The influence of plant type on green roof rainfall retention

Urban Ecosystems - Green roofs can mitigate the flood risk by reducing the volume of runoff through direct interception and subsequent evapotranspiration (ET), but the planting choices can...     

Salt tolerance of common green roof and green wall plants

Detrimental effects of road deicing salt on vegetation are well known and have been well studied, with the exception of typical green roof plants, which could experience damage on green roofs with public access and green walls near roadways in cold climates. Two studies were conducted comparing salt tolerance of five Sedum species, two Allium species and a mixture of turf grasses when exposed to six levels of salinity applied either as foliar spray or as liquid applications to the soil. A third study compared salt tolerance when plants were placed at three distances from a major highway. Response variables measured included survival, a health score from 0 to 5, and a growth index. Allium cernuum, A. senscens and S. ellecombianum were relatively tolerant of both saline spray and soil inundation at high saline concentrations in terms of survival, mean health scores, percentage of healthy plants and growth index. Sedum reflexum was much less tolerant of saline spray at higher salinity concentrations and soil inundation regardless of salinity levels. Distance from the road had no effect on plant survival rates but plants farthest from the road had higher mean health scores and a greater percentage of healthy plants than plants closer to the highway.     

Substrate influence on aromatic plant growth in extensive green roofs in a Mediterranean climate

Green roofs have been described as technical solutions to overcome urban environmental problems, such as decrease of vegetation and stormwater management. In the present study, two pilot 20 m² extensive green roofs were implemented in an urban Mediterranean region, at a 1st storey on a warehouse building structure, in order to test the adequacy of different substrates for supporting aromatic plants (Lavandula dentata, Helichrysum italicum, Satureja montana, Thymus caespititius and Thymus pseudolanuginosus). Experimental substrates included expanded clay and granulated cork as main components, supplemented with organic matter and crushed egg shell. A commercial substrate that obeys to FLL guidelines was also tested. Plant growth was assessed and compared within each platform. All experimental substrates proved to be adequate for vegetation growth, with the combination of 70% expanded clay, 15% organic matter and 15% crushed egg shell showing the best results regarding plant establishment and growth over time. Water runoff quality parameters - turbidity, pH, conductivity, NH₄ ⁺, NO₃ ^?, PO₄ ^3? - met standard values required for water reuse for non-potable purposes, such as toilet flushing or irrigation. Preliminary qualitative thermographic measurements comparing surface temperature of different plant species and the substrate showed that temperature of vegetation surface was lower than substrate, reinforcing green roofs benefits of lowering air temperature in their surroundings. The present research shows that aromatic vegetation combined with clay substrates are suitable for green roofs located in countries of the Mediterranean region.     

The impact of mosses on the growth of neighbouring vascular plants,substrate temperature and evapotranspiration on an extensive green roof

Currently the majority of vegetation used on shallow extensive green roofs are species of Sedum, which are able to survive in the harsh green roof environment. While mosses frequently colonize green roofs in Europe, intentional planting of mosses on green roofs is less common, especially in North America. Mosses may contribute to the ecosystem services provided by green roofs, and may act as facilitators of vascular plants. This study examined the effect of three different moss species on soil temperature, water loss rates and the growth of neighbouring vascular plant species. Overall, the presence of mosses in this experiment impacted the neighbour species differently, suggesting that mosses are best used in particular species combinations. One species of grass showed a net benefit of moss neighbours, suggesting that facilitation may be operating. Mosses reduced soil temperature relative to bare substrates; net evapotranspiration of green roof modules planted with mosses varied depending on the identity of moss and neighbour species.     

Vegetation on and around large-scale buildings positively influences native tropical bird abundance and bird species richness

High population growth in the tropics is driving urbanisation, removing diverse natural ecosystems. This is causing native species to suffer while introduced synanthropes flourish. City planners are developing urban greenspace networks, in part trying to address this issue. Architects contribute to these greenspace networks by designing elevated and ground level green spaces on large-scale buildings. However, little evidence is available on whether building green spaces support native fauna. This is true for birds in tropical Singapore that support important ecosystem services and have existence value. Therefore, in this study, we conducted bird surveys and statistical analyses to determine, if and how vegetation on three building green space types (ground gardens, roof gardens and green walls) have a positive impact on native or introduced bird species. We found that elevated greenery (roof gardens and green walls) on large-scale buildings supported a higher richness of birds and abundance of urban native birds than control roofs and walls without vegetation. Ground gardens supported similar levels of native species as roof gardens but also a larger proportion of generalist synanthropes. However, we found no tropical forest habitat specialists across any space type. Therefore, we recommend roof gardens and ground gardens as a potential space for urban natives outside of a less competitive ground-level urban environment. Our study also found certain building design elements (height of elevated space, presence of specific plants) supported different species groups. Therefore, we suggest that these ecological requirements for different species groups are considered when designing a building’s green space.

