nolan 2004; Durh-
man et al. 2006), but some taxa such as Sedum spp. are naturally found in these
conditions. However, even among succulents, substrate depth will influence total
coverage and coverage of individual species. In Pennsylvania, Thuring et al. (2010)
reported that white stonecrop ( S. album) and six-sided stonecrop ( S. sexangulare)
survived in 3 cm (1.2 in), but produced greater biomass at depths of 6 cm (2.4 in)
and 12 cm (4.7 in). Similarly, Getter and Rowe (2009) reported that the majority
of the 12 species of Sedum tested in Michigan exhibited greater growth and cover-
age at a depth of 7.0 cm (2.7 in) and 10.0 cm (4 in) compared to 4.0 cm (1.6 in).
At 5.0 cm (2 in) and 7.5 cm (3 in), Phedimus spurius and Sedum middendorfianum
were the dominant species, but at 2.5 cm (1 in), S. acre and S. album covered the
most area (Durhman et al. 2007).
Fig. 13.4  The Moos Lake water filtration plant in Willishofen, Zürich, Switzerland was built in
1914 and is home to 175 plant species, many of which are now endangered or rare. (Photo DB
Rowe)
324 B. Rowe
In Sweden, S. acre and S. album were dominant at a depth of 4 cm (1.6 in) while
the other succulents in the study, S. reflexum (syn. S. rupestre), S. sexangulare,
pink Mongolian stonecrop ( Hylotelephium ewersii), Chinese sedum ( Phedimus
floriferus), hybrid stonecrop ( Phedimus hybridus), Phedimus kamtschaticus (syn.
S. kamtschatium), and Caucasian stonecrop ( Phedimus spurius) grown in various
combinations had minimal coverage by the end of 3 years, generally 15 % or less for
all other species combined (Emilsson and Rolf 2005; Emilsson 2008). Development
over time varied depending on the original species mix planted, as well as substrate
composition.
As depth increases, the number of potential species expands to grasses, many
annual or herbaceous perennials, and even woody plants. Deeper substrates are ben-
eficial for both increased water holding capacity (Durhman et al. 2006; VanWoert
et al. 2005a; VanWoert et al. 2005b) and as a buffer for overwintering survival, as
shallow substrates are more subject to fluctuations in temperature (Boivin et al.
2001). As discussed above, Dunnett et al. (2008) reported the greatest survival,
diversity, size, and flowering performance of grasses and herbaceous perennials
occurred at a substrate depth of 20 cm (8 in) compared to a depth of 10 cm (4 in).
By the end of 5 years, all species survived at both depths, however, 14 of 15 species
maintained at least 50 % of their original numbers at 20 cm (8 in) whereas, only
eight did so at 10 cm (4 in). Likewise, in Southern Tuscany, most of the 20 Mediter-
ranean xerophytic species tested exhibited greater growth and cover at 20 cm (8 in)
relative to those grown at 15 cm (6 in) (Benvenuti and Bacci 2010). In addition
to greater moisture stress, temperatures in the shallower substrate [15 cm (6 in)]
reached a maximum of 90 °F (50 °C) and were on average 9 °F (5 °C) higher than
the 20 cm (8 in) deep substrate. This could be partially explained by the fact that
shallower substrate depths often have less coverage which exposes more substrate
to direct sun resulting in higher substrate temperatures (Getter et al. 2009a).
13.3.3 Substrate Moisture
Substrate moisture is a function of substrate composition and depth and is often the
limiting factor for plant survival on green roofs (Dvorak and Volder 2010). In the
Getter and Rowe (2009) study discussed above, mean volumetric moisture content
at the three substrate depths were correlated with plant growth and coverage. Simi-
larly, Thuring et al. (2010) reported that the herbaceous species tested were severely
affected by drought when grown in shallower substrates. In addition, Monterusso
et al. (2005) found that only four of 18 species of native herbaceous perennials and
grasses still existed after 3 years when grown at a 10 cm (4 in) depth without irriga-
tion. The majority of the plants tested were considered to be drought tolerant, but
their survival in a native environment relies on deep tap roots to obtain moisture.
Survival and persistence could have been improved by increasing substrate mois-
ture through changes in substrate composition, depth, or by providing irrigation.
325
13 Long-term Rooftop Plant Communities
However, deeper substrate depths that hold more moisture are not beneficial to
all plant species as long-term survival of stress tolerant species often depends on
shallow soil depths with limited moisture. Otherwise, species with greater growth
potential will outcompete them. This was even evident in the Rowe et al. (2012)
study as S. acre and S. album were dominant at 2.5 cm (1 in) whereas P. spurius
and S. middendorfianum were most prevalent at deeper depths. Similarly, Emilsson
(2008) reported that S. acre decreased in area of coverage after 2 years. This may
be because S. acre allocates a relatively small percentage of plant carbon to the root
system (Getter et al. 2009b) and these roots also tend to be shallow and less able to
compete for water. Increasing substrate depth is of no advantage to this species, as
it must then compete against more aggressive plants with greater biomass (Getter
and Rowe 2009). Likewise, in the Dunnett et al. (2008) study, Armeria maritima
performed better at 10 cm (4 in) relative to 20 cm (8 in). Armeria maritima is a self-
seeder and likely took advantage of the greater bare space at the shallower depth.
