RESEARCH PAPER An evaluation of the Lost World and Vertical Displacement hypotheses in the Chimantá Massif, Venezuelan Guayana.

To document the occurrence of vertical displacements of vegetation in the high plateaus of the Venezuelan Guayana ( tepuis) over the last c. 6000 years, and to discuss their significance for the origin of their flora, especially the endemism patterns observed in their flat summits. Two hypotheses have been proposed for the origin of the summit flora. One (the Lost World hypothesis) proposes a long history of evolution in isolation from the surrounding plains, while the other (the Vertical Displacement hypothesis) suggests that vertical movements of vegetation during the Pleistocene glacial-interglacial cycles would have resulted in floristic mixing within the lowlands, and genetic interchange among plateau summits. This work has been conducted on the flat summit of the Churí-tepui, in the Chimantá massif, at 5°15′ Lat. N and 62°01′ Long. W, around 2250 m altitude. Pollen analysis and radiocarbon dating of two peat outcrops, using modern analogue technique and numerical methods for palaeoecological interpretation were used. The replacement of a high-altitude plant community (a paramoid Chimantaea shrubland) by a lower elevation (< 2300 m) Stegolepis meadow, occurred about 2500 years before present (yr bp). This vegetation change is inferred to have resulted from a regional climatic shift to higher temperature and moisture. A subsequent decrease in temperature and moisture led to the establishment of present conditions after about 1450 yr bp. The highland vegetation of the tepuis responded to climate shifts with vertical displacements, supporting the hypothesis of vertical mixing. However, a physiographical analysis shows that around half of the tepuis would never have been connected by lowlands. Therefore, both hypotheses are needed to explain the origins of the summit flora in the tepuis. [ABSTRACT FROM AUTHOR]