Are lichens biocompasses? Revisiting an old prediction using new techniques

Abiotic factors influence not only vertical distributions of epiphytes, but also their radial positioning, such as around tree trunks. Due to planetary patterns of solar incidence, the equator-facing trunk portions receive direct sunlight, creating restrictive microclimatic conditions. Conversely, pole-facing portions remain shaded, providing optimal conditions for some epiphytes. Since trunk microhabitats are continuous, the distribution of epiphytes involves directional (compass-derived) data, requiring circular analytical methods. However, such assessments remain unexplored for avascular epiphytes. In the southern hemisphere, we investigated radial distribution of all lichen crustose and foliose thalli occurring on concentric trunks of a Syagrus oleracea population. We hypothesized that: (1) both lichen types will orient southward, the optimal zone; (2) lichen types do not differ in preferred direction, indicating a stronger microenvironmental than interaction filter; and (3) other variables (height, phorophyte size, canopy cover) do not affect orientation, which is primarily governed by azimuth. We address these questions using circular statistics, including preferred direction tests, between-sample comparisons, and linear-circular regressions. We surveyed 845 lichen thalli (180 crustose, 665 foliose). The lichens were non-uniformly distributed and oriented southward, with similar distributions. Radial orientation was unaffected by height, canopy cover, or phorophyte size, confirming azimuth as the primary determinant of orientation. These results demonstrate how abiotic conditions, driven by planetary patterns, indirectly determine lichen niche occupation on a fine radial dimension of trunks, offering new insights into epiphyte ecology.