Classification and distribution patterns of plant communities on Socotra Island, Yemen

Question What are the main plant communities and vegetation zones on Socotra Island in relation to climatic, geological and topographic factors? Location Socotra Island (Yemen). Methods A total of 318 releves were sampled along an altitudinal gradient. Floristic and environmental (topographic, geological and climatic) data were collected and analysed using numerical classification and NDMS ordination; an analysis of the correlation between plant communities and environmental factors was also performed. Results Eight types of woody vegetation, seven of shrubs, six of herbaceous and seven of halophytic vegetation were identified. Ordination revealed the importance of altitudinal and climatic gradients, as well as of geological substrata. Conclusions Four vegetation zones were identified. The first three are located in the arid region with altitude ranging from 0 to 1000 m and the fourth in the semi-arid region from 1000 to 1500 m a.s.l. Specifically they are: (1) an arid coastal plain mainly located on an alluvial substratum between 0 and 200 m, characterized by shrubland and grassland communities; (2) a transition zone from 200 to 400 m, between the alluvial substratum and the upper limestone area; (3) an arid limestone zone between 400 and 1000 m, interspersed with hills and plateaus; and (4) a semi-arid upper zone of the Haghier Mountains from 1000 to 1500 m on a granitic substratum.