Other species such as the succulents T. calycinum and T. parviflorum also depend
on bare space for long-term survival in climates such as that found in Michigan.
These species are perennials, but are killed by cold winter temperatures in Michigan
and reappear each year by reseeding. However, as the roof obtains 100 % coverage,
there is little open space for germination to continue from year to year and the spe-
cies eventually disappears (Getter et al. 2009a).
Supplemental irrigation can alleviate substrate moisture problems, but the use of
potable water on green roofs is often problematic. If irrigation is to be supplied, then
it should be done so with the most efficient and sustainable method for the particular
application (Rowe et al. 2014). Irrigation is critical when growing vegetables on
roofs (Whittinghill and Rowe 2012a; Whittinghill et al. 2013).
13.3.4 Microclimates, Roof Slope, Orientation, and Irradiance
Levels
Microclimates present on a roof will dramatically influence short and long-term
plant communities (see Chap. 3). They can be caused by variations in substrate
composition and depth as described above, or from variations in irradiance levels
due to shaded areas, roof slope, and roof orientation.
In the irradiance level (full sun versus full shade) study on the MSU Commu-
nication Arts Building described above it was found that regardless of depth, spe-
cies differed depending on sun exposure (Getter et al. 2009a). After four growing
seasons, heath sedge ( Carex flacca), was still present at 12 cm (4.7 in), but only in
the shade. After 9 years it has completely disappeared. Even though species mix
was changing among solar radiation levels, overall coverage was not significantly
different between sun and shade. Roof slope and orientation also influence sub-
strate moisture and thus plant communities. Getter et al. (2007) reported that water
retention was reduced by 10 % when slope increased from 2 to 25 %. Orientation is
also important as evapotranspiration increases with solar exposure. Köhler and Poll
326 B. Rowe
(2010) reported that the greatest plant coverage was found on north-facing sections
of the roof on the Paul-Lincke-Ufer Building in Berlin. The most dominant species
was Allium schoenoprasum while on south facing slopes Sedum spp. dominated.
Grasses were least competitive on west facing slopes.
One example of a roof designed to create various microclimates that in turn
promote diversity is the California Academy of Sciences in San Francisco (Hauser
2013). The seven domes create different microclimates due to variations in slope
and sun exposure and thus substrate moisture. This in turn influences the plant com-
munities that find their niche among the various microclimates where they have a
competitive advantage. The roof was originally planted in 15 cm (6 in) of substrate
with four perennial and five annual species uniformly spaced over the entire roof.
Today, there are approximately 70 native species thriving where the environmental
conditions are best for each individual species. This increase in species richness
is contrary to the decreases that occurred when only one substrate depth was em-
ployed on the other roofs described above.
Therefore, it seems logical that one way to increase plant diversity on green roofs
is to create multiple microclimates. If roof slope and orientation are not options
then variations in substrate depth and composition can be created. Because different
species can compete best in a specific environment, each species will find its niche
location where it has advantages over competing species. This will likely increase
the biodiversity potential and improve the species richness of the long-term plant
community as environmental conditions change over time and species increase and
decrease in numbers.
13.3.5 Initial Plant Choices, Functional Diversity and
Complexity, and Maintenance Practices
The plants present on a green roof at any given time also depend on what was
initially planted, the functional diversity and complexity of these species, and the
intensity or lack thereof of maintenance. Some plants may be originally chosen for
factors such as aesthetics, but may be ill suited for the particular environmental
conditions and destined to fail. Others may be too aggressive and will crowd out
everything else (Getter and Rowe 2009; Rowe et al 2012). If the overly aggressive
species was not planted to begin with, then the dynamic would be completely dif-
ferent.
Maintenance is also a major factor. If ‘weed’ species are removed on a regular
basis then they clearly will not be able to colonize a roof. In this case, weeds are
defined as any species that was not planted in the original design. Colonizing spe-
cies will also be influenced by the proximity to local seed sources. The height of the
roof and the surrounding landscape will influences seed sources (Chap. 15). Main-
tenance practices such as irrigation and fertility management are also major factors
as discussed in Chaps. 4 and 5.
327
13 Long-term Rooftop Plant Communities
There are also complex interactions among plants (Chap. 8). Nagase and Dunnett
(2010) studied how plant diversity on a green roof influenced survival by testing
combinations of three major taxonomic and functional plant groups that are com-
monly used for extensive green roofs (forbs, sedums and grasses). They concluded
that under drought conditions, combinations of species differing in functional diver-
sity and complexity exhibited greater survival rates and visual qualities than mono-
cultures. They attributed this result to the fact that plants of the same taxonomic
group compete for the same resources when grown together.
