To the casual observer, fuzzy-looking green growth on hard surfaces are mosses and all mosses are the same – understandably, it’s hard to distinguish small organisms beyond their superficial identifying traits, such as color, when viewed from a distance. Their uniqueness is hard to ignore on closer inspection – but they’re not just aesthetically distinct, different species are each equipped with specialised survival strategies as well. A guided tour by Professor Higuchi and Endo-san along the trails of the forests surrounding Shirakoma-no-ike (“Shirakoma-no-ike”) quickly revealed the diversity among these simple plants and dispelled some unanswered questions I had lingering from a trip to Yakushima’s Shiratani Unsuikyo.
I was keen to make another trip to a moss-covered forest after being captivated by the beauty of Yakushima’s Shiratani Unsuikyo earlier in 2016. With the absence of colourful flowers and wildlife, the spotlight was redirected to the moss-covered landscape. Nature had mastered the art of color-blocking there – moss-green, bark-brown and granite-grey, embellished with a hint of camellia-red. The viewing experience was thus simplified and hikers stood still to enjoy the tranquil environment. The rarity of moss-covered forests and the lack of flowers and wildlife naturally raised questions of why and how they came to be.
Photograph of a waterfall with moss-covered rocks and trees in Yakushima’s Shiratani Unsuikyo
It turns out that moss-covered forests aren’t all that rare after all. Although we catch only glimpses of moss in urban areas, they dominate the understory of temperate rainforests because it provides the perfect conditions for them to proliferate.
Like all plants, moss require water and sunlight to perform photosynthesis and smaller amounts of other nutrients to survive. Lack of a vascular system is the key factor that distinguishes mosses from other plants and limits its habitat options. Without roots that absorb water and nutrients from soil, and vascular tissues that transport the resources through its entire structure, most species of mosses are confined to damp and shady areas.
Temperate rainforests provide the optimal conditions for a wide range of moss species. The densely packed canopy of conifers let in just the right amount of sunlight – it blocks out enough sunlight so it doesn’t dry out mosses and makes life difficult for vascular herbaceous plants, yet lets in enough for mosses to perform photosynthesis. On the contrary, forests comprised of densely packed deciduous trees are much less suitable for mosses. While some species of mosses are able to grow through leaf litter to gain access to sunlight, many species are unable to do so and eventually die from lack of sunlight.
Photograph of a Northern Japanese Hemlock (Tsuga diversifolia) acorn and pine straw on a bed of Red-stemmed Feathermoss (Pleurozium schreberi) and Stairstep Moss (Hylocomium splendens).
Water is even more important for its survival. In addition to being a resource for photosynthesis, water contains the nutrients needed for growth and is vital to the reproductive success of mosses.
Photograph of ripples forming from afternoon showers in Shirakoma-no-ike
The diagram below illustrates the lifecycle of bryophytes, an umbrella term generally used for non-vascular land plants such as mosses, lichens and liverworts. Water is needed for sperm in the antheridium (male sex organs of mosses) to travel to the archegonium (female sexual organs of mosses) for fertilisation to occur, though moss-inhabiting microarthropods have recently been observed dispersing sperm as they move from one gametophyte to another.
Once fertilisation is complete, mosses enter the sporophyte stage of its lifecycle and produce haploid spores over the course of three to six months. Once the cap enclosing the sporangium is removed or falls off, the process starts again as the spores germinate and are able to attach itself to a wide variety of substrates, including those with hard surfaces such as rock or cement.
Photograph of Umasugigoke or Haircap Moss (Polytrichum commune) sporophytes and gametophytes in Shirakoma-no-ike
Shirakoma-no-ike is renowned as an important moss viewing site in Japan. No matter where you look, the forest understory is dominated by moss or other bryophytes, where tree trunks and large boulders serve as the most common forms of substrate. The forest is mainly comprised of coniferous species such as Northern Japanese Hemlock (Tsuga diversifolia) and Maries’ Fir (Abies mariesii), which are much more beneficial to moss than deciduous trees. Soil chemistry also plays a role in its successful colonisation of Shirakoma-no-ike. As a result of large amounts of volcanic ash that covered the area from a fairly recent (in the geological sense) eruption, the alkalinity of the soil increased and consequently became less hospitable for vascular herbaceous plants that would otherwise out-grow mosses. The presence of large boulders and a lack of a thick and nutrient-rich layer of humus further hinders the development of vascular herbaceous plants. Without the presence of competitors and natural predators, mosses were free to proliferate.
Photograph of moss-covered boulders and trees, standing and fallen, in Shirakome-no-ike
Despite being a haven for mosses, different moss species in Shirakoma-no-ike compete for finite resources as moss population peaks. Each species of moss has thus developed unique strategies to better acquire resources. Seitakasugi is a great example of a moss that has developed such strategies to increase its chances of survival. Its leaves are arranged so as to maximise exposure to sunlight and water. The tip of Seitakasugi male gametophytes are shaped concavely so when raindrops splash in the anthrodium, the sperm inside would be propelled further than if it had merely travelled to the neighbouring gametophyte through a running droplet of water.
Photograph of Seitakasugi (Pogonatum japonica)
Mosses have evolved through millennia to enhance their competitiveness but still strictly exist as epiphytes on trees, both dead and living. On the otherhand, wood decaying fungi that are commonly found inhabiting the same woody substrate as mosses acquire nutrients and energy by digesting the wood. Although there are no established mutualistic relationships between wood decaying fungi and moss, the latter indirectly supports the former’s efforts – wood decaying fungi require free water to release enzymes for the decay mechanism to occur and mosses help keep the surroundings moist.
Photograph of moss and fungi coexisting on decaying wood
Mosses also play other broader roles in its environment, including water retention and reducing erosion. Humans have long seen the merits of the plant and have made use of them. Traditionally, mosses were used for bedding, packing and even wound dressing. Although modern day uses are primarily for decorative purposes, a recent study showed the potential of using moss as bio-indicators of pollution.
From left to right, top to bottom: 1. Species of lichen known as Lobar linita; 2. Girgensohn’s bogmoss (Sphagnum girgensohnii); 3. Dicranum hamulosum; 4. Pseudobryum speciosum; 5. Species of liverwort known as Bazzania denudata; 6. Hypnum plicatulum; 7. Umasugigoke or Haircap Moss (Polytrichum commune); 8. Seitakasugi (Pogonatum japonica); 9. Stairstep Moss (Hylocomium splendens)
There’s much more than meets the eye with mosses and hopefully you’ll have a minute to take a closer look at those fuzzy-looking green plants the next time you’re in a temperate forest.