The value of air purification and carbon storage ecosystem services of park trees in Warsaw, Poland

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Introduction
The valuation of ecosystem services has a history dating back over a quarter of a century.The origin of this concept is usually associated with a paper by Robert Costanza and his team (COSTANZA ET AL., 1997), in which the term ecosystem service was defined as "the benefits people derive from an ecosystem".What the paper is arguably most famous for is the authors' attempt to assess, in monetary terms, the ecosystem services provided by nature worldwide.They estimated the value of the world's ecosystem services to be around 33 trillion USD per year -which was more than the global GDP at the time.The results of the study have been cited in some of the most prestigious scientific journals, such as Science (ROUSH, 1997).Likewise, it has been quoted by many renowned media outlets including the New York Times, Newsweek, Science News, National Public Radio, and the BBC (COSTANZA ET AL., 2017).The vast popularity of this paper contributed to an explosion of research and debates on valuing ecosystem services, that continues to this day.
Besides highlighting the importance of studying the monetary value of ecosystem services, Costanza and his team also made a first attempt at classifying ecosystem services -an important step in facilitating the comparison of results from different valuation studies.Their paper identified 17 types of ecosystem services.In subsequent research projects focusing on assessing ecosystem services (e.g., MEA, 2005;TEEB, 2010;CICES, 2013), the list proposed by Costanza and his team has been expanded upon and made more specific.Nowadays, the CICES list (Common International Classification of Ecosystem Services) is the reference list used in many studies (HAINES-YOUNG & POTSCHIN, 2018).This is also the list officially recommended by institutions in the European Union for research purposes.
The current version of the CICES list (v5.1) includes 90 ecosystem services.Of course, not all these services can be provided by all types of ecosystems.For instance, the service "provision of wild animals used as a source of energy" could perhaps be provided by woodlands or oceans, but not urban trees.Nevertheless, the value provided by urban trees can easily be assigned to other services included in the CICES.Studies assessing the monetary value of these services found, among others:  Trees provide raw materials such as fruits (CICES: 1.1.1.1)or wood fuels such as pellets or briquettes (CICES: 1. Evidently, trees provide a range of ecosystem services and many of them are of high economic value.It is important to conduct valuation studies, as quantification can raise environmental awareness and help policymakers to better manage urban greenery and achieve desired environmental improvements (RAUM ET AL., 2019).The study presented in this paper aimed to assess the value of the air purification and carbon storage ecosystem services provided by urban trees growing in Warsaw's public parks and garden squares.

State-of-the-art
The first step in the valuation of air purification and carbon storage ecosystem services is to estimate the amount of air pollutants removed by trees per year (g/year), as well as the total amount of carbon stored (kg).The second step is to estimate the costs that would have been incurred in the absence of these ecosystem services, i.e., how much more money would have to be spent on healthcare or fighting the harmful effects of climate change.This Avoided Cost Method is a well-established technique used to value ecosystem services by researchers across the globe.
An in situ study that aims to calculate the exact annual amount of pollutants absorbed by an individual tree growing in the city would be a very methodically complicated and capital-intensive process.The same is true for the carbon storage ecosystem service.When estimating the amount of carbon stored in tree tissues, it is found that about half of the dry mass of wood is made up of atmospheric carbon (NOWAK, 1994).However, when this is calculated for an individual tree, the tree is not removed, dried, and weighed to estimate how much carbon it has stored.Instead, models which were developed during studies under laboratory conditions are used to estimate these quantities (SZKOP, 2020a).The software tool i-Tree Eco can be used to estimate the amount of carbon stored in tree tissues (kg) and the annual amount of pollutants removed (kg/year) by an individual tree.By linking this data with the information about avoided costs due to the provision of ecosystem services, researchers can successfully estimate the economic value of such services.
The i-Tree Eco software was developed in the United States and has gained popularity abroad in recent years.Studies using this software have been performed in Canada (LEFRANÇOIS, 2015) In addition to the countries listed above, this software has also been used to perform several studies in Poland.Among others, it has been used to estimate the value of air purification and carbon storage ecosystem services provided by trees growing in Krasińskich Garden in Warsaw (SZKOP, 2016), trees growing in the Palace Park in Wilanow (ZAWOJSKA ET AL., 2016) and selected urban treen in the Wola district of Warsaw (SZKOP, 2021a).However, these were rather small-scale studies and did not allow for the verification of obtained values being either significantly higher, or lower, than the average in the region.
To date, no studies to assess the economic value of air purification and carbon storage ecosystem services across all the public parks and garden squares of a large city have been conducted in central and eastern Europe -including Poland.Moreover, the results of the studies conducted in other regions of the world may not be valid for central and eastern Europe's spatial context, e.g., due to a difference in tree species composition.The presented study aims to fill this knowledge gap.

