Epiphytes and ancient trees
Well-lit surfaces of bare wood (exposed by damage) and of bark provide suitable surfaces for the growth of specialist plants and micro-organisms – mosses, liverworts, lichens, algae and micro-fungi. The gradual build-up of these epiphytic (‘on the plant’) species into recognisable communities is a very extended process of colonisation, growth and reproduction over decades and even centuries before it culminates in the fullest diversity that can be attained in Britain and Ireland.
Thus, the richest sites tend to be those with the oldest trees but species-richness depends also on local conditions, particularly sunlight, humidity and shelter. Owing to the need for sunlight, it is the large old open-grown trees that support the richest epiphyte communities, rather than trees in closed-canopy woodland. Also, air pollution is a critically important negative factor, since it is poorly tolerated by most epiphytes.
During the slow development of epiphyte communities, there are changes in their species-composition as they progress towards maximum diversity, as can be seen by comparing two year-old twigs, older branches and old trunks. Also, communities on smooth-barked trees are different to those on trees with heavily furrowed bark, since the characteristics of different epiphytes (especially growth habit and life cycle) influence their ability to succeed on different kinds of bark in the long term.
The Lobarion is the famous community of ancient wood pastures, where it occurs on the trunks and major boughs. It comprises the spectacular foliose species such as the tree lungwort Lobaria pulmonaria and rarer closely-related species such as Lobaria virens. The reddish liverwort Frullania tamarisci is a good indicator of the community as it is often very visible from a distance. The Lobarion is confined to large old oak, ash and beech over much of lowland Britain, but the further west one travels it can increasingly be found on younger trees. In the east, where air pollution is worse, it is also generally less widespread than in the west. Even with reduced air pollution in eastern woods, it will take a very long time for this community to return.
Ancient trees and their lichens need active management:
- Keep ivy under control. Too much can shade out the lichen interest;
- Keep slurry, dung and artificial fertilizers away from the trees as these
materials all kill lichens;
- Site intensive livestock units, and slurry and dung stores, well away
from ancient trees;
- Consider tree work to prolong the life of the trees, where considered desirable.
Atmospheric pollution, combined with the loss of ancient trees, has all but led to the extinction of these beautiful organisms across most of lowland Britain.
This is the other classic ancient tree epiphyte community and is normally only found on trees at least 250-300 years old, and is mainly known from old pollard oaks, whose bark has become dry and brittle with age. The community is named for the major species in southern Britain … the crustose Lecanactis premnea. It is truly the ancient tree lichen community!
Francis Rose Ancient British woodlands and their epiphytes (British Wildlife 1993 5: 83-93)