Aims Ecosystems dominated by herbaceous plant communities are amongst the most diversified landscape units in the Guayana (Guiana) Shield region. This paper aims to present a synthetic overview of the wide array of herbaceous ecosystems found in the region in an attempt to furnish a more concrete baseline for a better understanding of the pattern of variation, and to clarify some of the differences that occur in the vegetation of the area.
Location The Guayana Shield region, and the area of north-eastern South America extending between the Orinoco River to the North and the Amazon River to the South (c. 8° N to 1° S).
Methods Floristic and ecological field data gathered from over 300 study sites located at different altitudinal levels in the Venezuelan Guayana and the northern Brazilian Amazon are evaluated and interpreted in the light of personal observations and existing literature.
Results The diversification pattern includes physiognomic as well as floristic variation and shows two opposing tendencies in relation to their altitudinal location: grass-dominated meadows (savannas) predominate in the macrothermic lowlands, whereas non-gramineous, broadleaved herbaceous communities are dominant in the mesothermic highlands of the Guayanan mountains. In detail, the biogeographic region of Guayana (Guiana) consists of three easily recognized altitudinal levels, each of which contains a highly characteristic set of landscape types with their associated ecosystems and endemic plant communities. Between 0 and c. 500 m a.s.l. the extensive macrothermic (mean annual temperature, MAT > 24 °C) lowlands are found, where an enormous – still largely unexplored – diversity of forest types represents the main plant cover of the plains, peneplains, glacis and piedmont slopes. Sparsely distributed within this forest cover are numerous herbaceous ecosystems, ranging from true grass savannas to unusual and poorly understood meadows developed on extremely oligotrophic white sandy soils dominated by non-gramineous genera of Rapateaceae and Xyridaceae. In the intermediate submesothermic (MAT 24–18 °C) Guayana uplands, extending roughly between 500 and 1500 m a.s.l., the grasslands of the Gran Sabana in south-eastern Venezuela reach their upper altitudinal limit. At the same altitudinal level, however, several distinct herbaceous communities are found, in which other genera of Rapateaceae, together with Bromeliaceae and Xyridaceae, are predominant. Finally, at the uppermost altitudinal level, i.e. in Pantepui (which includes the characteristically flat topped mountain summits (tepuis) of the Guayana highlands) between 1500 and 3000 m a.s.l., encompassing a range of meso- to submicrothermic temperature regimes (MAT 18–8 °C), the extensive herbaceous ecosystems are developed either on deep organic soils (peat) or on open sandstone surfaces. These high-tepui meadows present a considerable physiognomic diversification and are formed by a variety of endemic genera of the Rapateaceae, Bromeliaceae, Xyridaceae, Eriocaulaceae and Cyperaceae families, often dominated by locally endemic species on each of the larger tepui massifs. In contrast, grass dominated plant communities are very rare and restricted to only a few high-tepui sites.
Main conclusions A marked floristic and ecologic differentiation of herbaceous ecosystems in the Guayana Shield region can be recognized. The ecological differentiation results primarily from the wide spectrum of variations in the substrate found at the various altitudinal levels of the Guayana Shield region. A possible explanation for the present-day pattern of herbaceous vegetation types may be the following: Non-gramineous meadows representing ancient species pools of Guayana-centred families had evolved successful colonization strategies in occupying extremely nutrient poor sites at all altitudinal levels. In contrast, the more modern grass savannas, which preferentially occupy the peripheral Guayana landscapes, are restricted to richer soil conditions with better internal drainage and water retention conditions.