Road typology is associated with street-tree diversity, structure and composition in a rapidly urbanizing Indian city
This study of Guwahati, India, reveals that road typology significantly influences the diversity, structure, and composition of urban street trees, with larger roads supporting higher species richness than highways, while highlighting the need for diversified planting and road-specific management strategies to enhance ecological resilience in rapidly urbanizing cities.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the heart of a growing city, the trees lining the streets are more than just decoration; they are a living infrastructure that cools the air, filters pollution, and offers a place for wildlife to move through the concrete. Yet, as cities expand rapidly, the way these trees are chosen and planted often follows a rigid pattern. Urban planners frequently select a few hardy species that are easy to maintain, creating a landscape that looks uniform but lacks the resilience of a wild forest. This phenomenon, known as biotic homogenization, means that cities around the world are beginning to look more alike, with the same few types of trees dominating every block. Understanding how different types of roads influence which trees survive and thrive is crucial for city leaders who want to build greener, healthier environments. If the road itself dictates the forest, then managing urban nature requires looking at the street as a specific habitat, not just a line on a map.
In the rapidly expanding city of Guwahati, located in the northeastern part of India, a team of researchers set out to see how the type of road shapes the trees growing alongside it. They focused on the southern part of the city, a bustling area where residential neighborhoods, commercial hubs, and major transport routes intersect. To get a clear picture, the team walked along 143 different stretches of road, each measuring 200 meters long. They categorized these stretches into four distinct groups based on their width and traffic: wide highways, large main roads, medium-sized streets, and small residential lanes. Over the course of several months in 2022, they counted every tree with a trunk thick enough to measure, recording its species, its size, and its overall health. They were looking for patterns to see if the wider, busier roads held different kinds of trees compared to the quiet, narrow lanes.
The survey revealed a total of 1,455 trees representing 80 different species. While the city is home to a surprising variety of plants, the researchers found that a small handful of species dominated the landscape. Five specific types of trees made up nearly half of all the individuals counted, suggesting that planting choices have become quite repetitive. The most common tree found was Pongamia pinnata, followed closely by others like Monoon longifolium and Delonix regia. Interestingly, the study found that native species—those originally from the region—still make up the majority of the trees, accounting for about 70 percent of the total. This is a positive sign, as it indicates that local biodiversity has not been completely overwhelmed by imported, exotic plants, which is a common problem in many other urban centers.
However, the type of road mattered significantly. The researchers discovered that the widest highways, despite having the highest number of trees per kilometer, supported the least variety of species. These roads were dominated by a few specific types, creating a simplified forest structure. In contrast, the large main roads held the greatest diversity of species and the largest, most mature trees. The smaller residential lanes, while having fewer trees overall, showed a more even mix of different species and a higher proportion of native plants. The health of the trees also varied; trees on the large roads were generally in better condition and larger in size, while those on the highways and small lanes were often younger and smaller. This suggests that the large roads have retained older trees from earlier phases of the city's development, whereas the highways and small lanes are seeing more recent planting efforts.
The study also looked at the age of the trees by measuring the width of their trunks. They found that many of the most common species, such as Pongamia pinnata, were mostly young trees with thin trunks. This points to a recent wave of planting, likely part of new urban greening initiatives. In contrast, the largest trees, like the massive Samanea saman, were found mostly on the older, wider roads. These mature giants are vital because they provide the most shade and store the most carbon, but they are often the first to be removed when roads are widened for traffic. The researchers noted that while replacing a lost mature tree with a new sapling might keep the tree count the same, it takes decades for the new tree to provide the same ecological benefits as the old one.
Ultimately, the findings suggest that the road itself acts as a filter, deciding which trees can grow and how they develop. The study indicates that current planting practices in Guwahati are leading to a simplification of the urban forest, where a few species are used repeatedly across different areas. While the city still boasts a healthy number of native trees, the reliance on a narrow selection of species and the loss of mature canopy trees could make the urban forest more vulnerable to pests, diseases, and climate changes in the future. The researchers conclude that for cities like Guwahati to remain resilient, urban planners need to move beyond simple tree-counting goals. Instead, they should focus on protecting the large, old trees that already exist and diversifying the types of trees planted, tailoring these choices to the specific conditions of each road type to ensure a richer, more robust urban environment.
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