Pollen allergies have been rising in cities, but what causes them? Maud Bernard-Verdier and colleagues studied over fifty grasslands in Berlin to see what made allergies worse. They found that two factors increased the problem. The biggest effect was urbanisation, which increased allergenicity. A second factor was the presence of invasive species. The study, the first of its type, was recently published in the journal Ambio.
If you’re allergic to pollen, you’re not alone. Up to 40% of people suffer from pollen allergies, and they seem to be getting worse. A combination of warmer temperatures, increased carbon dioxide and urban living appears to be fuelling the problem. As a result, pollen is a leading cause of chronic respiratory diseases worldwide.
One of the oddities is that pollen sensitivity is increasing faster in urban areas than in rural locations. One explanation put forward is that urban inhabitants don’t get the exposure to allergens that rural people get, a variation on the hygiene hypothesis. Bernard-Verdier and colleagues set out to quantify precisely how urban grasslands affected pollen allergies.
The team examined grasslands in Berlin. Their survey took in two gradients. One was the degree of urbanisation, the other the presence of invasive species. The study examined fifty-six 4 × 4 m grassland plots. In their article, Bernard-Verdier and colleagues write: “These field sites were part of a collaborative research platform called the CityScapeLab Berlin, created as part of the Bridging in Biodiversity Science project, and spanned a wide range of habitats, from rural landscapes at the urban fringe to highly urbanised settings along road median strips or railroad tracks. They represent a standardised model ecosystem with reduced environmental heterogeneity since all these dry grasslands could be assigned to the same vegetation type (i.e. Sedo-Scleranthetea), following the phytosociological vegetation classification of Braun-Blanquet…”
“To our knowledge, this study is the first to provide a quantification of allergenicity at the plant community level for a standardised model ecosystem (i.e. dry grassland) along an urbanisation gradient. By combining community vegetation surveys with available data on species allergenic and phenological traits, we were able to compare grassland communities in terms of richness, abundance and turnover of allergenic species and their allergens.”
The team found that the more urbanised a grassland plot was, the more allergenic it was. Around 80% of allergenic species were native plants, but the non-native plants contributed to allergies in a couple of ways.
Firstly, they increased the diversity of allergens, meaning that a grassland had a greater opportunity to hold something that would get up your nose. The authors write: “Our finding that local factors—amount of built-up areas around the grassland patch and level of neophyte invasion—were significant predictors of allergenicity supports previous observations that urban pollen environments are determined by local habitat filters…”
A second factor is seasonality. The non-native plants expanded the season that allergens were airborne. So not only do urban grasslands have more variety, but they’re also a problem for longer. The authors refer back to research in AoB PLANTS showing why this might be the case.
Elizabeth Wolkovich and Elsa Cleland examined phenology, the timing of life stages, in invading plants. One way an invasive plant can side-step competition is if it can occupy a niche that native plants have left empty. Timing the release of your pollen is one way of doing this. If your pollen is released at a different time to the natives, and your flowers are open to receive it at a different time, you’re not going to get your flowers clogged up with incompatible pollen, and they’ll work more efficiently, helping you colonise your new home. But timing your pollen release at a different time means that, for hayfever sufferers, the pollen season is extended.
Bernard-Verdier and colleagues note that it’s not enough for a plant to be classified as allergenic or non-native to be a problem. They write: “[H]alf of non-natives classified as allergenics in Berlin grasslands were insect pollinated, thus, scoring low PAVs [Potential Allergenic Values], and non-natives were not on average more frequently or more severely allergenic than natives. Thus, high levels of plant invasions in themselves do not predict high allergenicity, and merely counting the number of allergenic species does not directly relate to potential allergenic value.”
The authors state that herbaceous plants are a challenge for managing allergies. Unlike planned and easily monitored plants, like trees, self-seeded grasses and weeds get everywhere. They can also be an established part of the environment and are not considered in urban management. But this is not the only tool planners can use.
Bernard-Verdier and colleagues write: “A second approach to mitigating urban allergens is at the habitat scale, adopting practices favouring low-allergenicity ecosystems. In our study, grasslands with the highest potential allergenic value were both very urban and highly invaded by neophytes. In these grasslands, neophytes were not themselves more allergenic, but rather acted as indicators of habitat conditions—i.e. an increase in man-made disturbances… —fostering novel plant communities rich in allergenic species, both native and non-native. Reducing disturbances and encouraging succession from pioneer ruderal vegetation or, alternatively, intensively managed lawns, to diverse semi-natural meadows within the city may decrease the local abundance and impact of major allergens… [R]educing mowing frequency is known to help maintain a diverse herbaceous layer, while allowing successional processes to replace ruderal weedy neophytes with a diversity of more competitive long-term natives.”
In light of this, projects like No Mow May could have immediate human benefits as well as improving local biodiversity.
The team conclude by reminding readers that allergenic plants are not the only problem in cities and may not even be the most significant factor in producing allergies. Both air pollution and urban warming contribute to the spread of allergens. Instead, they argue that their approach can help identify allergy hotspots. This way, people and the planet can still enjoy diversity in green spaces without being hampered by a reaction.
READ THE ARTICLE
Bernard-Verdier, M., Seitz, B., Buchholz, S., Kowarik, I., Lasunción Mejía, S. and Jeschke, J.M. (2022) “Grassland allergenicity increases with urbanisation and plant invasions,” Ambio. https://doi.org/10.1007/s13280-022-01741-z