In addition, Butler and Orians (2011) showed that the drought tolerant succulent,
S. album, could have a positive or negative influence on neighboring plants depend-
ing on substrate moisture content. When ample substrate moisture was present, S.
album had an adverse effect on growth of threadleaf giant hysop ( Agastache rup-
estris) and whorled milkweed ( Asclepias verticillata). In contrast, during drought
conditions the presence of S. album as an understory cover facilitated growth of
these more water dependent herbaceous plants. The favorable response during
drought is likely due to S. album shading the surface, reducing evaporation from
the substrate surface, and from a reduction in substrate temperatures (Butler and
Orians 2011). One might expect the same result for other plant species although the
use of S. album cover crop for green roof production of an assortment of vegetables
had no effect on crop yields (Whittinghill and Rowe 2012b). Vegetables tested were
tomatoes ( Lycopersicon esculentum), bush beans ( Phaseolus vulgaris), bush pick-
le hybrid cucumbers ( Cucumis sativus), sweet peppers ( Capsicum annuum), and
large-leaf Italian basil ( Ocimum basilicum). However, these plants were irrigated
regularly so water deficit conditions were never an issue.
13.4 The Long Term Ecological Research (LTER) Model
Applied to Green Roofs
Long-Term Ecological Research (LTER) is a National Science Foundation (NSF)
funded program that was created in 1980 (Callahan 1984; Kratz et al. 2003). The
research network of scientists currently includes 26 research sites studying ecology
over extended temporal and spatial scales. Long-term studies are important because
the natural world is dynamic and with climate change, patterns of natural variation
are occurring even faster. Plant communities take time to accumulate biomass, re-
spond to disturbances such as invasions of native and non-native species, weather
extremes, or disease and insect pressures, and there may be time lags between the
cause and effect of ecological changes. They can provide a baseline from which to
determine if an ecological system has changed over time and define the range of
natural variability, they allow us to assess relationships and interactions among vari-
ous components of the system, they allow us to detect cause and effect relationships
among slowly changing variables, and data gathered across multiple sites can lead
to stronger conclusions than those from single sites (Kratz et al. 2003).
328 B. Rowe
Although many would argue that placing plants on top of buildings in artificial
substrates is not a natural system, the same LTER concepts apply to green roofs.
One difference is that studies of natural landscapes could span decades, centuries,
or even thousands of years. Buildings do not last that long. Green roofs are limited
in time as most roofing membranes are replaced within 40–50 years. So what con-
stitutes a long-term study on a green roof? Regardless, studies that span years are
critical for making sound conclusions on long-term plant communities. The short
1 and 2 year studies that are common in the green roof literature do not really tell
us anything about what species will be populating a green roof in the future. These
short-term experiments are really just studies of plant establishment. However, the
prevalence of 1 or 2 year studies at single sites is not surprising as research fund-
ing is rarely guaranteed for more than a few years. Also, many studies are graduate
student projects, which cannot be dragged out for years and years. Even so, when
studies have been conducted for three or more years, conclusions drawn are often
dramatically different than what would have been concluded following just one or
two seasons. Three to 5 years seems sufficient to predict long-term plant communi-
ties on shallow roofs consisting of sedum. However, on deeper roofs or roofs where
species are allowed to colonize, then a much longer period of time is needed.
The few longer-term green roof studies where the original plantings were re-
corded in order to provide a baseline from which to work from all point to the
importance of long-term studies. As outlined above from the 7 year study on the
MSU Horticulture Teaching and Research Center, conclusions drawn at the end
of 2 years were significantly different than what was present following 7 years
(Durhman et al. 2007; Rowe et al. 2012). Similar results were drawn comparing
12 species of stonecrop in terms of absolute cover (Getter and Rowe 2008; Getter
and Rowe 2009). Likewise, Dunnett et al. (2008) emphasized the importance of
long-term monitoring of green roofs because of the changes that occurred in plant
communities from the first to the fifth year of their experiment with 15 herbaceous
perennials and grasses (Dunnett and Nolan 2004; Dunnett et al. 2008). In all of the
above studies, changes in plant community development occurred faster at shal-
lower substrate depths relative to deeper ones.
Setting up green roof research sites similar to the NSF LTER program would pro-
vide opportunities to follow changes to green roof habitats for longer periods of time
and also look at similarly designed roofs across geographic distances. Because of the
relatively short life spans of roofing membranes and modern buildings, it may be more
feasible to conduct replicated studies over numerous geographic locations with vary-
ing climates, etc. As with most research, the primary roadblock to doing so is funding.
13.5 Future Research Needs and Questions
Since there have only been a handful of green roof plant studies that were carried
out for more than 1 or 2 years, an obvious place to start is to initiate more of these
studies. One example is a study that was initiated in 2011 to evaluate establishment,
329
13 Long-term Rooftop Plant Communities
survival, and changes in plant community over time on the Molecular Plant Sci-
ences Building at Michigan State University (Fig. 13.5). Plugs of four grasses and
13 herbaceous perennials native to Michigan were installed at substrate depths of
10 cm (4 in) and 20 cm (8 in). Up to 45 plugs of each species were planted on 20 cm
(8 in) centers. Survival rates were recorded during June 2012 and as expected most
species experienced greater survival when grown in 20 cm (8 in) relative to those at
10 cm (4 in). The roof will continue to be sampled every year into the distant future
to record the presence of individual species.