Study area
The i-Tree Eco software requires field data to estimate the amount of air pollutants absorbed by trees per year (g/year) and the amount of carbon they store (kg).Data that relate to the size, species, health condition and location of the analyzed trees were obtained from the Greenery Office of the Capital City of Warsaw (ZZW) (ZZW, 2021).
Data from 29,165 trees growing in 41 selected urban green spaces (14 public parks and 27 garden squares) in central Warsaw were used in the analysis.This covers an area of approximately 234 ha.The location of the individual analyzed trees within the city boundary (zoomed-out view) (Fig. 1), the location of the public parks/garden squares (zoomed-in view) (Fig. 2), and the names and sizes of these areas (Table 1) are shown below.

Method
The amount of air pollutants absorbed by the analyzed trees per year (g/year) and the amount of carbon they store (kg) were estimated using the i-Tree Eco software.The analysis focused on three air pollutants: SO2, NO2 and PM2.5, as the software provides reliable estimates for these compounds.Once the air pollution removal and carbon storage ecosystem services were estimated in physical terms (units of mass), the ecosystem services were valuated (expressed in monetary terms).This was performed by evaluating the social costs that would have been incurred in the absence of these ecosystem services, i.e., the Avoided Cost Method approach.
The value of an ecosystem service is calculated by multiplying the obtained physical value of the service with the unit social cost.The unit social cost is an estimate of the economic costs, or damages, that would result when reducing the ecosystem service by one unit.For example, the unit social cost of a certain air pollutant, or CO2, is equal to the cost of emitting one additional kg, or ton, of that substance into the atmosphere, indicating the potential benefit of reducing emissions by the same amount.These unit social costs depend on many factors and are therefore not easy to calculate.Fortunately, many long-term research projects have already determined such values for the substances included in the presented study.
In this study, the benefits of improving air quality by removing SO2, NO2 and PM2.5 were calculated based on the unit social costs of these pollutants, which were estimated for various European research projects and summarized in the EXTERNEE DATABASE (2005).ExternE's methodology determines the unit social costs using an "impact pathway" approach.This means analyzing a series of events linking each activity under consideration (e.g., the emission of an additional ton of SO2) to its "effects" (e.g., impacts on human health, plants, physical assets, etc.) and then determining the monetary value of those impacts (e.g., the cost of treating diseases or renovation of building facades).The value of the unit social costs of air pollutants strongly depends, among other things, on how much of the population is exposed to the concentration of a given pollutant or what the costs of treating air pollution-related diseases are in a specific country.
To take these factors into account, the ExternE project values calculated for urbanized areas in Poland were used in this study.These were: 18.26 EUR/kg of NO2, 12.05 EUR/kg of SO2, and 56.70 EUR/kg of PM2.5 (at 2021 prices)1 .
The benefit of the carbon storage ecosystem service, as presented in the study, was calculated based on the unit social cost of carbon (SCC).This SCC value was determined by the Interagency Working Group on Social Cost of Greenhouse Gases, United States Government (IWG) (IWG, 2021).It shows the marginal cost of the impact caused by emitting one extra ton of CO2 and is equal to 51.00 EUR/ton of CO2 (162.22 EUR/ton of C) 2,3 .
In this study, the air purification service is estimated as a stream of benefits (EUR/year or EUR/ha/year) and the carbon storage service as a fixed value (EUR or EUR/ha).This is a deliberate decision by the authors, as trees remove air pollution each year and these pollutants will never return to the atmosphere in the same form.Therefore, presenting this value as a stream of benefits is justified.
The matter is more complex when it comes to the carbon storage ecosystem service.Trees sequester carbon every year, but all, or most, of that carbon will return to the atmosphere when they die and decompose.Thus, how much carbon a tree has sequestered in a given year is less important than the total amount of carbon stored in its tissues.The carbon storage social cost is estimated for the removal of the tree and therefore the release of all the carbon it has stored.For that reason, presenting this service as a fixed value is justified.
After estimating the value of the ecosystem services provided by the 29,165 trees, for which the necessary parameters were available, the results were extrapolated to all public parks and garden squares in Warsaw.According to the information obtained from ZZW (ZZW, 2021), Warsaw is covered by 1013.7 ha of public parks and 207 ha of garden squares.Based on the analysis carried out for the trees growing in the selected 14 public parks and 27 garden squares, the lower (first quartile) and upper (third quartile) estimates per hectare were calculated for each of the two area types.These values were then multiplied by the total area of public parks and garden squares in Warsaw to estimate the total value of air purification and carbon storage ecosystem services provided by all its urban trees.

