Interception and retention of macroscopic bryophyte fragments by branch substrates in a tropical cloud forest: an experimental and demographic approach

Although the significance of canopy plant communities to ecosystem function is well documented, the process by which such communities become established in trees remains poorly known. Colonization of tree surfaces by canopy-dwelling plants often begins with the establishment of bryophytes, so the conditions that affect the dispersal of bryophytes in the forest canopy merit study. We assessed success rates of one mechanism of bryophyte propagation, the aerial dispersal of macroscopic fragments, using an experimental approach. We quantified interception and retention of marked fragments released from a 36 cm×36 cm grid 50 cm above branches of saplings and mature trees of the species Ocotea tonduzii in a montane cloud forest in Costa Rica. Only 1% of bryophyte fragments dropped over sapling crowns in this manner were retained for the 6-month duration of the study, while branches in the forest canopy with intact epiphyte loads and branches that had been stripped of their epiphytes retained 24% and 5%, respectively. Our results suggest that larger-diameter branches and the presence of other epiphytes can both improve the retention of bryophyte fragments on canopy branches. Further work will be needed to address the relative roles of other dispersal mechanisms (spores, gemmae, microscopic bryophyte fragments) and the dynamics of growth and establishment of macroscopic bryophyte fragments following their interception. Received: 1 March 1999 / Accepted: 24 August 1999