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Valerie Stacey1,2 and Cassie J. Majetic Department of Biology Saint Mary’s College Notre Dame, IN 46556

Walter Carson2 Department of Biological Sciences University of Pittsburgh Pittsburgh, PA 15260


Although it is well known that lichens and bryophytes can be sensitive indicators of specific mi- crohabitats, it remains less clear how attributes such as host tree identity, tree size, directional location on a tree, and epiphyte co-occurrence impact the degree of lichen and bryophyte colonization. To address these questions, we sampled the percentage cover of lichens and bryophytes on 50 individuals each of Acer saccharum (sugar maple), Pinus resinosa (red pine), and Populus tremuloides (quaking aspen) in a northern hardwoods forest. The samples were taken in 500 cm2 areas on each tree, 1.5 meters above the ground, in each of the four cardinal directions for a total of 2,000 cm2 on each tree. Quaking aspen trees had a significantly higher percentage of bryophyte cover than sugar maple, whereas sugar maple had nearly five times more lichen cover than quaking aspen. Lichens and bryophytes were essentially absent (<0.1% cover) on red pine. For sugar maple, percentage cover of lichens was significantly negatively correlated with DBH (r = –0.30; p = 0.036). The north side of sugar maples had a significantly higher percentage of bryophyte cover than any of the other cardinal directions, and lichen and bryophyte cover were strongly negatively correlated on the north and east sides (r = –0.48, p<0.001; r = –0.39, p = 0.005, respectively). Our results demonstrated that the percentage cover of these two life forms varied strongly with host tree and that cardinal location can potentially mediate the degree to which these life forms covary. We suggest that bark chemistry and substrate texture, as well as cardinal directions underlie the patterns found in this study.

KEYWORDS: corticolous epiphytes, lichens, bryophytes, substrate texture, cardinal direction


The effects of microclimate (Campbell and Coxson, 2001) and substrate characteristics (Kuusinen 1995; Löbel and Rydin 2006; Käffer et al. 2016) on lichen and bryophyte abundance are well known; however, the influence of these factors on the biotic interactions between bryophytes and lichens is less well understood. These interactions may mediate the distribution and percentage cover3 of

1 Author for correspondence (

2 University of Notre Dame Environmental Research Center, Land O’ Lakes, WI 54540

3 Throughout this paper, the word “cover” will be used to mean “percentage cover.”


lichens and bryophytes. For instance, certain secondary metabolites produced by lichens completely inhibited the germination of many bryophyte species (Lawrey 1977). Such activity might increase lichen cover or persistence at the expense of competing species. Similarly, Jüriado et al. (2009) reported a significant negative correlation between bryophyte cover and lichen species diversity. Kuusinen (1995) found significantly higher species diversity and cover of bryophytes and lower lichen species diversity on Populus tremula than on Salix caprea, which suggested that the biotic interplay between epiphyte types impacts their contribution to total cover on different host species.

In addition to interactions among epiphytes, variation in physical and chemical characteristics of individual host trees will also affect the distribution of epiphytes. For example, smooth, homogenous bark, deep fissures, and loose-scaled bark are likely inimical to lichens and other epiphytes (Kuusinen 1995; Löbel and Rydin 2006). Käffer et al. (2016) found that lichen species growing on host trees with smooth bark typically exhibited low species richness and low cover. In contrast, rough bark with a higher water storage capacity typically enhanced epiphyte cover (Levia and Herwitz 2005). Bark pH may provide another axis of niche differentiation, as some epiphytes prefer high pH while others are sensitive to alkaline substrates (Jüriado et al. 2009; Jovan et al. 2012). Additionally, many studies have shown a positive correlation between lichen cover and tree size (diameter at breast height; DBH), as well as basal area (Li et al. 2015; Edman et al. 2007; Johansson and Ehrlén 2003; Dettki and Esseen 1998). This pattern is also consistent with bryophyte cover and DBH (Hazell et al. 1998). Finally, some preliminary work suggests potential impact of cardinal direction on epiphyte cover (Monge-Nájera et al. 2002) and species diversity (Kivistö and Kuusinen 2000). For instance, Monge-Nájera et al. (2002) pooled twenty years of lichen cover data in Costa Rica. Cover values by cardinal orientation were somewhat variable (west 17%, east 14%, north 13%, south 12%). The authors attributed this pattern to climatic variability, as winds in San José move from northeast to southwest and the western sides of trees generally receive less sun and wind during the dry season.

