GE-I-4: Ragweed pollen burden
Click to enlargeSource: Aleksandr Lesik / stock.adobe.com
2023 Monitoring Report on the German Strategy for Adaptation to Climate Change
Click to enlargeSource: Aleksandr Lesik / stock.adobe.com
GE-I-4: Ragweed pollen burden
The spreading and establishment of ragweed is presumably favoured by climate change. However, so far the results of pollen measurements do not indicate any significant trends. However, there were key loads where – owing to long-haul transportation and / or local plant populations – pollen concentrations were distinctly higher than the corresponding regional mean. The relevant maximum annual totals occurred predominantly in the eastern region.
The illustration GE-I-4 ’Ragweed pollen burden’ contains a biaxial chart. For the years from 2006 to 2021 the lines show the annual mean total of ragweed pollen in pollen per cubic metre air. A differentiation is made between the central, northern, eastern, southern and western regions. In addition, the mean pollen total across all regions is illustrated. A particularly high ragweed pollen burden occurred in 2014 in the northern, eastern and central regions. In the eastern region, the annual mean total amounted to just short of 225 ragweed pollen per cubic metre air. In the same year, the measuring station at Dresden in the eastern region also recorded the maximum annual mean pollen total amounting to 436 pollen per cubic metre air. For the years of 2006 to 2021 this is illustrated by means of dots in the respective colour code for each region. There are no significant trends discernible.
Introduced ragweed is highly allergenic
Apart from the indigenous allergenic plants, in view of the increasingly warm weather patterns, other plant species will become relevant as allergen producers – species that hitherto did not occur in Germany or occurred only here and there. For example, annual ragweed (Ambrosia artemisifolia) which emanates from North America – known as ragweed – has an extraordinarily high allergenic potential. Ragweed used to occur relatively seldom and inconsistently in Germany. Its occurrence did not start to increase until the beginning of the 1990s. Nowadays annual ragweed occurs in all Länder, and especially in eastern Germany, where it has started to form extensive, well established localised populations containing thousands of plants. This plant thrives in gardens, on uncultivated or fallow ground, in arable fields and cut-flower fields, in agricultural set-aside areas, on building sites and on road and path verges. Causes for the spreading of this species may be, for instance, the contamination with seed from wildlife food or cut-flower plots, and bird food containing ragweed seed material. Other causes might be the transportation of soil from affected areas in connection with building projects as well as the adhesion of seeds to agricultural machinery or mowers used on road verges. In order to counteract the proliferation of this species, via products such as birdfood, maximum content values were stipulated in an EU Directive in 2011 restricting the unwanted addition of ragweed to feedstuffs.
The fact that ragweed is able to spread and establish itself in Germany is, however, to a relevant extent, also attributed to climate change, because the annual plant can only achieve the seed maturity required for dispersal where the prevailing climate is warm or moderate with mild autumn months. Similar concerns exist with regard to the proliferation and establishment of other thermophilic plants with highly allergenic pollen, such as the Pellitory-of-the-wall (Parietaria officinalis or P. judaica) or the olive tree (Olea europaea).
In sensitised individuals, even minor ragweed pollen concentrations can provoke allergenic symptoms and can trigger asthma in up to a quarter of allergy sufferers23. Besides, there are reports of rare skin reactions after skin contact with the flower head or other components of this type of plant. Another important aspect of the establishment of ragweed is the fact that, once the plant is established, the period in which its pollen can be airborne is extended well into October24, as this is a late-flowering plant. The temporal extension of the period in which pollen is air-borne also extends the duration of the time in which affected individuals will suffer from related complaints. This adds to the stress for people who are prone to suffering from allergies.
In Germany, ragweed pollen – the same way as pollen from indigenous plants – are captured mostly in pollen traps within the PID measuring network. The time series illustrated here is based on nationwide surveys of 50 PID stations. As in the case of birch pollen (cf. Indicator GE-I-3) not all stations supply data every year, which is why the mean values are calculated on the basis of pollen-trap numbers which change from year to year.
Currently, the pollen concentration of ragweed is relatively low in Germany; the mean value across all measurements so far amounts to approximately 25 pollen per cubic metre of air annually. By comparison: For birch, the comparable value is 6,800 pollen per cubic metre air annually. However, in some years, values can arise at some stations which are 20 times higher than the mean value mentioned above. The stations which have reported peak values for those individual years are distinctly higher in the eastern part of Germany. These high pollen concentrations can – to some relevant extent – also be attributed to long-distance transportation from affected neighbouring countries. The peak value so far amounting to 436 pollen per cubic metre air annually was measured in Dresden in 2014.
So far there are no statistically significant trends. The same is true for all regions. In eastern Germany the regional mean values are typically higher than those of other regions, owing to influences described above which emanate from eastern neighbouring countries. The region in the area of Drebkau in south-eastern Brandenburg stands out owing to its special scenario: It can be described as a regional ragweed hotspot which is not illustrated here in terms of the pollen data shown. The nationwide high concentrations of ragweed pollen in 2014 were caused by persistent wind dispersal of pollen from the Hungarian basin during the plant’s flowering period, which was due to weather conditions with a south-easterly flow. This kind of south-east wind is one of rather infrequent weather phenomena. Ragweed is particularly wide-spread in Hungary and other neighbouring countries, especially in Slovakia, Romania, Serbia, Bosnia-Herzegovina and Croatia. The so-called ‘long-haul flights’ of pollen may emanate from those countries.
The pollen total measured does not permit any robust conclusions as to the risk of the population actually coming into contact with these pollen or in respect of developing sensitisation or allergic reactions. Nevertheless, for precautionary reasons, every effort should be made – subject to the rules of proportionality – to curb the further spread of various ragweed species in Germany. As far as any direct ragweed control and the elimination of ragweed populations is concerned, there are primarily mechanical options available. The most efficient method is to uproot the one-year old plant in June when it is easily recognised and it is easy to tell it apart from other plant species, and when it is not yet dispersing any pollen. If this type of manual control is not possible, it will be necessary to resort to mowing between June and September, at least four times at intervals, in order to weaken the plant to such an extent that it is no longer able to produce any shoots.
23 - Bergmann K.-C., Werchan D., Maurer M., Zuberbier T. 2008: Threshold value for Ambrosia pollen in nasal provocation: patients characterization. Allergo Journal 17: 375-376.
24 PID – Stiftung Deutscher Polleninformationsdienst 2023: Pollenflugkalender 5.0 für Deutschland 2016–2021: https://www.pollenstiftung.de/pollenvorhersage/pollenflugkalender.html
