What's 'cool' in the world of green façades? How plant choice influences the cooling properties of green walls

Green walls provide an option for reducing the thermal load on buildings, reducing the requirement for mechanised air conditioning and helping to mitigate urban heat islands. The range and extent of benefits can vary with green wall typology. This research investigated green façades utilising wall shrubs and climbing plants to reduce air temperature adjacent to, and surface temperatures of, brick walls. Artificial wall sections were used to provide replicated data sets in both outdoor and controlled environmental conditions. During periods of high solar irradiance outdoors, the presence of live Prunus laurocerasus plants placed against walls significantly reduced air and surface temperatures compared to blank walls, but also in comparison to excised (non-transpiring) plant sections. Largest temperature differentials were recorded mid-late afternoon, where air adjacent to vegetated walls was 3°C cooler than non-vegetated walls. Prunus also provided significant wall cooling in controlled environment studies, but was intermediate in its surface cooling capacity (6.3°C) compared to other species; Stachys and Hedera providing >7.0°C cooling. When evaluated on a per leaf area basis, however, other species demonstrated greater cooling potential with Fuchsia, Jasminum and Lonicera out-performing others. Not only was it evident that different species varied in their cooling capacity, but that the mechanisms for providing wall cooling varied between species. Fuchsia promoted evapo-transpiration cooling, whereas shade cooling was more important in Jasminum and Lonicera. Plant physiology and leaf area/morphology should be considered when selecting species to maximise cooling in green wall applications. © 2013 Elsevier Ltd

Cut-off in their prime? Response of two landscape shrubs to different levels of root pruning, during active and quiescent growth phases

Shrubs have an important role in the future design of urban landscapes. Due to city-densification and pressure on space, shrubs are increasingly preferred over trees for urban amenity plantings. In contrast to trees, however, relatively little information exists on how shrubs adapt to urban stress. This includes their responses to physical root injury, that might occur through trenching or transplanting activities. Two shrub taxa, Philadelphus coronarius ‘Aureus’ and Euonymus fortunei ‘Silver Queen’, were used to investigate the effects of severity and time of root injury on plant viability, and how additional fertiliser influenced recovery. A novel ‘split-pot’ system was developed to differentiate where root injury was induced. Results showed that both taxa were relatively resilient to root-pruning, although root injury was more detrimental during active growth than when plants were quiescent. This re-enforces the notion that transplanting of shrubs should be avoided in the summer. Shoot development was not more detrimentally affected by severe root-pruning compared to light pruning. There was also evidence that uniform severe pruning across the root-ball stimulated stronger root-regeneration compared to root systems differentially injured. No consistent response to fertiliser was noted. Results have implications for the resilience and management of shrubs within the urban landscape

Functional green roofs: Importance of plant choice in maximising summertime environmental cooling and substrate insulation potential

Green roof plants can reduce local air temperatures and heat load to a building in the summer, improving thermal comfort of residents. Little is known, however, about how different plants compare in their potential to provide these two ecosystem services. Consequently, this study investigated whether some plants can offer more potential summertime environmental cooling and substrate insulation than others. Over two summers (2012/2013), canopies of two succulent and four broad-leaved plant genotypes, with contrasting plant traits, were monitored alongside bare substrate in Reading, UK. Plants were studied outdoors within small plots (1.5 × 1.5 × 0.1 m). Continuous monitoring took place during warm days and nights and included variables (leaf surface temperatures) and fluxes (long-wave radiation, sensible heat flux and transpiration) that are indicative of cooling potential. The strength of substrate insulation was estimated by comparing the ground heat flux below the canopies to that of the bare substrate. Plant traits (leaf colour or thickness), structural parameters (height and leaf area index, LAI), radiative properties (albedo and emissivity), and stomatal conductance were also measured to help explain the differences in cooling potential among the species. Non-succulent canopies, in particular light-coloured ones, with high leaf stomatal conductance and high LAI provided maximum potential for substrate insulation and environmental cooling in hot periods, particularly compared to bare substrate and thick-leaved succulents. These results suggest that succulent plants are not best suited to provide significant summertime environmental cooling and substrate insulation and that others (e.g. Salvia and Stachys) might be preferable where the delivery of these benefits is a priority. Our findings highlight that, in addition to survival, aesthetics and cost, the plants’ ability to deliver a range of ecosystem services should be considered in the plant selection/green roof planning process

Functional urban ground-cover plants: identifying traits that promote rainwater retention and dissipation

Urban vegetation can influence urban hydrology and reduce the risk of flooding. Urban forestry studies have suggested that tree type and species choice affect the amount of rainwater intercepted and retained. Little information exists, however, for other landscape typologies, and the sorts of ground-cover plants that are best used to retain/detain rainwater during storm events. This is important as many urban spaces are too small to facilitate trees, but can accommodate roadside vegetation, buffer strips, rain gardens, green roofs and stormwater planters. Thus, this research aimed to determine how choice of ground-cover taxa affected rainwater interception and retention. Six model species with contrasting leaf morphologies were used to determine how well rainwater was intercepted, but also dissipated through evapotranspiration (ET). A pot-based system was used to determine how plant water balance changed during late summer in the UK, with the aim to understand how leaf traits affected hydrological processes. Plant choice was important, with fine-leaved taxa, Festuca glauca and Dianthus ‘Haytor White’ showing best rainwater interception and Festuca demonstrating highest rates of dissipation from the substrate. Overall, compared to non-planted pots, those with plants present were more effective at capturing water (by 2.3–3.0x), and evapo-transpiring water (by 2.5-4.0x). Results indicate that ground cover vegetation has potential to aid urban water management in those localities where space is limited for trees. Plant choice and community-structure should be considered, especially when there is a desire to dry out soil/substrate quickly and restore maximum soil moisture holding capacity

