The occurrence of cyanobacteria in the phyllosphere of several understory plants was investigated during a one and a half year study in a primary premontane rain forest in Costa pica. Altogether, seven species of epiphyllous cyanobacteria were identified and are discussed here. The ecology of the two most frequent species in the understory, Scytonema javanicum and Sc. hofmannii, was studied in further detail. Within the understory, the relative abundance of these species, as well as of epiphyllous bryophytes, was more influenced by air humidity than by light. The humidity, characterized as potential evaporation, also had influence on the pseudosuccession of the phyllosphere vegetation. While at moist sites bryophytes and both species of Scytonema appeared nearly simultaneously on 6-9-month-old leaves, at dryer sites Sc. javanicum, and especially Sc. hofmannii, appeared 6-9 months after bryophytes had established. On 13-15-month-old leaves bryophytes and Scytonema were usually well established. On average, 20-30% of the leaf area of 2-5-year old leaves of Spathacanthus hoffmannii (Acanthaceae, an abundant understory shrub) were covered by bryophytes, 2-3% by Sc. javanicum, and 0.1-0.2% by Sc. hofmannii. An influence of phorophyte species on the occurrence and abundance of these two species of Scytonema could not be detected. However, Sc. hofmannii was more frequent on leaves with higher bryophyte cover, while Sc. javanicum was independent from bryophytes. A comparison of leaves from the understory and from the canopy showed that the composition and abundance of cyanobacteria species changed with height above forest floor.
