Mount Merapi is one of tropical rainforest ecosystems in Indonesia, characterized by its high biodiversity, which is considered to be a significant source of germplasm (Wijayati & Rijanta, 2020). The unique characteristics of Mount Merapi ecosystem has led this area to be designated as a national park, with the objective of protecting its unique ecological features. Mount Merapi National Park (MMNP) was established in 2014 based on the (Forestry, 2014), with one of the main objectives to protect the habitat and biodiversity of flora and fauna living in the area (Wijayati & Rijanta, 2020).

However, Mount Merapi is one of the most active volcanoes in the world, with a short eruption period of 2 - 7 years(Wismaya et al., 2016). The high volcanic activity of Mount Merapi can have various impacts on the surrounding environment. Volcanic activity can release ash and gases into the atmosphere, which can be a source of air pollution. This volcanic ash and gas can affect the biogeochemical cycles of carbon, sulphur, and halogens in the ecosystem (Delmelle et al., 2015). This is exacerbated by anthropogenic activities emanating from the slopes of Mount Merapi. Therefore, air pollution monitoring is becoming increasingly important.

To monitor environmental pollution, organisms, such as lichens, can be used as bioindicator to conduct biomonitoring. Lichen is an organism resulting from the mutual symbiosis between algae and fungi (Untari, 2024). Lichen obtains its nutrients from the surrounding air, so it is sensitive to the quality of the surrounding air, therefore, can be used as a bioindicator organism for monitoring air quality (Bukabayeva et al., 2023). Lichen is a long-lived and slow-growing organism with an unchanging morphology, no roots and no protective structure, so it can passively absorb substances from the atmosphere throughout its life cycle (Zarabska-Bożejewicz, 2020).

Lichens can be used as bioindicators of air pollution in the form of heavy metals, organic compounds, and even radioactive elements (Bukabayeva et al., 2023). Monitoring using lichens is simple and informative because it can assess the ecological state of the environment along with the response of the organisms, it allows for a wider study area, and it is more effective and cost-effective (Matos et al., 2019).

This research was carried out in Gunung Bibi Forest of Mount Merapi National Park. Administratively, Gunung Bibi is located in Wonodoyo Village, Cepogo Subdistrict, Boyolali Regency, Central Java Province, Indonesia. Gunung Bibi Forest is included in the Sanctuary Zone, with functions to protect biodiversity, habitats, and ecosystems in the Mount Merapi National Park area. Therefore, ecosystems in the core zone are unaffected by human influence and anthropogenic activities. As an area included in the core zone, Gunung Bibi Forest has a high biodiversity value which is typical of the Mount Merapi ecosystem (Wijayati & Rijanta, 2020).

Despite the highly valued biodiversity, Gunung Bibi Forest is located in the area mostly affected by the eruption of Mount Merapi, with an air quality index category classified as unhealthy (Nugroho et al., 2023), and surrounded by areas having a high level of agricultural activities, predominantly involving agriculture and cattle farming, which serve as the primary means of subsistence for the local community (Statistik, 2023). Both conditions have the potential to affect the air quality on Gunung Bibi, therefore, air quality monitoring is recommended.

Given the remote location of Gunung Bibi, characterized by rugged topography and limited access to power supplies, the utilization of lichen as bioindicator for air quality monitoring could offer certain advantages in terms of efficiency when compared with instrumented monitoring techniques (Blett et al., 2003). As demonstrated by the lichen data collected, there are regions of particular concern, which make the data useful for further, more extensive and instrumented monitoring studies (Blett et al., 2003).