     

The exchangeability and leachability of metals from select green roof growth substrates

Batch extraction and leaching studies were conducted with potential green roof substrates (e.g., Axis, Arklayte, coal bottom ash, Haydite, Lassenite, lava rock, and composted pine bark). The results indicated that these materials would not likely be sources of Cr, Cu, Fe, Ni, or Zn and that Lassenite would be considered a source of Mn if the leachate concentrations were compared to USEPA drinking water standards for these elements. Lassenite would not be a source of Mn if the data was compared to a USEPA standard for Mn toxicity to aquatic life. All of the substrates tested leached Cd and/or Pb concentrations that exceeded the USEPA water quality standards at least once during the 6-month leaching study, so these materials may be potential sources of Cd and Pb in green roof storm water runoff. The leaching of Cu, Cd, Fe, Mn, Pb, and Zn was differentially influenced by time and/or the presence of a single Sedum hybridum ‘immergrauch’ plant. The leaching of Cd, Cu, and Pb displayed complex, three-way interactions between main effects (substrate type and the presence or absence of a plant) and between leaching events. For all substrates except Lassenite, the presence of a S. hybridum plant decreased the leaching of Pb over time. The leaching of Cd was generally enhanced by plants for most substrates with time. Collectively the results suggest that changes in the biogeochemical conditions within green roof systems may alter metal solubility, decreasing the leaching of some elements and increasing the leaching of others.     

Elevated phosphorus: dynamics during four years of green roof development

Green roofs are emerging engineered ecosystems that provide multiple benefits, but many are constructed with nutrient-rich substrate and have been found to leach out high levels of phosphorus (P) in runoff. It is unclear, however, how long green roofs act as sources of P or what mechanisms are responsible for these net losses. We measured P concentrations in runoff water over 4 years from a 1–5 year old extensive green roof in Cincinnati, OH, USA, produced a model to predict runoff P levels into the future, and validated predictions using runoff from 2 nearby extensive green roofs. P concentrations in runoff from the focal green roof were on par with heavily fertilized agroecosystems and displayed strong seasonal dynamics and a rapid decline over the 4-year study. Runoff measurements and changes in substrate P content over a 2-year period were used to estimate a mass balance for green roof P. P loss from the substrate was substantial (4.55 ± 2.3 g P/m²/yr), but only a small portion of the loss was attributable to leaching of P in runoff (0.19–0.65 g P/m²/yr). Missing P may be attributed to a combination of plant uptake and altered P form and binding strength, but further research is needed to precisely identify the mechanisms of P depletion. Our results also suggest that these and similar extensive green roofs are likely to act as environmentally significant sources of P for 10 or more years following roof installation, highlighting the need for reductions in initial substrate P content.     

Chemistry of growth medium and leachate from green roof systems in south-central Texas

Installation of intensive and extensive green roofs is becoming popular for reducing runoff from impervious surfaces in many cities around the world. Most studies on runoff quality from green roofs have been conducted in cooler northern climates. We examined the losses and gains of nutrients, cations and selected anions in planted and unplanted growth medium and compared these to initial growth medium (IGM) typically used for green roof modules in south-central Texas. Water extracts of growth medium and leachate from three replicates of unplanted growth medium and three planted species (Sedum kamtschaticum, Delosperma cooperi and Talinum calycinum) were examined. During the first 6 months after establishment we observed high losses of nitrate (25 to 44 mg kg⁻¹), dissolved organic carbon (DOC: 155 to 190 mg kg⁻¹) and nitrogen (DON: 9.0 to 11.2 mg kg⁻¹) and orthophosphate-P (1 to 2 mg kg⁻¹). Average leachate concentrations based on four rain events 6 months after establishment ranged from 0.3 to 6.6 mg L⁻¹ in planted modules and 6.3 mg L⁻¹ in unplanted modules for nitrate-N, 38 to 42 mg L⁻¹ in planted modules and 32 mg L⁻¹ in unplanted modules for DOC, 2.1 to 3.1 mg L⁻¹ in planted modules and 2.1 mg L⁻¹ in unplanted modules for DON and 0.27 to 0.37 mg L⁻¹ in planted modules and 0.40 mg L⁻¹ in unplanted modules for orthophosphate-P. We suggest that after the establishment of green roofs, leachate losses may contribute some runoff concentrations of nitrogen, carbon and phosphorous in urban areas.     

