Zygodon forsteri Mitten 1851

This moss tends to be found in association with older trees that have cavities in exposed roots, or where branches have broken away. These can catch rainwater and fill up with dead leaves. The water trickling out of these reservoirs forms a seepage track which can persist for years. In wet autumns, the moss can form large colonies, but in a dry summer, these can, apparently, disappear rapidly. The trees favoured by the moss are usually ancient beech pollards growing in a well-lit woodland. The practice of pollarding has become less common since the turn of the twentieth century, but has enjoyed a recent revival as the conservation benefits of this form of management have been realised.

Knothole moss is listed in the UK Biodiversity Action Plans and also included in English Nature's Species Recovery Programme. As it is rare it is vitally important to conserve the remaining colonies of the moss and, where possible, increase the populations.The sites where the moss is known to grow are now all within Sites of Special Scientific Importance (SSSIs). A management plan exists for conserving the species and is available to all managers of these sites. Surveys are also taking place to establish the true extent of this species and whether colonies can be grown in culture for potential re-introduction programmes.

Knothole moss is a small moss, and forms dark green cushions on tree trunks or exposed roots. It resembles other Zygodon species and the best identification for the non-expert eye is the dark, shiny, almost blackish-green colouring.

Knothole moss has very precise habitat requirements. It grows only in the raintracks on beech trees growing on acid soils in open, well-lit sites. Very occasionally it has been recorded on other species of tree. The raintracks are not fed with water from the canopy of the tree, but from pools of water held in a cavity in the exposed roots or truck of the tree. Colonies typically, grow around wound tissue such as knotholes giving the moss its common English name.

This species is widespread but rare across most of Europe. In Britain it is now restricted to three locations, the New Forest, Burnham Beeches in Buckinghamshire, and Epping Forest in Essex. Of these colonies, the one at Burnham Beeches is the largest, with much smaller populations at the other sites where it is restricted to a handful of trees.

Classified as Endangered in the UK, fully protected under the Wildlife and Countryside Act Schedule 8. Classified as Vulnerable in Europe.

Whilst never common, this moss is threatened by the felling of its host trees, possibly through safety considerations. The famous 'hurricane' of October 1987 that tore through southern England, also felled several host trees in Epping Forest. Another long-term issue is creating replacement pollarded trees for the future. A continuity of suitable trees is essential for the moss to survive at any one particular locality. Another, rather ironic factor, is the increase in competing species of moss which have benefited from the reduction of acid rainfall. These other mosses are suspected of crowding out the knothole moss. Moss collecting is another possible threat as a small colony of knothole moss could be removed completely by an irresponsible collector.

Zygodon forsteri là một loài Rêu trong họ Orthotrichaceae. Loài này được (Dicks. ex With.) Mitt. miêu tả khoa học đầu tiên năm 1851.