Searching for a trace of Artemisia campestris pollen in the air

The aim of the study was to determinate whether Artemisia campestris was present in the vicinity of 8 pollen monitoring stations in Poland by examining temporal variations in daily average airborne Artemisia pollen data recorded by Hirst type volumetric traps. Three day moving averages of airborne Artemisia pollen were examined by Spearman’s rank correlation test. Results show that Artemisia pollen seasons in Poland generally display similar unimodal patterns (correlation coefficients r > 0.900; P < 0.05). The only exception was the Artemisia pollen concentration noted in the outskirts of Poznań (Morasko), where the bimodal pattern was revealed. Correlations between Artemisia pollen data recorded at Poznań-Morasko and the other Polish sites were the lowest in the investigated dataset; this was particularly noticeable in the second part of pollen season (r ~0.730). We show that the typical bimodal pattern in Artemisia pollen seasons, which is characteristic of the presence of both A. vulgaris (first peak) and A. campestris (second peak), does not occur at the majority of sites in Poland and is restricted to the outskirts of Poznań. In fact, it was noted that the pollen monitoring site in Poznań-Centre, just 8 km from Morasko, only exhibited one peak (attributed to A. vulgaris). This shows that the influence of A. campestris on airborne pollen season curves is limited and can be largely disregarded. In addition, this study supports previous records showing that the spatial distribution of airborne Artemisia pollen within a city (urban-rural gradient) can vary markedly, depending on the species composition.


Introduction
Artemisia pollen allergens are an important cause of allergy in Europe [1,2].Several species of Artemisia grow in Poland, particularly A. vulgaris, A. campestris, A. absinthium, A. annua, and A. austriaca [3,4].Artemisia vulgaris, the most common species, is considered to be the main source of airborne pollen [3,5].It is a ruderal plant, with a rate of occurrence exceeding 60% in ruderal habitats (Motiekaitytė 2002, in Kazlauskas et al. [6]).As a consequence, airborne Artemisia pollen seasons in Central Europe usually exhibit a unimodal shape, with only one main peak during the season [7][8][9].However, a second peak in airborne concentrations of Artemisia pollen has been observed in the middle of September in southeastern parts of Europe [10].This second peak could occur frequently at some sites and is responsible for more than 30% of airborne Artemisia pollen recorded during the year.
This second peak is often attributed to the pollen release in A. scoparia and A. maritima that flower in autumn.These two taxa also grow in southern Poland, but their occurrence is only sporadically reflected in the curve of airborne pollen seasons.Late peaks in Artemisia pollen season have only been observed intermittently in Polish cities, like Rzeszów and Kraków, and the intensity of such late peaks are much lower than observed in Southern Europe.On the other hand, the influence of another Artemisia species on the airborne pollen curve has recently been observed in Poznań (western Poland) [3].Combinations of aerobiological, phenological and pollen production data revealed that the majority of airborne Artemisia pollen grains recorded in the second part of August are released by A. campestris.This species mainly occurs in semi-arid climates and prefers dry, sandy, open areas such as roadsides, woodland openings, old fields, wastelands, sand and gravel soils [3,11].In such habitats, A. campestris frequently creates a thick and wide conglomeration in vegetation patches.A single plant can produce as much as 65.3 × 106 pollen grains [3].As a result, a distinct increase in airborne Artemisia pollen concentrations was observed at the pollen monitoring station located in the outskirts of Poznań (Morasko) during second fortnight of August.In this semi-rural area, Artemisia pollen curves have a bimodal shape with two peaks recorded in August.
Artemisia campestris is a common species in Poland, occurring almost in the whole country [4], however its abundance has not been precisely established.Based on previous observations carried out in Poznań [3], which revealed that A. campestris was almost as common as A. vulgaris, and its pollen contributed markedly (30%) to total Artemisia pollen sum, it is expected that high densities of A. campestris will also impact on the curve of Artemisia pollen seasons in other cities.Airborne pollen counts may serve as a good bio-indicator of the flora composition and diversity providing important information about its distribution and abundance [12][13][14].Based on the pollen trace in aerobiological data it should be possible to determinate whether certain plant species occur in a particular area.The aim of this study was, therefore, to investigate whether A. campestris was present in the vicinity of 8 pollen monitoring stations in Poland by examining temporal variations in daily average airborne Artemisia pollen data.

