Plant Cuttings

Image of plate 72: Muscinae from Kunstformen der Natur by Ernst Haeckel (1904) is in the public domain.

Here at Cuttings HQ we are interested in all matters botanical, and all types of plants (and other photosynthetic organisms such as algae (Michael Guiry) and cyanobacteria), and fungi (Grace Brewer)). However, it is probably a fact that most of the blog items emanating from that august home of phytomiscellanea concern flowering plants. That’s a reflection of how much more prevalent stories are about those plants rather than any assumption about their greater importance in the grand scheme of things botanically. Still, as a general plant educator, it behoves Mr Cuttings to do his best and seek out stories about the under-represented groups of plants on this planet. Hence this blog post about moss (Ellen McHale).

In the context of the Plant Kingdom, moss is often described as a lower plant. That term is easily interpreted in a derogatory way to mean ‘of lesser importance’ than so-called ‘higher plants’, such as angiosperms. But, and quite apart from the difficulty in defining ‘importance’, that interpretation is misguided. Although mosses don’t reproduce by seeds, don’t have vascular tissues to transport food and water long-distances, nor have true leaves, stems, roots or flowers (e.g. here, and here), and are rather small (Elizabeth Humpert) – typically, 0.2–10 cm (0.1–3.9 in) tall – they are still marvellous, miniature photosynthetic organisms. Plus, they were present on the planet hundreds of millions of years ago (Robin Wall Kimmerer; Katie Field & Silvia Pressel, and long before flowering plants arrived (Ibrahim Sawal; Hervé Sauquet et al., Journal of Experimental Botany 73: 3840–3853, 2022; https://doi.org/10.1093/jxb/erac130).

If it means anything, since mosses appeared early on in the evolution of the Plant Kingdom, and therefore occupy a lower rung on that evolutionary ladder, lower indicates an acknowledgement of that position, not any assessment of their importance. Being more ancient than plants that evolved more recently (e.g. ferns, gymnosperms, and angiosperms), mosses consequently had an impact on the development of the land flora that followed their appearance. Furthermore, being lower – generally much shorter in height than ferns and flowering plants and therefore closer to the soil surface – moss is, and continues to be, in a position to have a direct impact upon the surface of the planet that they helped to colonise (Michael Marshall; David Beeson).

And interactions of moss with the soil is the thrust of the research undertaken by David Eldridge et al. (Nat. Geosci. 16: 430–438, 2023; https://doi.org/10.1038/s41561-023-01170-x)*. In summary, that international team of 50 investigators carried out “the most comprehensive global standardized field study to quantify how soil mosses influence 8 ecosystem services associated with 24 soil biodiversity and functional attributes across wide environmental gradients from all continents”. Beautifully brief, but a little bare for this blog’s readership; let’s put some flesh on those bones.

Ecosystem services (Reagan Pearce) are defined as: “the benefits provided by ecosystems that contribute to making human life both possible and worth living”, and include both direct and indirect contributions that provide for human wellbeing and quality of life. Scandalously anthropocentric, this definition nevertheless allows – and encourages – us to identify the many ways in which the natural environment supports the planet, and thereby helps to keep humans – and other living things – alive. Of the many ecosystem services that have been recognised to date, Eldridge et al. considered: soil biodiversity preservation; carbon sequestration [long-term storage of carbon (Noelle Eckley Selin)]; nutrient cycling; plant pathogen control; antibiotic resistance control; organic matter decomposition; microbial habitat; and biomass of symbiotic organisms.

Perhaps their stand-out discovery was that, globally, soil mosses potentially support 6.43 Gt [gigatonnes]** more carbon in the soil layer than do bare soils, i.e. “The amount of soil carbon associated with mosses is up to six times the annual global carbon emissions from any altered land use globally” (David Eldridge et al.). In other words, mossy areas*** are an enormous sink of carbon, which means that this material is not in the atmosphere contributing to further global warming (Sarah Fecht, Hannah Ritchie et al.). Preserving such mossy areas intact is therefore important in limiting excess CO₂ release into the atmosphere and thereby restraining increase in temperature from the associated global warming****.