Here, we ask the following questions:

1. Is there a positive or negative relationship between the cover of bryophytes and lichens? 2. To what degree do lichens and bryophytes vary among three common tree species? 3. Does the cover of lichens and bryophytes increase with host size or vary with cardinal direction? MATERIALS AND METHODS


This study was conducted at the University of Notre Dame Environmental Research Center (UNDERC), located in Land O’ Lakes, Wisconsin, during the summer of 2015. Aspen-birch, maple- beech-birch, and spruce-fir forests dominate this relatively undisturbed 3000 ha forest, which straddles the border between Vilas County, Wisconsin, and Gogebic County, Michigan (46°13’N; 89°32’W). This area is part of the hemlock-white pine-northern hardwoods region of the Eastern Deciduous Forest Biome (Delcourt and Delcourt 2000).


Study Species

Three species that commonly occur in northern hardwoods forests clearly represent diversity in bark characteristics that may influence epiphyte diversity and cover, and so were chosen as the focus for this study. The bark of Pinus resinosa (red pine) is soft, very loosely attached, and profusely and continuously sloughed off during its life. It is divided by shallow fissures into broad, flat ridges that are covered by thin, loose, light red-brown scales (Culberson 1955). The bark of Populus tremuloides (quaking aspen) is thin, often roughened by horizontal bands of circular wart-like excrescences, and frequently marked below the branches by large, dark scars. The bark of Acer saccharum (sugar maple) is thick and broken into deep, longitudinal furrows, with the surface separating into small, plate-like scales (Sargent 1961).

Sampling Method

We sampled 50 individuals each of sugar maple, red pine, and quaking aspen. We identified and numbered 30 different sites containing stands of sugar maple and quaking aspen. We then randomly selected 10 of these stands, using a random number generator, and haphazardly selected 5 individuals of each tree from each site. Because red pine was less common and restricted in its local distribution, we haphazardly sampled 25 of these trees at each of two sites. Haphazard sampling was necessary because locations identified randomly within each site did not always meet our sampling requirements. If the randomly selected location did not meet the requirements, we would continue to walk 10 meters in a randomly selected cardinal direction until requirements were met. To ensure independent sampling events, all sampled trees, both within and among species, were at least 10 m apart. We sampled only living trees that were at least 10 cm DBH (exact DBH was recorded for each tree sampled) and that were beneath a closed canopy and at least 40 m from a road or forest edge because edge effects affect epiphyte diversity (Rheault et al. 2003). We sampled lichens and bryophytes using a 10 ¥ 54 cm frame, subdivided into five 10 ¥ 10 cm quadrats (protocol described in Lovadi et al. 2012), to estimate the cover of epiphytic lichens and bryophytes. A smaller (5 ¥ 5 cm) square was used to aid in visual estimations of cover within each 10 ¥ 10 cm quadrat. We vertically placed the frame against each tree trunk

1.5 meters above the ground in each of the four cardinal directions, placing the bottom of the frame at the 1.5-meter mark; thus, a total area of 2000 cm2 was censused on each tree. If lichen was growing on top of bryophytes, the cover of both bryophyte and lichen visible on the surface was estimated. Analysis

To calculate cover, the number of cm2 covered by lichens or bryophytes in each of the quadrats was estimated. These values were then added together to determine the total cm2 covered by bryophytes or lichens in the sampling area. To determine cover, we divided total cm2 covered by total sampling area. This process was repeated on each of the four cardinal directions for each tree; average cover per tree was determined by averaging cover of all four cardinal directions.

