Street Trees in Far Cotton: Distribution and Changes Through Time

Street Trees in Far Cotton: Distribution and changes through time Ria Berry Abstract Street trees have been shown to be important for well-being, urban cooling and pollution reduction. However, in recent years there have been high profile cases of large number of street trees being removed in various UK cities. Historical positions of street trees in Far Cotton, Northampton were plotted in ArcGIS from historical maps and aerial photographs, and these were compared with GPS positions of current street trees in the area. There has been a reduction in the number of street trees of 58% since the 1960s. In addition trees on busy roads, where the pollution reduction would be most beneficial, are more likely to have been removed than those on residential streets. Careful choice of suitable street trees can greatly enhance the urban environment. There is currently no governmental air pollution monitoring in Far Cotton, and magnetic remanence studies on street trees can provide a valuable proxy. Introduction This report highlights the decline in the number of street trees in the Far Cotton area of Northampton and the significance of this change. This area has been brought to light by the residents of Penrhyn Road who have noticed a lot of trees being removed from their street. One resident, Alice Whitehead, started a movement called the ‘save our street trees’ project, which has raised concerns. The aim is to accurately map the change and show conclusively, just how significant of a change this is to Far Cotton and its residents. Street trees are so important as they provide many benefits, such as: ● Urban cooling. Without urban cooling, air temperatures in man-made, built up areas will start to rise. This can cause harm to vulnerable people in urban areas and cause great discomfort both physically and emotionally. Higher air temperatures in urban areas can also cause ozone to be formed at a ground level which is detrimental to human health (Monteiro, et al., 2019). Studies have shown that the urban cooling provided by urban greenspace (street trees included) saved the government £22M in annual energy consumption in London alone (Moss, et al., 2019). ● Improving the mental health and wellbeing of residents. Street trees have been shown to improve the lifestyle of residents by reducing stress (Knecht, 2004). ● Removing pollution particulates from the air. Street trees create a barrier between pedestrians and traffic by blocking the particulates produced. This means that harmful particulates can be removed from the air (Smith, 2012) and washed away in the rain, preventing them from getting into and harming the lungs of pedestrians (Nowak, et al., 2011) Methods The number of street trees was investigated through time using a combination of historical and aerial data, with the modern number of trees being mapped using GPS; allowing for accurate comparison. The definition of a street tree is a tree that lines the pavement, separating pedestrians from oncoming traffic. Mapping in ArcGIS To start with, data from the Far Cotton area was collected. This was done through Digimap; a combination of aerial photos and historical maps were used. All maps downloaded from Digimap were National Grid 1:1,250 and 1:2,500 OS open carto. ● Historical maps: 1880s, 1960s and1970s. ● Aerial photos: Present. For other years (1945, 2004, 2009, 2010, and 2018) aerial photos from Google Earth’s way back feature were used. These images were downloaded and georeferenced in ArcGIS. In ArcGIS (using an OS open carto base map), points were plotted to indicate the position of trees for each of these years. The years used were only those that mapped the street trees precisely; for some maps series (e.g.1930s) street trees were not mapped at all while in more recent maps (1990s onwards) trees were only indicated by a wooded area infill. However, the 1960s maps show all the street trees, as shown by irregular spacing [Figure 1]. When plotting the trees from the aerial images, shadows were used to infer the position of the base of the tree in cases where the trees were unclear due to the angle of the sun. GPS mapping Once the points had been mapped on ArcGIS, a Trimble Juno 3b GPS was used to go out and map every tree in the actual streets of Far Cotton to get a true representation of the modern day number. For this part, only trees that separated the road and the pavement were considered street trees. When adding the point, different attributes were noted [Table 1] to help see potential patterns in the planting of different street trees. Attribute Categories Type Deciduous, evergreen Habit Bushy, narrow, round Status Live, dead, stump, gone Size Small, medium, large Circumference In cm (used to later determine the age of the tree) Species Identified by photos taken (used later to determine the age of the tree) Trees were only mapped as ‘gone’ if there were signs in the pavement showing where a tree once was [Figure 2]. Data from the GPS was then uploaded to the GIS . In ArcGIS the current trees could then be mapped by their attributes (live, dead, stump or gone) [Figure 3]. Determining the ages of the trees Finally, the species of the street trees were recorded and from this, approximate ages were calculated; employing a variety of methods. A combination of diameter, growth factor of the species (Hungerford Virtual Museum & New Forest National Park), and girth (circumference) were used. The main method of age calculation however was done through a website (Benning) that gave an approximate age for the tree based on its girth (circumference). Once species and approx. age had been identified, they were mapped in ArcGIS [Figure 4 & Figure 5]. Results Overall change Highlighted by the maps created in ArcGIS, a clear decline in the number of street trees from the 1960s to now [Figure 6 & Table 2] is shown. Urban Development and the planting of street trees Using this data, the relationship between the planting of street trees and the development of urban areas in Far Cotton [Figure 7] was also investigated. This allowed an estimation of the number of street trees relative to the urbanised area (Table 2). It was also found that the urban tree numbers was greatest in the 1960s [Table 2]. Area ÷ #street trees = 1 street tree for every xm² of urban development Year Area m² #street trees xm² %Decrease in #street trees 1960s 852,848.3 457 1,866.2 58 Modern 1,026,096.0 193 5,316.6 When compared with the building age, there is evidence that street trees were being planted as the development of the urban areas progressed. However, because of the extent of the historical maps available, this pattern can only be seen in the 1960s [Figure 8]. This is shown through the increase in the number of street trees in the 60s coupled with the increase in urban development. Tree Species It was also found that there was no clear pattern in the planting of specific species of street tree. (aside from the large number of Ornamental Cherries in the south east) [Figure 4 & Figure 9]. Similarly, with tree age [Figure 5], there is no clear pattern. Discussion The number of street trees in Far Cotton has decreased significantly since the 1960s. It can be assumed, therefore, that street trees that have been removed are not being replaced [Table 2]. For example, one area was completely rebuilt between the 1960s and the 1970s [Figure 10], this would mean some of the street trees would have needed to be removed for construction. In this area, the biggest decline in street trees was between these ages. Similarly, there has been a reduction in the number of street trees on St. Leonard’s Road in the northside of Far Cotton. This road is a main road with a lot of shops,and so, a lot of road traffic. The maximum extent of street trees on this road was in the 1960s and has since declined [Table 3]. This change in the number of street trees could be because of long roots causing damage to the pavements, plumbing and houses (Urban Regeneration and Greenspace Partnership). Year #Street trees on St Overall % decrease Leonard’s Rd 1960s 43 1970s 22 97.7 2004 2 Now 1 It is hard to tell exactly when the biggest decline in street trees occurred and when they were planted. Photo evidence has shown that the number of trees on St Leonard’s Rd in the 1960s was of similar extent in the 1930s [Figure 11] through to the 1950s [Figure 12] (Far Cotton History Group). These historical photos also provide an accurate way of confirming the position of trees mapped from the data used. For example, the 2004 trees were difficult to plot due to the position of the shadows, meaning the precise position of the tree had to be interpreted [Figure 13]. However, photos taken from the 1998 flash floods (B.Adams) [Figure 14], confirm the position of the street trees plotted using the 2004 Google Earth aerial imagery. Using the data collected from 2004 and an image from 1998 of the same road, it can be assumed that there were three trees on St Leonards Rd in 1998 and one was removed some time between then and 2004. Evaluation The findings of this report show a definite decrease in the number of street trees in Far Cotton from the 1960s to now, and there is no evidence of effort to increase this number. The Far Cotton area was chosen specifically because there has been recent local news about the apparent decline in street trees.

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