Spotlight | Bryophytes and the Boundary Layer

30th November 2023
by Peter Morton

In this species spotlight I will not be focusing on just one species, rather touching on an entire family of nonvascular land plants, the bryophytes. This family includes mosses, liverworts, and hornworts. Mosses are something that I suspect most of us are aware of and often admire but know so little about.

Through meeting the County moss recorder for Buckinghamshire my interests were piqued. I purchased a fascinating book on the subject written by American Distinguished Teaching Professor of Environmental and Forest Biology, Robin Wall Kimmerer called ‘Gathering Moss’. This series of essays delves into the subject from a unique point of view and puts forward some insights into the nature of bryophytes. One really fascinated me.

Mosses are a primitive form of plant, which I suspect you did know. They are believed to be the first form of flora to etch out an existence on land, back when the world was a very different place. Algae were the dominant form of life in the seas and as the atmosphere changed and the ozone developed, blocking out UV, an opportunity arose, and the land was for the taking.  Terrestrial forms remained small because they lack the hygroscopic fibrils of cellulose and lignin, qualities which give other plants a strength, stability and hydrology that allows them to grow to sometimes staggering heights. Having risen from an aquatic existence, bryophytes set about their occupation of the globe using the building blocks they had at their disposal. These limitations define them but make them so tenacious.

For mosses to survive and thrive they need a set of certain conditions, defined by the presence of water. For moss to reproduce the male and female flowers must be connected by water. They need it to resist dehydration through their often-thin cell walls and for their cells to absorb dissolved nutrients. Because of this defining characteristic and link to their early aquatic existence they often inhabit the dark and damp corners of the world, but it’s not quite as simple as that.

There is a place, between earth and sky, known as the boundary layer. It is the all-encompassing point where everything solid ends and the air begins, which is in a constant state of reaction, dancing and swirling around everything it touches. Think of a time when you stood in a meadow on a breezy summer day, the air constant and gentle flowing around you, this is known as laminar flow. if you were to take a seat, now lower down, the steady breeze would be baffled and almost broken as the currents graze the meadow below and vortices form, creating waves atop the dancing blades of grass. This is known as turbulent flow. Now picture laying back and rest your head close to the ground – the breeze has all but stopped.  That is the boundary layer. Down at this level the air must work its way through every leaf and every stem and each time it does, so it swirls and slows; much in the same way a brook babbles around boulders. This turmoil in the air creates a special set of conditions. Free from the drying winds where warmth, humidity and rich air can mingle and play, this is where the mosses make their homes.

Everything everywhere has a boundary layer. Relative to the environment around, this layer can dramatically change. In a vast woodland where proud trees stand with countless branches holding the winds aloft, the broad boundary layer below can provide fine accommodation for some of our largest mosses reaching upwards of 10cm in size. Whereas on a wind battered mountainside or coastal cliff only the smallest and most stubborn mosses can survive hiding behind boulders from the wind or tucked in cracks on the rocky outcrops and only being a few millimetres tall. Their persistence doesn’t end there though – bryophytes’ love of the boundary layer has shaped their form and function. Their stems, leaves and proclivity for huddled communities helps shape the flow of air round and through them.

They have become masters of this environment, almost to the point of detriment. The air within them can fall so still that it would be almost impossible to set seed and spread forth new generations. However, this is a trivial matter when you understand air like mosses do. By growing setae which stand proud above their micro canopy the mosses can bring their microscopic spores up into the turbulent zone and cast the next generation to the winds where they will roam and settle, sometimes waiting up to 50 years for conditions to become suitable for germination.

Globally there are over 22,000 known varieties of bryophyte with over 1000 in the UK. They occupy every continent and nearly every niche imaginable.  They texture landscapes and create microcosms; they have grown constant and unyielding through millennia, by-standing extinctions, ice ages and the rise and fall of empires. These ancient archetypes are not just remnants of what was, but also champions of what is. Next time you find yourself out and about I recommend taking a moment in the boundary layer and admiring these underappreciated floral forefathers.

25th February 2024
by Pete Morton