In long-term studies it is important to record baseline plantings when the roof
was first installed in order to know what changes occur over time. However, older
existing roofs should to be sampled also even if it is not exactly known what existed
there in the beginning. Estimates can often be made based on the type of roof, loca-
tion, and who installed the roof. For example, even though it is not known exactly
what was planted on day one and the roof was overseeded and additional species
added the year after installation, the roof on the Church of Latter-day Saints Con-
vention Center described above should be reevaluated. The roof is now 14-years-old
and valuable information could be gleaned and compared to the original study. In
addition, changes in substrate composition should be looked at on this roof as well
as others. Since long-term studies may not always be possible, the LTER model
could still be followed by replicating studies over multiple geographic locations to
determine the role of site-specific environments on plant community development.
Plants species should be tested by themselves and in combination with multiple
Fig. 13.5  The Molecular Plant Sciences Building at Michigan State University is being used to
follow the green roof plant community over time. (Photo DB Rowe)
330 B. Rowe
species. Other factors that should be investigated include interactions among plant
species, the effects of roof maintenance (pulling weeds or allowing other species to
colonize), and how different plant combinations influence ecosystem services such
as stormwater management, heat flux, aesthetics, and the ability of the roof to pro-
vide habitat for wildlife. Common sense would suggest that increasing plant diver-
sity would increase the ability of a green roof to provide these services and reduce
the impact of environmental change (Cook-Patton and Bauerle 2012). Although
this statement is true for the most part, adding plant species without considering
their interactions may actually decrease services (Lundholm et al. 2010, MacIvor
et al. 2011). Research is needed to determine which combinations of species and
functional groups will complement each other and maximize services over time
(Chap. 8). It is a challenge to balance relative competition among species so that
more aggressive species do not dominate the community and reduce biodiversity.
Lastly, more roofs need to be installed where multiple microclimates are created
and then these interactions among the various microclimates need to be studied to
see how they influence long-term plant communities. Different microclimates were
achieved on the California Academy of Sciences Building in San Francisco due to
roof slope and sun exposure. Other options include varying substrate compositions
and depths on the same roof. All of these practices should increase plant diversity,
green roof function, and long-term success.
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Whittinghill L, Rowe DB (2012b) Vegetable production on extensive green roofs. Proc. of 10th
North American Green Roof Conference: Cities Alive, Chicago, IL. 17–20 October, 2012
Whittinghill L, Rowe DB, Cregg B (2013) Evaluation of vegetable production on extensive green
roofs. Agroecol Sustain Food Syst 37(4):465–484
Dr. Bradley Rowe has conducted green roof research at MSU since 2000, was the founding chair
of the Green Roofs for Healthy Cities research committee, and received the GRHC Excellence in
Research Award in 2008.
... Many green roofs in Europe and North America are designed using standards modeled after the German FLL guidelines (F.L.L. 2008). These standards impose a measure of consistency so that many green roofs have similar soil texture and depth, and initial plant species (Dunnett and Kingsbury 2004;Snodgrass and Snodgrass 2006;Rowe 2015). Several factors, including plant selection, soil composition and maintenance schedule, impact the long-term dynamics of the plant communities on green roofs (Rowe 2015). ...
... These standards impose a measure of consistency so that many green roofs have similar soil texture and depth, and initial plant species (Dunnett and Kingsbury 2004;Snodgrass and Snodgrass 2006;Rowe 2015). Several factors, including plant selection, soil composition and maintenance schedule, impact the long-term dynamics of the plant communities on green roofs (Rowe 2015). If these factors are relatively uniform when green roofs are constructed, the plant communities that result after many years may share similarities as well. ...
... Data from annual surveys could then be compared to older but similar roofs to determine the ideal maintenance plan needed to direct a specific successional pathway. Unfortunately, long-term vegetation surveys of green roofs carried out by experts with botanical knowledge are extremely rare (Rowe 2015). However, if green roof plant communities share a common trajectory, it may not be necessary to collect data from a site year after year. ...
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Green roofs may contribute to biodiversity conservation. However, these habitats are not typically planted to support biodiversity and are not monitored to see how communities change over time. Plants on green roofs must be adapted to high stress caused by drought and severe temperatures. Over several generations, plant functional diversity may decrease, as only species able to colonize and survive in harsh habitats persist. If particular traits aid in survival, careful species selection could direct the trajectory of green roof succession and ensure future biodiversity support. In this study, we used long-term vegetation surveys from six green roofs in northeast Germany to discern patterns of plant traits and functional diversity. To determine if site managers could predict future green roof diversity by extrapolating from a common pattern of change, we compared our results with analyses of a chronosequence (1–93 years since establishment) of 13 sites in the same region. We also measured site-level properties of the chronosequence sites to explore factors other than time that may contribute to functional diversity. We found that plant functional diversity generally increased during an initial establishment period but subsequently decreased. Mature communities were primarily made up of species highly tolerant of stress and repeated disturbance. Our results differed slightly between the long-term and chronosequence site types, highlighting the uniqueness of green roof communities. Site-specific factors, including building height, vegetated area, soil depth and soil water-holding capacity, likely contribute to plant functional diversity on green roofs.
