Investigating the Effect of Tree Planting Patterns (Evergreen and Deciduous) on the Improvement of Urban Microclimate

Industrialization and urbanization have increased air pollution, making environmental sustainability a critical focus in urban planning. In this context, trees are crucial in mitigating pollution by reducing carbon dioxide (CO2) levels, increasing relative humidity, and improving urban aesthetics. This study investigates the effects of different tree planting patterns (evergreen and deciduous) on key environmental parameters: relative humidity, CO2 concentration, and ambient radiation temperature. The research was conducted near the Faculty of Agriculture of Tabriz University in East Azerbaijan Province, Iran. Using ENVI-met software, the area was modeled on October 10, 2020, at a resolution of 2 by 2 meters. The simulated data, which included climate variables such as temperature, relative humidity, and CO2 concentration, was validated using field measurements. The results showed that tree planting patterns with continuous canopy cover significantly improved environmental conditions by reducing CO2 levels, increasing relative humidity, and reducing radiant temperature. Deciduous trees, including plane trees (Platanus orientalis) and ash trees (Fraxinus excelsior), outperformed evergreen species in reducing air pollution and improving environmental quality. This superior performance is attributed to their larger leaf area and canopy diameter, which improve their ability to capture CO2 and regulate microclimatic conditions. The study highlights the critical role of strategic urban tree planting, with a particular focus on deciduous tree species with dense foliage due to their ability to improve the urban microclimate, reduce air pollution, and enhance environmental quality. By implementing these strategies, urban planners can create healthier, greener, and more sustainable cities that are better equipped to adapt to climate challenges.
Extended Abstract
1-Introduction
Urbanization and urban development have increased the need for work and housing and exacerbated the unequal distribution of urban infrastructure, thermal stress, and the effects of urban heat islands. Rapid urban growth and population concentration have led to climatic changes and threatened human health, making urban well-being a research priority. Uncontrolled urbanization has altered climatic patterns such as temperature, precipitation, and relative humidity. Microclimatic parameters such as radiant temperature and relative humidity directly affect thermal comfort, and environmental materials and albedo influence the radiation and absorption of light. Urban green spaces play a crucial role in regulating the microclimate. Trees improve environmental health and thermal comfort by providing shade, evaporation, and transpiration, reducing temperature and increasing relative humidity. The impact of plant species on reducing carbon dioxide and air pollution is climate-dependent, and selecting appropriate species and planting patterns helps improve environmental conditions. This research examines the effect of planting patterns and canopy models on reducing temperature, carbon dioxide, and increasing relative humidity in cities and guides urban planners.
 2-Materials and Methods
This study simulates and analyzes the effects of different tree planting patterns on the microclimatic conditions and thermal comfort around the Faculty of Agriculture of Tabriz University in East Azerbaijan, Iran. For this purpose, the ENVI-met 4.4.6 software was used, which models three-dimensional climate changes with high resolution. Input data included regional meteorological information (air temperature, relative humidity, wind speed, and direction) and physical characteristics of trees (height, crown diameter, leaf type). Two tree planting scenarios were examined: (1) with 1-meter spacing between tree canopies and (2) without spacing (overlapping tree canopies). Twelve different combinations of common tree species in Tabriz (such as plane tree, elm, and silver cypress) were simulated. The simulations were carried out over 8 hours on a cold day (October 10, 2020). Model output included air temperature, relative humidity, carbon dioxide concentration, and mean radiant temperature (MRT). The accuracy of the ENVI-met model was validated using field data. The R² values for temperature and relative humidity were 0.96 and 0.89, respectively, indicating good model accuracy. The results showed that different tree combinations and planting patterns can help reduce temperature and improve thermal comfort. In addition, the cooling effects and microclimatic changes were analyzed through simulation and data analysis in different scenarios.
 3- Results and Discussion
The results of this study show that tree planting patterns have a significant impact on improving microclimatic conditions in urban environments. A comparison of two tree planting patterns showed that pattern #2 (trees without gaps between canopies) performed better than pattern #1 (trees with a distance of 1 meter between the treetops). Humidity (Perini et al., 2018). This difference is mainly attributed to the larger leaf surface area and higher CO2 reduction in sample #2. Deciduous trees (e.g. sycamore, ash, elm) were more effective at reducing CO2 than evergreen trees (e.g. black pine, silver cypress) and increasing relative humidity. These effects were more pronounced on cold days and in urban and residential areas. Increasing leaf surface area and tree height significantly contributes to improving environmental conditions, especially on cold days when deciduous trees are best at reducing temperature and increasing humidity. Overall, the results showed that Pattern #2, particularly the use of deciduous trees with larger leaf areas and canopy diameters, is the most effective solution for improving environmental conditions and reducing air pollution in urban areas. Deciduous trees play a more important role than evergreen trees in improving microclimate and thermal comfort, especially on cold days, by absorbing more CO2, reducing radiant temperature, and increasing relative humidity.
 
4- Conclusion
The results of this study showed that tree planting patterns and species types have a significant impact on improving urban microclimatic conditions. The use of deciduous trees with broad canopies, such as sycamore and ash, in overlapping planting patterns (Pattern No. 2) led to a reduction in CO2 concentration, an increase in relative humidity, and a decrease in radiant temperature. Deciduous trees, due to their higher sunlight absorption and greater evapotranspiration, performed better than evergreen trees. Selecting native species suited to the climate can also help reduce air pollution and urban heat islands. These findings can assist urban planners and landscape architects in designing urban green spaces. Moreover, the use of planting patterns with overlapping canopies to increase shade and reduce temperatures is recommended. In this regard, simulation tools like ENVI-met are effective for predicting environmental impacts.