Forest Pertubations and Biodiversity During the Last Ten Thousand Years
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Acru Oecologicu 19 (3) (1998) 295-302 /OElsevier, Paris Forest perturbations and biodiversity during the last ten thousand years in French Guiana Pierre Charles-Dominique I*,Patrick Blanc ', Denis Larpin 3, Marie-Pierre Ledru 4, Bernard Riéra Corinne Sarthou 5, Michel Servant 4, Christophe Tardy ' Laboratoire d'kologie génirale MNHN/URA 1183/CN~S,4 avenue du Petit Château, 91800 Brunoy,,France. Laboratoire de botanique tropicale, URA I I83/ChRS, 12 rue Cuvier 75005 Paris, France. 43 Laboratoire de biologie végétale, MNHW Service des Cultures rue Bufion, 75005 Pans, France. Laboratoire daformations surpmjicielh, Orstom, 32 avenue Han' ?hagnat, 931 43 Bdycé&x, France. 'Laboratoire de biologie végétale, M", 61 rue Buflon 75005 Paris, France. 'Institut de bokznique, URA 1477/CNRT,I63 rue Auguste Broussonet 34000 Montpelliq France. * Correspondingau tlior Received September 17,1996; revised March 25,1997; accepted September 1,1997 Abstract - Tropical forests can be described as a mosaic of juxtaposed eco-units corresponding to different stages of regMeration after treefals. However, these small-scale regeneration mechanisms alone cannot account for the different pattems of species distribution, plant communities and population structures found in this habitat. The presence of charcoal layers in the soil and the study of sediments along streams suggest that large-scale forest fires deeply affected the tropical forest vegetation, even in high rainfall areas such as French GuianarMany atypical plant dis- tribution and population structure pattems, in relation to what would be expected from present-time regeneration processes, can be explained by - these large-scale events which happened during the last few thousand years. O Elsevier, Paris Holocene drought events / palaeofires /pioneer plant fluctuations / soil charcoals 1. INTRODUCTION vegetation types, and some population and community structures, cannot easily be explained by these small- Tropical rain forests are made up of small juxta- scale forest dynamic processes. posed units of vegetation corresponding to different So far, the study of species distribution in tropical two stages of the regeneration process. This cyclic process ecosystems has been based on scales of time and is initialized by thé fall of a large tree (opening of a space: gap), enabling the establishment of light-demanding -on the local short-term scale (under 1 km2, 1- short-lived pioneer species, which are then replaced 100 years), the mechanisms of the forest regeneration little by little by mature forest species 116, 26, 571. processes and the key role of gap openings have been However, this process largely depends on the size of used to explain the structure and dynamics of the the gap, and redevelopment of pre-existing trees may forest mosaic [24, 25, 28, 36, 39, 561. also contribute, to various degrees, to the natural -on the continental and regional long-term scale regeneration of the gap. In addition, when a tall emer- (10 000-100 O00 O00 km2, 10 000-1 O00 O00 years), gent tree dies upright, other subtle replacement pro- the Holocene refuge theory provides the theoretical cesses may take place [7, 27, 541. The natural framework for the interpretation of global species dis- 32, evolution of these units organizes the entire forest tributions [11, 15, 31, 52, 551. architecture around light availability [35, 37, 541. The Both approaches can only bring satisfying answers analyses of plant populations in undisturbed forests at their own scale, and remain unable to explain give a good picture of this general regeneration pro- abnormal population and community organizations. cess, based on the repetitive succession of small Our approach focuses on an intermediate scale and events, distributed more or less evenly in space and uses both functional arguments and local evidence for time. However, some distributions of plant species and past events. i 1 . .. 296 P. Charles-Dominique et al. As different plant species have more or less delayed 2.2. Methods reactions after perturbations, the vegetation could retain scars left by major events even after a very long In order to locate the main forest types, the explo- time. This hypothesis suggests that ancient perturba- ration of the area (covered by aerial photographs) was tions of a much larger scale than ordinary gaps may extended over several kilometres, using footpaths radi- have left long-lasting scars still visible in present-time ating from the camp (figure I). Botanical inventories forest ecosystems. included one 87 ha core area [30], and several Evidence for such ancient large-scale perturbations transects distributed on its periphery [18]. Each tree has been discovered in soils and sediments, during the above 30cm DBH (10cm for focal species) was Ecofit research programme. This project includes marked, measured, mapped and identified to species, geologists, ecologists, palynologists and climatola- genus or family level. Botanical samples were col- gists who work together on the evolution of the inter- lected in