Essay: Using lichens as indicators of air quality at three sites in Bristol

Results
Lichen are often used as indicators of air quality and pollution levels as they are sensitive to atmospheric chemistry. This is because they lack roots, so obtain all their nutrients from the atmosphere (Wolseley et al., 2013). In order to quantify air quality, Nitrogen Air Quality Index (NAQI) is used, allowing comparison between sample sites. NAQI is calculated using a Lichen Indicator Score (LIS), comparing presence of Nitrogen sensitive (N-sensitive) and Nitrogen tolerant (N-tolerant) lichen species. The relationship between LIS and NAQI allows prediction of air quality using Figure 4 from Wolseley et al. 2013.
Of the three sites surveyed; Snuff Mills, Leigh Woods and Black Down, they all had similar NAQI values (see Table 1). However, due to cut off values, the air quality varied between sites. Both Snuff Mills and Leigh Woods had N polluted air quality with NAQI values of 0.94 and 0.96 respectively. Despite also having a high NAQI value of 0.84, Black Down is only ‘at risk’ of Nitrogen pollution. As well as pollution, there are other factors to consider in the measurement of air quality such as temperature and precipitation (Pinho et al., 2004).
Table 1: NAQI scores for each habitat sampled
Location LIS NAQI value Air Quality
Snuff Mills -0.417 0.94 N polluted
Leigh Woods -0.450 0.96 N polluted
Black Down -0.046 0.84 At risk
For Snuff Mills and Leigh Woods, 60 twigs were sampled, each acting as their own zone for analysis. Due to similar counts of zones with both N-sensitive and N-tolerant species, (26, 51 and 26, 53 respectively) the mean scores per twig were also similar. Both locations had a mean of 0.43 per twig for N-sensitive with 0.85 and 0.88 respectively for N-tolerant. In comparison, Black Down had 65 twigs sampled and contrasting counts of N-sensitive species (54), but similar N-tolerant (57). Black Down also had the same average per twig a Leigh Woods of 0.88 but a higher mean than both others for N-sensitive of 0.83.
Discussion
The results showed a varying level of pollution across the three sites. Black Down had the lowest NAQI value and so the lowest pollution level. Both Leigh Woods and Snuff Mills had similar levels of pollution shown by similar NAQI values. This variation in pollution could be explained by the location of the sample sites in terms of proximity to the city of Bristol and main roads. The highest area of pollution was Leigh Woods with NAQI = 0.96. Of the three sites Leigh Woods is the closest to the city centre and so would likely have the highest level of pollution from the passing traffic on the A369. Snuff Mills is also near the M32, a major motorway, with traffic emissions causing a high NAQI value of 0.94. These busy thoroughfares cause higher levels of sulphur and nitrogen oxides from vehicle emissions which affect air quality. In contrast, Black Down is amongst the Mendip Hills in an Area of Outstanding Natural Beauty (AONB). Therefore, the site is within protected landscapes with few roads to cause pollution, reflected in the lower NAQI score of 0.84. The results mostly aligned with expectations due to the location of the sites. However, the little difference in NAQI for Leigh Woods and Snuff Mills may not be significant, resulting from existing variation or collection differences. It could also be proposed that Leigh Woods would have a lower NAQI value than Snuff Mills, as it is a National Nature Reserve. However, the proximity to the city caused conflicting results to this expectation, with Leigh Woods having the highest NAQI score.
Despite the evidence presented for differing air quality across the three sites, there were flaws in the methodology. Firstly, the sites were sampled on different dates, and during the week of 25-29th January 2019 there were numerous weather fluctuations. As shown by Pinho et al., changes in temperature and precipitation can influence lichen biodiversity between collecting dates. A possible flaw to the actual species surveyed is the proposal that crustose lichen are not suitable for use as air pollution indicators. Crustose have a higher optimum tolerance whereas fruticose and foliose types have a higher sensitivity to pollution (Pinho et al., 2004). To improve this, only fruticose and foliose lichen could be sampled by three groups, one at each site and on the same day to prevent weather fluctuations.
In regard to the collection of the lichen samples, there are flaws in the method provided by Wolseley et al. which could introduce confounding factors into the distribution of species. Firstly in order to compare observations across sites, both on this small scale and internationally, the same tree species in study sites must be used (Conti & Cecchetti, 2001). This would enable homogenous comparisons to be made, but in the study the species of tree surveyed were not recorded as well as using twigs instead of branches or the trunk. When collected, the twigs were then removed from the site and not analysed until examined in the University of Bristol’s Life Sciences teaching laboratory. This transport could have caused samples to be damaged before examination, impacting the results. Analysing species in situ and using specified tree species could improve this but would add another layer of obdurate complexity to data collection and analysis.
Only using twigs with each as a zone could also affect the results due to a smaller surface area for lichen to grow or at a greater exposure so some species may only be present on tree trunks or bark. In order to negate this and with more time, a sampling grid could be placed on the trunks of trees and collect the samples in situ (Scerbo et al., 2002). Although this would be more time consuming, it could reduce the bias associated with collecting twigs from the most accessible branches as different species may grow higher up or in different areas on the tree. A further improvement or adjustment to the method could be to assess the quantitative amount of pollution in the samples. Chemical analysis of thallus tissues or measuring the activity of the lichen such as ATP production could be used. The sulphur or nitrogen content could be measured to determine the amount present in the atmosphere (Conti & Cecchetti, 2001). Although this would be a stronger measure of pollution, it would also be time and labour intensive.
In conclusion, the study showed the differences in air quality across the three sites by using NAQI values, a measure of nitrogen pollution. The results confirmed the expectations that areas closer to urbanised areas and roads had higher pollution than those in nature reserves further away. Despite flaws in methodology, the results allowed comparison within sites but would need further improvements or statistical analysis to be more conclusive. With more time and analysis, the study could be used to generalise the level of air pollution in urbanised areas and inform protection of the remaining green landscape.
2019-2-10-1549838618