Notwithstanding the rich biodiversity of Gunung Bibi, there has never been a comprehensive study of lichens diversity as a bioindicator in this area. Previous research on lichens in Gunung Bibi was limited to the identification of lichens species, such as the initial report on corticolous lichens by (Susilawati, 2013) and a subsequent study on the characterization of fruticose and foliose lichens by (Susilawati, 2017). Similar studies in other regions have demonstrated that lichens diversity is closely related to air pollution levels, particularly the presence of sulfur, nitrogen, and atmospheric heavy metals from both natural and anthropogenic activities (Agnan et al., 2017)(Zarabska-Bożejewicz, 2020)(Bukabayeva et al., 2023). However, no report has yet been found that addressed the question of a possible correlation between diversity in lichens samples and air quality in the studied area, to establish the possibility of lichens as air quality bioindicator. Therefore, to answer this question, a research was conducted to analyze the relationship between lichens species diversity and air quality in Gunung Bibi Forest of Mount Merapi National Park, and the factors influencing the air quality. The findings from this research are expected to provide deeper insights into the potential of lichens as environmental bioindicators in conservation areas, while serving as a basis for monitoring strategies and mitigating the impacts of air pollution in this region.

A total of 61 individual trees were sampled for lichens species at the 2 research stations, resulting in a total of 36 lichen species from 13 different families Table 1.

Parmeliaceae was the predominant family, with a total of 11 species, followed by Pertusariaceae (5 species), Physciaceae (4 species), Stereocaulaceae (3 species), Lecanoraceae (2 species), Graphidaceae (2 species), Phlyctidaceae (2 species), Monoblastiaceae (1 species), Chrysotrichaceae (1 species), Arthoniaceae (1 species), Collemataceae (1 species), Megalosporaceae (1 species), and Ramalinaceae (1 species).

Parmeliaceae is the most diverse group of lichens in the world, consisting of 2,765 species and 77 genera (Lücking et al., 2017), with a wide distribution. A study by (Karmacharya et al., 2022) showed similar results that Parmeliaceae is also the most dominant family in the Kathmandu Valley area, consisting of 8 genera and 20 species. In our study, the species that exhibited the highest Importance Value Index (IVI) values at Station I were Graphis scripta (72.01), Phlyctis argena (69.42), and Flavoparmelia caperata (34.16). Conversely, the highest IVI values at Station II were observed in Phlyctis argena (80.56), Cryptothecia striata (19.00), and Parmotrema perlatum (18.67). This finding indicated clear disparities in the prevalent lichens species at the two research stations, suggesting variations in lichens composition between the stations. Furthermore, G. scripta and P. argena are cosmopolitan lichens that demonstrate a high level of resistance to various environmental conditions, including atmospheric metal pollution (Agnan et al., 2017), thus explaining their dominance at both Station I and Station II. Both species also have a broad global distribution, G. scripta (Figure 2 a) has a distribution in tropical to subtropical areas, while P. argena (Figure 2 b.) has a distribution in Africa, Europe, North America, and Asia (Nash et al., 2002).

Station I recorded 20 lichens species from 8 different families, while Station II recorded 30 lichens species from 13 different families Table 1. The Shannon-Wiener Diversity Index showed that Station I had a level of lichens diversity in the "medium" category with a value of 2.557. Station II, on the other hand, had a level of lichens diversity in the "high" category with a value of 3.003. The observed discrepancy in the diversity of lichens samples collected from the two stations indicated a potential difference in the quality of ambient air at these locations.

In the early stages of exposure to air pollution, there will be an increase in the concentration of pollutants in the lichens thallus; as the pollution levels increase lichens may disappear completely, creating a condition known as 'lichens desert' (Bukabayeva et al., 2023). Therefore, the higher the air pollution in an environment, the lower the lichens diversity found in the environment (Muvidha, 2020).

In this particular instance, Station II exhibited a better air quality, as proven by the higher level of lichens diversity compared to that at Station I. In addition, Station I was characterized by a greater total number of lichens colonies; however, it has a comparatively lower lichens thallus coverage area compared to Station II Table 1.

Lichens thallus coverage area can be used as an indicator to assess the suitability of environmental conditions in which lichens live. Good environmental conditions allow lichens to grow optimally, so that lichens can be found intact with a large area of thallus coverage. The accumulation of air pollution in the thallus of lichens causes physiological and morphological changes in the lichens, resulting in a decrease in thallus coverage area and even fragmentation of originally intact thallus parts (Nasriyati et al., 2018). The reduction in thallus coverage area is one of the lichens responses to ambient air pollution (Nasriyati et al., 2018). This result strengthened the previous indication that Station II had a better air quality than that at Station I, which was indicated by higher lichens thallus coverage area at Station II.

