Cities; Urbanism; Architecture; Art
What is the “Street Tree Needs Index”? The “Street Tree Needs Index” is simply an area or neighbourhood based calculation that generates a local “Need” for street trees.
How do you decide the need of an area for street trees? The criteria informing the “Street Tree Needs Index” are potentially infinite. They may be potentially infinite, but no matter how many are chosen they should be clearly stated and they should be calculable. If they are calculable they are transparent for all to see.
In our “Street Tree Needs Index” for Dublin we have identified and quantified 12 criteria that could inform the next programme of city centre street tree planting. There is of course potential to add many more.
Each individual criterion is given a “Relative Weighting” according to their ascribed importance.
These weightings can be adjusted as each city feels fit. The weightings simply reflect the value of each criterion is given by the city. The planting of trees is positively correlated to each criterion.
|2||Children under 12||8|
|3||Older age groups 65 +||2|
|4||Proportion of Apartment/Flats||12|
|5||Lower Socio-Economic Groups||8|
|6||Proportion who Walk to work||8|
|7||Proportion household with no Car||4|
|8||Density of Population||8|
In our weighted criteria for Dublin the population of an area or neighbourhood receives 24% of the weighting. Put simply areas with more people should have more trees. The density of population of an area receives 8% of the relative weighting. This is a ‘reward’ for density, sustainability and is also potentially efficient. Areas with a high density of population tend to have more people walking about looking at and enjoying street trees.
The proportion of flats/apartments in an area receives 12% in our weighted criteria. Apartment dwellers, unlike residents of suburban style houses do not generally have private front and back gardens. They are more likely to be dependent on public goods and shared spaces for greenery. Our weighted criteria thus favour areas with high density of flats and apartments. Dublin, perhaps unlike many continental European cities has a high proportion of homes with front and back garden in the heart of the city.
Areas with high levels of vehicular pollution emissions can reasonably make the case that they need more trees to mitigate c02 and other pollutants. Street trees can also muffle noise pollution. We give vehicular pollution 16% in our weighted criteria.
Our criteria favour those areas where a higher proportion of residents walk to school or work and areas where households do not own a car.
We also favour areas with a higher proportion of lower socio-economic groups. Lower socio-economic groups are more likely to have fewer choices regarding alternative access to green space, trips to the countryside, or travel abroad. Street trees are an important public good. There is a good argument that the distribution and access to public goods should be ‘progressive’, that those most dependent on them for health or a sense of well being are favoured. We give the predominance of lower socio-economic groups 8% in our weighted criteria.
The criteria also favour areas with a high proportion of children under 12 and people over 65+. The presence of greenery, even seasonal tree canopy is known to have an important impact on child cognitive development. Similarly the presence of street greenery for the elderly, retirees is recognised medically and socially. Favouring areas with children and the elderly receives 10% in our weighted criteria.
To conclude our Weighting Criteria for Dublin favours areas where most people live, where households disproportionately live in apartments, that are disproportionately from lower socio-economic groups, that have no car, that walk to work, and that have high number of children . It also favours areas with high proportion of workers and areas that suffer from high levels of vehicle emissions.
We then plugged the data of our weighted criteria into our study area to generate a “Street Needs Index” for all our city centre and inner 43 neighbourhood or district areas. This generates a comparable numerical index. (Each district or neighbourhood street need is expressed as a percentage of the highest).
Having established a “Street Tree Needs Index” we simply compare this to the EXISTING STREET TREE numbers in each area. This gives us what we call the “Street Tree Deficiency Index”. This is a simple, transparent and comparative tool to assess the equity of the existing street tree distribution in a city. It should guide us where to plant next.
So what does our Dublin City “Street Tree Deficiency Index” reveal about Dublin?
Perhaps unsurprisingly there is enormous variation across the city centre and inner city.
The area with the HIGHEST Street Tree Deficiency is Ballybough East (Ballybough A DED). This area has some 317 residents per street tree – the lowest number of trees per person in the city.
The areas Street Needs Index is 0.72 – high, but not the highest in the city. (14 out the 44 areas have a higher Street Needs Index). However when a relative high street needs index is compared to lowest number of existing street trees person, Ballybough East has the Highest Street Tree Deficiency. As the area with the Highest Street Tree Deficiency – this is expressed as 100.
Ballybough needs more street trees. Any city planting programme should identify the best opportunities to plant there.
The area with the LOWEST Street Tree Deficiency is Ballbridge-Herbert Park area. (Pembroke West B). This area has some 6 residents per street tree – the highest number of trees per person in the city.
The areas Street Needs Index is 0.54 – is low (The lowest Street Needs Index).
So the area with the LOWEST Street Needs Index has the highest number of street trees per person. This is a significant public subsidy to an area. Ballsbridge-Herbert Park is also one of the wealthiest areas in the city. The area Street Tree Deficiency – is 1.7 (as expressed as proportion of 100)
Perhaps this is unsurprising to those who are familiar with the geography of the city. It may not be surprising but it is quantifiable.
So “playing with the figures” how many street trees would the city need to plant in Ballybough to reach the number of people per tree ratio in Ballsbridge-Herbert Park? That answer is straightforward a total of 537 additional street trees to be exact.
But given the significant differential NEEDS INDEX, how many street trees would the City have to plant in Ballybough for Ballybough and Ballsbridge to have the same “Street Tree Deficiency Index”. The answer is 639.
Another area with a very high Street Tree Deficiency is the Mountjoy Square west neighbourhood (Rotunda A DED). This area has some 224 residents per street tree – the second lowest number of trees per person in the city.
The areas Street Needs Index is 0.93 – high, the third highest in the city (only 2 out the 44 areas have a higher Street Needs Index). Combining a very high Street Needs Index and very low number of existing street trees person, Mountjoy Square west neighbourhood the 2nd Highest Street Tree Deficiency. (68.0)
Ballybough and Mountjoy are neighbours. There are both located in the north east inner city, an area historically associated with inter generational deprivation.
How many street trees would the city need to plant in Mountjoy Square West to reach the number of people per tree ratio in Ballsbridge-Herbert Park? The answer is 719 additional street trees.
But given the significant differential NEEDS INDEX, how many street trees would the City have to plant in Mountjoy Square neighbourhood for Mountjoy and Ballsbridge to have the same “Street Tree Deficiency Index”. The answer is 829.
Our study area focuses on Dublin’s inner urban area. This includes all lands inside Dublin Canal Rings and the Ballsbridge/Sandymount area. The inclusion of the Ballsbridge/Sandymount is because of the very high number of workers/employment in the area. The density of employment is one our 12 criterion for street tree planting.
Our study area has a total population of 146,000 – that is 27.7% of the population of the functional area of Dublin City Council (527,000).
The 2011 Census reveals that some 287,000 people work inside the functional boundary of Dublin City Council.
Study area includes 43 of the 162 DEDS (Small area statistics) that make up Dublin City Council.
There are 7,356 Street Trees in our Study Area.
Our ‘Open Data’ (excel file format): Inner City/City Centre Character Areas/Neighbourhoods, Street Tree numbers, Population, Persons per Street Tree, Our “Needs Index” (Composite index of Socio-Economic need for street trees (Relative Weightings) “Street Tree Deficiency Index” (Includes entire Census 2011 for our 41 inner city/city centre Character Neighbourhoods)