A Focus on Lichens and What They Need

The East Dartmoor National Nature Reserve (NNR) is special for many things, and one of them is lichens. Therefore, we were very fortunate that as part of the Festival of NNRs Granite Elements held an exhibition titled ‘Focus on Lichens: a Weekend of Art and Science‘ on 11th and 12th June at The Woodland Centre situated in Yarner Wood.

Please see the summary by Clare Benson (Granitite Elements).

“Lichens are all around us, but are easily overlooked. This exhibition was designed to draw attention to the hundreds of lichen species which thrive in the clean, damp air of East Dartmoor. It was one of the events marking the 70th anniversary of the Yarner Wood National Nature Reserve.

Granite Elements local art group got together again after lockdown for a project focusing on lichens, which are found in a fascinating variety of forms, patterns, and colours. Twelve members of the group made artists’ prints based on lichens, using techniques including linocut, collagraph, cyanotype, monoprint, wood engraving and embossing.

Each person stitched a set of prints into an individually designed cover. The prints and books were on display at the Woodland Centre, together with other lichen-inspired art and craft work by members of the group.

The exhibition also included a display of lichen-dyed wool samples from a project organised by Rachel Jones of Plantlife. We are aware of the rarity of many lichens, and only used small quantities of abundant species, collected from the ground or from dead branches. This project helped us learn to identify lichens, as it is important to use the right ones for different dye techniques. Some of the colours produced were amazing.

Over the weekend we had around 80 visitors, and many stayed a long time, taking it all in and showing real interest. It was great to be able to share our enthusiasm with others. Thanks to all the artists, and to Albert Knott and Natural England for enabling this event to happen.”

In the same week in June, Natural England and the Woodland Trust enabled volunteers and some students from Plymouth and Exeter universities to get involved with citizen science projects as part of  Plantlife’s Building Resilience in South West Woodlands. Please see other past blogs which describe this exercise: https://eastdartmoorwoods.org/category/building-resilience/

This project explores the need to let sunlight in through the woodland canopy, and the management our team undertake to support the lichen assemblage.

One important fact Clare touches on is that lichens also depend on the rain which Dartmoor receives and the clean air which mainly comes from the Atlantic. These two key factors are ever changing and at Yarner Wood we have been monitoring these changes for over fifty years.

A weather station has been situated on the NNR since the 1960s, and you can access the current weather data by visiting the Met Office website: https://www.metoffice.gov.uk/weather/forecast/gbvrg6vzc#?date=2022-06-25. The station can sometimes be mistaken for the air quality monitoring station which is on Yarner Wood Heath, but also collects important environmental information.

Read these three fascinating articles below, written by Dr Louisa Kramer (Ricardo Energy & Environment), and Dr. Y Sim Tang CEH.

“The Yarner Wood Nature Reserve has, throughout its lifetime, been an open-air laboratory. Monitoring of air pollution and weather has been carried out in the reserve for many years, and the Yarner Wood Automatic Urban and Rural Network (AURN) site has been an important part of this work, since it started operation in 1985. The site is also part of the National Ammonia Monitoring Network and the Acid Gas and Aerosol monitoring Network (AGAnet). Further information on the monitoring networks can be found on the UK-Air website[1].

The AURN monitoring station at Yarner Wood is a rural background site, located in an area which is partly wooded, partly open and comprises rural heath land. Figure 1 shows a photo of the self-contained, air-conditioned housing.

Figure 1 The AURN Monitoring Site at Yarner Wood

“This long-term monitoring station has been measuring ozone (O3) concentrations since June 1987 and nitrogen oxides (NOx)  – which consists of nitric oxide (NO) and nitrogen dioxide (NO2) – since September 2003.

Ozone concentrations at rural sites, such as Yarner Wood, are typically higher than concentrations measured in urban locations. Ozone is not directly emitted in any large quantities, instead it is classed as a “secondary pollutant” in the sense that it is mainly formed via reactions in the atmosphere involving volatile organic compounds (VOCs) and NOx (in the presence of sunlight). NOx can also remove ozone from the atmosphere, through the reaction between O3 and nitrogen oxide. In rural areas there is typically less NOx (therefore less nitric oxide) available to remove ozone, when compared to urban areas, therefore ozone levels can build up. In addition, ozone can be transported long distances from where it was formed.

