Author's personal copy Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) 165–173 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Paleoecology of the Guayana Highlands (northern South America): Holocene pollen record from the Eruoda-tepui, in the Chimantá massif Sandra Nogué a,b, Valentí Rull a,⁎, Encarni Montoya a,b, Otto Huber c, Teresa Vegas-Vilarrúbia d a CSIC-Botanic Institute of Barcelona, Palynology and Paleoecology, Pg. del Migdia s/n (Montjuïc), 08038 Barcelona, Spain b Department of Animal Biology, Plant Biology and Ecology; Autonomous University of Barcelona; Bellaterra, 08193 Barcelona, Spain c Trauttmansdorff Botanical Gardens, Via San Valentino 51a, Merano, Italy d Department of Ecology, University of Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain article info abstract Article history: The previously recorded vegetation constancy during most of the Holocene, atop some summits of the Received 4 March 2009 Guayana tabular mountains (or tepuis), led to the hypotheses of either environmental stability or site Received in revised form 13 July 2009 insensitivity. As high-mountain biomes are considered to be especially well suited for recording past Accepted 24 July 2009 environmental changes, a palynological study on the uppermost summit of the Chimantá massif was Available online 3 August 2009 designed to test its suitability for these purposes. A peat sequence was obtained spanning the last ~13.0 cal kyr BP, but an acceptable resolution for paleoecological reconstruction is available only for the last Keywords: ~4000 years. Around 4.3 cal kyr BP, the modern vegetation was established and has remained virtually Palynology Paleoclimatology unchanged until today; minor paleoenvironmental changes recorded in other sequences around 2.5 cal kyr Paleoecology BP were not detected here. The main paleoclimatic trends are in good agreement with other neotropical Holocene records, especially from Lake Valencia and the Cariaco Basin. It is concluded that high-altitude tepuian sites Neotropics are useful to record paleoenvironmental changes of moderate to high intensity but once a dense vegetation Guayana cover is established, gentle shifts remain hidden due to the capacity of plant communities to absorb the Tepuis changes. The best sites for paleoecological research atop the tepuis are those lying on or near altitudinal Vegetation constancy ecotones, especially between the meadows and the paramoid shrublands (~2200 m elevation). Sites within the meadow domain, as most well-studied so far, are relatively insensitive to Holocene paleoenvironmental changes. © 2009 Elsevier B.V. All rights reserved. 1. Introduction more reliable criteria for predicting the eventual future reactions to the projected global warming. The Guayana Highlands (GH), i.e. the summits of the typical Compared to other neotropical regions, the paleoecological and Guayana table mountains or tepuis, hold amazing biodiversity and paleoclimatic study of the GH are relatively recent. The first analyses endemism figures in all the organisms studied so far (e. g. Mayr and reported the absence of late Pleistocene sediments, leading to the Phelps, 1967; McDiarmid and Donnelly, 2005; Berry and Riina, 2005; hypothesis of extended aridity in the entire Guayana region before the Berry et al., 1995), and have been considered an important center of Holocene (Schubert and Fritz, 1985; Schubert et al., 1986). However, neotropical speciation (Funk and Brooks, 1990). The origin of such further records from the plains around the tepuis documented biotic features has been largely debated and different hypotheses temperature and moisture changes during the LGM and across the have been proposed, most of them related to Quaternary climatic Pleistocene/Holocene boundary (Bush et al., 2004; Rull, 2007). Atop changes (Mayr and Phelps, 1967; Huber, 1988; Rull, 2004a, 2005a). In the tepuis, the oldest sediments found so far are around 8 cal kyr BP spite of the lack of direct human disturbance (Huber, 1995b), the GH old; since then a more or less continuous record is available (Rull, biota seems to be seriously threatened by ongoing global warming, 1991). To date, the most remarkable findings are 1) a phase of which might result in the extinction of a considerable number of increased hydrological balance (as measured by the precipitation/ species by habitat loss if the IPCC predictions are realized (Rull and evapotranspiration ratio or P/ETP) between about 4.5 and 2.0 cal kyr Vegas-Vilarrúbia, 2006; Nogué et al., 2009). The study of ecological BP and recorded in the Guaiquinima massif, and 2) a slight increase in reorganizations linked to past climatic shifts is important to assess the temperature (~1 °C) detected in a tepui from the Chimantá massif response of the GH biota to environmental changes, thus providing (Churí-tepui), starting around 2.5 cal kyr BP (Rull, 2004b, c; 2005b). However, the results of further studies from other Chimantá tepuis ⁎ Corresponding author. (Acopán, Amurí and Toronó) were confusing because they did not E-mail address: [email protected] (V. Rull). record any significant vegetational or climatic change during the last 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.07.019 Author's personal copy 166 S. Nogué et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) 165–173 6000 cal kyr; instead, they just described minor reorganizations likely to have been associated with local ecological shifts (Rull, 2005c). From these studies, it was not clear if the lack of manifest vegetation changes was due to either climatic or ecological stability or to site insensitivity. The first possibility is not consistent with the results obtained in the Churí-tepui, where an upward shift of the ecotone was recorded (Rull, 2004b, c). Site insensitivity, on the contrary, seemed to be favored by the fact that the Acopán, Amurí and Toronó cores were obtained around 2000 m, where altitudinal shifts are more difficult to record due to the lack of an altitudinal ecotone in the vicinity (Rull, 2005c). To solve this dilemma, new studies were proposed on high- mountain localities, which are considered to be among the most sensitive biomes to environmental changes (Diaz and Bradley, 1977). Here, we report the results of pollen analysis from a peat core obtained in the highest peak of the Chimantá massif, the Euroda-tepui (~2700 m elevation), spanning from around 13 cal kyr BP to the present. 2. Study area The Guayana Highlands lie on the Precambrian Guayana Shield, in northern South America (Fig. 1), between about 0° and 7° Lat N and are developed on the quartzites/sandstones of the Roraima Group, with localized diabase intrusions (Briceño and Schubert, 1990). The tepuis are remnants of ancient erosion surfaces that were isolated after denudation due to the Gondwana breakup and the formation of the extensive Orinoco and Amazon River basins (Briceño and Schubert, 1990). The Chimantá massif is among the largest and highest tepuian complexes, with an area of about 900 km2 (~600 km2 in the summits), and almost a 2700 m altitude at its highest summit, the Eruoda-tepui. The massif is surrounded by the Gran Sabana (GS) midlands, which have around an 800 m elevation. The Chimantá summit is shaped by a combination of several internal and external tepuis separated by deep, densely vegetated internal valleys. The external tepuis are more exposed to the action of strong winds and fire events originating in the neighbor plains of the Gran Sabana. Climatic data for this area are scarce but sufficient enough to define its climate as mild and very wet (Galán, 1992). The annual average temperature is around 12 °C (at 2600 m elevation) and fairly constant throughout the year, with an adiabatic lapse rate of −0.6 °C/100 m altitude. Total annual rainfall (P)is3350mmandthetotal evapotranspiration (E) is 820 mm/year, giving a high hydrological balance of P/E ~4. Minimum precipitation occurs between January and March (60–100 mm/month), when NE trade winds are stronger and the Intertropical Convergence Zone (ITCZ) is displaced to the south. Maximum precipitation occurs between May and August, when trade winds are at their minimum intensity and the ITCZ is over the region (Galán, 1992). Fig. 1. Location map. A) Map of northern South America, showing the location of the As in the whole Guayana region, the vegetation shows an Guayana Highlands (square) and the Chimantá massif (open circle). Courtesy of NASA/ altitudinal pattern, from the midlands (500–1500 m elevation) to JPL Caltech. B) Radar image of the Chimantá massif, indicating the sites mentioned in the highlands (N1500 m) (Huber, 1992, 1995a). The Gran Sabana the text. The coring site is indicated by a star. Er = Eruoda, Ti = Tirepón, Ak = Akopán, plains (up to 800 m elevation) are covered by savanna vegetation, Am = Amurí, Ch = Churí, To = Toronó. Courtesy of ANAPRO Digital. C) Helicopter view of the southern cliffs of the Tirepón-tepui, in the vicinity of the Eruoda summit, as an dominated by grasses and gallery forests along the rivers and on example of the typical tepuian topography (photo V. Rull). humid slopes. The slopes, the transition between the Gran Sabana plains and the vertical cliffs of the Chimatá, extend from 800 m to 2000 m and are covered by evergreen upper mountain forests composed mainly of algae (Stigonema) and lichens (Cladonia, Cladina, dominated by Bonnetia (Bonnetiaceae),