Environmental Drivers of Foliar Particulate Accumulation, Leaf Structural and Biochemical Responses in Urban Trees in Delhi, India

Cities are often defined by their concrete and steel, but they are also shaped by the air that moves through them. In many parts of the world, this air carries a heavy load of tiny solid particles, a mixture of dust, soot, and smoke that settles on everything it touches. Trees are frequently planted in these environments to act as natural filters, catching these particles on their leaves and keeping them out of the air we breathe. However, a lingering question for city planners and ecologists is whether a tree that catches the most dust is also the healthiest tree. It is possible that a species might be excellent at trapping pollution but suffer physically from the burden, or conversely, that a resilient tree might not be the best at cleaning the air. Understanding the relationship between how much dust a leaf holds, how the leaf is built, and how the tree chemically reacts to the stress is essential for choosing the right plants to green our cities.

To explore this balance, researchers in Delhi, India, set out to examine ten different native tree species across four distinct urban locations. They worked during the summer and winter of 2025, collecting leaves from trees situated near busy roads and industrial areas, as well as in quieter spots. The team measured exactly how much coarse and fine particulate matter had settled on the surface of each leaf, counting the weight of the dust per unit of leaf area. Alongside this, they looked at the physical structure of the leaves, such as their thickness and how dense they were, and analyzed their internal chemistry, including levels of specific vitamins and pigments. They also calculated a standard measure known as the air pollution tolerance index, which estimates how well a plant can withstand polluted conditions.

The results revealed a stark difference between the seasons. During the summer, the leaves carried a moderate amount of dust, but by winter, the total amount of deposited particles on the leaves increased by approximately 3.5 times. The heaviest loads were found on trees located at the roadside and in industrial zones, where the air is most turbulent and polluted. Among the ten species tested, the Plumeria alba tree held the most dust on average, followed closely by the Ficus benghalensis and the Alstonia scholaris. In contrast, the Azadirachta indica and Aegle marmelos trees retained significantly less dust. The researchers found that the amount of dust a tree held was linked to specific chemical changes within the leaf, such as higher levels of ascorbic acid and a shift in the leaf's pH, but it was not simply a matter of the leaf being thicker or having a larger surface area.

Perhaps most importantly, the study showed that a tree's ability to accumulate dust and its ability to tolerate pollution are not the same thing. The air pollution tolerance index did not change significantly between seasons, suggesting that a tree's chemical resilience is a separate trait from how much dust it physically catches. This means that selecting a tree for a city park based solely on how much pollution it removes might not guarantee that the tree will survive the stress of that pollution. The researchers noted that species like the Ficus benghalensis, which had a dense, sturdy leaf structure, appeared to be a strong candidate for urban planting because it combined high dust retention with structural resilience. Ultimately, the study suggests that city planners should not rely on a single factor when choosing trees. Instead, the best approach involves looking at a combination of how much dust a species captures, how its leaves are built, and how its internal chemistry responds to the environment. By weighing these factors together, cities can select trees that are not only effective air filters but also robust enough to thrive in the challenging conditions of an urban landscape.