All statistical analyses were performed using R (R Core Team, 2015). Data were transformed and normalized using the logit function prior to all analyses. A Shapiro-Wilk test was performed to confirm normality (p > 0.05). No analyses were done on red pine because lichens and bryophytes were essentially absent. A one-way MANOVA was performed to test for significant differences between the average cover of lichens and bryophytes on sugar maple and quaking aspen trees. We used Pearson correlation coefficients to assess relationships between the average bryophyte and lichen coverage on each tree species, as well as the relationship of each to DBH and cardinal direction. We also used student’s t-test to further investigate the relationship between DBH and lichen and bryophyte cover. A repeated measure MANOVA and a series of paired t-tests were used to determine if there was a difference in bryophyte or lichen cover per cardinal direction for both tree species.


Our one-way MANOVA model was significant overall (F=47.7, p < 0.0001). Lichens covered three times more area on sugar maple than quaking aspen (F =77.26, p<0.0001; Figure 1). Quaking aspen had higher bryophyte cover than


FIGURE 1. Average percentage cover of bryophytes and lichens on 50 sugar maple (Acer saccharum) and 50 quaking aspen (Populus tremuloides) trees. Error bars represent standard error.

sugar maple, although this pattern was less pronounced than that seen for lichens (F = 4.51,p = 0.036; Figure 1). Lichens were nearly absent (0.03% cover) and bryophytes never occurred on red pine at our sites (1.5 meters above ground; compare to Figure 1). There was no correlation between bryophyte and lichen cover on quaking aspen (r = –0.122; p = 0.401; Figure 2); however, they were

FIGURE 2. Relationship between average percentage cover (APC) of lichens and of bryophytes on sugar maple (each dot represents one tree; r=-0.246; p=0.085) and quaking aspen (each triangle represents one tree; r=-0.122; p= 0.401).


FIGURE 3. Average percentage cover (APC) of lichens and bryophytes on each of four cardinal directions on sugar maple trees (each dot represents one tree). North: r= -0.48, p<0.001. East: r=-0.39, p=0.005. South: r=-0.13, p=0.378. West: r=-0.086, p=0.553.

negatively correlated on sugar maple (r = –0.246; p = 0.085; Figure 2), and this negative relationship strengthened sharply on north facing (r = –0.48, p<0.001) and east facing (r = –0.39, p = 0.005) sides of sugar maple trees (Figure 3). Furthermore, there was a significant difference in bryophyte cover per cardinal direction on sugar maple trees (F = 5.971, p<0.001; Figure 3) overall. Specifically, there was more on the north sides than the south sides (t= 4.14, p<0.0001; Figure 3). No significant relationships were found for cardinal direction and epiphyte cover on quaking aspen (north: r = –0.245, p = 0.09; east: r = –0.207, p = 0.15; south: r = –0.125, p = 0.39; west: r= –0.102, p= 0.48).

Bryophyte cover did not increase with DBH for either sugar maple (r=-0.19; p=0.19) or quaking aspen (bryophyte: r = 0.071; p = 0.62). Lichen cover decreased with DBH on sugar maple (r = –0.30; p=0.036), but not for quaking aspen (r= 0.40; p=0.78). There was a significant difference between the average lichen cover on the ten smallest and ten largest sugar maple trees, (p=0.037), though there was no difference between the ten largest and smallest quaking aspen trees (p=0.76).


We found that the overall cover of lichens and bryophytes varies with tree species. It is likely that this variation is related to bark characteristics (Culberson 1955), although the influence of stand-level characteristics, such as the age of


the stand, abundance of large sized trees, and canopy openings (Boudreault et al. 2008), which are often linked to species composition, cannot be ruled out (Mc- Cune 1993; Berryman and McCune 2006). Sugar maple, with thicker and rougher bark, has three times the cover of lichens and slightly less bryophyte cover than quaking aspen, with its smooth and homogenous bark. This suggests that these tree species differ in their suitability as substrates. Red pine bark, which lacked epiphytes, is scaly and regularly sloughs off the tree (Sargent 1961), suggesting that this type of bark is not stable enough to allow epiphytes to establish and grow. Indeed, Cáceres et al. (2007) concluded that, at least for tropical trees, lichens need a more stable substrate with “lower degree of shedding.”