Results
The study showed that the 29,165 examined trees remove an estimated 3,503 kg of NO2, 504 kg of PM2.5, and 375 kg of SO2 from the air each year.This service can be valued at approximately 91 thousand EUR/year.The average value of this service per tree is slightly more than 3 EUR/year.These urban trees store about 30 thousand tons of carbon in their tissues.If all the trees were cut down and the carbon stored in their tissues returned to the atmosphere as CO2, it would result in a social cost estimated at approximately 5 million EUR.The average value of this service per tree is around 170 EUR.It should be noted that, unlike the pollution removal ecosystem service, this is not the value of an annual stream of avoided costs (or benefits), but rather a fixed benefit of avoiding the cost of tree removal at a given point in time.Detailed valuation results are presented in Table 2.
The value of ecosystem services such as air purification and carbon storage is related to the size of a tree, which is related to the age of a tree.As a rule, the age of trees in urban areas does not have a normal distribution.Warsaw is no exception, as there are a lot more younger trees than old ones.This is because many newly planted trees do not survive to maturity, due to harsh urban conditions.Therefore, presenting the data by referring to the median, first and third quartiles (Q1 and Q3) may be more informative.This information is presented in Table 3.The median value of the air purification ecosystem service provided by the analyzed trees is very similar to the average value of 3 EUR/year.The value of this ecosystem service for half of the analyzed trees ranged from 0.9 to 4.6 EUR/year.
The median value of the carbon storage service provided by the analyzed trees is almost 97 EUR, therefore it is different from the average value.The value of this service for half of the analyzed trees ranged from 27.1 to 213.5 EUR.
These ecosystem service results were also investigated by type of area, and this is shown in Table 4 (public parks) and Table 5 (garden squares).
The study showed that the total value of the NO2, SO2 and PM2.5 pollution removal ecosystem service provided by all trees growing in the analyzed public park areas in Warsaw, is around 83 thousand EUR/year.On average, one hectare of the public park areas in Warsaw provides an air purification service to the estimated value of 408 EUR/ha/year.The median value is 392 EUR/ha/year.The value for half of the analyzed public parks ranged between 340 (Q1) and 489 EUR/ha/year (Q3).
The carbon storage ecosystem service provided by the trees growing in these parks has a total estimated value of around 4.5 million EUR.On average, one hectare of public park area in Warsaw provides this service to the estimated value of 22.4 thousand EUR/ha.The median value is 22.2 thousand EUR/ha.The value for half of the analyzed public parks ranged between 20.3 (Q1) and 25.9 thousand EUR/ha (Q3).It was found that the total value of the NO2, SO2 and PM2.5 pollution removal ecosystem service provided by all trees growing on the analyzed garden squares in Warsaw, is around 9.4 thousand EUR/year.On average, one hectare of garden squares in Warsaw provides this service to the estimated value of 347 EUR/ha/year.The median value is 310 EUR/ha/year.The value for half of the analyzed garden squares ranged between 234 (Q1) and 483 EUR/ha/year (Q3).
The carbon storage ecosystem service provided by the trees growing on these squares has a total estimated value of around 0.5 million EUR.On average, one hectare of garden squares in Warsaw provides a carbon storage service to the estimated value of 18.9 thousand EUR/ha.The median value is 19.4 thousand EUR/ha.The value for half of the analyzed public parks ranged between 13.1 (Q1) and 25.1 thousand EUR/ha (Q3).
The study covered an area of 14 public parks and 27 garden squares with a total size of around 234.3 ha.This constitutes less than 20% of the total area of Warsaw's public parks and garden squares, as public parks in Warsaw cover 1013.7 ha and garden squares cover 207.0 ha (ZZW, 2021).The values4 (EUR/ha or EUR/ha/year) obtained for the selected public parks and garden squares were thus extrapolated to represent the total area of public parks and garden squares in Warsaw, as shown in Table 6.The results suggest that the total value of the NO2, SO2 and PM2.5 air purification ecosystem service provided by all trees growing in public parks and garden squares in Warsaw, is between 393 and 560 thousand EUR/year.The value of the carbon storage service provided by these trees is between EUR 23.3 million and EUR 30.3 million.

