Diversity and complexity of the Araracuara sandstone flora and vegetation in the Colombian Amazon

Insular open vegetation of the western Guayana Shield in Colombia (c.150-1000 m) surrounded by NW Amazon rain forest (over 3000 mm annual precipitation) has been botanically unexplored until the early 1990¿s. During recent botanical exploration of the sandstone plateaus of the Araracuara region a total of 381 vascular species belonging to 205 genera and 77 families have been documented. This is much less than in the same altitudinal interval of the floristically rich core area of the Central Guayana Province in southern Venezuela. The lowermost sandstone plateaus of the Araracuara region (West Guayana Province) offer an array of different habitats for 1) pioneer-like herb vegetation (at least 18 plant communities) and 2) shrubland and low thin-stemmed forest (5 plant communities). Basically three types of sandstone habitats occur: gently sloping rock, concave and convex sandstone surfaces. Microtopography in combination with substrate thickness and organic matter content and degree of water supply have created a number of different microhabitats, which account for a large number of different plant communities. Low herbaceous vegetation structure consisting of main lifeforms such as chamaephytes, hemicryptophytes and therophytes combined with a limited number of species per vegetation type resulted in a low alpha diversity and a relatively high beta diversity (especially in open pioneer-like and herbaceous vegetation). Similar evidence with a high diversity of habitats has also been found in another landscape dominated by extreme environmental conditions: the high Andean paramo. In contrast to paramo West Guayanan sandstone flora and vegetation have a much longer historical record. The present highly specialized vegetation of West Guayana must have been derived by selection and adaptation to the harsh conditions of the equatorial sandstone surfaces with many different habitats. Plant strategies developed involve: 1) conservative use of limited amount of nutrients, 2) xeromorphic structures, 3) different photosynthesis pathways, 4) long distance seed dispersal by wind and fauna, 5) longtime investment in strategic growth forms such as branched and unbranched ground rosettes, caulescent rosettes, tufted
herbs with long needle like leaves or short thick coriaceous leaves, 6) development of many therophytic species adapted to cyclic flooding and drought.