How Mosses Survive Extreme Drought

Mosses belong to the bryophytes, an ancient lineage of non-vascular land plants that lack the specialised internal plumbing—such as xylem and phloem—found in higher vascular flora. Because they possess neither true roots to draw moisture from deep underground nor thick waxy cuticles to seal water inside their foliage, their internal moisture levels fluctuate in direct accordance with the surrounding atmosphere. In botanical terminology, this condition is known as poikilohydry. While a sudden drop in humidity causes most garden plants to wilt, suffer irreversible tissue damage, and eventually perish, many moss species can lose virtually all their free cellular water and yet survive indefinitely in a dormant, air-dry state.

This remarkable capacity is known as desiccation tolerance, a trait that distinguishes poikilohydric mosses from drought-avoiding plants like desert succulents, which store large internal reservoirs of liquid. When ambient humidity plummets, the metabolic machinery of desiccation-tolerant mosses winds down systematically until respiration and photosynthesis halt altogether. In this suspended state, often termed anhydrobiosis, the plant can endure extreme thermal variations and intense solar radiation. Upon the arrival of moisture—whether in the form of rainfall, melting frost, or merely dense morning fog—the dried tissues rehydrate within minutes. Physiological activity resumes with surprising speed, with photosynthetic carbon fixation often detectable within an hour of wetting.

At the microscopic scale, surviving near-total dehydration requires an intricate suite of biochemical defences to prevent cellular collapse. As water departs, cellular components are subjected to immense physical tension and the risk of lethal crystal formation. To counteract this, tolerant mosses accumulate high concentrations of non-reducing sugars, predominantly sucrose, alongside specialised stress-responsive proteins known as late embryogenesis abundant (LEA) proteins. Working in tandem, these molecules facilitate a physical transition called vitrification, wherein the remaining fluid in the cytoplasm solidifies into an amorphous, glass-like state rather than forming sharp, damaging ice or solute crystals. This biological glass immobilises cellular structures, immobilising enzymes and safeguarding membranes against mechanical shearing.

Curiously, the most perilous phase of a moss’s drought cycle is often not the drying itself, but the subsequent influx of water. Rapid rewetting can cause sudden osmotic shock, leading to membrane rupture, while the sudden restart of electron transport chains in chloroplasts generates harmful bursts of reactive oxygen species (ROS). Left unchecked, these chemically aggressive molecules inflict severe oxidative damage on cellular lipids, proteins, and genetic material. Resilient mosses mitigate this hazard by maintaining constitutive pools of antioxidant compounds—including glutathione, carotenoids, and ascorbate—as well as specialised enzymatic scavengers like superoxide dismutase. These defences neutralise toxic by-products before cellular repair mechanisms can complete their work.

Complementing these cellular safeguards are sophisticated morphological features that help mosses manage moisture physically. Many arid-zone species possess specialised hyaline hair points, or awns, projecting from the tips of their minute leaves. These translucent structures serve multiple functions: they reflect excess ultraviolet and visible sunlight, trap humid air close to the plant surface, and actively collect microscopic droplets from morning mist. Furthermore, as drying occurs, individual leaves frequently curl tightly against the stem or twist in tight spirals. This collective shrinkage reduces the total exposed surface area of the moss cushion, creating a stagnant boundary layer of air that substantially slows the rate of water loss.

In arid and semi-arid environments, desiccation-tolerant mosses play an indispensable role as primary components of biological soil crusts, or biocrusts. By anchoring loose sand and silt with their rhizoids—filamentous root-like structures—they create a cohesive surface layer that prevents severe wind and water erosion. Additionally, several desert mosses live in close association with nitrogen-fixing cyanobacteria, thereby enriching nutrient-poor soils and facilitating the colonisation of other plant species. Field observations indicate that disturbances to these moss-dominated crusts can destabilise entire local ecosystems, as the fragile biological mantle often takes decades to recover from mechanical damage caused by vehicles or livestock.

The extraordinary durability and physical properties of mosses have long attracted human interest. During major military conflicts in the nineteenth and early twentieth centuries, particularly during the First World War, peat moss was harvested extensively across northern regions to serve as a surgical dressing for wounded soldiers. Its exceptional capacity to absorb liquid—holding up to twenty times its own weight in water—combined with naturally occurring antimicrobial compounds made it an invaluable alternative to scarce cotton supplies. In contemporary science, researchers are investigating the biochemical mechanisms of moss desiccation tolerance to develop novel techniques for preserving biological materials, such as pharmaceuticals and fragile vaccines, without relying on continuous refrigeration.