Influential environmental factors of lichen diversity on multiple scales in northwestern quebec 

Background to the Lichens

Lichens are a complex symbiotic relationship. In its most basic form, this consists of a fungi (mycobiont) and an algae or cyanobacteria (photobiont). Once lichenized, the mycobiont provides a structure that protects the photobiont from desiccation, and most herbivory through the production of chemical compounds. The photobiont provides energy for the mycobiont generated from photosynthesis. This allows the partners to survive in habitats that would otherwise be inhospitable to them. Lichens that have both algae and cyanobacteria photobionts, the latter in specialized structures called cephalodia, are called tripartite lichens. When one mycobiont can form a symbiosis with either algae or cyanobacteria, the different morphotypes are known as photosymbiodemes. Within the range of one lichen species, the mycobiont may associate with different photobionts from separate clades (O’Brien, Miadlikowska, & Lutzoni, 2013; Yahr, Vilgalys, & DePriest, 2006). Likely, this is because algae and cyanobacteria from different clades are more successful in different parts of a region and are either preferentially chosen by the mycobiont or are simply the most available for lichenization (Yahr et al., 2006).

Mycobionts may also lichenize ‘non-compatible’ algae as a means to survive until compatible algae are found. Most mycobionts come from the Ascomycetes, but research shows that a secondary fungal partner from the Basidiomycetes is imbedded in the peripheral cortex of many lichens (Spribrille et al., 2016). However, after a more recent study failed to find these Basidiomycetes in the majority of their lichen samples, lichenologists are still trying to understand when, where, in what abundance, and under what circumstances these tertiary fungal partners appear (Lendemer et al, 2019). Additionally, bacteria are known to live on and in lichens, and have been proposed as important symbiotic partners (Grube & Berg, 2009). Together, these many disparate parts create the whole organism of the lichen, sometimes referred to as the holobiont, which can function as its own miniature ecosystem (Hawksworth & Grube, 2020). The form of the lichen holobiont comes in three basic growth forms: fruticose lichens, which grow erect or pendant and have no discernable lower or upper surface; foliose lichens, which are flattened and have a recognizable upper and lower surface; and crustose lichens, which form crusts over a substrate and the lower surface of which has no cortex (a cuticle or skin-like structure) but rather comes into direct contact with the substrate. Fruticose and foliose lichens are also often grouped together under the term macrolichens. This differentiates them from the crustose, or microlichens, whose identifying structures are generally not visible with the naked eye. As they usually lack easily recognizable features and require more microscopy work, they are more difficult to identify to species. For this reason, many studies that are limited by time or funding focus only on macrolichens.

Lichens on Alpha, Beta, and Gamma Scales The scale at which lichen diversity is measured matters. For example, Humphrey et al. (2002), found 42% of lichen species only once in their plots, a phenomenon termed “local rarity”, but which they attributed to the insufficient size of their plots to “capture a representative sample”. In other words, this “local rarity” would disappear if the lichen diversity were sampled on a larger scale. Additionally, particular factors may affect lichen diversity differently at smaller versus larger scales. An example of an environmental factor that affects lichen diversity on different scales is humidity. With humidity, a pattern emerges in which moister and more humid regions and habitats have greater lichen diversity (Coyle & Hurlbert, 2016). However, when comparing different microhabitats within a given habitat, lichens in the moister and more humid microhabitats are more likely to be outcompeted by bryophytes (Boudreault et al., 2008). The above-mentioned example does not use ‘scale’ in the sense of a numeric distance, however. Distance in terms of meters or other similar measurement systems does not always make as much ecological sense as the environmental difference between microhabitats, habitats, and regions. It is therefore easier to explore patterns of lichen diversity by using the concepts of alpha, beta, and gamma scales. Some papers will describe alpha scale as differences on what might be termed a single microhabitat – for example, changes in lichen diversity between the canopy, trunk, and base of a single tree. Beta scale is then the changes in lichen diversity between different microhabitats within the same habitat. This study however will define alpha scale as changes between microhabitats within a habitat. Beta scale then refers to changes between different habitats, and gamma scale as changes across the study region. ‘Changes’ more specifically means increases or decreases in lichen species richness, environmental factors, or the interaction of the two.

