Vacant urban plots are anything but a waste

Within many urban environments, one can come across vacant parcels of land. Such land parcels may be in residential, commercial, or industrial areas, and oftentimes they will be home to, because of the lack of disturbance and management of the land, plants. Some of these plants will, of course, be trees (of varying ages – depends on how long the land has been vacant for). In certain cities, the amount of vacant land may be particularly high, with declining fortunes of landowners, a worsening economy, and a change in demographics, being three drivers of the abandonment of land. In Detroit, USA, for example, around 32% of all urban land is currently vacant.Of course, in many aspects, this is not good, though is there a silver lining to such vacant plots? The study investigated in this post (referenced at the bottom) seeks to establish just that, in terms of ascertaining how valuable vacant land plots are as green infrastructure.

The study area for the investigation was the city of Roanoke, Virginia, USA. This city was chosen as it has a wide variety of different vacant land parcels, of which many are old industrial (manufacturing / factory) plots that were abandoned as the economy declined and technologies changed. In total, the vacant areas of land were split into five different categories, as shown in the below table. Overall, 114 vacant plots, both under public and private ownership, were sampled.

vacant land types city tree
The five types of vacant land, as defined by the authors.

Of these land types, an area amounting to nearly 30% of the entire city was considered to be classed as vacant. Granted, some areas are intentionally vacant, as they are floodplains, wetland areas, and so on. The table shown below gives more of an indication of what land types there are in the city, and how much of the total city area they comprise.

vacant land area tree
Total land area of the different vacant land types.

Within these vacant plots, there was a total of 210,000 trees (an average of 30.6% canopy cover, in these vacant areas); of which nearly 41% of the trees were quite small (below 15.2cm in diameter). The total number of trees are split as follows: (1) derelict: 25,725; (2) natural: 26,514; (3) post-industrial: 7,488; (4) transportation-related: 28.923, and; (5) unattended with vegetation: 121,613. Because of the species of trees found on the vacant sites (see below), it can be recognised that this data means many trees are young. This, in turn, means that the ecosystem services (filtering air pollutants, sequestering carbon, catching rainwater, cooling the urban environment, and so on) provided by the trees are rather limited, though as the authors note there is scope for the services to increase in time, as the trees mature. Of course, there are older trees within these vacant plots, and nearly 6% of all of the trees had a DBH of over 76.cm, though the population is heavily skewed in favour of young trees. It is also worth noting that the invasive tree of heaven (Ailanthus altissima) features rather readily, and this may be undesirable on an ecological level. In fact, invasive non-native plants are perhaps more likely to be able to colonise vacant land plots than native ones, in many instances – particularly on brownfield sites.

tree species vacant land urban
A breakdown of the tree species found within the vacant plots. The three main species are American elm (16.5%), tree of heaven (12.3%), and box elder (6.7%).

Combined, the trees within the vacant plots do however have many benefits. First and foremost, they have a very positive impact upon air quality, by filtering out harmful pollutants that, as a result, has a monetary value attributed to it (as shown below). For example, nearly 30 metric tons of moderately-sized particulate matter is removed by the vegetation every year, and this equates to financial benefits of $450,000 (nearly $16,000 per metric ton).

trees pollution removal urban
The removal of pollutants by trees growing within vacant land parcels. The bars represent total pollution removal per year, whilst the triangles relate to the financial benefits.

Beyond the removal of air pollutants, the trees growing within vacant land also have a marked benefit when it comes to the sequestration of carbon. As carbon dioxide is considered by some (but not all) scientists as being a marked cause of climate change, some will promote the utilisation of trees for their assimilative capacity when it comes to storing carbon. In the study area, the 210,000 trees were found to sequester a gross total of 2,090 tons of carbon each year, which has a monetary value of $164,000. Granted, not all species are equal in this regard, and it is the American elm and the tree of heaven that top the charts for carbon sequestration (see the below graph), with the elm being far above anything else in this regard. Curiously, whilst the box elder is the third most common tree species within vacant plots, it does not rank third for carbon sequestration. When the authors looked at total carbon storage, which is what carbon has been stored over the trees’ lifetimes, a total of nearly 100,000 tons had been stored, which is valued at $7.6m. This value does however vary across vacant land types, with much of the carbon being stored in areas unattended that possess vegetation, though this may very likely be down to the fact that this land type has over half of the total 210,000 trees.

carbon sequestration trees vacant land
The total amount of caron sequestered, each year, by some of tree species found within vacant plots. Each metric ton of carbon sequestered is valued at $78.5.

These vacant land-borne trees also reduce the expenditure incurred by residents of the city, and particularly when it comes to heating or cooling of property. Each year, the trees reduced, by over $210,000, the expenses paid to heat and cool property. Such savings translate into indirect benefits for limiting air pollution, as this reduces cost is associated with reduced energy bills. This obviously means less energy was used, which implies that individuals emit (via consumption) fewer total pollutants, each year.

There also exists a structural value to the vegetation within these vacant plots, and for the city’s trees this amounts to $169m. Again, it is the unattended land that possesses vegetation that bears most of this value ($111m), as such land has by far the most trees, though other land types also have important financial values. The below table details exactly this.

structural value vacant land tree
The structural value of the trees within each land type, compared with the number of trees and the total cover of trees within each land type.

What I find particularly interesting about this study is that it suggests that we need to adopt a broader approach to how we view vacant areas of alnd within the city. Often, people will refer to areas of abandoned land within industrial and residential areas as wastelands, though according to this research they are anything but. Even though post-industrial and derelict land types themselves feature only to a small degree within the urban environment, they can certainly support tree populations, and the longer these areas remain void the better, from an environmental standpoint. Granted, there is a pursuit to re-build the economy after the banks managed to royally muck it all up and run off with billions in bailouts from the taxpayer, so such sites may be ear-marked for development if there is the justification, though at the same time there is huge opportunity to almost re-wild inner-city areas. Can there be a balance between the economy and the environment, therefore? If these sites are developed, perhaps there exists scope to retain some areas of trees. It’s also important to recognise the value these trees have by merely existing, and whilst it’s not a value that can be ‘cashed in’, it is nonetheless a value that should be properly recognised.

Source: Kim, G., Miller, P., & Nowak, D. (2015) Assessing urban vacant land ecosystem services: Urban vacant land as green infrastructure in the City of Roanoke, Virginia. Urban Forestry & Urban Greening. 14 (3). p519-526.

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Vacant urban plots are anything but a waste

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