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Background

Lichen are not plants, but rather a symbiotic relationship between fungi and algae and/or cyanobacteria. Macrolichen (leafy or bushy lichen) that grow on trees obtain all their nutrients from the surrounding air, moisture, and rain, which means that they also absorb many of the chemicals and pollutants present in the air and can serve as mini living air quality monitors, or bioindicators. You can learn more about lichen and their main functional groups in this research note

This page will hopefully be helpful to folks at various stages in the air quality study design process. Anyone just starting to design an air quality study or reassessing an ongoing study can use this wiki to assess whether bioindicators can and should be integrated into the study.

Use Cases and Limitations

Use Cases

There are many benefits to using lichen to assess air quality compared to traditional instrumentation. Namely, biomonitoring with lichens tends to be lower cost than installing monitoring instruments or collecting samples, and there is a database of baseline "clean" lichen community composition in Forest Service plots for comparison. Furthermore, lichen have a wide geographic range and can monitor air quality in locations not amenable to sensors or equipment, making them suitable for assessing relative air quality in a region.

Limitations and Considerations

However, you should be aware of the limitations and considerations of using lichens for air quality biomonitoring. As with all bioindicators, lichen composition and health are influenced by factors other than the parameter of interest, which in this case is air quality, so other environmental variables such as temperature, relative humidity, available substrates, elevation, and precipitation need to be collected and used to isolate the pollution signal. In other words, multivariate analysis is often required. Another consideration is that lichens do not absorb particulates, and there are limited methods to use lichen to assess PM.

In addition, while academic research on this topic is abundant, to our knowledge, there is no known precedent for a large-scale US community science project using lichen as air quality bioindicators, and US regulatory agencies are often unfamiliar with lichen monitoring data. Thus, it is critical to clearly communicate monitoring design and/or to convert lichen data into pollutant concentration units familiar to regulators through co-location with instrumented monitors and calibration.

Currently, lichen are most useful as low-cost screening tools to identify pollution hotspots and prioritize placement of more expensive equipment.

Source: US Forest Service, Air Pollution-Related Lichen Monitoring in National Parks, Forests, and Refuges: Guidelines for Studies Intended for Regulatory and Management Purposes (2003)

Methods

Categories of Methods

	Elemental Analysis	Community Analysis	Physiological Assessment	Transplant Study
Description	Collect lichen samples for laboratory analysis to determine identifies and concentrations of elements of interest	Collect and compare information on richness, abundance, and presence/absence of indicator species over geographic space or time.	Analyze changes in lichen anatomy and photosynthetic ability of naturally occurring lichen or transplanted specimens in response to exposure to pollutants. Examples of parameters to measure include chlorophyll concentrations, chlorophyll fluorescence, cell membrane integrity, CO2 exchange capacity.	Transplanting healthy lichen to a test area and measuring physiological response and/or elemental accumulation.
Expertise Required	Lichen Identification, Laboratory Analysis, Results interpretation	Lichen Identification	Lichen Identification	Lichen Collection and Transplantation
Equipment	Hand lens, Special laboratory-grade collection bags	Hand lens	Varies, but can include: Spectrometer IR camera Regular camera Nail polish remover	Artificial substrates such as plastic air filters and burlap
Cost	Lab testing cost varies; $50-$250 per sample depending on processing steps required and elements to test for. Laboratory-grade specimen bags can be expensive	Hand lens cost $5-$20		Burlap bags can be obtained at low or no cost. Local farmers may give away used bags for free.
Considerations	Lab turnaround times can take 6-12 months, and it is difficult to determine how long pollutants have accumulated in a tissue sample.

Specific Methods

Community Analysis

Physiological Assessment

Elemental Analysis

Other Guidance for Study Design

Determining Lichen Species of Interest The number of species and the species considered in the study depend on several factors. - Study method

• Elemental analyses, physiological studies, and transplant studies usually focus on one species of lichen to increase comparability across samples.
• Community analyses usually involve several species of lichen because they rely on differences in the composition of lichen across samples to indicate air quality differentials. However, it is possible to conduct a simpler version of community analysis by focusing on assessing presence and/or abundance of one or a few species of lichen.

• Abundance: In single-species studies, a fairly abundant species of lichen in the area should be selected to ensure availability at sufficient sampling plots.
• Sensitivity to pollutant(s) of interest
• Identifiability

Spatial Sampling When collecting data on epiphytic lichen, to characterize an area, it is impractical to survey all the trees in the entire area of interest. So how do you survey a subset of the trees while making sure that the subset (also known as sample), can adequately represent the entire area? Enter: spatial sampling.

First, you need to determine what is the parameter you are sampling–is it a fixed area, like a city block or a grid, or is it a single tree?

Then, you can consider the main sampling strategies:

• Random: Every sample has an equal chance of being chosen. You would label each subarea (e.g. a city block) with an ID and put all the IDs into a random number generator to select your desired number of samples
• Stratified: Separate the population into groups with similar attributes (for example, separating blocks by their cardinal direction, or separating trees by species), then conduct random sampling on each group. Note that the attribute you use to create the groups should influence the lichen characteristic(s) you are interested in. For example, separating your population into areas with blue houses and areas with red houses have no impact on lichen characteristics, whereas, separating highly shaded areas and very sunny areas would have more of an impact.
• Systematic: Similar to random sampling, but instead of picking random IDs, you will select every n-th ID.
• Convenience: Taking samples that are the most accessible. Beware: this technique is easy, but it’s not random and not representative of the population. This can be useful in the early stages of your research, for example to get a sense of the quantity of lichen available in your area.

After determining your sampling method, you will want to collect a "pilot" sample to test out your fieldwork technique(s) and inform a final sample size. You can then use the instructions laid out in the "Sampling Size" section to estimate a sample size for a given margin of error.

Converting Observations to Air Quality Indicators

Consulting Experts

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Questions

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