Perspective EssayDistribution of ornamental urban trees and their influence on airborne pollen in the SW of Iberian Peninsula
Introduction
Urban green infrastructure is a concept that includes ornamental street trees and all other vegetation in urban environments. It is a concept that has been developed in the last two decades, and it is clear that urban green infrastructure can improve the quality of life for citizens (Breuste, Artmann, Li, & Xie, 2015). The benefits from ornamental street trees and urban green spaces are widely alleged in literature (Mullaney, Lucke, & Trueman, 2015; Tyrväinen, Pauleit, & Seeland, 2005) for the following reasons: (a) providing aesthetic beauty and visual relief (Shackleton, Chinyimba, Hebinck, Shackleton, & Kaoma, 2015), (b) the protection of pedestrians from the sun (Gillner, Vogt, Tharang, Dettmann, & Roloff, 2015), (c) offering habitats for birds and other animals in urban landscapes (Idilfitri, Sulaiman, & Suriani-Salleh, 2014), (d) lowering urban temperatures and mitigating the intensity of urban heat island effects (Ashby, 2001; Flatley, 2001; Norton et al., 2015; Yu & Nyuk, 2006), (e) acting as noise filters (Fang & Ling, 2005) and capturing particulate matter, carbon dioxide, ozone and other air pollutants (Whittinghill, Bradley, Schutzki, & Cregg, 2014), (f) improving the willingness to work and work performance (Kaplan, 1993; Wolf, 2003), (g) regulating ecosystem services by increasing water infiltration (Haase & Nuissl, 2007), (h) crime reduction in cities with more developed green zones (Kuo & Sullivan, 2001), and (i) in some countries, providing economic benefits for residents; for example, in Lisbon, for every $1 invested in tree management, residents receive $4.48 in energy savings (Soares et al., 2011).
Nevertheless, in spite of the numerous beneficial aspects of ornamental plants in urban landscapes, the pollination of these plants generates allergies in an increasing number of people each year (Cariñanos & Casares, 2011). Although there are differences among countries, the incidence and prevalence of asthma and rhinitis is increasing worldwide (Pawankar, Canonica, Holgate, Lockey, & Blaiss, 2013; Pawankar, 2014). The prevalence of allergies in the world population is between 10 and 40% as estimated by the WAO (World Allergy Organization). It has been noted that people living in urban environment suffer more pollen allergies than that from rural areas (D’Amato et al., 2007; Ogren, 2002) and it has been suggested that this may be originated by the interaction of air pollutants with airborne pollen (Cariñanos et al., 2002; Cariñanos, Galán, Alcázar, & Domínguez, 2007). Pollens from ornamental trees such as Olea europaea, Cupressus and Platanus are highly allergenic in the Mediterranean area (D’Amato et al., 2007). The impact on humans needs to be considered from two perspectives: the town design and the choice of ornamental species (Staffolani, Velasco-Jiménez, Galán, & Hruska, 2011). In that regard Cariñanos, Casares, and Quesada (2014) proposed an allergenicity index to manage the design of urban green spaces. That useful index is calculated for a determined area and shows the relationship between a product of different values and the surface analysed. The product includes the number of trees, allergenic potential based on 5 categories, type of pollination (separated into 4 categories), the length of the main pollen season period (separated into 3 categories) and a measure of the canopy volume to estimate pollen production. The index is named IUGZA (Urban Green Zone Allergenicity Index).
Because of the demographic trends of populations moving from rural to urban areas, cities are becoming bigger (Schmidheiny & Suedekum, 2015), and the development of green areas is a growing concern for urban planning designers (Rupprecht, Byrne, Garden, & Hero, 2015). This means that more people may be affected by ornamental tree pollen allergies. When thinking about this concern, it is relevant to consider the criteria for the selection of urban trees because the issue of allergies from ornamental trees is rarely discussed in this context. According to Sæbø et al. (2005), it should be considered that trees with allergy-inducing pollen could be avoided. To assess that problem correctly, it is necessary to take into account the abundance of ornamental trees, their distribution, and spore sampler locations because the location of the pollen source is essential for the interpretation of airborne pollen records. Although some work has been performed on a larger scale (Fernández-Rodríguez et al., 2014b; Oteros et al., 2015; Rojo, Rapp, Lara, Fernández-González, & Pérez-Badia, 2015) there are no studies that take into account the distance of the pollen sources from the spore samplers.
