Green Roofs for Urban Biodiversity:

Latornell SymposiumGavin Miller

TRCA Flora BiologistNovember 2010

Green Roof Research to Date• Hydrology

storm water quantity attenuationstorm water quality improvement

• Energy conservationinsulationevapotranspirative cooling

• Urban agriculture & open space

How can a green roof support biodiversity?

• Target suitable species & communitiesSite conditions (extensive green roof)

Analogous ecosystems in nature

• Two roofs looked at: York & KortrightFlora (and fauna)

• Recommendations

Typical Conditions

• Shallow substrate (about 15 cm)• Heat, wind, dryness (York irrigated)• Nutrient leaching• More similarity to primary succession• Fairly low vegetation cover• Tough conditions favour more unusual

conservative species in long term

Analogue ecosystem - AlvarFlat limestone pavement with thin soil – closest analogue to most extensive green roofs. Globally significant habitat.

Analogue ecosystem – Sand Barren

Dry, exposed sites with minimal organic matter

Analogue ecosystem – Mineral FenConstant mineral-rich calcareous ground-water seepage; fairly low nutrient status. May be associated with old gravel pits. Similar to irrigated green roof.

York University Green Roof

• Built in 2001 – study in 2004-05• Substrate volcanic pumice from BC• Some compost, peat, sand, clay• 14 cm depth• Two main sections each with own seed mix• Smaller sections• Regular irrigation.

York University Green Roof

Roof sections• Ornamental flower seed mix• Grass seed mix• Smaller sections quickly surveyed: mostly

flower mix, one unseeded• Mostly non-native species (showy semi-

wild flowers and fescue grasses)• Seed mixes skewed results

Results – York University study

• 91 species of vascular plant in 2004• 110 species in 2005• 37% native / 63% exotic• Flower roof 62 species in 2005 (29% native)• Grass roof 62 species in 2005 (42% native)• One small unseeded section 36 spp. (50% native)• High moss & liverwort cover (30 - 40%); discontinuous

vascular plant cover

Vegetation Cover – York U Roof

Floristic Quality• Biodiversity still fairly low• Untested but fairly consistent slight

increase from 2004 to 2005• Coefficient of conservatism (0-10 scale)• Roof as a whole 2.8• Flower roof mean 1.9-2.5• Grass roof mean 1.2-1.3• High quality prairie mean >4

Seed mix still dominant after 4 yrs

A closer look• Some sensitive species were found• Mostly colonized spontaneously• 7 species of TRCA regional concern; 8

with CC of 5 or more• Most of these spontaneous; 2 in seed mix• Familiar invasive species in area (e.g.

thistle, buckthorn) did not colonize well!

York University Roof - Asters

York University Roof - Orchids

the birds and the bees…

Fauna – bird and bee observations• Six species of birds observed• Birds seemed to favour adjacent ground

habitats• 21 species of bees in 2004 and 27 in 2005

on a few York University sites including green roof

• Green roof appears to support similar bee diversity as on-ground old-field.

Limitations to York U study• Very short time frame (2 seasons)• New green roof cannot be justly compared

with high quality prairie• Study wasn’t able to compare green roof

with comparable surface habitat• Green roof is small – only one in vicinity, so

no cumulative effects• Roof was seeded with non-native seed mix!

Archetype Sustainable House – Kortright Centre

Kortright Green Roof• A new roof, no pre-existing seed mix• Allowed for experimentation with native

species• Small area, but open to public viewing and

interpretation• Deliberately chose alvar species• Combination of salvaged plants from

Kawarthas and nursery-propagated.

At time of planting - April 2009

Kortright Roof – some species• Nodding wild onion (Allium cernuum)• Wild chives (Allium schoenoprasum)• Prairie smoke (Geum triflorum)• Columbine (Aquilegia canadensis)• Hairy beard-tongue (Penstemon hirsutus)• Prickly-pear (Opuntia humifusa) 1 plant• Prairie ragwort (Packera paupercula)• Bristle-stalked sedge (Carex eburnea).

About a month later…

A year later – August 2010

Good showings a year later byChivesNodding wild onionFalse pennyroyalPrairie ragwortPrairie smokeColumbineBristle-stalked sedge

• Some of these species are sensitive.

Limitations & Opportunities - Kortright• Qualitative – no actual study• Small size• Need to track performance of individual

species.

Easily accessibleClean slateNearby comparisons – Earthrangers.

Sedums or native plants?Sedum roof, Earthrangers Alvar roof, Kortright

The potential…• 20% of the City of Toronto surface is roof

tops• 16% of Don watershed (2002) is natural

cover (forest, wetland, meadow)• Therefore green roofs are essential for

improving health of urban watershed.

Aid to existing or target natural cover

Role of TRCA re. Green Roofs• Support programs like City of Toronto’s• Standard setting for hydrological

performance and native biodiversity• TRCA has interest due to storm water

mitigation and natural heritage• TRCA has hydrological and biological

expertise for restoration design.

Recommendations• Reduce fertilizer – substrate can

contribute to phosphate loading• seed mix – ensure native and document

sourcing – don’t just use sedum• could rely more on what seeds naturally• target certain suitable flora of conservation

concern for recovery• decisions about target community,

substrate depth, irrigation, etc.

Next steps• long-term monitoring of different roofs

substrate types, depths, irrigationnative versus sedumplantings versus spontaneousinclude invertebrates

• 5 – 7 year minimum to get clear monitoring signal.