Figure 2 Number of hours O3 concentrations at Yarner Wood were in the “Moderate” or higher pollution band between 1988 and 2020.

“Figure 2 shows the number of hours each year when O3 concentrations were in the “Moderate” or above pollution band, based on the Daily Air Quality Index [2] between 1988 (the first full year of O3 measurements) and 2020. Overall, there has been a decrease in moderate or higher pollution at Yarner Wood over the period 1988 to 2020. The decrease is likely to be related to an overall reduction in ozone precursors (NOx and VOCs). However, there are some years when an increase in O3 pollution is observed, which often coincide with summer heatwaves, (such as those in 1995, 2003 and 2006) as hot, sunny, and calm conditions are conducive to ozone formation. It is therefore important to continue monitoring ozone and its precursors in the UK to understand the potential impact of climate change on ground level ozone.

Figure 3 below shows the long-term trend in O3 concentrations at Yarner Wood since measurements began. The trend analysis was performed using the Theil-Sen statistical method in Openair[3]. The trend is calculated from monthly averaged data based on the hourly measurements. The function includes an option remove the seasonal variation (de-seasonalise) in the data, for a clearer trend. This option also applies an interpolation method to fill in any gaps in the data.

Between 1988 and 2020, the trend analysis shows that O3 concentrations have slightly increased by 0.07 µgm-3 per year, on average at Yarner Wood. However, the trend is not statistically significant (p-value = 0.1, indicated by ‘+’ at the top of the plot).  

The primary source of nitrogen oxides is from fuel combustion processes. Road transport is the largest contributor to NOx in the UK, accounting for 33% of total NOx emissions in 2019[4]. As mentioned above, NOx levels in rural areas are typically lower than in urban areas, as there is less impact from road traffic emissions. Figure 4 and Figure 5 shows that at Yarner Wood, a decreasing trend in NOx of 0.26 µgm-3 per year, and NO2 of 0.21 µgm-3 per year, on average, has been observed between 2004 and 2020 (both statistically significant at the 0.001 level), suggesting that reductions in nitrogen oxide emissions are reducing NOx concentrations measured at this rural site.

Figure 6, Figure 7 and Figure 8 show smoothed trend plots of monthly mean concentrations at Yarner Wood, for O3, NOx and NO2 respectively. The smooth trends were calculated using the SmoothTrend function in Openair. Similarly to the Theil-Sen trend analysis, the data has been de-seasonalised first.  For ozone, concentrations have not changed considerably since 1988. For NOx and NO2, a downward trend is observed, particularly between 2004 – 2012, though it has flattened off in later years.

Figure 3 De-seasonalised Trends in Ozone at Yarner Wood
Figure 4 De-seasonalised Trend in NOx at Yarner Wood
Figure 5 De-seasonalised Trend in NO2 at Yarner Wood
Figure 6 Smoothed De-seasonalised Trend Plot of Monthly Mean Ozone Concentration at Yarner Wood
Figure 7 Smoothed De-seasonalised Trend Plot of Monthly Mean NOx Concentration at Yarner Wood
Figure 8 Smoothed De-seasonalised Trend Plot of Monthly Mean NO2 Concentration at Yarner Wood

[1] https://uk-air.defra.gov.uk/

[2] https://uk-air.defra.gov.uk/air-pollution/daqi?view=more-info

[3] https://ukair.defra.gov.uk/data/openair.

[4] https://www.gov.uk/government/statistics/emissions-of-air-pollutants/emissions-of-air-pollutants-in-the-uk-nitrogen-oxides-nox”

Author: Dr Louisa Kramer, Ricardo Energy & Environment

Sulphur Dioxide (SO2): Changes over time

The time series of annual mean sulphur dioxide (SO2) concentrations measured at Yarner Wood (Figure 1) shows how ambient concentrations of SO2 have decreased since 1987.

