Moisture and Mosses
Mosses are at their best when leaves are fully hydrated. The nuances of the shades of their verdant color vary among bryophyte types from neon chartreuse to brilliant emerald to deep, dark, almost black, depths of greens. Many bryophytes dramatically change appearance from their moist to dry states.
However, other mosses and liverworts may look similar with the naked eye in both dry and wet conditions. Since mosses absorb moisture quickly, this transformation can occur right before your eyes. The green chorophyll becomes more intense while robustness and size of the leaves increases as they unfurl when re-moistened.
Moisture is essential for the reproductive cycle of mosses. In the gametophytic stage, mosses have sex. The male sperm swims to the female egg for fertilization. In vegetative reproduction, the little gemmae get splashed out of cups to form their own new plants using water as a vehicle for dispersal.
Without a cuticle, and with only one cell layer, these non-vascular plants absorb primarily through their leaves. In addition, external water movement occurs with the substrate surface through a series of capillaries. Certain bryophytes have specialized transport cells. These cells (haploid and leptoid) could be compared to the internal systems (xylem and phloem) of vascular plants. Polytrichum moss species have rigid stems and possess these internal water transportation qualities similar to more complex plants.
Mosses can absorb water like a sponge but some only a little at a time while others, like Sphagnums, can absorb up from 20-30 times their weight. The absorptive properties of mosses allow extensive colonies to provide water filtration slowing down the rush of stormwater and giving it a chance to reach the soil. Mosses can reduce the impact of flash-flooding or heavy rainstorms as erosion control plants (Climacium and Polytrichum).
Although some mosses like to be really wet consistently, others may prefer to dry out. A considerable number of mosses will not tolerate constantly “wet feet.” In contrast, aquatic mosses can actually live under water. Fontinalis was recently found to be growing at 1000 ft depths in Yellowstone Lake near a geo-thermal vent.
Mosses absorb moisture and nutrients through their leaves. In other words, they drink rainwater and eat dust particles. Their entire sustenance is derived through this simple diet.
Although all mosses require some moisture to survive, needs can be met through living in microclimates that provide moisture niches. Crevices, cracks, nooks and crannies are desirable micro-habitats.
Mosses have botanical mechanisms for tolerating dormant, dry states. Some types even need to dry out between periods of heavy moisture.
In arid habitats, bryophytes and other cryptogams form what are called “biological soil crusts.” During intense and sudden downpours, they absorb water and hold the soil in place preventing or reducing erosion.
Resilient to Minor Pollutants
Since mosses can absorb water so easily through their leaves, correspondingly contaminants in the water particles including pollutants or undesirable amounts of minerals, could affect mosses either in natural settings or gardens. However, in this moss gardener's experience, supplemental watering, from typical urban sources with their array of chemicals, has NEVER harmed any of my moss installations. In my professional opinion, it’s okay to use tap water. If you feel the necessity of further precaution, you can add a water filter to your water source.
Of interest is a recent study about air pollution and mosses/lichens as indicators of environmental toxins. Scientists in Oregon were delighted at this new way to measure harmful pollutants. As a horticulturist, this tolerance of mosses indicates the amazing abilities of mosses to survive. Sure seems like another reason to sing praises for intentionally planting mosses to me!!!