... Rooftop microclimates are extreme growing environments for plants, and are subject to high wind speeds, temperatures and solar irradiance. As a result, when choosing plants, practitioners tend to target drought-tolerant succulent species in the genus Sedum (family: Crassulaceae) which is the most commonly used green roof plant, and the industry standard in North America and Europe (Getter and Rowe, 2006;Rowe, 2015). Green roofs planted with Sedum are effective at surface cooling, for example, MacIvor and Lundholm (2011) found they contributed to a reduction in substrate temperature during peak summertime hours by up to 2.39°C, compared to a conventional asphalt rooftop in Halifax, Nova Scotia. ...
... Studies that explore green roof ecosystem services typically span no longer than two years (Rowe, 2015). In practical application, green roofs are intended to operate in-situ in the long-term and are therefore subject to ecological succession dynamics and climatic variability (Getter et al., 2009). ...
Article
The thermoregulation of buildings and cities by green roofs is a primary driver of their integration into urban environments. In warm seasons, green roofs cool buildings (thereby reduce interior air conditioning costs), and cities (impervious surfaces contribute to urban heat islands and vegetation mitigates contributions by conventional roof surfaces). In cool seasons, green roofs insulate buildings by reducing heat flux through the roof surface. Here we investigate thermoregulation services provided by extensive green roofs in warm and cool seasons from temperature data points recorded at 5-minute intervals over a four-year period, and from modules containing either Sedum or perennial grasses and herbaceous flowers, mineral- or organic-based substrate, 10 cm or 15 cm substrate depth, and supplemental irrigation or none. We demonstrate that Sedum outperformed a mixture of perennial grasses and herbaceous flowers over the total inter-annual survey period. The meadow mixture was more dependent on supplemental irrigation than Sedum, but more susceptible to inter-annual climate variability. Our findings point to the durability of Sedum as a plant for extensive green roof cooling, as well as the importance of plant selection and identifying traits that match not just microclimatic conditions in summer, but also in winter.
... Arthropods and microorganisms begin to colonize these sites almost immediately, and are often brought in with the planting material or growing substrate (MacIvor and Ksiazek 2015, Molineux et al. 2014. To date, most studies of biota on green roofs have been carried out for very short time periods (Rowe 2015) and long-term monitoring is rare. Community assembly and diversity patterns on green roofs may resemble other urban habitats or may lack comparable reference sites (Dunnett 2015), and exhibit unique patterns due to their highly engineered state. ...
... Zhang et al. 2013), where ecological succession can be altered, suppressed, or completely arrested (Collins et al. 2000). Our results support those of other green roof studies conducted over shorter time-frames (Bates 2013, Carlisle and Piana 2015, Dvorak and Volder 2010, Rowe 2015. It is possible that minimally maintained green roofs follow site-specific successional trajectories that are difficult to distinguish without additional replicates and longer observation periods (Matthews 2015, Prach et al. 2001. ...
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Cities can support biodiversity and provide the ecosystem services upon which life depends. Green roofs are increasingly common in cities and could be designed to increase biodiversity, but community assembly and succession patterns on green roofs are poorly documented. We used long-term vegetation surveys at 6 extensive green roofs and sampled a 1-93-year chronosequence at 13 extensive green roofs in northeast Germany to determine if plant and arthropod diversity increased over time in a deterministic pattern. We also explored abiotic factors that may contribute to community diversity on green roofs. We found that vegetation cover increased over time, but beyond the first 2 years, vegetation richness and diversity did not. There is no evidence for broadly applicable patterns of succession of plant communities on green roofs. Although the abundance, richness , and diversity of arthropods increased slightly over time, this trend was not statistically significant for ants, bees, beetles, or spiders. The size of the vegetated area of the roof, the conditions of the growing substrate, species richness and diversity of the vegetation, and the proportion of ground-level green space surrounding the roof at 0.5-km and 1.0-km radii were associated with increased arthropod abundance, richness, and diversity. We conclude that community diversity on green roofs is highly variable and dependent on several biotic and abiotic factors that are not consistent among extensive green roofs. Community successional patterns are not conserved; thus, each green roof may support a novel community and contribute to urban biodiversity.
... Second, monitoring organisms of conservation concern in remnant or restored urban patches and collecting offspring for propagation under similar conditions could create an urban adapted stock for future restoration efforts. For example, the Swiss meadow orchid communities remaining on 100+ year old green roofs at the Moos Water Treatment facility in Switzerland provide habitat for rare orchid species, and the seeds of these plants are now used in community restoration around urban Zürich (Rowe, 2015). This latter case represents one example of the successful integration of urban evolutionary ecology and conservation, and could be used as a model for other urban conservation efforts. ...