Also shown (as dotted lines) are the estimated total annual emission of SO2 (data from the NAEI, in kilotonnes (kt)). This is plotted against the axis on the right.

The annual mean concentration of SO2 shows a steady decrease, consistent with the downward trend in the amount of total SO2 emitted.

Air sampling methods used at Yarner Wood to measure SO2 concentrations are also shown above the time series plot. Over time, the methods and exposure periods used were changed to match the sensitivity required to measure the continued decline in SO2 concentrations over time. 

Figure 1: Time series in Sulphur dioxide (SO2) annual mean concentrations measured at Yarner Wood and national emissions in UK.

Data source:

SO2 concentration data, non-automatic networks (https://uk-air.defra.gov.uk/data/)

SO2 emissions data (https://naei.beis.gov.uk/data/)

Ammonia (NH3): Changes over time

The time series of annual mean ammonia (NH3) concentrations measured at Yarner Wood (Figure 2) shows how ambient concentrations of NH3 have remained relatively unchanged since 1997. Between 2015 and 2020, there is a very small decrease (by ~ 0.1 µg NH3 m-3), but there is greater uncertainty with data across this period owing to lower data capture (< 75 %).

NH3 concentrations at Yarner are below the UNECE Critical levels of 1 µg NH3 m-3 (annual mean) for the protection of lichens and bryophytes.       

Also shown (as dotted lines) are the estimated total annual emission of NH3 (data from the NAEI, in kilotonnes (kt)). This is plotted against the axis on the right.

NH3 emissions show a very small decline between 1996 and 2008 and remained relatively unchanged until 2014, when emissions started to rise slowly again. The trend in national NH3 emissions are not matched by changes in monitored NH3 concentrations at Yarner Wood.

Figure 2: Time series in ammonia (NH3) annual mean concentrations measured at Yarner Wood and national emissions in UK. Note: 1996 data are not shown since monitoring started in September 1996. *Data capture < 75 %.

Data source:

NH3 concentration data, non-automatic networks (https://uk-air.defra.gov.uk/data/)

NH3 emissions data (https://naei.beis.gov.uk/data/)

Ammonia (NH3), acid gas and aerosol: Changes over time

Figure 3 shows the time series of annual mean ammonia (NH3), acid gas (nitric acid (HNO3), sulphur dioxide (SO2), hydrochloric acid (HCl)) and inorganic aerosol concentrations and percentage contribution of the individual components to the total measured (by mass) at Yarner Wood.

This shows how ambient concentrations of NH3 have remained relatively unchanged compared with other components.

It also shows that the increasing dominance of nitrogen over sulphur. The sum of reactive nitrogen (NH3-N, HNO3-N, NH4+-N and NO3-N) contributed 38 %  to the total mass measured in 2020, compared with just 7 % from the sum of sulphur (SO2-S and SO42--S)

A considerable fraction of the aerosol components measured was made up of sea salt (Na+ and Cl), with 52 % contributions to the total mass measured in 2020 from the sum of Na and Cl.

Figure 3: (Left) Annual mean gas and aerosol concentrations, and (right) Percentage composition of gas and aerosol components measured at Yarner Wood, as part of the UK Acid Gas and Aerosol monitoring Network (AGANet) between 2000 and 2020. Note: 1999 data are not shown since the network started in September 1999. *Annual Data capture < 75 %.

Data source:

NAMN and AGANet data, non-automatic networks (https://uk-air.defra.gov.uk/data/)”

Author: Dr. Y Sim Tang CEH

Author: Dr, Y Sim Tang CEH

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  1. […] Yarner has been actively studied for several decades, with focus on the woodland interest plus weekly butterfly counts and daily moth recording. Weather trends have been recorded since the 60s, and air quality is also measured, with trends published in a previous East Dartmoor NNR blog: A Focus on Lichens and What They Need | East Dartmoor Woods blog […]