Using big data to improve ecotype matching for Magnolias in urban forestry

Trees play major roles in many aspects of urban life, supporting ecosystems, regulating temperature and soil hydrology, and even affecting human health. At the scale of the urban forest, the qualities of these individual trees become powerful tools for mitigating the effects of, and adapting to climate change and for this reason attempts to select the right tree for the right place has been a long-term research field. To date, most urban forestry practitioners rely upon specialist horticultural texts (the heuristic literature) to inform species selection whilst the majority of research is grounded in trait-based investigations into plant physiology (the experimental literature). However, both of these literature types have shortcomings: the experimental literature only addresses a small proportion of the plants that practitioners might be interested in whilst the data in the heuristic (obtained through practice) literature tends to be either too general or inconsistent. To overcome these problems we used big datasets of species distribution and climate (which we term the observational literature) in a case study genus to examine the climatic niches that species occupy in their natural range. We found that contrary to reports in the heuristic literature, Magnolia species vary significantly in their climatic adaptations, occupying specific niches that are constrained by trade-offs between water availability and energy. The results show that not only is ecotype matching between naturally-distributed populations and urban environments possible but that it may be more powerful and faster than traditional research. We anticipate that our findings could be used to rapidly screen the world’s woody flora and rapidly communicate evidence to nurseries and plant specifiers. Furthermore this research improves the potential for urban forests to contribute to global environmental challenges such as species migration and ex-situ conservation

Bringing fronts back : a research agenda to investigate the health and well-being impacts of front gardens

This perspective article proposes an agenda to investigate the impacts of front gardens (yards) on health and well-being. As front gardens are increasingly being paved over, significant ecological benefits will be lost. In addition, urban green infrastructure has a measurable role to play in addressing major public health issues related to mental health, chronic stress, inflammatory, and cardiovascular diseases. The social value of front gardens remains un-quantified. Future research can contribute to assessing the psycho-socio-cultural value of front gardens, and fostering healthy environments for people to live, work, and play in. A better understanding of the health impacts of front gardens can provide a relatable argument to protect permeable surfaces and spaces for nature, as well as to discourage the general public from paving over their front gardens. We propose eight central research questions to be addressed in future work, and elaborate on further variables, lines of inquiry, and suggested intervention trials and observational studies. Potential research findings will have implications for decision-making in fields of horticulture, landscape architecture, urban planning, and public health

Rewilding in the garden : are garden hybrid plants (cultivars) less resilient to the effects of hydrological extremes than their parent species? A case study with Primula

Urban green infrastructure, such as gardens, can mitigate some of the consequences of climate change, e.g. reducing flash-flooding or urban heat islands. Green infrastructure, however, may itself be vulnerable to a changing climate, and not all garden and landscape plant taxa will remain viable under weather scenarios predicted for the future. It has been suggested that cultivated forms of garden plants (hybrids and selected varieties) particularly, will be susceptible to enhanced stress associated with more frequent flooding, drought and rapid oscillations between these hydrological extremes; thus potentially limiting the range of taxa that can be used in gardens in the future. This research explored this concept by evaluating cultivated forms of the common garden plant – Primula, and testing whether these were less resilient to the effects of hydrological extremes than their progenitor species, Primula vulgaris. The results support this hypothesis and demonstrated that cultivated taxa were more susceptible to the hydrological stresses imposed than Primula vulgaris. Interestingly though, those cultivars that superficially resembled the parent species (Primula ‘Cottage Cream’) showed more stress tolerance than others with larger or more ornamental flowers, suggesting a ‘gradient of susceptibility’ within the hybrids. The notion that the most flamboyant cultivars are sacrificing stress tolerance for traits linked with aesthetics is discussed. The data, albeit on one genus only, has implications for the design of gardens/ornamental landscapes for the future and calls for more attention within breeding programmes to enhance abiotic stress tolerance within garden and landscape plants

Where the wild things are! Do urban green spaces with greater avian biodiversity promote more positive emotions in humans?

Urban green space can help mitigate the negative impacts of urban living and provide positive effects on citizens’ mood, health and well-being. Questions remain, however, as to whether all types of green space are equally beneficial, and if not, what landscape forms or key features optimise the desired benefits. For example, it has been cited that urban landscapes rich in wildlife (high biodiversity) may promote more positive emotions and enhance well-being. This research utilised a mobile phone App, employed to assess people’s emotions when they entered any one of 945 green spaces within the city of Sheffield, UK. Emotional responses were correlated to key traits of the individual green spaces, including levels of biodiversity the participant perceived around them. For a subsample of these green spaces, actual levels of biodiversity were assessed through avian and habitat surveys. Results demonstrated strong correlations between levels of avian biodiversity within a green space and human emotional response to that space. Respondents reported being happier in sites with greater avian biodiversity (p < 0.01, r = 0.78) and a greater variety of habitats (p < 0.02, r = 0.72). Relationships were strengthened when emotions were linked to perceptions of overall biodiversity (p < 0.001, r = 0.89). So, when participants thought the site was wildlife rich, they reported more positive emotions, even when actual avian biodiversity levels were not necessarily enhanced. The data strengthens the arguments that nature enhances well-being through positive affect, and that increased ‘engagement with nature’ may help support human health within urban environments. The results have strong implications for city planning with respect to the design, management and use of city green spaces