Influence of vegetation composition on runoff in two simulated green roof experiments

Despite the fact that green roofs are based upon living systems, the majority of published research literature contains little specific information on the contribution of plants to the various functions and properties of green roofs. Furthermore, there has been little investigation of the influence of the composition of vegetation on the physical properties of a green roof system. This paper reviews previously published material that throws light on the role of vegetation composition on green roof function, with particular regard to rainwater runoff. Two experiments at the University of Sheffield, UK, are considered in detail: (a) An outdoor lysimeter experiment that investigated the quantity of runoff from trays containing 100 mm of growing medium and combinations of grasses and forbs, together with bare substrate, and (b) a greenhouse experiment using simulated rainfall to estimate the amount of rainfall intercepted by different vegetation types. In both cases the vegetation ranged from simple monocultures of forbs and grasses through to complex mixtures of both. In both cases, the composition of the vegetation was found to significantly affect both the amount of water retained and released from the system.     

Trends in bird species richness,abundance and biomass along a tropical urbanization gradient

Impacts of urbanization on biodiversity are commonly studied using urbanization gradients which provide a space-for-time substitution in estimating consequences of urban expansion. Rates of urbanization and human population growth are high in tropical countries of the developing world, which also hold most of the world’s biodiversity hot-spots, yet few studies have considered biodiversity trends along urban gradients in these regions. Bird communities across a gradient of nine sites in Uganda, from the city centre of Kampala to outlying rural locations, were studied over a six year period. These sites were ordered along an urbanization gradient using Principle Components Analysis based on habitat variables estimated at each site. Bird species richness showed a decrease from rural to urban sites, a trend especially evident in forest birds. There was no clear pattern in total abundance, total biomass or biomass per individual along the gradient. However, this latter result was heavily influenced by a colony of Marabou Storks at one site. When this species was omitted, there was evidence of a positive trend with urbanization, showing that as species richness decreased, the bird community was increasingly dominated by larger species with increasing urbanization, which were mainly scavengers able to exploit human refuse. These results provide further support for the negative impacts of urbanization on species richness, but also demonstrate trends in abundance and biomass are variable across different regions. In particular, the increasing dominance of larger species in urban areas may be relevant to certain geographic and/or socioeconomic contexts.     

Relation between green spaces and bird community structure in an urban area in Southeast Brazil

Increased urbanization typically leads to an increase in abundance of a few species and a reduction in bird species richness. Understanding the structure of biotic communities in urban areas will allow us to propose management techniques and to decrease conflicts between wild species and human beings. The objective of this study was to describe the structure of the bird community in an urban ecosystem. The study was carried out in the city of Taubaté in southeastern Brazil. Point-counts were established in areas with different levels of tree density ranging from urban green spaces to predominantly built-up areas. We looked for a correlation between the richness/abundance of birds and the size of the area surveyed, the number of houses, the number of tree species and the number of individual trees. The results of multiple regression showed that bird richness had a direct relationship with vegetation complexity. The abundance and diversity of tree species were better predictors of bird species than the number of houses and size of the area surveyed. We discuss implications of this study for conservation and management of bird diversity in urban areas, such as the need to increase green areas containing a large diversity of native plant species.     

Buzzing on top: Linking wild bee diversity,abundance and traits with green roof qualities

Green roofs are potentially valuable habitats for plants and animals in urban areas. Wild bees are important pollinators for crops and wild plants and may be enhanced by anthropogenic structures, but little is known about wild bees on green roofs in cities. This study investigates the effects of green roof qualities (floral resources, substrate character and depth, roof height and age) on wild bee diversity, abundance and traits (nesting type, sociality, pollen specialisation, body size) on green roofs in Vienna. Nine green roofs were sampled monthly between March and September 2014 by a semi quantitative approach. Wild bees were collected in pre-defined sub-areas for the same amount of time and floral resources were recorded. Over all green roofs, 992 individuals belonging to 90 wild bee species were observed. Wild bee diversity and abundance was strongly positively affected by increasing forage availability and fine substrates. Wild bees on roofs were characteristically solitary, polylectic and 8.3–11.2 mm. Regarding nesting type, the percentage of above-ground nesting bees was higher compared to the common species composition in Middle Europe. Ground-nesting wild bees were mainly eusocial, smaller (6.4–9.6 mm) and positively affected by roofs with fine substrates. During June, when forage availability by wildflowers on roofs was “low” (5–15% flower coverage), flowering Sedum species were an important forage resource. We conclude that wild bee diversity and abundance on green roofs are enhanced by floral resources. Furthermore, the installations of areas with finer and deeper substrates benefit ground nesting and eusocial wild bees.     

