Macrolichen abundance at the species level in 42 sites across a subarctic (56°N) to arctic (62°N) latitudinal gradient in Nunavik (Québec, Canada). Data was collected in the dominant vegetation types of six regions sampled along this gradient.

From south to north, we visited Clearwater Lake (CL; 56°20'N, 74°27'W), Boniface River (BR; 57°45'N, 76°10'W), Le Roy Lake (LL; 58°29'N, 75°28'W), Payne Lake (PL; 59°32’N, 74°63'W), Chukotat River (CR; 61°18'N, 75°55'W) and Deception Bay (DB; 62°05'N, 74°16'W). For each region, the most dominant vegetation types were identified according to their land surface cover, based on the 2018 Québec Northern Vegetation Map produced by the Ministère des Forêts, de la Faune et des Parcs (MFFP) of the Government of Québec. Three sites in each of the dominant vegetation types were randomly selected in a radius of five kilometers around each region’s research station prior to fieldwork (Figure 1b). Minimal distance between sampling sites was set to 250 meters. For each polygon of the vegetation map, a 250 m-wide buffer zone was excluded to ensure that sampling was conducted in the central section of the polygon. The final site selection consisted in a total of 42 sites and included three sites of each of the following dominant vegetation types: lichen woodland, open subarctic heathland (thereafter open area), and shrubland in Clearwater Lake and Boniface River (the two southernmost regions), open area and shrubland in LeRoy Lake and Payne Lake, herbaceous tundra in Chukotat River, and herbaceous tundra and shrubland in Deception Bay.

For each of the study sites, a 30 m x 40 m quadrat was positioned in a representative portion of the site. Twenty 1 m-long linear surveys were randomly positioned along parallel transects placed every two meters along the largest (40 m) side of the quadrat . Macrolichens (fruticose, foliose and squamulose lichens) were identified to the species level according to the floras of Brodo, Sharnoff, & Sharnoff, (2001), Hinds & Hinds (2007), and Thomson & Brehmer, (1984). Crustose lichens were not identified. Terricolous and saxicolous lichen cover was measured by species for each 10 cm segment along every 1 m-long linear survey. The measurements made on these ten segments were summed to obtain a precise estimate of each species cover to the nearest centimeter on the 1-m long survey. Species for which cover was < 1 cm were assigned a cover of 0.5 cm. Vegetation cover was also evaluated for the following plant functional groups: erect shrubs (> 15 cm-high), prostrate shrubs (< 15 cm-high), graminoids, forbs and bryophytes. For each 1 m-long transect, we measured the height and evaluated the canopy closure of erect shrubs, using the following cover classes: 0-25%, 25-50%, 50-75%, and 75-100%. Soil organic layer thickness was measured at the center of every survey transect and five soil samples were collected systematically at every site. Soil pH was evaluated using the mean pH value of the five soil samples collected during the field campain. Soil samples were first dried in paper bags at room temperature during the field campain and soil pH was measured using a pH-meter by suspending soil material in distilled water with a 1:1 ratio once back at Laval University (Eckert & Sims, 1995). For each site, we also summed the cover of each vegetation functional group measured in the field and calculated the mean value of the soil organic layer thickness.

Cladonia arbuscula Wallr. and Cladonia mitis Sandst. were both identified as C. mitis, since the PD (phenylethylenediamine) spot test necessary to distinguish both species was not performed during fieldwork. Cladonia sulphurina Michx. and Cladonia deformis Hoffm. were also recorded together as C. sulphurina, since the distinction between these two species is mainly based on the presence of squamatic acid revealed by UV light, which was not used during fieldwork. Cladonia borealis Stenroos and Cladonia coccifera Willd. were both identified as C. borealis because they are morphologically very similar. For the same reason, Cladonia stygia Ruoss and Cladonia rangiferina Wigg. were pooled. Cladonia trassii Ahti and Cladonia stricta Nyl. were also pooled because taxonomic distinction between the two species is still unclear. Finally, Stereocaulon species were only identified to the genus level due to the unclear taxonomy of the genus (Lavoie et al., 2020). Other specimens that could not be identified directly in the field were collected and brought back at the Louis-Marie Herbarium (Laval University, Québec, Canada) for further identification using dissecting microscopes and chemical spot tests. The cover of primary thallus of Cladonia species was measured but all specimens were recorded as Cladonia spp. and were therefore not included in species richness analyses. Crustose lichens cover was measured but species were not identified, and thereby only included in the abundance analyses.