On the occurrence of bryophytes and macrolichens in different lowland rain forest types at Mabura Hill, Guyana

A floristic and ecological study of bryophytes and macrolichens in different lowland rain forest types around Mabura Hill, Guyana, South America, yielded 170 species: 52 mosses, 82 liverworts and 36 macrolichens. Lejeuneaceae account for about 30% of the species and are the dominant cryptogamic family of the lowland rain forest. Special attention was paid to the flora of the forest canopy, by using mountaineering techniques. It appeared that 50% of the bryophyte species and 86% of the macrolichens occurred exclusively in the canopy. Dry evergreen 'walaba' forest on white sand is particularly rich in lichens whereas the more humid 'mixed' forest on loamy soil is characterized by a rather rich liverwort flora. More species are exclusive to the mixed forest than to dry evergreen forest due to the 'canopy effect', i.e. the occurrence of xerophytic species in the outer canopy of both dry and humid forests. Furthermore, canopy species have wider vertical distributions on trees in the dry evergreen forest than in the mixed forest, due to the more open canopy foliage of the dry evergreen forest. Epiphytic bryophytes and lichens are often neglected in rain forest studies as their main occurrence is in the forest canopy (Gradstein et al. 1990). In the Guianas, one of the last areas of the world covered largely by virgin lowland rain forest, floristics and ecology of rain forest bryophytes and lichens were investigated at Moraballi Creek, Guyana, by Richards (1954) and in the Kabalebo area in western Suriname by Florschütz-de Waard & Bekker (1987). These studies were performed at forest ground level and did not include a detailed analysis of the forest canopy. In the present study, focusing on rain forests at Mabura Hill, Guyana, whole standing trees have been inventoried by using mountaineering techniques for access into the canopy (ter Steege & Cornelissen 1988). The Mabura Hill area is situated between the Essequibo and Demerara River, ca 180 km SSE of Georgetown (520' N, 5810' W) at an altitude less than 100 m. above sea level. Rainfall is between 3000 and 4000 mm annually and mainly in the periods May-August and December-January (Fanshawe 1952). Average air temperatures are between 27 and 30 C and relative air humidity rarely drops below 60 % in the forest. The main rain forest types around Mabura Hill are (Richards 1952; ter Steege & Cornelissen 1990): 1. Walaba forest (ca 30 m high): dry evergreen