The poor air quality at Station I may have been caused by anthropogenic activities since the location of Station I is closer to residential areas and the agricultural land of the residents, which heightened the impact of agricultural activities to air quality at Station I. Agricultural activities, such as the use of organic and mineral fertilizers and the burning of organic materials, can release nitrogen compounds into the air, for example in the form of ammonia (NH₃), which can increase the pH of tree bark when deposited (Zarabska-Bożejewicz, 2020).

Furthermore, high atmospheric nitrogen levels lead to changes in lichens diversity and abundance, such as an increase in nitrophilic lichens species and a decrease in the abundance of non-nitrophilic species (Zarabska-Bożejewicz, 2020). In non-nitrophilic species, excessive nitrogen exposure leads to a decrease in chlorophyll A and ergosterol (Zarabska-Bożejewicz, 2020).

Most of the people in Cepogo Subdistrict work as farmers. According to (Statistik, 2023), Cepogo Subdistrict is the subdistrict with the largest number of individual agricultural businesses in Boyolali District, which includes food crops, horticulture, plantations, and livestock farming.

As for the livestock subsector, there are a total of 9,478 households that raise livestock, including beef cattle, dairy cattle, and goats (Statistik, 2023), which makes Cepogo Subdistrict one of the largest contributors to cattle farming in Boyolali Regency. In the management of livestock waste, there are still farmers who do not treat the wastes, but directly dispose the wastes in the backyard of the house, in the river around the settlements, or even directly dispose the wastes in their gardens and fields without prior treatment (Pertiwi et al., 2022). The activity of disposing agricultural wastes without treatment can produce gas emissions that can pollute the environment, such as ammonia (NH₃), hydrogen sulfide (H₂S), carbon dioxide (CO₂), and methane gas (CH₄) (Pertiwi et al., 2022).

Aerobic organic wastes treatment (composting) is one of the sources of NH₃ emissions (Zarabska-Bożejewicz, 2020). The deposition rate of ammonia (NH₃) is rapid, causing most of ammonia to be easily distributed close to the emission source. About 10% of NH₃ can be found within 100 m of the emission source, and when dissolved in water and ionized to NH₄⁺,ammonia can be transported as far as 1,000 km (Zarabska-Bożejewicz, 2020). Therefore, ammonia resulted from agricultural activity is highly potential to contribute to air pollution.

Based on their adaptation ability to the deposition level and the availability of atmospheric nitrogen, lichens can be grouped into three types: oligotrophs, mesotrophs, and eutrophs. Oligotrophs are lichens that grow only in nutrient-poor environments, mesotrophs have intermediate nutrient requirements, and eutrophs are lichens that thrive in nutrient-rich environments (McCune & Geiser, 2009).

Oligotrophs are most abundant in areas with average N deposition of 0.5 - 4.2 kg N/ha/year and become less abundant as N deposition increases above 4.2 kg N/ha/year, mesotrophic lichens prefer areas with N deposition of 4.2 - 8.0 kg N/ha/year, and eutrophic lichens can tolerate N deposition up to 8.0 kg N/ha/year (Service, 2025).

Eutrophic lichens species are also known as nitrophilus species which are characterized by being nitrogen-tolerant species that benefit from nitrogen eutrophication (Zarabska-Bożejewicz, 2020). This categorization is generally applied to the macrolichens group, which consists of foliose, fruticose, and larger squamulose lichens (McCune & Geiser, 2009). Macrolichens are often used as bioindicator species for air quality due to their large surface area, easiness for observation, and greater sensitivity than microlichens, such as crustose lichens (Pasaribu et al., 2023).