Aerobiological analysis
Daily average airborne Artemisia pollen data were collected at nine pollen monitoring sites located in Poland (two in Poznań), by volumetric pollen traps of the Hirst design [15] situated at roof level (Tab.1).Detailed characteristics of pollen monitoring stations are presented in Tab. 1. Slides were counted following the methods described in literature, i.e., by four horizontal lines (in Poznań, Kraków, Sosnowiec, Wrocław, Lublin, Łódź and Szczecin) or 12 longitudinal transects (in Rzeszów) [16,17].These two counting methods have been shown to produce comparable results [18].Daily average (00:00-24:00 h) airborne Artemisia pollen counts were converted into concentrations (grains/m 3 ).

Statistical analysis
The averaged Artemisia pollen curves were prepared according the method described in Bogawski et al. [3].In short, Tab. 1 Information about the nine pollen-monitoring stations used in this study.

Szczecin
3-day moving averages of daily average airborne Artemisia pollen data were calculated for every pollen season.These pollen curves were normalized by attributing a value of 1 to the peak day that occurred before 15th of August (flowering of A. vulgaris).If the peak day occurred after 15th of August, i.e., the period when A. campestris starts flowering [3], the second day with the highest pollen concentration (before 15th of August) has been chosen as a peak day.This happened 4 times: in Poznań (Morasko campus on the outskirts of the city) in 2006 and 2014, and in Rzeszów in 2005 and 2006.This manipulation has been applied so that the peak day attributed to A. vulgaris (which mainly flowers until the middle of August) can be calculated separately from the peak day of A. campestris.Next, the pollen season curves were set together and centred to selected peak day.The general mean pollen season curve has been calculated for every station.Spearman's correlation coefficient was used to analyze the relationships between Artemisia pollen seasons curved recorded at different cities.To avoid days with very low pollen concentrations, the length of pollen data sets were limited to 43 days with the highest pollen levels, i.e., three weeks before peak day (pre-peak period, 21 days) and three weeks after peak days (post-peak period, 21 days).Following pollen data sets have been compared: (i) whole data sets (43 days); (ii) pre-peak and post-peak periods (both n = 21 days).
The statistical significance of differences between correlation coefficients has been tested using website software Statistics Calculators version 3.0 (http://www.danielsoper.com/stat-calc3).In addition the peak records during 2005-2014 have been compared between pollen monitoring sites.This time period has been chosen as this is a common period for all investigated station (except Rzeszów, 2005-2013).

Results
Averaged curves of airborne Artemisia pollen seasons had very similar unimodal and symmetrical pattern in most cities.The only exception was the aerobiological station in Poznań-Morasko (Fig. 1).In most cases, the Spearman correlation coefficients between pollen season curves at all investigated cities exceeded r = 0.900 (P < 0.05; Tab. 2).Whereas the pollen season curve in Poznań-Morasko had a bimodal shape and calculated correlation coefficients were much lower and, in most cases, differences between coefficients were statistically significant (r = 0.644-0.892;P > 0.05).Interestingly, the Spearman correlation coefficient between Artemisia pollen levels recorded at two monitoring stations in Poznań was significantly lower (r = 0.841) than between pollen concentrations at Poznań-Centre and other cities (r ~0.950, P > 0.05; except Łódź P = 0.907).In the prepeak period, the correlation coefficients between Artemisia pollen levels at all sites (including Poznań-Morasko) were exceeded r > 0.930.In contrast, when the post-peak period was considered, the correlation coefficient between pollen season curves at Poznań-Morasko and other investigated sites dropped to r = 0.720-0.730(the difference between correlation coefficients were statistically significant, P < 0.05).

