Vegetation and environmental constancy in the Neotropical Guayana Highlands during the last 6000 years?

The aim of this paper is to document the Holocene (6000 years BP to the present) vegetation trends on the summit of three tepuis (table mountains), from one of the largest and highest tepuian massifs of the Neotropical Guayana region, the Chimantá, situated in Venezuela. The tepui summits are almost pristine and unique sites to record natural forcings and ecosystem responses. Here, pollen analysis and radiocarbon dating of four peat sequences obtained with a manual Hiller borer are presented, and compared with modern analogues from surface samples, for interpretation. Highland tepuian meadows have been the dominant vegetation type throughout the time interval studied. The sequences studied exhibited different minor vegetation patterns in time, recording primarily local vegetation dynamics, such as lateral variations in the forest-meadow ecotones, and quantitative shifts in the dominant meadow taxa. Moderate climatic shifts formerly reported for other localities were not recorded here, probably because the intermediate altitude and the geomorphological characteristics of the sites studied made them insensitive to subtle regional changes in temperature and moisture, which are hidden by local vegetation shifts. The results of the present study allowed estimation of the magnitude of formerly reported vegetation shifts in the same massif. In the studied sites, the constancy in the vegetation through time cannot be considered only as the result of a high degree of climatic stability, but also the consequence of site insensitivity.