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Urban ecosystems are rapidly expanding throughout the world, but how urban growth affects the evolutionary ecology of species living in urban areas remains largely unknown. Urban ecology has advanced our understanding of how the development of cities and towns changes environmental conditions and alters ecological processes and patterns. However, despite decades of research in urban ecology, the extent to which urbanization influences evolutionary and eco‐evolutionary change has received little attention. The nascent field of urban evolutionary ecology seeks to understand how urbanization affects the evolution of populations, and how those evolutionary changes in turn influence the ecological dynamics of populations, communities, and ecosystems. Following a brief history of this emerging field, this Perspective article provides a research agenda and roadmap for future research aimed at advancing our understanding of the interplay between ecology and evolution of urban‐dwelling organisms. We identify six key questions that, if addressed, would significantly increase our understanding of how urbanization influences evolutionary processes. These questions consider how urbanization affects non‐adaptive evolution, natural selection, and convergent evolution, in addition to the role of urban environmental heterogeneity on species evolution, and the roles of phenotypic plasticity vs adaptation on species’ abundance in cities. Our final question examines the impact of urbanization on evolutionary diversification. For each of these six questions, we suggest avenues for future research that will help advance the field of urban evolutionary ecology. Lastly, we highlight the importance of integrating urban evolutionary ecology into urban planning, conservation practice, pest management, and public engagement. This article is protected by copyright. All rights reserved.
... Few ecological studies are carried out on green roof communities, indeed as green roofs are dynamics systems, the plant community establishment is subject of changes, due to climate and nutrient availability (Rowe, 2015). The monitoring of such plant communities may evidence the possible changes in the ecological structure and composition over time and plant species that respond to environmental conditions at different time and/or re-establish from a seed bank, contribute to improve the long-term performance of green roofs (Cook-Patton and Bauerle, 2012). ...
Article
Green roofs are roof free spaces where living organisms can find an appropriate habitat to colonise. The establishment of plant species with different functionality can enhance biodiversity and provide ecosystem services. However, drought and nutrient availability can affect the plant development. The extensive green roof was set up in Pisa (Italy) in 2014, 12 modules of 10 cm depth were filled with three substrates composed of compost from municipal mixed waste, pelletised paper sludge, and commercial tephra product (Vulcaflor), as follows: Vulcaflor + compost, Vulcaflor + pellet + compost, and Vulcaflor + pellet, characterised by decreasing level of nitrogen content. The species planted in 2014 were chosen from the herbaceous spontaneous vegetation of urban and rural swards not often mowed, plus two sedum species. After the establishment phase, the green roof community was progressively dominated by Sedum species and other species were seeded in 2016. In 2018-19 the plant functional types and the community structure were monitored. Besides seasonal fluctuations, nitrogen shaped the composition of the community, and Sedum species showed high cover values in nitrogen-richer substrates. Annual forbs colonised the plots with a lower nitrogen content. In summer, the number of species drastically fell, and Sedum album was dominant in the three substrates. Seedling recruitment regenerated the community in the cooler season, increasing the diversity in the poor substrate. The scarcity of nitrogen led to the development of stress-tolerator annuals increasing the biodiversity in the rainy-cool season. Annual species constitute a transient seed bank which enables the system to regenerate when rain follows periods of heat and drought.
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How green roofs can be designed to be resilient to changes in climate. The article provides details and examples from Ecoregional Green Roofs: Theory and Application in the Western USA and Canada
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This chapter introduces ecoregional green roofs by discussing the development of native plant communities, the history of modern green roofs, and some observations about ecoregional green roofs. It examines the development of the natural vegetation in the western U.S. and Canada and the kinds of plant communities that make up ecoregions appropriate for different forms of green roofs. The history of green roof origins and the development of ecoregional green roofs provide insight into the growth of the modern green roof industry in Europe and North America. Original intentions for green roofs can be misguided, as design decisions or maintenance practices can be out of line with the vegetation selected, or the microclimate of the roof. Several early examples of built ecoregional green roofs highlight successes and lessons learned. Although the conceptual framework laid out in Chap. 1 (and Chap. 2) can be applied anywhere, the climate characteristics for green roofs growing west of the 100th meridian provide background and rationale for the targeted regions of this book. Our knowledge and research literature is only beginning to include the analysis of ecoregional green roofs located in cities where plants experience prolonged exposure to heat and drought, or both.
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This chapter investigates the theoretical background of environmental and ecological factors that can be used to inform the design of ecoregional green roofs. Ecoregions are defined by major or minor delineations of plant communities and their interactions with other resident or transient organisms. By observing and learning how native vegetation adapts and thrives in its natural settings, green roof researchers, educators, and designers can learn how to make decisions about the resourceful use of native vegetation on green roofs. This chapter discusses how green roofs must respond to environmental factors such as heat stress, drought, and varied slope and soil conditions, and how these factors can inform the design of green roofs with native vegetation. The chapter ends with a discussion regarding how ecoregions are defined in this book and are employed in the case studies in Part II of this book.