Lichens in Peatlands In one bog, Pearson (1969) estimated that lichens contributed between 25-46% of the primary productivity. This suggests that lichens are of very high importance to peatland systems, yet the literature still reveals large gaps in our knowledge of peatland lichens, especially in Eastern Canada. The first of those gaps important to this study is the basic understanding of lichen diversity within peatlands and among different types of peatlands. The second is a better comprehension of the environmental factors that affect lichen diversity in peatlands. However, in order to understand lichens in peatlands, we must first start by understanding what a peatland is, and why they are important. In Canada, a peatland is a habitat with at least 40 centimeters of peat, peat being organic material decomposing in an anoxic environment (NWWG 1997). Peatlands are important because they store large amounts of carbon, between 41.5 and 489 Pg depending on the source consulted (Vasander & Kettunen, 2006). Additionally, peatlands make up 13% of Canada’s land cover, making them an important habitat on a national scale (Warner and Asada, 2006). Peatlands can also be split into many different categories. As fens and bogs had the highest overall percentage of total peatland studied in the three sectors, they are the most important categories for this study. The difference between the two is that fens receive groundwater as their main water source, while bogs’ main water source is from rain. Returning to lichens, the literature shows that bogs are more lichen diverse than fens across Canada (Warner and Asada, 2006).

On closer inspection, however, of the five papers cited in Warner and Asada (2006) for the mid-Boreal region (to which the Eeyou Istchee belongs) only one is from Quebec – the others are all from Alberta and its environs (Beilman, 2001; Chee and Vitt, 1989; Karlin and Bliss, 1984; Vitt and Chee, 1990; Garneau, 2001). Additionally, lichens are either not considered or included peripherally to other objectives, and there are some difficulties interpreting the lichen data. Several write about the treatment of lichen nomenclature in their methodology, but then don’t report any lichen species, yet never state if this was because they never found any lichens or for another reason (Beilman, 2001; Chee and Vitt, 1989; Karlin and Bliss, 1984; Vitt and Chee, 1990). Garneau (2001) reports several species of lichens that are preferential to bogs, but there is no information on how this determination was made. 1.4 Lichens and Hydration Sources Like all other organisms, lichens need water in order to survive. Unlike other organisms, their high area to biomass ratio helps them to utilize hydration sources that would ordinarily be inaccessible (Gauslaa, 2014). In addition to rain, lichens are able to obtain hydration from humidity, fog, and dew. These different hydration sources take on different levels of importance for different lichen morphologies and habitats, however. Cyanolichens need liquid water in the form of rain or dew, while those with green algae as a photobiont are more likely to be able to utilize humid air (Gauslaa, 2014).

While foliose lichens appear to almost exclusively use rain as a hydration source, fruticose and alectorioid lichens can use dew and humid air to a far greater extent (Gauslaa, 2014). While humidity seems to be more important to lichens in shaded canopies, rain becomes more important at the top of the canopy and dew in forest gaps (Gauslaa, 2014). On the landscape, rain can be of greater importance on hilltops, while humidity gains in importance in ravines or northern slopes; dew gains precedence as a hydration source in the toe of the slope or on open land (Gauslaa, 2014). There is some interaction between the different sources of hydration of course. Humid air occurs after rain, and dew is more likely to form when the relative air humidity is high. Being able to utilize a greater range of hydration sources may be evolutionarily advantageous to lichens, but there are tradeoffs between rain, humidity, and dew. Lichens experience a phenomenon called suprasaturation depression, often shortened to suprasaturation. This means that when the lichen thallus reaches an internal water content above a certain amount – what exactly that amount is differs with the species – there will be a decrease in photosynthetic activation due to increased diffusion resistances (Lange, 1980).

The subsequent loss of photosynthetic activity can be detrimental to the lichen. While suprasaturation is common after rain, it is less common with dew and rare with humidity (Gauslaa, 2014). Despite the fact that it does not come with the, however, only 3-23% of “realized [photosynthtic] activity” occurred when lichens were hydrated from humid air alone (Cabraijic et al, 2010). This may be due to the fact that it takes considerably longer to become hydrated under humid air conditions. High water contents also were never documented under hydration from humid air alone, which might not have been enough for photosynthetic activity even though suprasaturation did not occur (Cabrajic et al, 2010). The study by Cabrajic et al (2010) did not test the tradeoff with suprasaturation, however. Additionally, in habitats or areas where high humidity is consistent and lasts for a prolonged period of time, humid air could still be a significant source of hydration. Cabrajic et al (2010) also hypothesized that humid air is important in extending the hydration period after a rain.