I came across this small desert on top of the wall of the bridge that carries Linton Road across Braybrooke Terrace (where the cars below are parked) in Hastings today.
I have watched plants like this colonise bare stone. Often they start as the tiniest pieces along a seam or crack that retains a little more water than areas nearby, then spread out over a few years to form small cushions. Eventually they may join up and make a thin layer of soil where vascular plants can get a foothold and in no time at all you have a forest.
The cushions even at the stage they are at in the picture are often well populated with fauna such as springtails, nematodes, black fungus gnat larvae and the larvae of the parthenogenetic midge
Bryophaenocladius furcatus. All these must be able to withstand long periods of desiccation when the moss cushions dry up in summer.
I have been reading
Animate Earth by Stephan Harding (2006) and the following passage on life during interglacial periods seemed to be illustrated by these mosses: "Plants grow well in the new high carbon dioxide atmosphere. They send their roots deep in search of nutrients, cracking open rocks with sheer brute force and with the subtle but relentless dissolving powers of their acidic chemical exudations. One can almost hear the gentle grinding noise of the increased weathering as plants all over the planet pummel and pulverize the rock, releasing nutrients on a scale unknown during the time of ice. Myriads of phosphorus, iron, silicon, calcium atoms are captured by plant roots to be sucked up into the growing green biosphere which, in its heedles growth, draws out more and more carbon dioxide from the atmosphere."
On 20 December I walked in the frost in Hargate Forest south east of Tunbridge Wells. It is an extensive area of pine, broad-leaved wood and heath with some valley mires, streams and a gill. Well worth a visit at any time of year and free access as it belongs to the Woodland Trust.