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Chapter 11 provides a synthesis of and commentary about ecoregional green roofs informed by the case studies in Part II. Discussions address critical factors outlined in Chap. 2 such as the conservation of native plant communities, the use of native vegetation on green roofs, biodiversity, maintenance, water for irrigation, and microclimates. This chapter also discusses how ecoregional green roofs have been used as a part of therapeutic and biophilic design, how green roofs play an important role in the development of green and sustainable architecture, integrated site design, and parameters implicit to LEED, the Living Building Challenge, and SITES programs. We also discuss the use, integration, and potential expansion of ecoregional green roofs at various scales as well as landscape structures such as corridors, patches, and matrices. The chapter concludes with a look at future opportunities for green roofs. We discuss research gaps, where new knowledge and research are needed. We also note opportunities related to policies, education, industry support, and innovation.
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Green roofs have received much attention in recent years due to their ability to retain rainwater, increase urban diversity, and mitigate climate change in cities. This interdisciplinary study was carried out on three historical green roofs covering bunkers in Wrocław, located in southwestern Poland. It presents the results of a three-year investigation of the water storage of these roofs. The study also presents soil conditions and spontaneous vegetation after their functioning for over 100 years. The soils covering the bunkers are made of sandy, sandy-loam, and loamy-sand deposits. This historical construction ensures good drainage and runoff of rainwater, and is able to absorb torrential rainfall ranging from 100 to 150 mm. It provides suitable conditions for vegetation growth, and forest communities with layers formed there. In their synanthropic flora, species of European deciduous forests dominate, which are characteristic of fresh or moist and eutrophic soils with a neutral reaction. Some invasive species, such as Robinia pseudoacacia, Padus serotina, and Impatiens parviflora, also occur with high abundance. Nowadays, historical green roofs on fortifications, although they have lost their primary military role, are of historical and natural value. These roofs can promote the nonmilitary functions of historical fortifications in order to strengthen the ties between nature and heritage. Protecting and monitoring historical green roofs should be included in the elements of the process of sustainable development and the conservation of these structures in order to mitigate climate change in the outskirts of the city. For this, it is necessary to ensure proper conservational protection, which, in addition to maintaining the original structure, profiles, and layout of the building, should include protection of their natural value.
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Plants suitable for extensive green roofs must tolerate extreme rooftop conditions, and the substrates in which they grow must fulfill horticultural and structural requirements. Deeper substrates may retain more water for plants during dry periods, but will also weigh more, especially when near saturation. A study in central Pennsylvania was conducted to evaluate the influence of substrate type and depth on establishment of five green roof plants. Two stonecrops [white stonecrop (Sedum album) and tasteless stonecrop (Sedum sexangulare)], one ice plant (Delosperma nubigenum), and two herbaceous perennials [maiden pink (Dianthus deltoides) and saxifrage pink (Petrorhagia saxifraga)] were planted in three depths (30, 60, and 120 mm) of two commercially available green roof substrates (expanded shale and expanded clay). Study flats inside a plasticulture tunnel received three drought treatments (no drought, 2 weeks early drought, and 2 weeks late drought). The two stonecrops performed well under most conditions, although tasteless stonecrop was stunted by early drought. Ice plant only grew well when provided with water. When subjected to any drought, the herbaceous perennials had the fewest survivors in the expanded shale. Saxifrage pink flowered profusely wherever it survived. The study plants were most affected by substrate depth, except for maiden pink, which responded solely to drought. When subjected to early drought conditions, the herbaceous perennials did not survive in 30 mm of either substrate, or in 60 mm of expanded shale. Although the stonecrops performed well in 60 mm of substrate when subjected to drought, their performance was superior in the expanded clay compared with shale.
Article
Because of greater interest in green roofs in the United States, it is critical to increase the number and geographic range of proven plant resources for long-term survival on rooftops. Successful plant taxa for extensive green roofs must establish themselves quickly, provide high groundcover density, and tolerate extreme environmental conditions. Furthermore, dead load weight restrictions on many buildings may limit the substrate depth that can be applied. The objective of this study was to evaluate the effect of substrate depth on initial establishment and survival of 25 succulent plant taxa for green roof applications in the midwestern United States. Survival, initial growth, and rate of coverage were compared for plants grown in three substrate depths (2.5, 5.0, and 7.5 cm) on 24 roof platforms. Plant coverage was determined from image analysis of weekly digital photographs. Results indicate deeper substrates promote greater survival and growth; however, in the shallowest depth of 2.5 cm, several species continued to persist. Of the 25 species initially planted, only 47% survived in the deepest substrate of 7.5 cm. Recommended species at the depths tested for climates similar to southern Michigan include Phedimus spurious Raf. 'Leningrad White', Sedum acre L., S. album L. 'Bella d'Inverno', S. middendorffianum L., S. reflexum L., S. sediforme J., and S. spurium Bieb. 'Summer Glory'. Subsidiary species that are present at specific substrate depths but may not exhibit an ability to cover large areas include S. dasyphyllum L. 'Burnatii', S. dasyphyllum L. 'Lilac Mound', S. diffusum W., S. hispanicum L., and S. kamtschaticum Fisch. The primary deterrent for these subsidiary species was little to no survival at 2.5 cm. Deeper substrates promoted greater survival and growth for nearly all species tested.
