|Green Roof Research in Halifax
Jeremy Lundholm & Scott MacIvor
Most of the research we do involves extensive green roofs. Extensive green roofs are typically defined as having shallow substrates (usually 8”/20cm or less of growing medium). The reason we are mainly focused on extensive roofs is that there is greater potential application on a greater range of buildings for shallow-substrate green roofs that minimize additional load on the building. That said, there are more than 50 green roofs that have been established in Halifax over the last 30 years, and most of these are intensive roofs. We are certainly interested in exploring the benefits of these roofs as well. In general, intensive roofs provide all the same benefits as extensive roofs, and typically in greater quantities.
1. Plant Species Evaluation
We have tested 20-30 plant species for extensive green roofs in Nova Scotia. Plants used on shallow-substrate (extensive) green roofs need to be both drought- and frost-tolerant. Many of the species favoured in the green roof industry are succulents native to Europe and Asia, especially Sedums. These plants originate mostly in rocky or alpine areas and are highly stress-tolerant. In Nova Scotia, we are primarily evaluating native plants from the coastal barrens. Coastal barrens feature shallow soils, high winds, salt-spray and other conditions that cause stress for plants. The dominant vegetation is mainly creeping shrubs, but there are forbs, grasses and even a few succulents. So far, the species showing the best survival and growth characteristics are not the main species on the barrens, but rather, disturbance-tolerant species that can grow quickly (Solidago bicolor and Danthonia spicata, for example). The slow-growing creeping shrubs, such as Empetrum nigrum, Vaccinium vitis-idaea and Gaultheria procumbens, may eventually be useful on local green roofs but it may take a long time for enough cover to establish. In the meantime, quickly spreading species may take over the roof. On the other hand, the creeping shrub Sibbaldiopsis tridentata grows fairly quickly and has several other features that make it a great green roof plant (flowers that attract pollinators among others). The species we are using do not represent a random sample of coastal barrens species, rather, we are limited by the species that we can easily propagate from seed.
2. Quantification of Ecosystem Services
The ecosystem services provided by green roofs that have received the most research attention roofs are summer cooling and stormwater capture. In our work, we quantify cooling by evaluating heat flux through the roof using heat flux transducers (Figure 1), however, it is easier to get replicated temperature measurements that heat flux estimates, so we often use these as an index of cooling: improved performance equals lower roof surface temperatures, because the temperature differential between the inside of the building (usually constant) and the outside is directly proportional to the amount of heat flowing into the building (a cooler roof means a smaller temperature differential between the roof surface and the interior). We have quantified stormwater capture in two ways: in a facility with 12 replicated green roof panels (including two conventional roof controls) we quantify flow rates using flowmeters and a datalogger. For better replication, so that we can test many different kinds of vegetation, we weigh modules in our modular green roof system [Figure], directly add a known amount of water (corresponding to various storm magnitudes) and re-weigh to determine how much water is retained.
Green roofs provide many other services, such as the provision of habitat for other organisms, winter thermal benefits, air pollution reduction, carbon sequestration, aesthetic and
Figure 1. Built-in green roof system, showing locations of sensors.
psychological benefits. We are already exploring the role of green roofs in Halifax in providing habitat for insects (see Habitat Provisioning section below), and we plan to extend these studies to winter benefits (already ongoing) and carbon capture. We are also interested in determining which plant species or mixtures best resist colonization by weeds and tree seedlings: the less invasible the system is, the less maintenance that is required.
Overall, green roofs represent a living ecosystem that provides many different services, most of which cannot be provided by competing technologies that do not include a vegetation component. For example, a white, or reflective roof may, in some situations, provide greater summer roof cooling than a green roof, but have little benefit in winter, and have no positive impact on stormwater management, habitat provisioning or urban aesthetics. Thicker insulation under a roof is also an optimal way to improve both winter and summer thermal performance, but again, fails to provide any of the other benefits of green roofs. We don’t necessarily expect green roofs to outperform any particular competing roof technology for a singular benefit, but overall, there is no other use of roof space that provides such a range of significant services.
Figure 2. Modular green roof system
3. Role of Vegetation and Effects of Biodiversity
While few researchers have examined the effects of different kinds of plants and vegetation on the performance of green roof services, this is one of the major emphases in our lab. To date, we have found that the kind of plants on the roof makes a large difference on the performance of summer cooling and stormwater capture. Preliminary results suggest that different species perform different services optimally, for example, Deschampsia flexuosa performs well in roof cooling but green roofs with this species are among the worst at capturing stormwater (likely because the conservative water-use and growth form result in the substrate beneath this species retaining more water between rain events than other species, with the consequence that less stormwater can be captured when it next rains).
