Plants, and urban trees in particular, play a significant role in multiple dimensions of human wellbeing [1-5]. However, research in this area has tended to focus on the role of greenspace in general and its impact on a specific domain of wellbeing or a single health condition [3,6-7], or health driver, e.g. ozone or peak temperature [8-9]. Recent focus on urban trees, health and resilience has ignited interest in the impact of trees specifically [10-12], however, this can prove to be difficult, as most studies use a single, overall measure of greenness, the normalised difference vegetation index (NDVI) [3]. All types of vegetation are seen as being the same in the NDVI, and it is not possible to differentiate different types of vegetation [13]. In this study, we attempt ways to separate out the effects of trees and better understand their contribution to wellbeing.
Urban greenspace and trees, including street trees play a recognised role in Singaporeans wellbeing, including air quality mitigation, noise mitigation, shading, mobility, and ecosystem services [14-16]. The greening of Singapore has been an aspiration since independence, with Lee Kwan Yew proposing Singapore as a Garden City [17]. More recently, in 2020, Singapore has adopted a vision of being a City in Nature [18-19]. However, a combination of shifting and conflicting policies (e.g. temperature reduction, to air quality improvement, to biodiversity and wellbeing), disconnected planning and development agencies and an increasing understanding of the different effectors on wellbeing by trees means that there have been limited attempts to connect trees and greenspaces to a holistic view of wellbeing. Until recently, Singapore has demonstrated significant coordination with “City in the Garden” vision [20] by integrating green spaces into the urban fabric to enhance liveability, promote health, and support biodiversity. Examples include the Park Connector Network (https://pcn.nparks.gov.sg/aboutrecreationalconnectivity/) linking parks and nature reserves, Gardens by the Bay (https://www.gardensbythebay.com/sg/en/about-us/our-gardens-story/sustainability-efforts.html) showcasing diverse plant species, the OneMillionTrees campaign (https://www.nparks.gov.sg/treessg/one-million-trees-movement/about-the-movement) promoting canopy coverage and connections between people and their natural environment and guides to Skyrise Greenery that include methods to integrate trees within, and on, multi-storey buildings in roof gardens and sky terraces, supported by standards e.g. [21].
Singapore is considered a ‘green’ city (see Han 2017). Older areas, e.g., Rochor, Downtown and Singapore River, have lower levels of green cover than newer regions, e.g., Punggol and Seng Kang [17]. Further, private residential estates have high green cover per capita, while public housing estates have moderate to low green cover per capita due to the high population, and not necessarily the overall green cover which can be relatively high (due to planted road buffers and skyrise greenery) [17]. Further, the park provision ratio (PPR) is low to moderate when compared to other cities in Asia-Pacific, North America, and Europe [17]. The rate of change over the past five years has been greater than the past 50 years, driven by the emergence of more greenery on buildings [23-24], including the rise of urban farming and denser greenery along roads complemented by replacement of street carparking with plantings).
Recent policies around trees, urban greening, and wellbeing include Urban Tree canopy cover standards, Green Infrastructure Frameworks and Biodiversity Net Gain targets which complement a range of ‘Green Factor’ or ‘Greening Factor’ Indices’. These Singaporean policies are also reflected globally, both directly as tree canopy cover targets [25] and indirectly, e.g. trees providing ‘shade and shelter’ within the ‘Healthy Streets Indicators’ applied in Australasia and Europe (e.g. Healthy Streets | Making streets healthy places for everyone). To help contextualise improving greenspace for human and environmental health, internationally, a 3-30-300 guideline [11] has been designed and may be applied to Singapore. The 3-30-300 guideline provides a more standardised guide that can be used anywhere. However, it has predominantly been used in temperate cities, with only one study on citizen preferences in a tropical urban environment [26]. With the data that we have, we partially apply the 3-30-300 guideline to Singapore. Despite a significant amount of greenspace across Singapore, residents still travel extensively to access greenspace across the city, potentially questioning some of the tennets of the 3-30-300 guideline. This may be due to the lack of proximate greenspace (viewable and within 300m from home or work), or the preference for destination greenspace over smaller, local greenspaces. Whatever their status, Singaporeans seek out greenspace and trees [12] in ever increasing numbers over the past few years.
When thinking about greenspace and trees in Singapore, there has been a significant body of work on ecosystem services in Singapore [27-29] This work has examined freshwater, food, and raw material provision, as well as regulating and protecting functions [27]. Richards and Edwards [28] examined shade provision, while Law et al. [29] examined equity of e.g., PM10 removal, runoff retention, temperature change, and noise abatement. Noisy environments, with resultant sleep disturbance, physiological and psychological impacts have been shown to reduce overall wellbeing [30-32].
In fact, De Kluizenaar et al. [33] note that exposure to environmental noise has been associated with a range of human health impacts, and studies on environmental noise in Singapore have been very limited [34]. One recent study [35] has examined tree charateristics (canopy) and the effects on environmental noise in an urban environment, finding that crown area and height were strongly associated with noise levels. Wickramathilaka et al. [36] found up to 3 db reduction in traffic noise in three story buildings with a tree barrier. This study will complement this sparse existing work, and also provide evidence for the ability of trees to positively impact on this aspect of human wellbeing.
In this paper, we aim to connect a web of greenspace, and tree related wellbeing measures in Singapore, adding value, impact, and knowledge. We intend to do this through gaining and providing a deeper understanding of the applicability of the 3-30-300 guideline in Singapore; understanding the impacts (and benefits) of ‘layering’ greenery in cities, particularly regarding sound mitigation, along with the challenges associated with assessing these greenspaces; and how greenspaces and trees affect the way in which people move around cities. Further, we contribute an alternative framework to demonstrate the value of integrating remote sensing, ecology, and social science.