Overall, David Eldridge et al. concluded that “Mosses are especially important for supporting multiple ecosystem services where vascular-plant cover is low”. This is not to say that other plants don’t contribute to ecosystem services [they do!], but underlines the hitherto largely-unappreciated role played by mosses in this most important regard.

In respect of soil biodiversity and functional attributes, Eldridge et al. examined: maintenance of soil biodiversity (specifically, the richness of fungi, bacteria, protists, and invertebrates); carbon sequestration (total organic carbon within the soil); nutrient cycling (soil totals of N [nitrogen], P [phosphorus], Cu [copper], Mg [magnesium], Mn [manganese], Zn [zinc], Fe [iron], and K [potassium] – all of which are essential nutrients for plants); organic matter decomposition indices (soil-located extracellular enzyme activities related to cycles of C [carbon], N, and P, content of glucose, and lignin, and basal respiration); microbial habitat (biomasses of fungi and bacteria); plant–soil symbiosis (biomass of arbuscular mycorrhizal fungi); antibiotic resistance control; and soil-borne plant pathogen control. Amongst their findings were that, compared to bare soils, soils beneath mosses are associated with greater carbon sequestration, pool sizes for key nutrients N, P, and Mg, and organic matter decomposition rates; and a lower proportion of [i.e. greater ‘control’ over…] soil-borne plant pathogens. All of which catalogue of moss-associated benefits is pretty impressive – and previously either unknown or underappreciated.

Not too surprisingly, the team’s data show that the contribution of mosses to several ecosystem services is significant, and global. Mosses may be small, but their influence is huge, and “likely to be associated with their well-known capacity to influence surface microclimates and their litter inputs compared with bare soils” (Eldridge et al.). In that respect, the team consider the contributions of mosses to several ecosystem services to be similar to those of vascular plants. However, and importantly, because moss-covered soil is found in areas of the planet where influence from vascular plants is limited, mosses played additional roles to those of vascular plants in supporting ecosystem services. Consequently, and as a result of that intensive and extensive study, the important contribution to soil life and function provided by mosses leads Eldridge et al. to conclude that these plants need to be conserved to support healthy soils. Which sentiment emphasises the oft-made point that size isn’t important, since, despite their comparative smallness, mosses ‘punch well above their weight’, and should never be … overlooked(!)*****.

* For more interpretation of that research, visit the SciTech Daily site, and the news item by Lachlan Gilbert.

** A gigatonne “is equivalent to one billion metric tons, 2.2 trillion pounds, or 10,000 fully-loaded U.S. aircraft carriers”, “roughly twice the mass of all of the people in the world”, or “1000 million tonnes = 10¹⁵ grams”.

*** Eldridge et al. (2023) estimate that mosses cover over 9.4 × 10⁶ km² of the planet, an area similar to Canada, China or the United States of America.

**** Having mentioned global warming and mosses together, it’s appropriate to say a little about work by Ruoyang Hu et al. (Cell 186: 3558–3576, 2023; https://doi.org/10.1016/j.cell.2023.07.003). Studying the moss genus Takakia (JM Glime) on the Tibetan Plateau (Diptarka Ghosh), they’ve found that it has the “largest number of fast-evolving genes under positive selection”. However, while this indicates rapid and successful adaptation to its high elevation environment, the team are concerned that it won’t be able to continue to adapt quickly enough to survive the environmental changes associated with climate warming. Or, as so well put by Hu et al., “Following nearly 400 million years of evolution and resilience, this species is now facing extinction.” For more on this research, see here, Ashley Strickland, and Jude Coleman.

***** Ed. – Ground-hugging mosses are clearly much more in touch with the earth – and in tune with the Earth – than us humans…