This study shows that the oceanic lichen, Bryoria bicolor, have declined severely the last 25 years, in the county of Östergötland. The lichen could not be rediscovered in over 50% of the sites where it had been previously observed. It is also declining in Norway and Finland, where it is classified as Near Threatened and Endangered in respective country. Similar results of declining lichens have been recorded by Hauck et al. (2013), where an overall loss in epiphytic lichen diversity have occurred in north-western Germany since the 19th century. The reasons for the decline of B. bicolor is still unclear after this study. However, some environmental factors are more likely to explain the decline of the species. One of these factors is humidity. This study indicated that B. bicolor is sensitive towards changes in the microclimate, and especially when humidity is affected. Other factors that cannot be ruled out of affecting B. bicolor are climate change and air pollution.

Climate change

Climate change can have profound effects on lichen survival, as lichens are both highly dependent on its climate and tends to respond sensitively towards global warming (Aptroot & van Herk, 2007). As B. bicolor is a suboceanic lichen, it demands a climate with high humidity (SLU Artdatabanken, n.d.). A higher annual precipitation would therefore be beneficial for the lichen. However, the annual temperature is continuing to increase in Sweden which also risks drier conditions in many habitats. This would benefit drought-loving lichens and most likely be harmful for moist-loving lichens, such as B. bicolor (Viles & Cutler, 2012). 

Air pollution

There is a well-established connection between lichens and air pollution, as many lichens are sensitive towards SO2. Lichens are therefore useful indicators for a habitats’ air quality and for human health (van Herk et al., 2002). B. bicolor has continued to decline in the western parts of Sweden, even though sulfur dioxide compounds have decreased dramatically during the last decade (Sunnanquist, 2007). As Sunnanquist (2007) did not see any significant environmental changes except a large canopy cover at the visited sites, air pollution by nitrogen compounds might be a reason for population decline. However, this study showed significant environmental changes, where B. bicolor was more commonly found in areas with higher humidity. This is indicating that the population decline is not happening randomly, which it would have if it was caused by air pollution. 

Forest fragmentation

Another factor that might have caused population decline of B. bicolor is old-growth forest fragmentation, which is causing the species to remain in small populations in the old growth forest fragments. There are higher risks of extinction for species living in smaller population sizes (Mace et al., 2008; Scheidegger & Werth, 2009; Öckinger & Nilsson, 2010; Wootton & Pfister, 2013), as they become more vulnerable towards stochastic genetic and ecological processes (Wootton & Pfister, 2013).

Interspecific competition

Differences in proportion of moss and lichen between absent and present sites were seen in this study. Amount of moss had a negative effect on survival of B. bicolor and amount of lichen had a positive effect. The negative effect of a higher abundance of moss on the presence of B. bicolor could be explained by competition. Epiphytes tend to compete for both light and moisture. There will therefore occur an interspecific competition between moss and lichens. Even though lichens are better at tolerating changes in moisture, they are still not as strong as moss species in wetter areas (Sales et al., 2016). 

Cardinal direction

B. bicolor did also show clear preferences regarding cardinal direction of the boulder it occurs on. The majority of the occurrences were found in a northern or north-eastern direction. One reason for this is that north-facing boulders tend to promote a more humid climate with lower temperatures and more shade. Other attributes that a north-facing boulder can contribute with is protection from weather elements, such as precipitation, sun and wind (Lewis & Śliwa, 2012). 


A higher inclination of the boulder showed tendencies on the occurrence of B. bicolor, though not significant. Small differences between absent and present sites could be seen as B. bicolor showed a slightly higher occurrence on boulders with a higher inclination. There are studies that have shown that inclination of the growth area can affect lichens (Barkman, 1969; Johansson et al., 2009). According to Martínez et al. (2014), a more inclined growth area will contain higher water availability, which can increase the occurrence of epiphytes. 

