Among the city and land use planners, the vegetation height in the cities is usually not considered as an interesting indicator. Vegetation vertical structure is important for biodiversity and ecosystem service provision. For example the air passing above, and not through, vegetation is not filtered; barriers should be high enough and porous enough to let the air through, but solid enough to allow the air to pass close to the surface.
In cities, however, while variation in the spatial extent and distribution of vegetation has been widely investigated, vertical vegetation structure and its potential drivers have not. It simply might be a useful tool describing the ecological state of various urban areas and districts.
Different land use functions have different needs and boundary conditions with regard to urban green space. During the development of the Helsinki Green Factor tools the city officials, land-use planners, and experts found it very important to develop land-use specific target levels for the Helsinki Green Factor (“viherkerroin”).
Each land-use class has an area based target level that sets the goal for the green factor score in the horizontal scale. In this study we aim to provide some very initial data in case someone wants to consider also the vertical scale.
Height of Vegetation in Helsinki
In this statistical analysis, we aim to analyze the current height of vegetation in Helsinki using two different approaches. In the first one, the average height of vegetation is analyzed within districts – it shows distribution in certain continuous areas.
The second approach shows statistics about the vegetation height in different city detail planning zones (“asemakaava”) derived from Combined Detailed Plan Map of The City of Helsinki (Helsingin kaupunkiympäristön toimiala, 2017). This statistics doesn’t cover continuous area, but connect areas within the same zone across the city. For the height determination we used the open source laser scanning data (National Land Survey of Finland, 2015) generalized to 3 x 3 m grid.
The city of Helsinki is divided into various subdivisions and statistical units. In this study, we used the division into 34 districts to visualize the differences in vegetation height. The differences are clearly visible.
The highest vegetation in Helsinki is in the Tuomarinkylä district (over 11 m in average) while the lowest average vegetation height is in Malmi district (6,9 m). The average height in Helsinki is 7,5 m.
Even larger differences can be found when we divide the city into 145 neighborhoods. The vegetation range is almost eight meters. While in the neighborhood of Hermanninranta the average height is about 4,5 m, which is clearly associated with the “young and developing” state of the area.
On the other hand at Haltiala the average value is almost three times higher (12,4 m) as Haltiala represents the old parts of the Helsinki central park (“Helsingin keskuspuisto”). Everyone who ever visited those areas will agree that both of the neighborhoods are very different if we consider the scale of ecosystem services that is provided.
As mentioned above, the second approach to analyze vegetation height was based on the building related zoning areas of detailed land use plans. These zones don’t cover continuous area, but in each zone buildings have usually similar use – center, mixed use (C), block of flats (AK), semi-detached and detached housing (AP), recreation and tourist areas (R), retail (K) and traffic (L). Here we can point out, that the AP zone is represented the most – we can find over 19 000 different AP plots in the Helsinki.
The second most represented zone, about 7 000 different occurrences, is the AK zone. AP and AK together represent more than 95 % of the building related zoned city area. As shown in the graph, the highest average height of vegetation is in zone R, about 9 m. This fact is very easy to explain, because R stands for recreation and tourist areas, which are supposed to offer good “green” environment for spending free time.
On the other hand, zone L1 represent the opposite side of chart, with average height about 6,2 m. That is also not surprising since L includes traffic areas, where the vegetation is usually quite young and sparse.
And again if we take the air quality as a factor, we might question why we are ending up having the lowest average height among the traffic intense zones. The analysis also shows that the abundance of high trees in Helsinki is pretty low,, while the vegetation under 10 m height prevails.
The vegetation height heterogeneity is associated with habitat diversity and species richness not speaking about the aesthetic effect that it provides to human. Understanding how vegetation vertical structure varies across cities and identifying the potential drivers of this variation might improve the management of urban vegetation for biodiversity and ecosystem services (e.g. air quality improvement).
The management of urban vegetation should not focus solely on the horizontal scale – amount of tree cover or green space present across cities. We should also focus on identifying where interventions to improve vegetation vertical complexity are required and how can we, as urban planners, affect on it.
The study was concluded by Denisa Kujovská, Bachelor’s degree programme student at the Czech University of Life Sciences Prague (CULS), during her Erasmus+ traineeship at FCG in the summer 2017.
Read also (in Finnish)
Viherkerroinmenetelmä antaa bonuspisteitä syötävistä kasveista tai perhosniityistä
Jan Tvrdy, Senior Advisor, Urban and Strategic Planning