The Northern Hemisphere moss Tetraphis pellucida inhabits decaying wood, in particular coniferous wood. In its forest habitats this species may produce abundant spores and gemmae. Decaying wood is a rather unstable and relatively short-lived substrate, with fragments liable to fall off at any time to expose new, bare wood. Tetraphis pellucida is a very successful exploiter of such a habitat, with reproduction predominantly vegetative in low density colonies of this moss, but sexual in high density colonies
In low density colonies there is a good likelihood of much bare wood in the immediate neighbourhood. In such a circumstance reproduction by short distance dispersal of vegetative propagules would allow the moss to exploit much of that bare area fairly quickly. Once a colony becomes fairly dense there is much less chance of any exploitable space nearby. Hence vegetative propagules would serve little purpose, whereas the production of spores, allowing the colonisation of more distant substrates, would be a better policy. By the way, don't think of too "distant" a dispersal range for spore dispersal. In both laboratory experiments and field observations many spores were found to land within a few centimetres of their originating sporophytes, but field observations showed spores landing over 50 centimetres away from the originating sporophytes – and some spores possibly up to two metres away. Laboratory experiments showed that simulated rainfall or mechanical disturbance of the gametophytes could disperse gemmae to 10 centimetres. It is of course possible that in the wild gemmae could be carried further, perhaps washed downhill by surface water flow or picked up by invertebrates.
Studies have shown that Tetraphis pellucida spores are less efficient than the much larger gemmae at occupying exposed wood, with perhaps 1 in 800,000 spores producing a new plant whereas about 1 in 10 gemmae are successful. However, in its forest habitats suitable decaying stumps or fallen trunks are often spaced further apart than the distances the gemmae are able to be easily dispersed so spores still play a very important role.
Given the mix of vegetative and sexual strategies it would be possible for the Tetraphis colonies on two nearby stumps to be showing two quite different behaviours – one with a sparse colony producing abundant gemmae, while the other, densely covered, is producing abundant sporophytes. In fact sexually and vegetatively reproducing colonies may be found on the same stump, the colonies being fairly close together but not intermingling. Note also that since successful sexual reproduction demands the existence of a dense colony, it is clear that in this species successful sexual reproduction depends on an earlier success in vegetative reproduction.
While Tetraphis pellucida is a very successful exploiter of bare wood, neither spores nor gemmae allow the moss to invade an area already occupied by a well-established bryophyte colony. Hence it is critical that Tetraphis propagules quickly occupy bare wood before other bryophytes become established. The two methods, used in the way described above, give Tetraphis pellucida considerable flexibility in exploiting neighbouring as well as more distant areas. Its main competitors (the mosses Hypnum imponens and Brotherella recurvans and the leafy liverwort Bazzania trilobata) lack that flexibility. However Tetraphis pellucida is extremely vulnerable to competition and may be overgrown by bryophytes with a creeping habit and crowded out by species with a tufty growth habit. Tetraphis therefore relies on a reasonable level of disturbance in dead wood substrates for continued presence in an area. Such disturbances may have various causes – for example, a passing animal knocking a chunk of bryophyte-covered wood from a dead stump to expose a fresh surface or fungal decay within the stump finally being such that part of the stump is sufficiently weakened to fall away. A reduction in disturbance frequency would favour other bryophyte species. Taken to an extreme, if disturbance to dead wood stopped completely in some area, Tetraphis pellucida would disappear from that area.
It has been suggested that with increasing colony density it becomes easier for sperm to be carried from male to female organs by capillary action, and hence sporophyte production would become more likely. Another factor that may promote sporophyte production in dense colonies is the fact that the sex ratio is dynamic. In sparse colonies there is a bias towards female branches. With increasing colony density there is an increasing percentage of male branches. With increased sperm production there would be an increase in the chances of fertilization and hence of sporophyte production.
The bryologist Robin Kimmerer, who has studied Tetraphis pellucida extensively, has called the moss "...a sequential hermaphrodite, changing its gender from female to male as the colony gets crowded...".