We have also tested whether increasing the number of species or increasing the diversity of life-forms on the roof can improve the performance of various green roof services. In our experiments, we have controlled the number of individual plants but altered the number of life-form groups. The different life-forms we have used so far include: succulents, grasses, tall forbs, creeping forbs and creeping shrubs. We have some evidence that the number of life-forms makes a difference: after the first growing season, most monocultures we tested did not make the roof cooler than substrate-alone controls (all membranes and growing medium of the green roof system, but not the plants), but four of the treatments in which we mixted life-forms were cooler than the controls. By the third growing season, the diversity effects were stronger, but many monocultures also performed well. It appears that planting a mixture of species, especially including multiple life-forms such as grasses, succulents and forbs can be beneficial compared to many of the monocultures. We are actively investigating these issues in several ongoing experiments.
4. Habitat Provisioning
Although green roofs should never be considered a substitute for ground level green space when construction of a building is inevitable, replacing the conventional roof surface with plants and growth medium will increase local vegetated surface area and improve the connection between scattered habitat space for urban flora and fauna. Moreover, using planting practices that increase functional diversity (ie. phenology, canopy cover, floral diversity, natives) will improve the resilience of our urban ecosystems to short-term environmental impacts. Working with local communities to use similar techniques on their roofs, gardens and parks will not only increase education and awareness, but also increase movement of species through out urban ecosystems, and sustain their populations long-term. The optimization of habitat provisioning on green roofs is gaining support as researchers begin to acknowledge the value of urban biodiversity and elucidate how the services provided by urban fauna, like insects and birds (ie. pollination, plant and insect pest control, decomposition) can be monetized.
Using different collecting protocols, we’ve characterized insect richness from our Green Roof Testing Facility over the past two years. Over 200 species of insect were identified from the Facility between May and October 2008. We expect that the number of species collected in the 2009 field season will be considerably higher (once we’ve analyzed the data. For the 2009 season, we are characterizing insect biodiversity (richness and abundance) and have included four more green roofs from downtown Halifax, and adjacent ground level green spaces into the analysis. Insect biodiversity will be compared between green roof and ground level space, along with vegetation richness, green space surface area, and a number of other environmental characteristics. Although data entry and identification continue, initial findings have included new insect records for Nova Scotia and the Maritimes, as well as the discovery of a large population of Phospaenus hemipterus (Coleoptera: Lampyridae) occupying a green roof and cemetery across the street. This species of firefly is endangered across its native European range, and is the only diurnal and flightless species known in the world. Cities are generally thought to be unsuitable for all but the most pesky and invasive insect species. Our research into habitat provisioning of green roofs has clearly demonstrated that urban habitats are capable of supporting a large diversity of insects, including numerous uncommon species.
5. Food Production
Food production from roofs mainly involves intensive roofs, where, given enough soil, you can grow practically anything possible in a ground-level garden in the same climate, but there is potential for extensive roof gardens to provide food as well. Chives (Allium schoenoprasum) are a common component of extensive roofs in Germany (Köhler 2006) and there are a number of other herbs that can probably grown in shallow soils on roofs. In moist climates, there may be potential to develop crops or edible wild plants that can be used for food. We are currently evaluating four berry plants on shallow extensive roofs (~6cm of growing medium): wild strawberry (Fragaria vesca), large cranberry (Vaccinium macrocarpon), low-bush blueberry (Vaccinium angustifolium) and mountain cranberry (lingonberry, partrigeberry) (Vaccinium vitis-idaea). Of these, wild strawberry and large cranberry have some potential: strawberry is currently (summer 2009) producing fruit on the roof. Large cranberry does very well in our system, but has not yet produced flowers in three years of growth on the roof. The others are too slow-growing to warrant interest yet, but we have more work to do in testing better growing media for these species. While our research group is very interested in food production from roofs, please remember that this is a plant ecology research lab, and our funding is tied to questions that are clearly ecological in nature. However, we are happy to partner with other organizations more capable of forwarding a green roof food production agenda.
6. New Projects
Saint Mary’s University is constructing a new green roof on our newest building, the Atrium. This building also features an indoor living wall which filters the air, resulting in lower heating and cooling costs (because less outdoor air needs to be imported and brought to room temperature). On the Atrium roof, we will be further quantifying the ecosystem services provided by green roofs. We will use this roof, and the other facilities on the library roof to determine the relationships between plant traits and green roof functions, the mechanisms by which greater plant diversity may improve green roof performance, and examine the long-term coexistence of plant species.