This spring the levels of exotic tree pollen such as Birch and Poplar trees have recorded very high concentrations in the Canberra air this year - why is this so?
Canberra's urban streetscapes comes to life in spring with the diversity and beauty of the many exotic trees - mostly derived from Europe and other parts of the northern hemisphere - as the appearance of new buds and leaves green the landscape. The flowers of many of these species, such as birch, poplars, and oak trees produce catkins - or cylindrical flower cluster that contain may flowers and is wind-pollinated. Many of these trees produce abundant quantities of pollen that are known to cause hay fever for some people and we are only just beginning to understand how the pollen seasons differ from year to year.
How much pollen do trees produce during the spring season and does the amount vary from year to year? Studies in the northern hemisphere have shown that weather conditions and location of flowering plants can influence the amount of pollen produced by a single tree through time (Jato V et al., 2007). Here I have plotted the daily pollen counts for 3 common exotic trees in Canberra - Birch (Betula sp.), Poplar (Populus sp.) and Pine (Pinus sp.) for the months of September and October for 2017 and 2018 (Figure 1). The figure shows a significant increase in the abundance of pollen being released into the atmosphere from these species. In 2017 there were only 5 days (~8%) during September and October when daily counts for total tree pollen exceeded 100 grains/m3 (EXTREME) compared to 30 days (~50%) of EXTREME tree pollen counts during the same period for 2018. Why did this happen?
Figure 1: Spring (Sept-Oct) daily pollen counts and pollen season period for Birch, Poplars, and Pine trees in Canberra for 2017 and 2018. Cumulative rainfall data (Bureau of Meteorology) is plotted for each season to compare the relative moisture availability during these two seasons.
One possible explanation may be the different weather conditions prior to and during the flowering period of each year. Overall the temperature (max. and min) during this period of 2017 and 2018 are similar and do not appear to be a major contributing factor to the observed difference in pollen productivity. Mean minimum and maximum temperatures for 2017 are 5 degrees C and 20.7 degrees C, respectively, compared to 4.1 degrees C and 19.2 degrees C during the same period for 2018. The rainfall plots in Figure 1 show a significant rise in rainfall (~40mm) during the first 2 weeks of September in 2018 compared to 2017 and this may have increased the water availability for many of Canberra's street trees during a critical period of flower formation resulting in a much greater level of pollen production in 2018 compared to the very dry early spring in 2017. The key factor influencing pollen productivity in exotic tree species in Canberra appears to be the amount of rainfall received during the early part of spring (or prior to the beginning of the trees pollen season). Further research is needed to confirm this observation, but a greater understanding of how the pollen production of different tree species varies in our region will help to better predict variations in pollen production in the future.