order to complete the identifications and tropical ecosystems during the last ten thousand years. were deposited in the Orstom herbarium (Cayenne) The tropical rain forest, thought to have remained and ia the National Museum of Natural History her- stable since the last glacial event, has in fact under- barium (Paris). gone deep modifications. A vibration core drill [23] was used for the core boring. Core samples were processed for seed and pollen contents following the usual method [19]. Char- 2. MATERIAL AND METHODS coal fragments were collected by sifting the soil of 50 x 50 cm pits in successive 5 cm-thick layers, to 2.1. The study area depths of 60 to 90 cm. The fragments were identified by studying vessel anatomy and comparing with a ref- This study was undertaken on the Les Nouragues erence collection [49]. The fragments (charcoal and research station (French Guiana), in pristine tropical seeds) were dated at Beta Analytic (Miami) and rain forest 100 km inland (4‘5, N, 54”40’ W). Annual Utrecht University, and ages converted into conven- O00 rainfall totals over 3 mm distributed over about tional 14cages. 280 days, with two drier periods of unequal duration: the main ‘dry season’ in September-October, and the much shorter ‘little summer’ around March. The rain- 3. RESULTS fall data place French Guiana in the ‘hyperhumid’ forest zone [34], along with the Andean foothills Overall, the Guianan forest is relatively homoge- < between the Rio Negro, Putumayo and Maranon neous at the scale of the main vegetation types, and is catchment areas. ~ organized as a mosaic of functional units evolving The area had been inhabited by Nouragues Native from one stage to another. This spatial organization Amerindians until the 18th century [12, pers. com.], fits well with the general mechanisms of natural regen- but on the actual research station grounds, the remains eration studied locally [7, 363, but a number of small of human presence have all been dated between 1 O00 units, enclosed in the extent of the forest, appear to and 1 500 BP [49]. It is possible that othsmore recent undergo distinct processes (rock savannas, low forests, or older remains are still to be found. The area has liana forests, bamboo thickets). These vegetation types been left untouched during the entire colonial era, are considered ‘atypical’ because they display partic- until the creation of the research station in 1986. ular structures and are relatively uncommon (they are sometimes absent over 50 to lOOkm). Some species The region is covered by primary forest to a canopy local distributions, which do not conform to the height of 30-45 m, with a few emergents reaching over In some places, this high forest is punc- general pattern found in the forest, are also termed 60 m. ‘atypical’. The interpretation of such ‘anomalies’ must tually replaced by other vegetation types, such as the ‘rock-savannas’, the low forests found around insel- take into account past situations, and this is why we bergs, the ‘cambrouzes’ bamboo thicket patches, and have attempted, through sediment analyses, to retrace the sometimes extensive liana forests. The occurrence the past events which may have deeply modified the of markedly different vegetation types contributes to a dynamics of the forest ecosystem. high species diversity. So far, 1 250 higher plant spe- Results will be presented in two sections. The fist cies have been identified on the 3 kmz surrounding the will focus on past events as they appear through the research station [30]. In the high forest sectors, botan- analyses of sediments (which record past events). The ical inventories have yielded between 160 and 260 tree second will examine how atypical vegetation types can species per hectare (trees above 10 cm DBH), and give evidence for environmental conditions which these values place Les Nouragues among the regions have since disappeared (traces of the past in the orga- with the highest species diversities [30, 38,411. nization of present living organisms). Acta Oecologica * F Forest perturbationsin French Guiana 297 Figure 1. Map of the study area - c: camp; 1: Rock savanna; 2 Low forest; 3: Bamboo thickets; 4: Liana forest; 5: Pino palm swamp; dotted lines: trails and paths; continuous lines: level curves (every 20 m, from 60 to 400 m), large dotted line: separation of hydrologic basins. VOL 19 (3) 1998 . .. - . .. 298 P. Charles-Dominiqueet al. 3.1. The main ancient large-scale perturbations of . The study of a pino palm swamp, a little further the forest upstream and lodged in a plateau depression, also displays traces of several periods of intensive erosion. Paheofires In this case, layers of sand and gravel several 3.1.1. centimetres thick have been found included within The charcoal layers found in the soil are the most clay-organic sediments or peat. The reconstitution of obvious indicators of palaeofires. They were first the soil profiles reveals up to three such layers one found by Soubies [47] in Southem Amazonia, and above the other, with a slight slope towards the center have been recently discovered in French Guiana on of the swamp, suggesting the transport of material by many sites.