Substratum preference, spore output and temporal variation in sporophyte production of the epixylic moss Buxbaumia viridis

Occupancy and sporophyte numbers of Buxbaumia viridis on patches of decaying wood were investigated during a 4-year period (1996-99). Temporal variation in sporophyte number was investigated in relation to precipitation. Spore number per sporophyte was counted and spore number per substratum patch area and forest area were calculated. To predict the occupancy of B. viridis on decaying logs and stumps in a forest, the patch size (suitable wood area) was the most important variable. Decomposition stage was included in the model as a quality factor, expressing the higher and more stable humidity in late wood-decay stages. The suitable wood area of each patch was determined first and foremost by the stage of decomposition and diameter of the decaying wood. Only 16% of all wood patches considered suitable for production of sporophytes were occupied., The probability of a patch being occupied increased linearly with patch size to approximately 7 dm(2); above this size the probability of being colonized was close to one. Both the number of sporophytes and the number of occupied patches were correlated with precipitation amounts during the summer months, with a reduction of occupied patches of 73% in the dry year 1999, compared with mean values for 1996-98. Spore number per sporophyte was correlated with length and width of the capsule, and varied between 1.4 and 9.0 million, with a mean value of 6.0 million. It is suggested that the rarity of B. viridis is caused by a low probability of patches being occupied because of their short longevity and small size, together with the facts that the species is dioicous, short-lived, sensitive to desiccation of the substratum, and has a gametophyte that is so minute it cannot compete with larger bryophytes.