Population biology of the clonal moss Hylocomium splendens in Norwegian boreal spruce forests. 5. Vertical dynamics of individual shoot segments

Demographic information was obtained for 12528 mature segments (for which dry weight was estimated and vertical position in the bryophyte carpet recorded) and 3109 regenerated growing points for the perennial clonal moss Hylocomium splendens, recorded in Norwegian boreal spruce forests during a 6-year period. Branching frequency varied with vertical position in the bryophyte carpet. Termination risk (probability of producing no offspring) was highest (44%) for buried segments, lowest (12%) for segments at intermediate vertical positions, and also high (26%) for emergent segments (due to increasing exposure to external mortality agents). Segment size increased from low levels in the bryophyte carpet to a maximum ca 2-10 mm below the top of the bryophyte carpet. This intermediate level was interpreted as the optimal compromise between incoming radiation (attenuating downwards) and microclimatic moisture conditions (improving downwards). Size-corrected fitness, the number of offspring emerging From a mature segment within one year after maturation after allowance for differences in size, was lower for buried and emergent segments than for segments at intermediate positions. Small emergent segments were apparently liable to suffer from vitality reductions due to desiccation. The vertical position of a daughter segment depended on that of its parent segment, but also showed considerable stochastic variation. Burial acted as a strong sink for small segments regardless of vertical position. No evidence was found for species-specific differences in the way pleurocarpous bryophytes interact, but reduced Vertical mobility of H. splendens when growing among acrocarps indicated that growth-form is an important determinant of bryophyte interactions. Evidence was found for vertical layering of the bryophyte carpet according to dominant type of interactions among individuals: none (environmental stress) above and at toe, facilitation [a (+, +) interaction] at intermediate levels because of favourable water relationships in closed stands, and amensalism [a (0, -) interaction] from higher-situated segments that deprive lower-situated segments access to light at lower relative levels. The intensity of amensalism increased downwards in the bryophyte carpet as indicated by a reinforced size hierarchy. The tendency for small H. splendens segments to become buried and lost from the population by amensalism is likely to represent a general mechanism for interactions between bryophyte species and succession in bryophyte-dominated stands. Population effects of climatic and local environmental factors (favourability vs stress), disturbance and apparently random events are discussed with reference to their impact on the relative sizes of subpopulations acting as sources (due to facilitation) and sinks (due to amensalism).