Airborne pollutants and tree phenology

Trees, whether in the urban or rural setting, provide a huge array of ecosystem services. A service that is of particular relevance to human activity is the ability for trees to filter out harmful airborne pollutants, such as ozone and nitrogen oxides. Of course, the ability for a tree to filter such pollutants is determined, to a large degree, by its phenology and morphology, and this phenology and morphology is considered to be influenced by the airborne pollutants themselves (ironically!).

In order to add substance to this premise, the authors of this study looked at how ozone, nitrogen dioxide, nitrogen oxides, and particulate matter, influenced the onset of flowering and leaf emergence in the tree species Aesculus hippocastanum (horse chestnut – red dots), Betula pendula (silver birch – white dots), and Corylus avellana (common hazel – yellow dots). For silver birch, the effects of air pollution on leaf morphology were also observed. The study was undertaken, during 2010, in the German city of Munich, and across the urban-rural gradient. The below map shows rather specific site locations.

A map of all study locations (dots relate to those mentioned above, though white dots with black dots within are sites where ozone and nitrogen dioxide were measured. In terms of the colours on the map behind: urban (red), forest / pasture (green), arable (yellow), and rivers / lakes (blue).

Data relating to air pollution was largely sourced from pre-existing surveys completed across Europe in the early 2000s, though site-specific measurements were taken at some of the sites where silver birch were studied (as marked by the white dots with black dots inside. Therefore, comparisons could be drawn between short- and long-term differences in ozone and nitrogen dioxide concrentrations within the air, and such comparisons could then be related to tree phenology and leaf morphology of silver birch present at those sites.

With regards to results that displayed marked significance, for silver birch it was identified that increased long-term ozone levels within the urban landscape very significantly delayed (p<0.01) the onset of flowering, and (to a lesser degree – P<0.05) full flowering, in spring, in addition to the delaying (p<0.05) of the first leaves unfolding. This may very well be because birch are considered sensitive to ozone. However, when rural sites were included with the urban sites, ozone did not significantly delay a silver birch’s phenological timings. The authors remark that this is peculiar, because rural locations tended to have higher ozone levels than urban locations. Therefore, perhaps other factors influence upon the flowering phenology of birch, at the same time. In the short-term however, ozone was not found to be significant at any site, though because ozone levels fluctuate greatly on a daily basis, adverse impacts on phenology may only be discernible over a more elongated time period.

For common hazel, nitrogen dioxide, nitrogen oxides, and particulate matter (of varying sizes) within urban sites delayed (p<0.05) full flowering (though moderately-sized particulate matter had a very significant delaying effect – p<0.01), and when rural sites were included nitrogen dioxide, nitrogen oxides, and only the very small particulate matter had the same impact. From these results, it can be suggested that common hazel’s maturing flowers are sensitive to such pollutants (notably nitrogen dioxide and the nitrogen oxides), and more so than young flowers. Because these pollutants are more common in urban locations, it is of little surprise that common hazel within such settings had more of a delay in full flowering.

Horse chestnut was observed to have massively delayed (p<0.001) full flowering in both urban and rural locations, where ozone levels were elevated, and also very significantly (p<0.01) delayed flowering onset. The fact this delay spanned across both urban and rural sites is interesting, as unlike birch it suggests that the higher average ozone levels in rural settings does have as much of an impact as ozone from urban settings. The below table outlines this all in greater detail, as well as non-significant results.

Degrees of significance: *** (P<0.001), ** (p<0.01), and * (p<0.05). Heading meanings: BBCH 61 (onset of flowering), BBCH 65 (full flowering), BBCH 10 (bud break), and BBCH 11 (first leaves fully unfold). Negative results within the table suggest that flowering and leaf emergence may even be hastened, though not to any significant degree.

In terms of how pollution impacted upon leaf morphology of silver birch, no correlations were found in rural sites. In the urban setting, only one result displayed significance, and suggested that an increased amount of airborne nitrogen oxides increase surface leaf area.

To conclude, we can certainly observe that airborne pollutants can have a significant effect upon tree phenology, though it is clear that this will vary between species (and that is hardly breaking news). Some tree species may be more sensitive to ozone, for instance, whilst others particulate matter. Concerningly however, this suggests that ecological processes reliant upon tree phenology timing with the phenology of insects, birds, and so on, may be adversely impacted, and notably in urban settings. As the authors remark, the onset of pollination by insects may be stalled, whilst the emergence of insects ready to pollinate is not stalled. Additionally, for species that unfurl their leaves later in settings with elevated levels of certain pollutants, such as silver birch and atmospheric ozone, they may subsequently filter out less of that very same pollutant during their ‘active’ period, which may facilitate in the gradual accumulation of that same pollutant on a local level, which in turn has a progressively stalling impact upon leaf emergence (a negative-feedback loop). On an amenity level, which is not ecologically concerning, but concerning nonetheless (and to a lesser degree), such delayed flowering may also impact upon residents’ perceptions of the changing seasons.

An emerging flower of the horse chestnut, in an urban park. How exactly may air pollution have impacted upon the timing of such flower emergence?

Source: Jochner, S., Markevych, I., Beck, I., Traidl-Hoffmann, C., Heinrich, J., & Menzel, A. (2015) The effects of short-and long-term air pollutants on plant phenology and leaf characteristics. Environmental Pollution. 206 (1). p382-389.

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Airborne pollutants and tree phenology

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