Article
Green roofs, or vegetative or living roofs, are an emerging technology in the United States. Because environmental conditions are often more extreme on rooftops, many xerophytic plants, especially Sedum, are ideal for extensive green roofs because they are physiologically and morphologically adapted to withstand drought. A greenhouse experiment was conducted to determine the effect of watering regimens on plant stress as measured by chlorophyll fluorescence (Fv/Fm), biomass accumulation, substrate moisture, and evapotransipiration on succulent plants on Sedum acre L., S. reflexum L., S. kamtschaticum ellacombianum Fisch., and non-Crassulacean acid metabolism (CAM) plants of Schizachyrium scoparium Nash and Coreopsis lanceolata L. Plants were grown at a substrate depth of 7.5 cm. Results indicate even after the 4-month period, Sedum spp. survived and maintained active photosynthetic metabolism to a greater extent than Schizachyrium and Coreopsis. Furthermore, when Sedum was watered after 28 days of drought, chlorophyll fluorescence (Fv/F m) values recovered to values characteristic of the 2 days between watering (DBW) treatment. In contrast, the non-CAM plants required watering frequency every other day to survive and maintain active growth and development. Regardless of species, the greatest increase in total biomass accumulation and fastest growth occurred under the 2 DBW regimens.
Article
Green roofs are an increasingly common, environmentally responsible building practice in the United States and abroad. They represent a new and growing market for the horticulture field, but require vegetation tolerant of harsh environmental conditions. Historically, Sedum species have been the most commonly used plants because, with proper species selection, they are tolerant of extreme temperatures, high winds, low fertility, and a limited water supply. A greenhouse study was conducted to determine how water availability influences growth and survival of a mixture of Sedum spp. on a green roof drainage system. Results indicate that substrate volumetric moisture content can be reduced to 0 m3·m-3 within 1 day after watering depending on substrate depth and composition. Deeper substrates provided additional growth with sufficient water, but also required additional irrigation because of the higher evapotranspiration rates resulting from the greater biomass. Over the 88 day study, water was required at least once every 14 days to support growth in green roof substrates with a 2-cm media depth. However, substrates with a 6-cm media depth could do so with a watering only once every 28 days. Although vegetation was still viable after 88 days of drought, water should be applied at least once every 28 days for typical green roof substrates and more frequently for shallower substrates to sustain growth. The ability of Sedum to withstand extended drought conditions makes it ideal for shallow green roof systems.
Article
Green roof technology in the United States is in the early development stage and several issues must be addressed before green roofs become more wide-spread in the U.S. Among these issues is the need to define growing substrates that are lightweight, permanent, and can sustain plant health without leaching nutrients that may harm the environment. High levels of substrate organic matter are not recommended because the organic matter will decompose, resulting in substrate shrinkage, and can leach nutrients such as nitrogen (N) and phosphorus (P) in the runoff. The same runoff problems can occur when fertilizer is applied. Also, in the midwestern U.S., there is a great deal of interest in utilizing native species and recreating natural prairies on rooftops. Since most of these native species are not succulents, it is not known if they can survive on shallow, extensive green roofs without irrigation. Five planting substrate compositions containing 60%, 70%, 80%, 90%, and 100% of heat-expanded slate (PermaTill) were used to evaluate the establishment, growth, and survival of two stonecrops (Sedum spp.) and six nonsucculent natives to the midwestern U.S. prairie over a period of 3 years. A second study evaluated these same plant types that were supplied with four levels of controlled-release fertilizer. Both studies were conducted at ground level in interlocking modular units (36 × 36 inches) designed for green roof applications containing 10 cm of substrate. Higher levels of heat-expanded slate in the substrate generally resulted hi slightly less growth and lower visual ratings across all species. By May 2004, all plants of smooth aster (Aster laevis), horsemint (Monarda punctata), black-eyed susan (Rudbeckiet hirta), and showy goldenrod (Solidago speciosa) were dead. To a lesser degree, half of the lanceleaf coreopsis (Coreopsis lanteolata) survived in 60% and 70% heat-expanded slate, but only a third of the plants survived in 80%, 90%, or 100%. Regardless of substrate composition, both 'Difrusum' stonecrop (S. middendorffianum) and 'Royal Pink' stonecrop (S. spurium) achieved 100% coverage by June 2002 and maintained this coverage into 2004. In the fertility study, plants that received low fertilizer rates generally produced the least amount of growth. However, water availability was a key factor. A greater number of smooth aster, junegrass (Koeleria macrantha), and showy goldenrod plants survived when they were not fertilized. Presumably, these plants could survive drought conditions for a longer period of time since they had less biomass to maintain. However, by the end of three growing seasons, all three nonsucculent natives also were dead. Overall results suggest that a moderately high level of heat-expanded slate (about 80%) and a relatively low level of controlled-release fertilizer (50 g·m-2 per year) can be utilized for green roof applications when growing succulents such as stonecrop. However, the nonsucculents used in this study require deeper substrates, additional organic matter, or supplemental irrigation. By reducing the amount of organic matter in the substrate and by applying the minimal amount of fertilizer to maintain plant health, potential contaminated discharge of N, P, and other nutrients from green roofs is likely to be reduced considerably while still maintaining plant health.