In this regard, several works have been performed on a large scale, more than 20 km, using back trajectories as a tool (Fernández-Rodríguez, Skjøth et al., 2014; Fernández-Rodríguez et al., 2015; Hernández-Ceballos, García-Mozo, & Galán, 2014a; Hernández-Ceballos, Skjøth, García-Mozo, Bolívar, & Galán, 2014 ; Oteros et al., 2015; Rojo et al., 2015). Some studies have compared the results of sensors separated between 1 and 3 km (Fernández-Rodríguez, Tormo-Molina, Maya-Manzano, Silva-Palacios, & Gonzalo-Garijo, 2014; Giorato, Bordin, Gemignani, Turatello, & Marcer, 2003; Irdi, Jones, & White, 2002; Khattab & Levetin, 2008; Tormo-Molina, Maya-Manzano, Fernández-Rodríguez, Gonzalo-Garijo, & Silva-Palacios, 2013), and other studies have used distances between 5 and 20 km (Arobba et al., 2000; Velasco-Jiménez, Alcázar, Domínguez-Vilches, & Galán, 2013). However, few studies have taken into account the distance of spore samplers from pollen sources over short distances in cities (Fernández-Rodríguez, Tormo-Molina et al., 2014b,c; Velasco-Jiménez et al., 2013, Velasco-Jiménez et al., 2014). Ornamental street trees play a minor role in atmospheric phenomena such as urban heat islands or climate change (Endlicher, 2011) in urban areas, where more than 50% of the world population lives (Heilig, 2012). These ornamental trees also receive a comparable attention among urban green infrastructure studies and urban aerobiological papers that are related mainly to the study of local source airborne pollen and its transport over short distances (Fernández-Rodríguez, Tormo-Molina et al., 2014b; Velasco-Jiménez et al., 2014). The study area has been aerobiologically monitored from 2011 using volumetric spore samplers. Natural or cultivated pollen sources are the main focus of the study and this paper deals with urban green areas as they are the closest to the majority of the population and the pollen traps used.
The aims of the present study were to analyse the relationship between the density or abundance of ornamental trees and airborne pollen records from three urban environments in the SW of Iberian Peninsula, and to assess the relationship between the distance of pollen sources from spore samplers and the pollen count captured. Furthermore, we emphasize the relevance of selecting tree species for ornamental purposes in designing urban environments considering their pollen allergenicity as a selection criterion with the intention of providing better air quality.
Section snippets
Study area
The study was conducted in three cities located in the southwest of the Iberian Peninsula: Plasencia (PL), Don Benito (DB) and Zafra (ZA), which were separated by a maximum distance of 182 km (PL-ZA), and a minimum of 77 km (DB-ZA), they are located in three main different environments in the region of Extremadura (Spain). The respective heights above sea level appear in Table 1. The general vegetation, or the most frequent land cover around the cities, was holm oak and cork oak trees, extensive
Ornamental trees
A total of 17635 trees were counted, belonging to 67 species (Table 2). The density of ornamental trees per inhabitants for each locality was 0.17 (DB), 0.15 (PL), and 0.30 (ZA). The density of inhabitants per tree was 5.9 (DB), 6.6 (PL), and 3.3 (ZA) [values of trees per 1000 inhabitants: 171 (DB), 152 (PL) and 300 (ZA)]. The 20 most abundant ornamental species of trees appear in Fig. 1; those include 86% of the total number of trees. Five species represent more than 50% of the total number of
Discussion
The relation between inhabitants, or population, and the number of ornamental trees is a measure that can be compared in different cities. In this study the localities had a higher number of trees per inhabitant than in other cities in the Iberian Peninsula or United States (0.13 Beja, 0.07 Lisbon, Alburquerque 0.04), lower (Berkely 0.35, Santa Monica 0.32), or similar (Charlotte 0.14) (Fernandes & Carvalho, 2004; Flatley, 2001; Soares et al., 2011). Compared with the average values from Europe
Conclusions
The average concentration of airborne pollen did not show great differences between the three localities studied, ranging 93–102 g m−3. It is not possible to properly estimate the amount of pollen from ornamental trees separately from natural vegetation. Notwithstanding, pollen from exclusively ornamental sources (Arecaceae, Casuarina, Cupressaceae and Platanus) represented approximately 10% of the total pollen recorded. Moreover, the most abundant tree pollen (Quercus) came mainly from natural
Acknowledgements
This work was conducted thanks to the Regional Government of the Junta de Extremadura in Spain (Project Number PRI06A190) and the European Social Fund (Project number PRI BS10008, Research Group FundGR15060).
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