Boulder-rich terrain

A boulder-rich terrain did show a significantly negative effect on the survival of B. bicolor. This was surprising as these types of habitats usually benefit lichen species, both in biodiversity and the preservation of the ecosystem (Spitale & Nascimbene, 2012). It has also been shown that B. bicolor prefer to grow on rocks and tree branches in mountain habitats (Jörgensen et al., 2012), which contradicts the results from this study. However, this variable was analysed within 20 metres of the growth area. As the lichen tended to grow next to wetlands, it might have decreased the proportion of boulder-rich ground, as the wetlands contained less boulders. Almost all sites were located in forests with a large proportion of both mountains and boulders. 


No differences in openness could be found between absent and present sites of B. bicolor. Nevertheless, openness and an increased light exposure have shown effects on lichens, as it can affect the microclimate with drier conditions (Phinney et al., 2020). B. bicolor will however not be exposed to direct sunlight as they tend to grow in a northern cardinal direction. This will protect them from losing to much water and becoming bleached. 

Proportion of Hypogymnia vittata

One variable that showed a small difference between absent and present sites was proportion of Hypogymnia vittata. The positive relationship between presence of H. vittata and B. bicolor could imply that H. vittata has similar habitat demands and might be experiencing a similar decline as B. bicolor

Landscape composition

There were no significant effects on landscape composition at larger scales. Neither clear-cuts or open water within 200 and 500 metres had any effects on B. bicolor. This was unexpected as it was thought that lichens, and particularly B. bicolor, would have negative effects of clear-cuts and positive effects of open water. It has been shown that disturbances, such as forestry, have short-term negative effects on lichens (Johansson, 2008). 

Open water

A close proximity to open water was expected to have a significant effect on B. bicolor as water can increase air humidity (Lidén, 2009), which has shown to benefit oceanic lichens (Jørgensen, 1996). Overall, open water close to where lichens tend to grow is beneficial for maintaining a higher air humidity. Proximity towards open water is often beneficial for lichens, but streams will also have this effect on them, and especially for B. bicolor (Lidén, 2009). Thus, another analysis of open water and streams in the vicinity of the lichen would be interesting. It has also been shown that wetlands and swamp forests have similar positive effects as open water, with a higher air humidity (Lidén, 2009). This might have affected the results as many sites where B. bicolor was present contained wetlands instead of open water.  

Tree diameters

Tree diameters were measured to collect information about tree continuity at the different sites. However, tree diameter did not show any effect on the survival of B. bicolor within 20 metres the growth area. These results were also unexpected, as longer tree continuity has been shown to have a positive effect on lichen survival. Older forests will contain higher levels of microclimatic humidity, which is beneficial for many rare and threatened lichen species (Tibell, 1992). Additionally, in Coppins & Coppins (2005), a long ecological continuity was shown to be a key factor for a higher lichen biodiversity. One reason for the unexpected results in this study is that many sites were located in wetlands. The correlation between tree age and tree diameter is weaker in these types of habitats, i.e., the trees might have been older even though the measurements were small. 

Tree continuity and edge effect

Even though tree continuity creates a stable climate with higher humidity, clear-cuts and other disturbances in the proximity can cause instabilities of the biotic and abiotic conditions within the older forest. Edge effect has been shown to change the microclimate by increasing solar radiation and wind exposure, which can decrease the overall air humidity in the area. As B. bicolor prefers to grow in a northern cardinal direction, the lichen might be less affected by edge effects than lichens growing in a southern cardinal direction.). Time since the disturbance happened is also a factor when studying lichen survival. If the edge is younger, there might not be any effect on the lichen community, as lichens tend to both live longer and be slow growing (Kivistö & Kuusinen, 2000). Since B. bicolor is growing in a northern direction with less prominent microclimatic changes, the effects from nearby clear-cuts and edges might not be recognisable yet. This might be the reason why clear-cuts within 200 and 500 metres did not show any effect on B. bicolor

In conclusion, this study highlights a severe decline of the rare lichen Bryoria bicolor over the last 25 years in the county of Östergötland. The specific reason behind the decline remains unclear, but environmental changes in humidity is a likely contributor. Further research at a larger range is necessary to fully understand and address the threats to the survival of B. bicolor.