How Tree Shape Changes Pollen Spread in City Airflow

Spring pollen can feel inescapable in cities. Yet how pollen travels through the air may be driven less by how much a tree emits and more by the tree’s physical form.

Researchers have found that tree species disperse pollen through urban air in markedly different ways.

Each tree’s architecture - from leaf spacing to how thickly the canopy is packed - alters how wind threads through branches and, in turn, how far pollen grains are carried.

The results indicate that pollen exposure is not distributed evenly across a neighbourhood. Instead, airflow around individual trees can produce localised hotspots where pavements, courtyards and adjacent roads are hit by much denser pollen clouds than nearby areas.

Mapping those dynamics could help cities rethink which street trees they plant - and where - as allergy seasons continue to lengthen.

Model shows how pollen spreads

As air flows around an urban tree’s branches, it breaks apart and recombines into shifting streams that transport pollen unevenly.

Using detailed airflow simulations, Talib Dbouk at the University of Rouen Normandy showed that these currents vary sharply according to the structure of the tree.

As a result, an oak and a linden release pollen into surrounding city air in distinctly different patterns, even when the wind conditions are comparable.

Spotting that difference is a key step towards understanding how such airflow patterns develop and why some trees disperse pollen farther than others.

Tree wakes reshape pollen paths

Once wind has passed a tree, it leaves a wake - a disturbed trail of air that continues beyond the canopy.

“The wake of a tree is very complex,” said Dbouk. Wind speed, wind direction, leaf crowding and seasonal leaf loss can all determine whether pollen grains clump together, drop out, or continue drifting.

Tree form strongly governs this airflow. Within the canopy, porosity - the open space between leaves and branches - controls how readily air can move through.

Tightly packed foliage blocks more air, whereas larger gaps let wind enter, split and re-form inside the canopy before it exits behind the tree. These contrasts reshape the wake and the route pollen grains follow after leaving the branches.

Because leaf area varies over the year, the very same tree may disperse pollen differently in spring than in summer, so residents can experience changing exposure from one tree over time.

Tracking pollen from the branch

Before tree pollen can be transported through city air, wind must first pull each grain free from the flower cluster where it is produced.

The model represented this by estimating the minute force required to detach a grain and then following its movement through the airflow around the canopy.

Because pollen grains are almost impossible to see in open air, ground-based measurements often miss this initial motion. Simulations can capture those hidden movements and uncover patterns that field observations struggle to detect.

To check the model, the team compared the airflow outputs with measurements collected around a real oak tree, and then carried out five validation tests using shapes ranging from solid objects to porous structures.

After those tests aligned with earlier data, the researchers applied the model to a living Tilia cordata linden tree, mirroring the complicated forms found in real urban settings rather than simplified wind-tunnel set-ups.

What the linden tree revealed

When the researchers examined the linden in detail, they observed a notable effect after wind moved through its canopy. The air behind the tree rapidly became turbulent - swirling, erratic currents that re-energised the flow.

These rotating pockets of air can keep pollen aloft for longer. Rather than settling quickly on the ground, grains are lifted, mixed and transported farther through the surrounding air.

This behaviour also differed from what the team observed around the oak. The contrast underscored that pollen transport depends strongly on each tree’s structure, not merely on random changes in wind.

Put another way, city planners cannot assume that all leafy street trees distribute pollen in broadly the same manner.

Allergy seasons are growing longer

Pollen is already a growing public health concern, in part because climate change is extending allergy seasons.

Across North America, pollen seasons have become about 20 days longer in recent decades. Over the same period, airborne pollen levels have risen by 21 percent, meaning the lengthening seasons are also delivering more pollen into the air.

When pollen is inhaled, the immune system can overreact, triggering allergic rhinitis - the familiar combination of sneezing, blocked nose and itchy eyes.

These reactions affect millions of people each year. According to the Centers for Disease Control and Prevention (CDC), pollen-related medical costs now exceed $3 billion annually in the United States.

Using tree placement to reduce pollen

Street design could use this type of modelling to reconsider where trees sit in relation to benches, entrances, schools and bus stops.

Species selection matters as well, because one tree might keep pollen close to a pavement while another carries it farther along the street.

“These important parameters inform the risk assessment and future mitigation policies aimed at limiting the exposure of people to allergic airborne pollen grains,” Dbouk said.

This is not an argument for removing urban trees, since cities depend on them for many benefits; rather, placement becomes a part of health planning.

From tree models to city planning

Predicting future pollen exposure will require more than analysing a single tree, because buildings, narrow streets and variable winds continually reshape how pollen moves through a city.

Later versions of the model could connect multiple trees with day-to-day weather patterns and seasonal leaf changes to produce neighbourhood-scale exposure maps.

Researchers also aim to extend the framework to track when pollen that has settled is lifted back into the air after later gusts disturb it.

Together, these developments could shift a tree-scale research tool into something city agencies can use before planting plans are signed off.

The work also recasts pollen exposure as more than a simple seasonal total. Instead, it becomes a street-by-street airflow issue shaped by urban design.

As cities warm and allergy seasons continue to lengthen, models like this may help planners cut the number of people exposed to the densest pollen plumes.