How a Tree's Shape Determines Where Its Pollen Ends Up in a City

American Institute of Physics

Allergy season is getting longer. Climate change is extending the pollination window for many plant species, exposing urban populations to airborne allergens for more weeks each year. But the amount of pollen in any given spot depends not just on which trees are nearby but on how wind moves through and around them. A new computational study has begun to quantify those dynamics at the level of individual trees.

The invisible wake behind a tree

When wind hits a tree, the result is aerodynamically complex. The air does not simply flow around the trunk like water around a rock. It penetrates the canopy, interacts with branches and leaves, decelerates, accelerates, and generates turbulence on the lee side. The pattern of that turbulence determines where pollen detached from the tree ends up.

"The wake of a tree is very complex," said Talib Dbouk, one of the study's authors and a researcher at Embry-Riddle Aeronautical University. Within that wake, multiple parameters interact: tree type, leaf area density (which changes with the season), wind speed, and wind direction all affect pollen transport.

Most pollen grains are invisible to the naked eye, making direct experimental measurement of dispersal patterns extremely difficult. The research team, which included scientists from the University of Rouen Normandy and the University of Lille in France, turned instead to computational fluid dynamics.

Modeling porosity and pollen detachment

The researchers developed an advanced simulation that models a tree's porosity, the percentage of the volume enclosed by its canopy boundaries that is actually occupied by tree material versus open air. They incorporated an algorithm sensitive to the small aerodynamic forces needed to detach individual pollen grains, accounting for the adhesion that holds pollen to anthers until wind overcomes it.

They validated their approach against previously studied structures, including an oak tree with existing experimental data. Once simulations matched real measurements, they applied the method to other species, including a real linden tree (Tilia cordata) located in the Rouen Normandy region of France.

Species shape dispersal patterns

The comparison between linden and oak trees made the central point clearly: tree type and topology, meaning its shape, leaf area density, and porosity, lead to distinct pollen dispersion dynamics in the surrounding environment. The linden tree generated regeneration of turbulence close to its canopy, a common outcome when wind passes through canopy-type structures, but the spatial pattern differed from the oak.

This matters for urban planning. A row of oaks along a boulevard will produce a different pollen exposure profile for pedestrians than the same row planted with lindens. The concentration peaks, the downwind dispersal distances, and the height at which pollen travels all depend on which species is planted and where.

What these simulations can tell city planners

The practical application is straightforward, even if the underlying physics is not. Cities routinely make decisions about which tree species to plant, where to locate parks, and how to design green corridors connecting urban spaces. Those decisions currently account for factors like shade, aesthetics, root damage to sidewalks, and drought tolerance. Pollen dispersal rarely enters the calculation.

"This work provides quantitative insight that can inform urban planning decisions, where public authorities can better orientate the management of green spaces in urban spaces," Dbouk said. The simulations could help planners position high-pollen species away from pedestrian zones, schools, and hospital entrances, or select lower-pollen species for areas with high foot traffic.

Significant modeling limitations

The current work models individual trees in isolation or small configurations. Real urban environments involve rows of trees, buildings that channel and redirect wind, heat islands that create updrafts, and seasonal changes in leaf density as trees grow and shed foliage. Scaling from single-tree simulations to neighborhood-level pollen exposure predictions will require substantially more computational power and more complex environmental modeling.

The simulations also assume steady-state wind conditions. Real urban wind is gusty, variable in direction, and strongly influenced by building geometry. How these transient conditions affect pollen dispersal is not captured in the current framework.

The researchers acknowledge these gaps and plan to expand their models to address larger-scale pollen dynamics in urban environments. But even the current single-tree results highlight a factor that urban planners have largely overlooked: the species-specific aerodynamics of pollen dispersal.

Climate change raises the stakes

As growing seasons lengthen and temperatures rise, many tree species are producing more pollen over longer periods. Some species are also expanding their range into previously cooler regions, introducing new allergen sources to populations with no prior exposure. In this context, understanding exactly how pollen moves through urban landscapes becomes not just an academic exercise but a public health tool.

The researchers' framework, even in its early form, represents a step toward evidence-based decisions about urban greenery that balance the clear benefits of urban trees against their role as allergen sources.