In the study area, a total of 17 macrolichens species were found Table 2. There were six eutrophic lichens species, four of which were found at both stations (Flavoparmelia caperata, Parmotrema perlatum, Punctelia perreticulata, Ramalina fraxinea), and two species were found at only one each at one of the stations (Usnea subfloridana at Station I and Parmotrema xanthinum at Station II). As for mesotrophic and oligotrophic lichens species, a total of six species were found. Out of six species, five of them were found only at Station II (Collema subflaccidum, Physcia adscendens, Physcia stellaris, Hypotrachyna revoluta, Parmeliopsis ambigua), and there was only one species found at Station I (Usnea glabrescens).

Our study showed that there were differences in lichens composition between the two research stations. These results suggested that there was a decrease in the diversity of non-nitrophilic lichens species at Station I, which was thought to be caused by high nitrogen emissions from agricultural activities. However, further research is needed to confirm this finding.

Nitrophilus species were mostly found at both stations, Station I and II. This result indicated that both stations had high atmospheric nitrogen deposition. The high nitrogen deposition may have been caused by agricultural activities in the vicinity of the research site.

Conversely, nitrogen levels at Station II may tend to be lower than at Station I, which was indicated by the presence of oligotrophic and mesotrophic lichens species at Station II, such as Collema subflaccidum, Physcia adscendens, Physcia stellaris, Hypotrachyna revoluta, and Parmeliopsis ambiguaFigure 3. These species were only found at Station II, so it is noteworthy that some of these species were used as bioindicator species in this study. Of the five species, Physcia stellaris (Figure 3 c) was the species having the highest IVI value of 12.42% Table 1, making it the strongest candidate for bioindicator species in this study. Physcia stellaris was also used as a bioindicator species in a study by (Susan et al., 2017) in the Midwest area of the United States to investigate the levels of contaminant elements in the area.

Thallus morphology of the lichens found at the research site can be categorized into 3 types, namely crustose, foliose and fruticose. Crustose lichens have thallus morphology such as a layer of crust that lives firmly attached to the surface of the substrate. Foliose lichens have a leaf-like thallus morphology, composed of thallus that will develop into lobes, and are equipped with rhizine structures functioning as attachment and food absorption apparatus. Fruticose lichens have a shrub-like thallus morphology with ribbon-like branches (Muvidha, 2020); (Untari, 2024).

In Stations I and II, crustose lichens were found to be the most dominant category, followed by foliose and fruticose lichens Figure 4. Crustose lichens are the most resistant to environmental influences because they require less water and can store water optimally due to their strong attachment to the substrate (Pasaribu et al., 2023). The strong and tight attachment also makes crustose lichens resistant to environmental stresses, such as temperature, drought, UV radiation, pollution and herbivores (Pasaribu et al., 2023). Foliose and fruticose lichens are more susceptible to environmental stresses. Foliose lichens can only be found under certain environmental conditions, for example in still natural habitats (Pasaribu et al., 2023). On the other hand, fruticose lichens are the most sensitive to environmental conditions, including air pollution (Roziaty, 2016). A study by (Sujetoviene, 2010) showed that foliose and fruticose lichens are commonly found in areas with low levels of pollution, while crustose lichens are found in areas with high levels of pollution.

The most abundant lichens found at Stations I and II was lichens attached to Klewer tree (Engelhardtia spicata) with a total of 51 occurrences Table 3. The bark type of Klewer tree which is classified as rough with furrows of < 1 cm in size seems to be an easier substrate for lichens to grow on compared to other trees having a smoother bark type. Lichens propagules are more easily trapped and grow on trees having rough bark-texture with shallow and small furrows (Susilawati & Kasiamdari, 2021). Bark type with deep cracks causes the bark to be unstable and fragile, so that it peels off easily, for example in Pinus merkusii (Susilawati & Kasiamdari, 2021). However, our study demonstrated a relatively high prevalence of lichens growing on the bark of Pinus merkusii tree (12 thalli at Station I and 8 thalli at Station II).

This finding suggested that bark exfoliation may impede lichens growth, yet it does not entirely preclude it. This phenomenon can be attributed to the fact that deep cracks in the bark are capable of maintaining adequate moisture levels in the substrate, thereby facilitating the penetration of rhizines in foliose lichen (Susilawati & Kasiamdari, 2021). This, in turn, provides a foundation for the growth of crustose, foliose, and fruticose lichens.

