Influence of microclimate on the occurrence of cyanobacteria in the phyllosphere in a premontane rain forest of Costa Rica

The occurrence of cyanobacteria in the phyllosphere of several understory p lants was investigated during a one and a half year study in a primary prem ontane rain forest in Costa pica. Altogether, seven species of epiphyllous cyanobacteria were identified and are discussed here. The ecology of the tw o most frequent species in the understory, Scytonema javanicum and Sc. hofm annii, was studied in further detail. Within the understory, the relative a bundance of these species, as well as of epiphyllous bryophytes, was more i nfluenced by air humidity than by light. The humidity, characterized as pot ential evaporation, also had influence on the pseudosuccession of the phyll osphere vegetation. While at moist sites bryophytes and both species of Scy tonema appeared nearly simultaneously on 6-9-month-old leaves, at dryer sit es Sc. javanicum, and especially Sc. hofmannii, appeared 6-9 months after b ryophytes had established. On 13-15-month-old leaves bryophytes and Scytone ma were usually well established. On average, 20-30% of the leaf area of 2- 5-year old leaves of Spathacanthus hoffmannii (Acanthaceae, an abundant und erstory 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. Howe ver, Sc. hofmannii was more frequent on leaves with higher bryophyte cover, while Sc. javanicum was independent from bryophytes. A comparison of leave s from the understory and from the canopy showed that the composition and a bundance of cyanobacteria species changed with height above forest floor.