Allergies in the City: what would a plant ecologist do?

Last year, while I was in the field trying to figure out how plants and insects are coping with urban life in Berlin, a visiting medical student from Spain, Sara Mejia Lanunciòn, came along with a question about the allergic risk of exotic plants. I have to admit, I had not given it any thought until then, but I was curious immediately.

Allergies are on the rise in cities, and since our future is looking to be more and more urbanised, pollen allergies and associated chronic respiratory disease are one of the top health issues of our time. For decades already, medical research has investigated the intricate and often mysterious workings of allergies, and generally agree that urban human populations are becoming more sensitive to pollen due to various factors such as cross-sensitization by air pollution, lack of childhood exposure, or a stressful lifestyle. This human-centred medical approach has not, however, given much space to the plant side of things, that is, the side responsible for producing the allergens.

Naturally, as a plant ecologist interested in functional diversity, my immediate thought was: how is the turnover in composition and functional diversity of urban vegetation changing allergenic pollen production? Can we measure allergen richness and diversity?

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Railroad tracks overgrown with birch trees in the Südgelande Urban Nature Park in Berlin, Germany

The biggest culprits in cities are often trees, such as birch or willow which have very allergenic pollen and are commonly found all over in cities all over Europe. But trees are mostly planted, and the choice of tree species can nowadays be made to minimize allergies. Spontaneous urban vegetation, however, is another story.

Most spontaneous urban vegetation does not consist of trees, but rather grasses, forbs, and bushes. They occupy road borders, railroad tracks, vacant lots, un-managed lawns, gardens, or any area left unattended. A city like Berlin is full of wild green places growing in the many interstice and abandoned areas still abundant in such a sprawling city with a unique history combining bombings, socioeconomic upheavals, and a fondness for big beautiful garden-like cemeteries. Berlin has embraced its wild urban nature in a number of great urban parks where trees and grasses are left to grow spontaneously between abandoned railroad tracks, with only minimal management. Berlin ecologists have famously been pioneers in urban ecology since the 1950’s, with, as the story goes, landlocked West Berlin ecologists turning to the only nature they could access within the wall. At least this is what I heard here…

Ostkreuz
Ruderal vegetation around a regional train station, Ostkreuz, Berlin

Despite decades of research in urban ecology, we are still far from understanding how the urban environment shapes biodiversity. Some things we do know: 1) exotic species tend to be abundant in cities; 2) Some types of native plants, categorized as urban adapters or exploiters, are able to thrive in urban areas, while others are left behind; 3) the urban environment, in particular a warmer micro-climate called the heat island effect, has a major influence on plant biology and ecosystem functioning, increasing productivity and shifting phenology. But we are still mostly ignorant of the rules and functioning of these so-called novel urban ecosystems, i.e. urban assemblages of exotic and native species in artificial, polluted and distributed urban environments. Current projects like the BIBS project (Bridging in Biodiversity Science –website) are large collaborative efforts trying to get to the bottom of this.

Tempelhofer Feld
One of our sites is in an old airport field turned public city park, Tempelhofer Feld, Berlin

Using vegetation surveys conducted by colleagues from the urban ecology lab at the Berlin Technical University (website), I attempted to describe the allergenic properties of urban grassland communities with the same mindset I would adopt in any study looking at functional trait diversity in plant communities. We collected as much information available online about allergenic properties of plants, i.e. “allergenic traits”. It turns out that there are many sources, and most freely accessible! Some sources reference clinical assessments of allergenic severity (e.g. http://www.pollenlibrary.com/), while others list our current knowledge of the allergen molecules found in the pollen of each species (e.g. http://www.allergen.org/ ). The later are the better curated ones, including reliable listings by the WHO, and provide so many details that for now my study has only barely scratched the surface. We then looked at the number and diversity of allergenic species and their allergenic molecules in urban grassland communities, and how these varied with urbanisation and the level of invasion by recently arrived exotic plants.

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A single plant species may produce several different allergenic pollen molecules.

Here are our latest results in a nutshell:

Exotic-rich urban grassland communities do not harbour more allergenic species than rural grasslands, however, they produce a higher diversity of allergen molecules. These new molecules are not contributed only by the exotics entering the communities, but also by the type of native plant species thriving in novel urban grasslands.

allergenic molecule diversity
Increase in allergenic molecule diversity in novel urban grasslands

This increase in allergen diversity is important because any additional allergenic molecule in the air is likely to touch new people in the urban population who were not previously allergic. In other words, a broader spectrum of allergenic molecules will affect a broader spectrum of people, thereby contributing to an increase in the prevalence of allergies in cities.

Identifying in advance the families of allergenic molecules which are novel to a region may be crucial for urban nature management.  One successful example is the case of Ragwort, which is severely allergenic not just through its pollen but also by simple contact with the skin. The species has been monitored via an online citizen science project since 2006 in Berlin, permitting the targeted removal of populations as soon as they were reported. The initiative has been successfully until now, reducing since 2013 the populations to only a few pockets of invasion.

Other aspects of plant functioning, such as flowering phenology, may affect not only the quality but also the timing and quantity of pollen production. We are currently trying to go further in identifying phylogenetic or functional groups associated with types of allergen production, or families of allergenic molecules.

I have presented some of these results earlier this month at the 2018 annual meeting of the ESA in New Orleans this year (find my presentation here: https://f1000research.com/slides/7-1375 ), and will be happy to discuss this ongoing project next week at the Neobiota 2018 conference in Dublin where I am doing a flash presentation and a poster.

Come and ask me about the project!

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