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Ecosystem services provided by trees in the city: Assessing the cooling capacity by measuring the dendrometric parameters (trunk diameter growth)

The acquisition of micro-dendrometric, tensiometric and thermal measurements was made during the summer of 2016 and 2017 at the rate of one measurement every 30 minutes

Today, in an attempt to combat the effects of urban heat islands (UHI), the planners and public policy makers have qualitative information available to enable them to come up with recommendations for planting trees without being able to assess their real impact.

There are many bibliographical references available for information about the services provided for trees grown in an urban landscape. However, there are very few publications available that provide statistical data and an objective quantification of how the presence of trees has a direct effect on heat mitigation in the city. The principal research studies into these effects are contained in a review paper written by Emmanuel Boutefeu (2011) and in the theses of Cécile de Munck (2013) and Wissal Selmi (2016).

Today, in an attempt to combat the effects of urban heat islands (UHI), the planners and public policy makers have qualitative information available to enable them to come up with recommendations for planting trees without being able to assess their real impact.

Within this context, it would appear necessary to invest in a quantitative study of the ecosystem services provided by trees grown in the city, particularly their cooling capacity during the summer periods.

The newly developed rue Garibaldi - 3rd arrondisement of Lyons, has been selected as a trial site for collecting data within the context of the massive European H2020 project. The Metropolitan Area of Lyons is a partner in this project for validating and disseminating the technologies used to acquire localised bioclimatic data on how the UHI effect can be mitigated by the natural cooling of the vegetation.

The prime objective of the study is to formulate a protocol for identifying the degree of thermal comfort induced by the presence of the trees at the Garibaldi site and analyse the parameters that can exert an influence. Micro-dendrometric sensors of the Pépi-Piaf type were installed to provide continuous measurements during the summers of 2016 and 2017 of the level of water stress and tree growth and thus deduce the physiological state of the trees monitored. Sensors for measuring the air temperature, plant water status and soil temperature were also installed to evaluate the level of heat mitigation vis-à-vis the level of available soil water.

 

Rue Garibaldo transformed

The rue Garibaldi, long ago designed as an urban express­way that crosses the city of Lyons from the Parc de la Tête d’Or in the north to the Parc Blandan in the south, has under­gone a major transformation since 2013. This project has focused on the introduction of trees and other greenery and the number of traffic lanes was reduced. The section affec­ted by the experiment now has almost half a hectare of green space along its 800-m length. The old plane trees have been conserved alongside more drought-resistant species.

 

Measuring devices

Micro-dendrometric measurements using the PépiPiaf system. Just like all plants, the trees promote their growth by absorbing water through the roots and trans­porting it through their organs due to the transpiration of the leaves, which acts as the driving force for the sap rising through the xylem. At the leaves, the water, before becoming vaporised in the substomatal chambers, is part of the process of photosynthesis, producing the sugar molecules that are going to nourish plant growth. Thus, this transpiration and photosynthesis activity varies in the course of the day and during the season, this being affected by the climate (climate change) and also the water resources available in the soil. The PEPIPIAF micro-dendrometric system developed by INRA - UMR PIAF (French National Institute of Agricultural Research) is a displacement sensor, which allows the user to measure and memorise (micrometric sensitivity) variations in the radial growth of a part of the tree in order to determine the leaf/water transpiration potential and the water constraints that affect them.

The water balance of all parts of the tree at any given moment is, therefore, the difference between the water absorbed through the roots, the water transpired through the leaves and the variations in the amount of water stored within the plant (e.g. for the manufacture of new cells or cellular plasmolysis). Therefore, the bark cells at a given level participate towards the transpiration stream according to the changes in the plant water balance throughout the day. At daybreak, the opening of the stomata leads to the transpiration of the tree. The water lost through transpiration is, at any given time, greater than the water absorbed through the roots, even in the presence of a wet soil. During this phase, the diameter of the different parts of the tree decreases. As illustrated in the graph below, this degrowth phase corresponds to the maximum contraction range (MCR) which reflects the intensity of transpiration.

At night-time, in the absence of any transpiration, the rehy­dration of the tree allows for a recovery in the stem diameter accompanied by a growth of the latter, which corresponds to the apparent daily increment (ADI). This growth gain takes place when the level of water available and the climatic conditions are favourable for photosynthesis.

The PepiPiaf system includes an LVDT (Linear Variable Differential Transformer) displacement sensor and a wireless data logging and transmission unit installed high up on the branch of an adult plane tree.

Description of the other measuring sensors. On the same plane tree equipped with the PepiPiaf system, a Watermark tensiometer-type sensor device is put in place to monitor the availability of the soil water.

The temperature sensors are installed in the crown of the tree in question and on a lamp post located about 20 metres from this tree.

 

Results

The micro-dendrometric, tensiometric and thermal measure­ments were conducted during the summer of the years 2016 and 2017 with a measurement taken every 30 minutes.

Only the results for the month of August in these two seasons are shown below.