Lichens diversity and abundance are influenced by environmental parameters, such as air temperature, humidity, light intensity, and nutrients in the place where lichens live (Pasaribu et al., 2023). Our study observed that Station I had a higher range of air temperature (19.9 - 30.6 °C) with an average of 24.0 °C, while Station II had a lower range of air temperature (20.4 - 25.8 °C) with an average of 22.8 °C Figure 5.

According to (Murningsih & Mafazaa, 2016), the optimal temperature for lichens growth is below 40 °C. Air temperature above 45 °C can damage the chlorophyll pigment contained in the lichens, thus disrupting the photosynthetic process of the lichens. Nonetheless, the mean air temperature measurements obtained from Stations I and II in our study were consistently lower than the maximum temperature threshold that could be tolerated by lichens.

Humidity is one of the factors that greatly affects the ability of lichens to absorb water, nutrients and pollutants contained in the air (Murningsih & Mafazaa, 2016). (K et al., 1996) stated that lichens can still grow and photosynthesize in habitat conditions classified as very humid, with humidity reaching 85%. Humidity more than 85% can reduce the efficiency of lichens photosynthesis by 35 - 49% (Hadiyati et al. 2017). Station I exhibited a lower range of humidity, ranging from 52.6 - 85.6% with an average of 69.66%, while station II demonstrated a higher range, ranging from 64.9 - 88.2% with an average of 76.65% Figure 5. The humidity measurements between the two stations showed an average humidity that is below the maximum tolerance of lichens. However, Station II relatively had a consistent higher humidity that influence lichens growth.

Light intensity is the factor influencing the ability of lichens phycobionts to perform photosynthesis. Lichens require the lowest light intensity of 1,025 lux to be able to photosynthesize (Murningsih & Mafazaa, 2016). Station I exhibited a higher range of light intensity (313 - 5,655 lux) with an average of 1,956.8 lux, while station II demonstrated a lower range (306 - 4,297 lux) with an average of 1,343.20 lux Figure 5. This finding indicated that the average light intensity at these two stations exceeded the minimum tolerance level of the lichens. The relationship between nitrogen-sensitivity and lichens species suggested that the lichens species found in our study are more likely to be well-adapted to habitats with higher levels of sun exposure (Zarabska-Bożejewicz, 2020). Therefore, conditions involving lower humidity, higher temperatures and higher light availability may be beneficial for nitrophilic lichens species and limit the growth of non-nitrophilic species (Pinho et al., 2012).

The disparities in air temperature, humidity, and light intensity measurements between the two research stations can be attributed to the distinct vegetation composition observed at each location. Station II had vegetation with higher tree density and denser canopy cover, resulting in lower light intensity, lower air temperature, and higher air humidity. According to (Yulianti et al., 2022), areas with open canopies are known to produce higher light intensity, higher air temperature, and lower air humidity, which in turn affects lichens richness. Furthermore, the disparity in environmental parameters between the two stations can be attributed to altitude differences. Higher altitude is associated with lower temperatures and higher relative humidity. Research of (Ismail et al., 2024) on lichens diversity and richness across varying altitudes indicated that higher elevations are more conducive to lichens growth, owing to the fact that low temperatures and high humidity prevent desiccation.

The diversity of lichens species found in the study area consists of 36 species from 13 different families. Lichens composition at the two stations differs, indicating disparities in air quality between the two stations. Station II (1,600 – 1,700 masl) exhibited indications of better air quality in comparison to Station I (1,500 – 1,600 masl), which was distinguished by a higher diversity index value, as well as a greater lichens thallus coverage area. The difference in air quality between the two research stations may have been caused by nitrogen emissions from agricultural activities that limit the diversity and abundance of non-nitrophilic lichens species. Lichens species diversity and abundance are influenced by environmental factors, such as air temperature, humidity, light intensity and host tree bark type.