Lichens are also sensitive to bark attributes other than texture, such as pH (Hauck et al. 2011) and water holding capacity (Levia and Herwitz 2005). Culberson (1955) generated a gradient of bark characteristics that he believed to have strong impacts on cover and host choice of epiphytic vegetative communities found on several focal tree species, including red pine. Red pine ranked the least suitable for colonization in hardness, water holding capacity, and pH, whereas members of the genera Acer and Populus were ranked the most suitable for colonization. Thus, at least for our three tree species, the system developed by Culberson (1955) predicted the relative lichen and bryophyte cover that we found; to our knowledge, few other studies have explored this possibility (Jüri- ado et al. 2009). Our findings suggest that Culberson’s approach, developed more than 60 years ago, retains its utility and should be more widely applied and expanded to further elucidate underlying mechanisms.

We found a negative relationship between bryophyte and lichen cover on sugar maple, the magnitude of which varied with cardinal direction. As bryophytes often prefer locations with more shade and moisture, and lichens can typically withstand harsher climatic conditions (e.g., more exposure to wind and sunlight), this relationship may be a result of the different climatic conditions at each cardinal direction. The north sides of trees likely receive less sunlight and thus potentially make bryophytes stronger competitors than lichens on the north side. However, we also saw negative correlations between lichen and bryophyte abundance on the east side of sugar maple trees. Additionally, no significant relationships between bryophyte and lichen cover were found on any cardinal direction of quaking aspen hosts. These results suggest that the level of competition between lichens and bryophytes may depend on both the host species as well as the directional location of the epiphytes on the host. Furthermore, the level of competition may vary depending on the specific species involved. Because little research on the influence of cardinal direction on lichen-bryophyte interactions has been published, our results suggest this as an intriguing direction for future studies.

Tree size (DBH) in sugar maple had no discernible effect on bryophyte cover and only a modest negative correlation with lichen cover, in contrast to previous studies that have shown positive correlations between lichen cover and DBH, as well as basal area, on a variety of tree species (Li et al. 2015; Edman et al. 2007; Monge-Nájera et al. 2002; Johansson and Ehrlén 2003; Dettki and Esseen1998). The lack of stronger correlations in this study is likely due to a relatively small


variation in maximum diameter in the stands studied (ranging from 15cm to 41.2cm in sugar maple and from 15.4 cm to 39.1 cm in quaking aspen).

Overall, these variation patterns suggest that interactions among lichens and bryophytes may be substrate-specific and fairly nuanced, as they are for lichen- lichen interactions (as described in Armstrong and Welch 2007). We propose that these lichen-lichen interactions could be used in future studies to describe the relationship between corticolous bryophytes and lichens. Additionally, lichen- bryophyte interactions may be species-specific (e.g., Colesie et al. 2012; Jüriado et al. 2012), a possibility not fully addressed in the current study. Future research exploring these interactions on a species-specific level would greatly improve our understanding of the nuanced patterns seen here.

Understanding the dynamics of lichen-bryophyte interactions provides further insight into early successional stages of ecosystems and nutrient cycling in all forest types. Future study should generate controlled experiments, as there are climatic factors involved at a microscopic scale in the development of both bryophytes and lichens that are challenging, if not impossible, to delineate in a field setting. Given that lichen-bryophyte interactions are likely to develop slowly over time (although there is potential for faster development with fast- growing species such as quaking aspen), further research evaluating these important relationships will require long-term studies using larger sample sizes and experimental approaches. We also suggest that these interactions be analyzed on a species-specific level, and across multiple temporal and spatial scales (exploring both substrate and stand level characteristics) to more fully understand their complex nature.


This research was generously funded by the Bernard J. Hank Family Endowment. We thank the staff at the University of Notre Dame Environmental Research Center, G. Belovsky, M. Cramer, H. Madson, J. Hart, and S. Small for field assistance and support. We also thank J. Ralston for assistance with statistical analysis, as well as L. Kloepper for feedback on an earlier version of this manuscript.


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