“There is a low mist in the woods–It is a good day to study lichens.” Henry David Thoreau, A Year in Thoreau’s Journal: 1851.
Above: Brownish monk’s-hood lichen (Hypogymnia vittata) on a mossy rock wall in an old-growth forest, eastern Norway.
Lichens all share a common “lifestyle” – whether you call it a symbiosis, parasitism, a collective of productive fungal farmers, or teams of brilliant algal architects, this lifestyle has no doubt been a successful strategy for survival. From miniscule Arctic extremophiles growing within solid rock to conspicuous meters-long cascading strands of Methuselah’s beard (Usnea longissima) festooning a veteran conifer canopy, lichens’ beautifully sophisticated biological machinery has allowed colonization of virtually every terrestrial habitat on Earth. Over their quarter of a billion year evolutionary story, they have developed the tools to thrive on every continent: from the inter-tidal to alpine zones, and from aquatic to desert habitats.
Cover image article: (Free to read online for a year.)
Phinney, N. H., Ellis, C. J., & Asplund, J. (2022). Trait-based response of lichens to large-scale patterns of climate and forest availability in Norway. Journal of Biogeography, 49, 286–298. https://doi.org/10.1111/jbi.14297
Unlike vascular plants, lichens are “poikilohydric,” meaning they are unable to actively regulate their water uptake and storage. To cope with a diverse range of environmental demands, these often alien-looking organisms have developed striking variation in physiology, anatomy, morphology and architecture. These “functional traits” represent direct operative links to their environment at both micro-and macroclimate scales, offering a window into each thallus’ unique survival scheme. Documenting the intimate relationship between lichen and environment is crucial in improving our understanding of how lichen communities assemble in nature and how they adapt in a changing climate.
With a diverse and dramatic landscape and climate – from boreal rainforests to alpine heathland and arctic tundra – Norway is an impeccable biogeographic laboratory for such investigations. Here, we were encouraged to ask questions about why certain lichen groups occur where they do. We used likely drivers, such as precipitation, temperature, and forest cover to predict current distributions of traits. In doing so, we found that some traits, such as types of photobionts (photosynthetic lichen components, i.e., green algae, cyanobacteria, or both), appear to respond well to broadscale environmental filtering, making their distributions reasonably predicable. Lichens with cyanobacterial photobionts (cyanolichens) and those with green algal partners (chlorolichens) have unique physiological tolerances that restrict them to a certain climate space. For example, the majority of cyanolichens are found in oceanic habitats in Norway but become scarce in the high Arctic. Why? Unsurprisingly, because this group requires liquid water for photosynthesis, so their affinity for the rainy, western coast makes sense. On the other hand, many chlorolichens are connoisseurs of non-rain water sources, such as fog or high humidity – a secret Thoreau seemed privy to. Some chlorolichens (i.e., trebouxioid) can even maintain activity in temperatures well below 0° C and, lo and behold, they are liberally scattered throughout a seemingly inhospitable Arctic “wasteland”.
But are some trait distributions more predictive than others? Growth form, for instance, while being intimately coupled to environmental stressors, shows relatively weaker relationships to climate at the macroscale. Perhaps these categories – although widely applied – are actually too broadly-defined to consistently represent meaningful functional relationships to their environment: growth forms contain a massive amount of physiologically relevant variation, which cannot be adequately captured in such a sweeping generalisation. Even within single tree canopies, hair or beard lichen thalli, for example, can show considerable variability in their morphology, despite being in the same growth form category. Trait categories can also be nested within or interact with each other so that their unique effect may be masked as part of a complex mosaic of traits. In future studies, we hope to discover how combinations of traits might contribute to an ecological response, as any given combination might dictate an organism’s fitness and, ultimately, explains how they distribute across space and time.
Although often overlooked, lichens are clearly key players in the ecosystems around us. They modify climate, provide food and habitat for both micro- and macrofauna and are informative environmental and bioclimatic indicators (not to mention they are exquisite champions of evolution). By better understanding the mechanisms by which lichens operate at the thallus level and how large-scale climate patterns drive their growth and distribution, we can more accurately predict how lichen communities will change in a drastically warmer northern climate.
Nathan H. Phinney: Postdoctoral researcher, Dept. of Biological Sciences, University of Bergen, Norway
Many thanks to the coauthors, Christopher Ellis and Johan Asplund for the comments on the above text and for joining me on this fun lichen ride.