Old growth mature forest types and their floristic composition along the altitudinal gradient on Silhouette Island (Seychelles) the telescoping effect on a continental mid-oceanic island
Senterre, Bruno; Gerlach, Justin; Mougal, James; Matatiken, Denis
The granitic Seychelles are the only mid-oceanic islands of continental origin. Botanists have long focused on taxonomy, and plant communities were described in a qualitative way, based on simple observation. Therefore the altitudinal belts, their floristic characteristics and distribution are still poorly understood and conservation efforts focus mainly on species-centred actions. Here we describe a quantitative study of plant communities and indicator species differentiated along the altitudinal gradient on Silhouette, the most pristine and second highest (740 m) island of this archipelago.Twelve plots were sampled from 80 m to 640 m above sea level. Each plot contains three nested subplots corresponding to three forest strata: 50 x 10 m for all individuals of the tree layer (i.e. trunk diameter > 5 cm), 50 x 4 m for the shrub layer (ligneous > 90 cm height), and 50 x 4 m for estimation of abundance-dominance coefficient in the herbaceous stratum (all herbaceous plants, plus ligneous plants < 90 cm height). The results are summarized in a two-way table. Both indicator species and vegetation types are discussed in relation to previous studies in Seychelles. Three altitudinal belts are distinguished: lowland, submontane and lower montane rain forests. Most of the best indicator species are found in the understory, especially ferns. The submontane belt develops at about 350 m and turns into a typical lower montane belt at ca. 550 m. Such transition zones occur respectively at about 900 and 1500 m in most of the tropical mountains, illustrating here a perfect example of the “telescoping effect”. Although the flora of Seychelles is relatively species poor, the strong characterisation of these altitudinal belts is unusual compared to younger islands in the Pacific, and may be a result of the longer evolution of its flora.