Tree Trunks as Hosts for Other Plants
The trunks and branches of ancient trees in damp areas of the British Isles are often festooned with flowerless plants of intricate shapes and subtle colours.
Look at the trunks and branches of different trees in a forest or a dense wood, and you will notice that the bark surface is often covered with mosses or lichens. Look closer, and you will see that these growths are very small and beautifully coloured flowerless or ‘lower’ plants.
Their existence, when you think about it, is quite remarkable for tree trunks are continually shedding both their outer bark and the plants that live on it. Tree trunks are also dry places, so you might imagine that few plants could live on them. Nevertheless, many plants have evolved to cope with tree trunk living, probably to escape competition from the undergrowth on the forest floor.
These plants are collectively known as ‘epiphytes’. Most plants draw up food and water from the, but epiphytes absorb their nutrients from the air around them; the lushest growths of epiphytes are therefore found on trees in the humid western parts of the British Isles where the air is often saturated with water vapour from fog or rain.
Delicate mosses and leafy liverworts form green mats on the relatively moist bark near the tree base; their minute leaves are often pleated or rolled to trap water. In the drier air higher up the trunk and on the branches, compact cushion-forming mosses-with upright stems (like a thick pile carpet) which trap water between them-are more common.
Lichens are either crustose and pressed to the bark or foliose – forming strange branching outgrowths. They festoon branches in woods of the West Country in particular, but most of them are too sensitive to pollution and aridity to thrive in many areas of the British Isles. Like mosses, they too absorb water over their whole surface, and anything dissolved in the water is absorbed with it, including pollutants such as sulphur dioxide and carbon monoxide produced by burning fuel or traffic fumes. In heavily polluted areas Pleurococcus, a tough microscopic alga which tinges tree trunks a dirty green colour, is the only epiphyte to be found.
Where epiphytes are abundant it is interesting to note how different species grow on different bark types. The trunks of old trees grow relatively slowly, giving epiphytes an opportunity to gain a strong foothold. You see many in the deep fissures of oak and poplar in particular, but only rarely in the rapidly flaking bark of sycamores.
On areas of damaged bark there is often a distinctive epiphyte flora because the tree is exuding nitrogen-rich substances. In the rain tracks running down the barks of beech and elm trees, epiphytes that can withstand the forceful water flow benefit from the extra nourishment such as bird droppings dissolved in water. Some rare mosses grow in the axils of large branches. If dust and humus collect there, then even non-epiphytic plants like ferns may grow.