Conclusions and recommendations
This study assessed the monetary value of ecosystem services provided by trees growing in public parks and garden squares in Warsaw.It focused on two ecosystem services: carbon storage and air purification.To conduct the valuation, the Avoided Cost Method together with i-Tree Eco software was used.This approach to valuation has been used in many studies across the globe, as well as in Poland.Among others, it has been used to estimate the value of air purification and carbon storage ecosystem services provided by trees growing in: the Krasińskich Garden in Warsaw (SZKOP, 2016), the Palace Park in Wilanów (ZAWOJSKA ET AL., 2016) and selected urban trees in the Wola district of Warsaw (SZKOP, 2021a).The results obtained in this study are consistent, in order of magnitude, with these previous studies.Unfortunately, the previous studies were smallscale and did not allow for the verification of values being either significantly higher, or lower, than the average for the region.The presented study improves on this, by conducting valuations on a much larger sample of 41 areas with trees (14 public parks and 27 garden squares) in Warsaw.
The findings presented here showed that, on average, one hectare of the public park in Warsaw provides an air purification ecosystem service to the value of 408 EUR/ha/year, while one hectare of garden square provides this service to the value of 347 EUR/ha/year.Regarding the carbon storage ecosystem service, it was found that one hectare of public park provides this service at an average value of 22.4 thousand EUR/ha and one hectare of garden square provides this service at an average value of 18.9 thousand EUR/ha.By extrapolating these results to represent the entire city, it was estimated that the total monetary value of the air purification ecosystem service provided by trees growing in all public parks and garden squares in Warsaw, ranges from 393 to 560 thousand EUR/year.The value of the carbon storage service ranges from 23.3 to 30.2 million EUR.
To date, no previous studies have estimated the economic value of air purification and carbon storage ecosystem services for such a large urban area.This applies not only to Poland, but also to the entire region of central and eastern Europe.This knowledge gap was identified and filled by the results of the presented study.Other researchers and decision-makers can now successfully use these findings to estimate the approximate monetary value of selected ecosystem services provided by public parks and garden squares in other central and eastern European cities.To facilitate this application, the results presented in this study were given values in units per hectare (EUR/ha or EUR/ha/year) and lower and upper estimates (Q1 and Q3) were also shown.
When interpreting the results of the conducted monetary valuation, one should bear in mind the study's main limitation.Though it is the largest of its kind to be conducted in Poland and in central and eastern Europe, the results provide only fragmentary information on the value of ecosystem services provided by urban trees in a large city.This is because: 1) The study did not cover the air purification ecosystem service fully.It focused on NO2, SO2 and PM2.5 pollutant removal, while the removal of other pollutants such as O3 or PM10 was not investigated.This is due to the methodological issues of such an investigation, i.e., the lack of reliable pollutant removal models and information on the pollutants' unit social costs -both of which are needed to perform the Avoided Cost Method valuation process.
2) The study focused only on public parks and garden squares, when many other urban green spaces were omitted.These include street trees, housing estate greenery, urban forests, cemeteries, allotment gardens and private gardens, to name a few.This means that the total value of air purification and carbon storage ecosystem services would be much higher if calculated for all green spaces in the city.
3) The study focused only on two ecosystem services, while trees provide a range of ecosystem services.Services that were not investigated include supplying raw materials, transpiration and shading, windbreak, runoff control, the reduction of noise pollution and noise masking, as well as aesthetic and recreational benefits.The latter service should especially be investigated, as existing international studies (BERTRAM ET AL., 2017;CHIESURA, 2004) indicate that recreation benefits could be much higher than the benefits from other ecosystem services provided by urban trees.
Notwithstanding the above-mentioned limitations, it should be noted that the presented study is an essential first step towards estimating the broadest possible range of ecosystem services provided by urban ecosystems in central and eastern European cities, but the topic requires further investigation.Future studies should aim to cover the air purification ecosystem service fully, by taking into account more pollutants.Other than just public parks and garden squares, a broader focus on more urban green spaces is also necessary.Finally, ecosystem services other than the air purification and carbon storage provided by urban trees, should also be assessed.

Fig. 1 .
Fig. 1.Location of trees included in the analysis

Table 1 .
Urban green areas included in the analysis

Table 2 .
Total and average economic values of the analyzed ecosystem services for the n=29,165 trees included in the analysis

Table 3 .
Economic value of the analyzed ecosystem services for the trees included in the n=29,165 analysis, expressed by quartiles

Table 4 .
Economic value of the analyzed ecosystem services across 14 public parks in Warsaw

Table 5 .
Economic value of the studied ecosystem services across 27 garden squares in Warsaw

Table 6 .
Results of data extrapolation for the whole of the Warsaw area