Discussion
Bogawski et al. [3] found that the bimodal curve of the Artemisia pollen season recorded in Poznań-Morasko was Tab. 2 Spearman correlation coefficient between Artemisia pollen seasons recorded at nine pollen monitoring stations in Poland (all correlations are statistically significant, P < 0.05).
The statistical significance of differences between correlation coefficients calculated for Poznań-Morasko and other sites has been marked ( P < 0.05).Note: only correlation coefficients in rows have been compared with each other (indicated by "→").
related to the flowering of two Artemisia species, i.e., A. vulgaris (first peak) and A. campestris (second peak).Our study shows that such patterns in Artemisia pollen seasons are not widely observed in other sites in Poland.Indeed, the airborne Artemisia pollen season recorded in the centre of Poznań, which was located just 8 km from Poznań-Morasko, only exhibited one peak and was similar to Artemisia pollen seasons recorded at the other sites included in the study.
Pollen monitoring stations are often located in city centres, and it is assumed that one station is usually enough to express the aerobiological situation in whole city area [19].Several studies that compared the aerobiological situations in two sites within the same city revealed that, in general, inter-site variation in the most common pollen types was not high.The highest differences were observed in relation to herbaceous [20] and/or ornamental plants [19,21,22].The latter are usually planted in parks, squares, gardens, and along streets, and are not observed in large number outside of urban areas.The urbanophilous and thermophilous plants, with a distribution restricted mainly to city centres [23], often show similar aerobiological patterns.
Conversely, pollen grains of urbanophobic species (plants unable to live in urban areas) might be underrepresented in the city centre.A. campestris prefers open areas (roadsides, old fields, wastelands) and as such could be considered as an urbanophobes.In Poznań, A. campestris grows mainly in the semi-rural areas and its very rare in the city centre (Paweł Bogawski, personal observation, 2014).This may explain why the second peak in daily average airborne Artemisia pollen concentrations was not observed in Poznań city centre.Similarly, the unimodal pollen season pattern recorded in the other monitoring stations also suggests that airborne Artemisia pollen grains are primarily released by A. vulgaris (the most common species in Poland) and the contributions of A. campestris is limited.
In this context, it is worth noting that although Artemisia species belong to anemophilous plants, their pollen grains are not as well adopted to wind transport as other pollen grains that are smaller and lighter (e.g., Betula or Ambrosia) [24][25][26][27][28]. Spieksma et al. [29] showed that the presence of nearby stands of Artemisia (within 100 m) had a very strong influence on the amount of pollen recorded by the trap.In such localities, the level of Artemisia pollen at ground level might be more than 5 times higher than at roof level.Local sources of Artemisia, therefore, strongly modify pollen season curves and the effect of remote sources can be largely disregarded.This indicates that the spatial and temporal distribution of airborne Artemisia pollen grains within the city may vary markedly, depending on the floral vegetation composition.It should be stressed that different Artemisia species have different ecological and phenological features [3,12,30] as well as produce different amounts of pollen [3].That, in turn, may have an important impact on people suffering from Artemisia pollen allergy, as there is a risk that they could be exposed to different levels of allergenic pollen in different city districts.

Conclusion
Artemisia pollen seasons recorded in urban areas in Poland exhibited only one single peak, which was attributed to the flowering of A. vulgaris.The importance of A. campestris pollen was only reflected in one semi-rural location in Poznań, where the airborne Artemisia pollen season had a bimodal pattern.Spatial variations in land use strongly influence species composition, which in turn affects the distribution of pollen in the air.In the case of Artemisia, which produces high numbers of pollen but is poorly adapted to wind transport, local differences in pollen exposure might be very high.This specific nature of Artemisia pollen should be considered in allergy prophylaxis.

Fig. 1
Fig. 1 Comparison of Artemisia pollen season curves recorded at nine pollen monitoring stations in Poland.The table below the figure shows the real values of Artemisia pollen level at peak days recorded during the decade 2005-2014, i.e., the common period for all stations (except Rzeszów, 2005-2013).