Urban bird community composition influenced by size of urban green spaces,presence of native forest,and urbanization

Urban Ecosystems - Understanding what determines wildlife species richness and community composition in urban green spaces is important for wildlife management and urban planning. Numerous studies...     

Seasonal patterns of urban bird diversity in a Mediterranean coastal city: the positive role of open green spaces

Urban Ecosystems - Urbanization causes rapid changes in the landscape and land use, exerting a significant pressure on bird communities. The effect of urbanization on bird diversity has been widely...     

Influence of substrate depth and vegetation type on temperature and water runoff mitigation by extensive green roofs: shrubs versus herbaceous plants

The relative contribution of substrate depth and vegetation type on temperature mitigation and stormwater runoff reduction was studied in an experimental green roof in North eastern Italy. Two substrate depths (120 and 200?mm) and two vegetation types (herbaceous plants and shrubs, respectively) were used, and compared to control modules with similar substrate depths but left bare of vegetation. Experimental observations showed that: a) green roofs substantially reduce thermal load over the rooftop, with significant effects of substrate depth and no apparent impact of vegetation type; b) thermal effects are strongly influenced by substrate water content; c) green roofs strongly reduce water runoff with significant substrate x vegetation effects. Our data suggest that green roof design addressed to optimization of the thermal functions should take into account adequate planning of substrate depth. Moreover, our data show that vegetated modules out-competed medium-only ones in terms of runoff reduction capacity, in accordance with some previous studies. Both shrub-vegetated and herbaceous modules intercepted and stored more than 90% rainfall during intense precipitation events, with no significant difference between the two vegetation types despite different substrate depths.     

Introduced and native plant species composition of vacant unmanaged green roofs in New York City

Urban Ecosystems - Vacant spaces, such as unplanted, unmanaged, or infrequently accessed green roofs, may play a critical role in sustaining biodiversity in cities. Initial plant composition of...     

Habitat patch size and tree species richness shape the bird community in urban green spaces of rapidly urbanizing Himalayan foothill region of India

Urban Ecosystems - Rapid urbanization is emerging as one of the leading threats to the biodiversity globally. But is especially a cause of concern for tropical countries which are urbanizing much...     

The dynamics of planted and colonising species on a green roof over six growing seasons 2001–2006: influence of substrate depth

Fifteen herbaceous perennial grass and herb species were planted into experimental green roofs in spring 2001. The species differed widely in their origins, heights, flowering times, life spans and growth forms but all were typical of dry and nutrient-stressed habitats. Three individuals of each species of a standardised size were randomly assigned to a planting grid at 20 cm apart in each experimental replicate plot at substrate depth of either 100 or 200 mm. Each treatment was replicated three times. During each growing season, the mean height and spread of each individual was recorded, together with flowering performance and % vegetation cover. In addition the numbers and % covers of all spontaneous colonised species were recorded. Greatest survival, diversity, size and flowering performance of planted species occurred at 200 mm depth. Bare ground and moss cover was greatest at 100 mm, as was diversity of colonising species. Differences between the early years and the final years of the experiment indicate the need for long-term monitoring of green roofs in addition to short-term experiments.     

Relationship of bird richness,abundance and assemblage to the built environment in a small island tropical urban setting: a Suva,Fiji case study

There have been few studies investigating the relationship between the built environment and the status of bird distributions in small island tropical urban areas. We present a study investigating the relationship between bird species richness, abundance and assemblage to the built environment in Suva, Fiji. Field surveys were taken at 54 randomly selected sites throughout the city, stratified by three building density classes and the central business district (CBD). At each site bird counts were recorded, along with environmental data such as average building height, within a 150 m radius. Land-use information was obtained from screen digitized high-resolution satellite imagery within the same radius. Distance to undeveloped patches of land within the urban area was calculated using a GIS. Analysis of the effects of the built environment was carried out for all species, and for exotic and native species separately. Abundance of exotics was significantly higher in the central business district (CBD) than all other urban density classes, and significantly higher than natives in all other density classes. We found a negative relationship between native species richness and distance to undeveloped patches, but no relationship for exotics. Species assemblage was not related to urban density class. We conclude that the status of native and exotic bird species in Suva is similar to what has been found in urban areas in temperate climates, and conservation efforts should focus on minimizing the amount of heavily urbanized “core areas” and protecting undeveloped areas of forested vegetation to improve bird biodiversity in small tropical islands cities.