CCMA8: UHI mitigation strategies IV : vegetation management processes
The urban tree as a tool to mitigate the urban heat island in Mexico City: a simple phenomenological model
1Instituto de Ecología, UNAM, Mexico; 2Centro de Ciencias de la Atmósfera, UNAM
The urban heat island (UHI) is a kind of thermal pollution in cities, although it is mainly a nocturnal phenomena also appears during the day in Mexico City. This pollution may affect the human thermal comfort which possibly influence human productivity in the spring-summer period. This heat excess is compensated by air conditioning systems implying an extra energy consumption and a feedback to the UHI. A simple phenomenological model based on the energy balance was developed to generate theoretical support of the UHI mitigation in Mexico City focused on the latent heat flux change by increasing tree coverage. Although it was necessary to establish an UHI update, hourly data of the urban energy balance components were generated in a typical residential/commercial neighborhood of Mexico City, and then parameterized using easily measured variables (air temperature, humidity, pressure, visibility). Transpiration and canopy conductance was measured every hour in four tree species using sap flow technique and parameterized by the envelope function method. Averaged values of net radiation, energy storage, sensible and latent (QE) heat flux were around 449, 224, 153 and 72 Wm-2, respectively. Daily tree transpiration ranged 3.64─4.35 Ld-1. Reducing air temperature by 1°C in the studied area using Eucaliptus camaldulensis (QE=48.0 Wm-2) required 138 trees ha-1 while a reduction of 2°C by Liquidambar styraciflua (QE=102 Wm-2) needed 67 trees ha-1. However, it is possible to establish more than four species to support biodiversity. This results shows that the simple reforestation of the city cannot mitigate UHI adequately, but requires to choose the most appropriate tree species to solve this problem. Also it is imperative to include these types of studies in the urban development planning and failure to do so, the UHI would be a dangerous problem in the future due to the rapid and anarchic growth of the city.
Evaluating climate-related ecosystem services of urban tree stands in Szeged (Hungary)
University of Szeged, Hungary
In the context of climate change, it is more and more important to form and evaluate the different urban climate adaptation strategies in a planning-oriented manner, so as to implement them easily in urban environmental management processes. One solution for that can be the evaluation of ecosystem services of urban trees and green spaces. This recently developed methodology focuses on the contribution of the environmental functions of the ecosystems to human well-being. Some of these services can be expressed in monetary value, which makes it easier to compare the significance of these attributes to other goals in urban planning. In our contribution, we present the results of an individual-based evaluation of two climate-related ecosystem services (carbon sequestration, air pollution removal) of the tree stands of the centre of our city of Szeged (Hungary), by the adaptation of a targeted model developed in the U.S. (i-Tree Eco). The baseline tree cadastre database was made and is maintained in strong correspondence with the urban environmental management company of the city. The calculalations are based on allometric relationships between measurable parameters of tree size and condition and state indicators of the referring services (e.g. leaf area, total biomass). Our results highlight the importance of tree condition, which may strongly affect service provision (huge parts of the stands in good condition belong to non-indigenous species). Therefore, tolerance of urban circumstances (determined e.g. with this type of analyses) should be taken into consideration during species selection. Meanwhile, old trees in good condition have a distinguished role from the point of view of climate-related services as well (besides biodiversity aspects). The total economic value of the two investigated services are comparable with the planting and maintenance costs. The presented results on a per-tree basis can form a base for developing spatial-based assessments methodologies, in order to fulfil the requirements of the policy objectives on mapping ecosystem services on a national level (EU Biodiversity Strategy 2020).