Is 'Life-Form' a Useful Concept in Bryophyte Ecology?

Bryophytes are a successful group of non-flowering land plants that mostly avoid competition with tracheophytes. Many bryophytes grow on substrata that are impenetrable to tracheophyte roots and tolerate frequent desiccation and poor conditions. Most species exhibit clonal or colonial life-forms. These rather than the individual shoots or thalli are the ecologically functional units. Approximately eleven distinct life-forms can be recognized although the schemes of no two authors agree. Bryophyte life-forms can be interpreted as recurring arrangements of the photosynthetic tissues that minimize evaporative water loss and maximize primary production. However, other factors such as prevention of photoinhibition and scavenging of cloudwater may also be important. Many species show plasticity of life-form according to environmental conditions. Aerodynamically competent colonies are probably fashioned passively because protruding individuals are more prone to desiccation than those remaining within the laminar boundary layer. Alterations of density and shoot and leaf orientation appear to be regulated by phytochrome-mediated growth responses to shading. The different life-forms can be arranged in sequences reflecting water availability and light intensity in different habitats but fewer categories are represented on soils than on hard substrata because of negative interactions (stress and disturbance) with tracheophytes. Despite the strong correlation of life-forms with moisture and light conditions, there are limitations to their use as a framework in ecological studies of bryophytes. Different categories of life-form have been loosely applied and an improved system based on objective criteria is required. Also, life-forms have not evolved independently of other attributes of life-strategy. Colonial life-forms confer a competitive advantage in pre-empting space in the patchy habitats occupied by bryophytes; moreover some types, like 'dendroids', may be specialists at capturing space occupied by other species. Life-forms may sometimes also be interpreted, especially in the more highly productive species like Brachythecium rutabulum. as adaptations for foraging. Internal movement of nutrients from favourably. placed ramets may be beneficial to the competitive fitness of the whole colony. Life-forms and breeding systems in bryophytes are unlikely to have evolved independently as dioicous taxa tend to form unisexual colonies with low chances of fertilization. It is concluded that life-form is a useful concept because of the exceptionally high dependence of bryophytes on transient external water supplies; however, major advances in understanding will depend upon investigation of the relationships of life-form to other attributes of bryophyte life-strategy.