Thermic Attenuation on Concrete Sidewalk under Urban Trees . Case Study : Santa Marta – Colombia

Background and purpose: Urban trees provide a number of services including shade and thermal attenuation. This is related to morphological and physiological characteristics of trees and may vary between species and even between individuals of the same species. The aim of this work was to identify thermic attenuations on concrete sidewalks under six tropical urban trees with six different types of shadows. Materials and methods: In Santa Marta City, Colombia (10o12 ́20” N, 74o13 ́33” W, 10 meters above sea level and 31oC temperature), we selected six trees (species) with different types of shade, and they are evaluated for soil temperature and the temperature in the shade and off throughout the day for four different days of the year. ANOVA and t-tests were performed with R program in order to identify the influence of the specie, the day, the hour and the position (at the thermic comfort level, surface temperature) on the temperature results obtained. urban surface materials [11-13]. For example, asphalt roads can reach temperatures over 70oC without shadow effect causing a reduction in the life cycle of this kind of surface and inducing other effects such as the release of toxic – mutagenic gases [14, 15]. The aim of this work was to identify thermic attenuations on concrete sidewalks under six tropical urban trees with six different types of shadows in Santa Marta city, Colombia. MATERIALS AND METHODS This research was developed in Santa Marta City, Colombia (10o12 ́20” N, 74o13 ́33” W, 10 meters above sea level) (Figure 1), which has an average temperature of 32.6 oC (Figure 2), and 443 mm of rain per year (Figure 3) [16]. Results and conclusion: Some trees have the most translucent shadows most likely due to nictinastic movements and consequently less temperature attenuation. On the other hand, other trees have denser shadows and can generate more substantial thermic attenuations. Regarding temperature data, the hour of the day shows the greatest influence on the variability of air temperature and the species shows the greatest influence on the variability of surface temperature. Honey berry (Meliccoca bijugatus) and Malay almond (Terminalia catappa) trees have denser shadows and can generate more substantial thermic attenuations. Tree physiology can play an important role in temperature attenuation in cities as a result of shadow effects and can be applied as a criterion to select urban trees in tropical cities.

Results and conclusion: Some trees have the most translucent shadows most likely due to nictinastic movements and consequently less temperature attenuation. On the other hand, other trees have denser shadows and can generate more substantial thermic attenuations. Regarding temperature data, the hour of the day shows the greatest influence on the variability of air temperature and the species shows the greatest influence on the variability of surface temperature. Honey berry (Meliccoca bijugatus) and Malay almond (Terminalia catappa) trees have denser shadows and can generate more substantial thermic attenuations. Tree physiology can play an important role in temperature attenuation in cities as a result of shadow effects and can be applied as a criterion to select urban trees in tropical cities.

INTRODUCTION
Urban trees provide a number of services including carbon and pollutants capture [1] shade, thermal attenuation and ultraviolet (UV) protection [2,3] as well as hydric regulation [4].
Heat islands are a consequence of direct sun light effects over city structures. These structures take sun energy and convert it into heat, generating an increase of average city temperature in comparison with nearby rural areas [5]. Trees can reduce temperature by thermic attenuation depending on the shape of their shadows and other physiologic factors such evapotranspiration [6] and nastic movements [7]. Thermic attenuation by trees as an ecosystem service for whole city [8] can range from 0.5°C to 1.4ºC [9,10]. These attenuations increase the service life of

RESULTS AND DISCUSSION
The average comfort level temperature was 30.3ºC and ranged from 30.1ºC to 34ºC. For the temperature measured at the sidewalk, the average value with shadow effect was 29.8ºC and ranged from 26.0ºC and 40.7ºC, while the average value without shadow effect was 40.8ºC and ranged from 40.3ºC to 77.2ºC. (  Temperature data were collected at the thermic comfort level (1.5 m) beneath the trees and on the sidewalk at points affected and not affected by the tree's shadow, for day we took temperature with eleven measurements per hour (each 5 sec for a minute) between 8:00 a.m. and 5:00 p.m. over 4 days (March 13 and 26 and April 6 and 7), for a total of 5 720 values. For each tree we started with air temperature and in second place we took the surface temperature under the shadow. Finally we took the surface temperature in direct sun light. The value of temperature was taken with an air thermometer (Sper Scientific Mini Environmental Quality Meter), with a resolution of ±1.2°C ambient, and surface thermometer (Extech® High Temperature IR Thermometer), with a resolution of 0.1°F/°C. ANOVA and t-tests were performed with R program in order to identify the influence of the specie, the day, the hour and the position (at the thermic comfort level, surface temperature) on the temperature results obtained.
For temperature collected we applied ANOVA and t-test. ANOVA results indicated that the species (kind of shadow) show the greatest influence on entire temperature variability, followed by the hour of the day, the date and the position (with or without shadow) (Figure 19). The kind of shadow is a primary factor for tree services associated with      [16]. T-test results indicated that the air temperature under the shadow of Malay almond 1 tree vary significantly (p-value < 0.05) from the air temperature results under the shadows of the perature only the Malay almond 2 and Honey berry give same significant of similar conditions ( Table 4). The values of air temperature are significant similar for 13:00 and 16:00 hours, 12:00 and 17:00 hours and between 13:00 and 15:00 hours ( Table 5). The values of surface temperature are similar significantly between 11:00 to 13:00 hours and between 13:00 and 16:00 hours (Table 6).
Finally about the surface temperature, all trees are significantly different in refer to surface with shadow (direct sunlight).
offer not significantly difference for other species, the species of Redwood tree and Rain tree significantly similar, the Malay almond 1 tree was significantly different about other trees; the Malay almond 2 tree was not similar to Redwood tree and at the same time significantly different about other trees, however the Redwood tree was significantly different about other trees; Honey mesquite tree was significantly different about other trees as such the Rain tree ( Table 3). The Honey mesquite, Redwood tree and Rain tree trees have nictinastic movements and consequently less temperature attenuation. About the surface tem-

REFERENCES CONCLUSIONS
For urban surfaces (sidewalks) we found a range of 50ºC between the lowest value (26ºC under shadow effect) and the highest value (77.2ºC under direct sunlight), conditions that can generate positive effects on heat islands and, in the case of asphalt pavements, even a control of toxic vapours and increase in life cycle durability.
Honey mesquite (Prosopis juliflora), Redwood tree (Platysmincium pinatum) and Rain tree (Enterolobium ciclocarpum) trees have the most translucent shadows most likely due to nictinastic movements and conse-quently less temperature attenuation. On the other hand, Honey berry (Meliccoca bijugatus) and Malay almond (Terminalia catappa) trees have denser shadows and can generate more substantial thermic attenuations. Finally found that there are significant differences between the shadow temperature control (sun light direct) and any temperature under the surface shadow tree.
This information suggests that tree physiology can play an important role in temperature attenuation in cities as a result of shadow effects and can be applied as a criterion to select urban trees in tropical cities.