Allergens in the City: is urban nature creating new health risks?

Ambrosia ssp. can be highly allergenic to humans.

Allergens in the City: is urban nature creating new health risks?

Author: Dr. Maud Bernard-Verdier, FU Berlin

In 2017, when the BIBS project was still young, a medical bachelor student from Spain, Sara Mejia Lanunciòn, visited our group (Jonathan Jeschke’s AG: the Ecological Novelty group) to do a small research project. She was interested in combining medical issues with ecology, and she thought that allergenic exotic plants would be a good way to go. As a plant community ecologist focusing on more  fundamental questions of community assembly and plant evolution, 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 become 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, resulting in impressive global collaborative efforts (http://www.worldallergy.org/). Research now generally agrees that urban human populations are becoming more sensitive to pollen due to various factors such as cross-sensitization by air pollution or other allergens, lack of childhood exposure, or a stressful lifestyle.

This human-centred medical approach has not, however, given quite as much thought to the plant side of things, that is: which plants are producing pollen in the cities, what type of allergens do they produce, and when? So typically, from the perspective of a plant community ecologist interested in plant functional diversity, my immediate thought was:

Is vegetation becoming more allergenic with increasing urbanisation?

Arguably, the biggest culprits for pollen allergies in cities are trees, with birch and willow trees commonly found everywhere across most cities in Europe or the US, and producing very large quantities of pollen in spring. Though producing smaller amounts of pollen, grasses and certain weeds (e.g. Ambrosia spp.) can be as allergenic, if not more, than trees. These small herbaceous plants also tend to be more diverse, more difficult to map, and more difficult to control. Accordingly, much of the research on allergenic urban plants has focused on trees. City trees are mostly planted, and while landscape architects of the past might have ignored allergy considerations, the choice of tree species can nowadays be made to minimize allergies. At least in theory. Spontaneous urban vegetation, however, is another story.

Spontaneous woody and herbaceous vegetation in Berlin, Germany. Left: Birch trees are left to grow spontaneously among abandoned railroad tracks in the urban nature park of Südgelände. Right: Grasslands on the old abandoned airport of Tempelhof are lightly managed by mowing once a year.

Most spontaneous urban vegetation is not made of trees, but mostly of grasses, forbs, and small shrubs. 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…

Despite decades of research in the field of 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 plants are able to thrive in urban areas, while others are left behind; 3) the urban environment, in particular the warmer micro-climate called the urban heat island, has a major influence on plant biology and ecosystem functioning, increasing productivity and shifting phenology. But we are still mostly ignorant of the assembly of these so-called novel urban ecosystems, i.e. urban assemblages of exotic and native species in artificial, polluted and distributed man-made environments. Our current projects in BIBS are precisely trying to get to the bottom of this. 

Using vegetation surveys conducted by Birgit Seitz and collaborators from the Urban Ecology lab at the Berlin Technical University, 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.

I first got a few students involved to help me collect as much information available online about allergenic properties of plants. Sara had laid the foundation in 2017 with a first collection of data, but it turned out that there are many (many) sources, and most are freely accessible online! Several online databases list our current knowledge of the allergen molecules found in the pollen of each species. Some are strictly curated (WHO data: http://www.allergen.org/), while others aggregate all published allergenic proteins, with varying degrees of clinical support (http://www.allergome.org/). The variety and heterogeneity of this new world of medical data means that it took me an extra couple of years to clean up and make sense of it. At first a side project, I worked on it whenever I had time off from the lab or the field. It paid off, as we now have assembled some exciting results in a manuscript that will hopefully be published soon.

Allergen molecules identified in the pollen of the native forb Artemisia vulgaris.

It turns out that grasslands in the urban core do tend to be more allergenic than the less urban ones. We show that this happens in three ways: 1) the composition of grasslands shifts with urbanisation, incorporating a higher cover of allergenic plants, mostly exotic ones; 2) those exotic allergenic plants tend to collectively produce a broader chemical diversity of pollen allergens with urbanisation, and 3) exotic plants flower later and extend the allergy season later into autumn in the urban core.

It was surprising to find that exotic plants contributed so clearly to patterns of change in allergenicity in cities. Exotics in Berlin grasslands are not more often allergenic than natives, and both categories comprise of roughly 30% of allergenic species. The large majority (80%) of allergenic species in these grasslands are in fact native species. However, native allergenics did not appear to change in abundance or diversity with urbanisation, whereas exotics did.

Moreover, exotic plants, and in particular those more recently introduced in the last five centuries (i.e. neophtyes), contributed a remarkably diverse pool of allergen molecules (figure 3). For 10 allergenic plant species, natives contribute on average 12 different biochemical families of allergens, while neophytes contribute twice as much. Neophytes are still continuously being introduced from different regions of the globe, and the diversity of their evolutionary origins may be one of the reason they contribute such a high diversity of allergens.

Seasonal distribution of allergenic species and allergen families in Berlin grasslands along the urbanization gradient.

Grassland sites are clustered into three balanced groups (n = number of grassland plots) according to the percentage of impervious surface in a 500 m buffer: <7 % for near-rural; between 7 % and 30 % for low-urban; >30 % for high-urban.

(a-c) Number of allergenic species flowering each month. Species are divided by floristic status: natives (green), older exotics (e.g. archaeophytes in orange), and neophytes (red).

(d-f) Potential number of unique biochemical allergen families produced per month. Because some biochemical families were redundant between floristic status groups, the total number of unique allergen families (grey line) is not equal to the three groups.

So should we be worried about the health risk of urban grasslands?

Urban nature and biodiversity are generally beneficial, providing a wide range of ecosystem functions and services, but there may be a need to balance these with the risk of possible ecosystem “disservices” such as allergens. However, things are not so simple. The medical allergy literature tells us that the effect of allergens in the environment is far from straightforward. While allergies can only develop when an allergen is present in the environment, and allergy symptoms get worse with increased allergen doses, a lack of sufficient allergen exposure, especially during childhood, may also be a problem. This is why immunotherapy works, and this counterintuitive dose-response makes things more complicated. Some medical researchers have voiced the concern that our increasingly urban population may in fact not be getting enough allergens. A recent study in New Zealand showed that growing up in a highly diverse and green environmental was associated to lower allergy prevalence. However, this positive effect appeared to be counteracted if the vegetation was exotic! This might have been the result of the exotics themselves, or the fact that exotic invaded vegetation tends to be less diverse.

Overall, in spite of contradictory patterns, a consensus seems to emerge that exposure to a diverse and green environment is rather a good thing to prevent allergies. In Berlin, allergen diversity did not decrease with urbanisation, but the proportion of green areas did so drastically. A main cause for increased allergies in cities is air pollution, which can be alleviated by an abundant and diverse urban vegetation. Therefore greener cities favouring a diversity of spontaneous native vegetation remain a desirable goal. A better understanding of how urbanisation impacts plant community assembly may provide guidelines for managing a more diverse and green city. Certain management measures may need to be implemented to reduce the impact of major allergens, such as reducing the abundance of invasive and late blooming exotics, and providing pollen traps such as water bodies or forested areas.

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