Association between local airborne tree pollen composition and surrounding land cover across different spatial scales in Northern Belgium
Graphical abstract
Introduction
Nature and urban green spaces provide ecosystem services associated with numerous health benefits (Twohig-Bennett and Jones, 2018). In urban areas, people have a reduced exposure to nature (Cox et al., 2018) and experience increased symptoms of asthma and allergies (von Hertzen and Haahtela, 2006). Therefore, urban green spaces are of utmost importance for improving physical (Braubach et al., 2017) and mental health (Barton and Rogerson, 2017; Bratman et al., 2019). However, nature and green spaces can also be a source of aeroallergens, especially when allergenic pollen producing tree species are present (Pecero-Casimiro et al., 2019).
Airborne pollen from many wind-pollinated plant species have allergenic potential. Worldwide allergenic pollen exacerbate allergies in up to 25 % of the population (Passali et al., 2018). It is estimated that 100 million Europeans suffer from allergic rhinitis, yet 45 % of this group remains undiagnosed (The European Academy of Allergy and Clinical Immunology (EAACI), 2016). Several factors such as meteorological conditions (e.g. wind speed and direction, humidity and precipitation) (Borycka and Kasprzyk, 2018), presence and type of landscape elements and infrastructure may influence the pollen dispersal and persistence in the atmosphere (Rojo et al., 2015). Long term measurements in Brussels, the capital city of Belgium, show increasing trends in pollen concentrations associated with increasing temperature and radiation and inverse associations with relative humidity and rainfall (Bruffaerts et al., 2018).
In European aerobiological networks, airborne pollen concentrations are monitored by the Hirst method at building roof-level, i.e. 10−20 meters above ground (Galán et al., 2014), optimal for homogeneous measurements representative for an approximate 25 km radius area (Oteros et al., 2019; Rojo et al., 2019). Depending on the local landscape, topography and climate, the measured pollen composition can even be relevant for a 50 km radius area (Gehrig, 2019). Pollen can be transported over long distances, contributing to an extension of the pollen season (Bogawski et al., 2019a). Short-distance transport, however, contributes to the most important pollen peaks (Rojo and Pérez-Badia, 2015). Nevertheless, standardized measurements are not taken at ground level, and as such outside the regular human breathing zone, potential local variations in pollen composition at lower height are poorly taken into account (Hjort et al., 2016; Rojo et al., 2019; Werchan et al., 2017). Peel et al. (2013) have shown that the actual pollen-dose can differ strongly from the measured regional pollen concentration.
State of the art birch pollen dispersion models still have difficulty taking into account fine-scale patterns of birch habitation (Kurganskiy et al., 2020). Local tree composition is expected to have an effect on the local pollen exposure (Weinberger et al., 2016) and to be a risk factor for tree pollen allergic sensitization (Lovasi et al., 2013). A better understanding of the drivers of local pollen composition could lead to improved urban green management and better health outcomes (Weinberger et al., 2016). While in palynology the link between regional vegetation and pollen has been widely studied (Fletcher and Thomas, 2007), we aim to contribute to the understanding of current health risks of poorly measured and modeled local pollen compositions.
The overall objective of this study is to measure local variations in airborne tree pollen composition by passive sampling at 2 m height above ground-level. We study how the passive measurements correspond to standardized sampling measurements regarding airborne pollen composition as well as timing of detection during the season. Then we want to identify the environmental factors of the surrounding landscape that drive the local airborne pollen composition. In addition, we test multiple spatial scales to find at which spatial scales the environment affects the local airborne tree pollen composition the most and are thus of relevance for exposure studies.
Section snippets
Study area
The study was conducted in Flanders, the northernmost of the three administrative regions of Belgium, as well as in the Brussels-Capital Region, which is geographically enclosed within the Flanders region. Flanders has an area of 13,522 km² and a population density of 482 inhabitants per km². The Brussels-Capital Region has an area of 162 km² and a population density of 7442 inhabitants per km². The climate according to Köppen is a maritime temperate climate (Cfb) (Peel et al., 2007).
Passive samplers
The design
Pollen measurements
Pollen of the 12 target tree genera were detected and quantified in concentrations ranging between ∼1 to >1000 grains cm−2 month-1 during the study period (Fig. 3). Quercus and Betula were measured at all sites in the highest quantities (130–1324 pollen cm-2 month -1). Platanus (13 sites out of 14), Aesculus (2 sites) and Fagus (6 sites) pollen were the only taxa that were not recorded at all sampling sites.
Comparison of the passive pollen samplers with the reference stations
The passive sampling measurements (lower panels in Fig. 4, Fig. 5, Fig. 6) showed that,
Local variation of pollen composition
Monitoring at the reference stations in Brussels and Genk showed that 2017 was a year of low pollen amounts for Alnus, Betula, and Corylus. Acording to the Belgian Aerobiological Surveillance Network the pollen season of 2017 was rather weak for these trigger trees (Sciensano, 2017). Quercus and Betula, two pollen taxa known to be transported over long distances (Maya-Manzano et al., 2017a,b; Skjøth et al., 2015), were measured at all sites in the highest quantities (Fig. 3). Airborne pollen
Conclusion
Passive sampling of airborne pollen demonstrated local variations in airborne tree pollen composition. Urban green spaces, agricultural areas, forests, shrub lands, grasslands and wet land cover types were characterized by marked airborne tree pollen compositions. Associations between local tree pollen composition were driven by landscape characteristics at the meso-scale (1−5 km). The effect of the meso-scale implies that not only green spaces within cities but also around urban areas are
CRediT authorship contribution statement
Michiel Stas: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Raf Aerts: Conceptualization, Writing - review & editing, Visualization. Marijke Hendrickx: Conceptualization, Writing - review & editing, Funding acquisition. Nicolas Bruffaerts: Conceptualization, Methodology, Writing - review & editing. Nicolas Dendoncker: Writing - review & editing, Funding acquisition. Lucie Hoebeke: Methodology, Writing - review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was carried out in the framework of the RespirIT project, which has been supported by a project grant from the Belgian Science Policy Office BELSPO (grant nr. BR/154/A1/RespirIT).
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