Assessing urban ecosystem services provided by urban trees in Strasbourg City

 Strasbourg city is located in North East France. The city’s population reached about 275 000 inhabitants in 2011 (INSEE,2011). The climate of Strasbourg is continental with an average monthly temperature of 2◦C in January and 19◦C in June. The mean annual precipitation is 665 mm (Meteo France website: http://www.meteofrance.com/accueil).

The Strasbourg Eurometropolis website (EMS) mentioned that the city has 400 ha of parks and is the only European city with protected alluvial forests around its out-skirts, with three great forests: Neuhof (757 ha); Robertsau (493 ha) and Rohrschollen (309 ha) (EMS, 2013).

This study is the first order estimation of urban ecosystem services in France by using i-Tree Eco model. 

The aim is to assess ecosystem services provided by trees by characterizing the structure of urban trees in Strasbourg city.

Quantified ecosystem services: Air pollution removal; carbon storage and sequestration

Tree structure informations are collected based on field survey conducted from April to July 2013 and obtained from 228 plots randomly ditributed within municipal green spaces of Strasbourg. At each 0.04 ha circular plot  general information (date, plot address, GPS coordinates, land use, tree and shrub cover, ground cover andplantable space) were recorded as well as individual tree data on species, diameter at breast height (DBH), total tree height, crown width, height to base of live crown, crown light exposure, crown dieback percent, percent crown missing.

Hourly meteo-rological data (e.g., wind speed, sky cover, temperature, liquidprecipitation, etc.)  and the hourly pollution concentration  were collected from the “Meteo France”, the National Oceanic and Atmospheric Administration and the regional Air quality Agency (ASPA).

Although the model was initially developed for US cities, i-Tree can be used worldwide, but requires data collection, input and formatting prior to use. There some uncertainties by could be minimized with better and more local input data. 

 Trees with diameters less than 15.2 cm constitute 44 % of the population and about 15 % of Strasbourg trees population has diameters greater than 70 cm. The majority of large trees (with diameter greater than 54 cm) are located in agricultural and vacant class.

- The three most common species are European beech (Fagus sylvatica) (12.0%), European filbert (Corylus avellena) (11.90%), and European ash (Fraxinus excelsior) (10.0%).

- The results of i-Tree Eco model show that municipal greenspace’s leaf area is 90.76 km2.

Trees in public green spaces in Strasbourg removed about 88.23 t per year of pollutants from July 2012 to June 2013: 1.20 t per year for CO; 13.84 t per year for NO2; 55.88 t per year for O3; 11.77 t per year for PM10coarse; 4.51 t per year for PM2.5and 1.04 per year for SO2.

Trees in Strasbourg remove about 7% of the emitted PM10 coarse.

Urban trees in Strasbourg store about 128 000 t (58 909 kg/ha) of carbon and sequester about 4059 tC/year. 

Urban trees help to mitigate air pollution, but they are one of many potential solutions to this problem. Reducing emissions from the source prevents pollutant emissions and trees should not be used as alternate solution to emission reduction, but rather as complementary one

In Strasbourg city, a wide range of street trees are regularly pruned to give them a geometric design and for safety reasons. As this pruning is done, managers should consider local traffic emissions to design canopy structure that is healthy with maximum leaf area, it allows for dispersion of local pollutants or limit dispersion toward sidewalks where people are often exposed to pollutant emissions.

Strasbourg University

ASPA Agency

Wissal Selmi

wissalselmi@yahoo.fr

Selmi W et al., 2016. Air pollution removal by trees in public green spaces in Strasbourg city, France, Urban forestry and urban greening, 192-201. 

Selmi W., 2014. Évaluation des Services écosystémiques rendus par la végétation urbaine. Application d’approches d’évaluations à La Ville De Strasbourg. Ph.D Thesis. University of Strasbourg