Tree density linked to wealth disparity – review
The benefits of biodiversity in new residential developments in urban areas are examined in a report which lists the drawbacks of inappropriately planted areas.
Horticulturalist and author of The Irish Gardener Peter Dowdall believes that planting should be measured as a cost-benefit line, and not merely a decoration, and landscape should be designated as statutory infrastructure.
A literature review produced by Natural Capital Ireland in collaboration with the Irish Green Building Council brings together evidence showing the environmental, social, and economic benefits.
It finds that nature-based solutions (NbS) are seen as cost-effective, locally adapted, and innovative approaches, with an outsized impact.
Ireland plans to building 300,000 new homes between 2025 and 2030), while simultaneously over 90% of protected habitats are in poor, unfavourable status, it states.
In 2016, researchers from the UCD School of Geography found a correlation with wealth disparity.
Deprivation
By mapping the extent of street trees across Dublin (60,000 out of 100,000 trees total), they found a direct relationship between the paucity of street trees and levels of deprivation.
Wealthier areas have significantly more trees (Kavanagh and Flynn, 2016).
In Ballsbridge, Dublin 4, for every tree there are only 8-10 residents, while in Dublin 8 this rises to one tree for every 30 residents.
In Ballybough, Dublin 3, there is one tree for every 200 residents.
A 2020 study which looked at the impact of green spaces on Dublin property prices found that a 10% increase in park space within 2km of a dwelling was associated with a 5.5% increase in price.
A British cost-benefit analysis showed a positive impact of £6,495 per person living within 500m of a small new park in an urban area, including:
- Less depression (£750),
- Higher cognitive development (£150),
- Reduced mortality (£2,999),
- Reduced crime, and
- Higher house price (£3,375) (Create Streets and Berkeley Group, 2023).
A 2015 London-based study explored the correlation between urban street tree density and anti-depressant prescription rates.
Researchers found that the boroughs of London that had a higher density of street trees were associated with lower anti-depressant prescription rates (even after controlling for potential confounders).
The review also finds that investments in natural capital including trees, wetlands, sustainable drainage systems (SuDS), rain gardens, green roofs, and biodiverse green spaces can generate significant long-term returns.
Property value
If nature in residential development can improve biodiversity, flood resilience, public health, water management, and property value, Dowdall asks why planting is often one of the first budgets to be reduced
The report examines the need for appropriately chosen trees individually planted with proper subsoil, top soil and drainage in place.
The evergreen oak Quercus ilex is best for shade as temperatures increase, for example.
The report argues that nature should be understood as essential infrastructure and a strategic asset, not simply as an optional amenity or planning requirement.
Dowdall adds that this reflects what he has seen in his career in horticulture.
“Planting is too often considered towards the end of a project, after the supposedly ‘essential’ decisions have been made,” he said.
“When budgets tighten, it can then become relatively easy to reduce or remove because the outcomes it is expected to deliver were never clearly identified at the beginning.”
Green interventions
The difficulty is no longer in demonstrating that horticultural and green interventions can create benefits, Dowdall points out.
The real challenge is translating that evidence into the commissioning, design, delivery, maintenance, and measurement of actual projects in Ireland.
One well-designed horticultural intervention can simultaneously support biodiversity, manage rainfall, reduce heat, improve wellbeing, and create a more attractive and valuable place, Dowdall adds.
But unless these benefits are identified and demonstrated, planting will remain vulnerable to being treated as decoration and, ultimately, disregarded.
He asks how Ireland can design, deliver, and measure horticultural and green interventions so that their wider benefits become credible, demonstrable and investable.
The perspectives from local authorities, developers, planners, landscape and horticultural professionals, researchers, public-health specialists and those working in natural-capital investment, will be invaluable, the horticulturalist said.