 

Evaluation of the transpiration capacity of the tree

Soil-Tree-Atmosphere water movement. The monitoring of the micro­metric variations of the diameter revealed a growth gain during the two measuring periods (August 2016 and August 2017). This growth gain is represented by the trends shown with the dashed lines in the graphs. It generally demonstrates a good photosynthetic functioning of the tree. A tree in a good state of health is a tree that actively participates in the coo­ling process. When the curve shows a downward trend, this demonstrates that the tree is suffering from plant water stress. A lack of water in the soil is the most commonreasonfor stress, significantly af­fec­ting photosyn­the­sis and transpiration.

The tensiometric measurements and rain gauge readings are recorded in figures 3 and 4. During the period under review, the variations in the soil water tensions reveal a good level of rooting activity of the plane trees and also as a result of the apparently stress-free water status. The rainfall received has largely contributed towards maintaining the tree’s rooting environment in this comfortable stress-free condition.

Evaluation of the tree’s transpiration capacity. The typical characteristics of the micrometric variations shown in the green curves (fig. 1 and 2) reveal perio­dically the dips that correspond to the maximum daily contraction amplitude (MDCA). Each dip in the curve reflects the contribution of the water reserves of the bark cells towards transpiration. The sap flux linked to the transpiration, therefore, represents the tree’s cooling capacity. The more the curve oscillates during the day between a maximum diameter and a minimum diameter, without any loss of growth, the more the tree transpires and consequently contributes towards the mitigation of the surrounding temperature. This qualitative evaluation, therefore, corresponds to achieving optimal physiological functioning for cooling the ambient air. In absolute terms, the cumulated losses in the diameter calculated between 06:00 h and 14:00 h is 332 microns in August 2016 and 128 microns in August 2017. If we compare the MDCA of the two seasons, we can see that the transpiration is quasi-regular in 2016 while the plane tree tended to transpire more at the end of the month in 2017.

These differences in the transpiration potential are linked to a number of factors, with the soil water conditions and the phenological stage of the tree being important factors. During the month of August, the growth of the plane tree slows. The water is moved through the tree essentially by the pull of transpiration which, in turn, depends on the climatic demand and water available in the soil.

The soil water conditions monitored by the tensiometer have been shown to be favourable for the transpiration activity of the tree during these two seasons (Fig. 3 and 4).

 

Improving thermal comfort 

The transpiration of the tree con-tributes towards a drop in tempe­rature in its close environ­ment. The differences in tem­perature recorded in the crown of the tree compared with those recorded close to the lamp post situated at around 20 m from the tree and those of the Bron meteorological station, are respectively 1.08°C and 1.78°C in 2016 and 0.81°C et 2.33°C in 2017.

The maximum difference of 8.09°C was recorded during the day on 18/08/2017 when the temperature reported by the Bron station was 32.80°, while that recorded at the tree was 24.65°C.  However, the distribution of maximum temperatures throughout the month for each season only showed slight variations in these temperature differences (fig. 5 and 6). The heat reduction generated by the presence of the tree was recorded on each date.

 

Conclusion

The PepiPiaf micro-dendrometric measurement of the varia­tions in the diameter of a tree gives a good indication of its growth, level of water stress and, therefore, ultimately the intensity of its transpiration. The values obtained during the first study allowed us to define how the transpiration flow contributes towards a reduction in heat and, consequently, towards combatting the urban heat island effects. However, this qualitative approach requires the implementation of a wider protocol, formulating a number of modalities and repeats for quantifying the cooling capacity and establishing the correlation between the transpiration of the tree and the temperature drop in the proximity compared with those of the nearest meteorological station.

This benefit provided by the cooling capacity of the tree is not the only ecosystem service of the urban tree. This can be added to a range of other useful services: storing carbon, air purification, refuge of biodiversity, soil protection and water infiltration…

 

To learn more about the principles of use of the PepiPiaf micro-dendrometer, please refer to these main articles of reference:

• AMEGLIO T., B. MORANDINI, CORELLI-GRAPPADELLI L., NAOR A., 2015 - Observer l’arbre plutôt que le sol. Réussir Fruits & Légumes 355, supplément Irrigation Arbo (Novembre 2015),  4-5.

• AMEGLIO T, DUSOTOIT-COUCAUD A, GUILLOT V, COSTE D, ADAM B., 2009 - PépiPIAF : une nouvelle génération de biocapteurs pour le pilotage d’une arboriculture de précision. 2ème conférence sur l’entretien des espaces verts, jardins, gazons, forêts, zones aquatiques et autres zones non agricoles, 2009, Angers, France. pp.10. 〈hal-00964672>

• DAUDET F.A., AMÉGLIO T., COCHARD H., ARCHILLA O., L. LACOINTE, 2005 - Experimental analysis of the role of water and carbon in the tree trunk diameter. Journal of Experimental Botany 56 (409), 135-144. (DOI: 10.1093 / jxb / eri026)

• AMÉGLIO T., CRUIZIAT P., 1992. - Daily variations of stem and branch diameter: short overview from a developed example. NATO ASI Series, Vol. H64. Mechanics of Swelling. Ed T.K. KARALIS. Springer-Verlag Berlin Heidelberg 1992, 193-204