Acru Oecologicu 19 (3) (1998) 295-302 /OElsevier, Paris

Forest perturbations and biodiversity during the last ten thousand years in

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, 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 (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 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

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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 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 ) 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).

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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.

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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. They appear to be independent of human surface water. One of these layers, with its upper limit occupation levels: Tardy et al. [49] showed that palae- at -85 cm, has been dated under 1 390 BP (UT86). Another, located between -131 and -140cm [19], ofire levels may occur undemeath human occupation O00 levels, antedating these by several thousand years. The would be about 3 years old (OBDY 1416). Today, presence of Man in. America is attested since the swamp does not receive any coarse particles during 30 O00 BP at least, and agriculture since 6 O00 BP [9], heavy rains, and these sand and gravel alluvions could but evidence for tropical rain forest agriculture is only only have been deposited during intensive soil erosion found since 2 500 BP ïn Meso-America [20]. In periods, as in large open areas after massive simulta- French Guiana, intensive archaeological searches neous destruction of the forest cover. The same goes undertaken in the Petit Saut area over 275 archaeolog- for the alluvions accumulated further down along the stream bed. At the scale of such a small catchment ical sites 1331 have not yielded any remains anterior to 2000 BP, while palaeofires appeared a long time area, such erosiodalluvion cannot be explained out- before this, during the periods 10000--8000BP, side extensive deforestation, such as those generated O00 by palaeofires . 6 000-4 O00 BP and 2 BP to the present time. 3.1.3. Human versus climatic origin of palaeofires 3.12-Erosion

' In all tropical rain forests, air humidity remains high Today, the physical erosion of the forest floor is mini- and does not allow the development of forest fires. The mal under primary forest cover (under 1 myear-l on traditional agriculture in these habitats uses the 'slash- average, [40]). The 30-45 m stratified vegetation con- and-bum' method, i.e. the felling of trees and then the siderably reduces thejmpact of heavy rains. Moreover, burning of the dead dry matter at the end of the dry the forest floor is entirely colonized by a dense mat of season. Sometimes, the dry season is too subdued to surface roots which form a protective mesh under the allow the dead wood to dry and no fire can be ignited, litter. Most of therain water drains into the ground whereby the piece of land is temporarily lost for cul- through the soil. Chemical dilution (24 kghayear-") tivation. In contrast, on exceptionally dry years, the and the export of fine particles (190 kghayear-', [13, fires set to clear the dead wood may propagate into the pers. CO".]) are the main pathways of soil erosion. surrounding forest, as it happened in southem Bra- Heavy rains notably intensify the flow of small streams zilian Amazonia in 1988, when over 80 O00 km2 were and rivulets, but very small amounts of solid matter are destroyed [Ronchail pers. comm.]. Such incidents actually tom from the forest floor, and the"water flow have not, in recent history, been recorded in hyper- merely reorganizes ancient sediments. Large-size ele- humid forests. However, the. widespread presence of ments are never driven from the forest floor into the charcoal layers indicate that several large-scale forest stream beds. However, remains of alluvial terraces that fires have occurred. could only have been formed during periods of inten- One possibility is that the great forest fires recorded sive erosion were found in Les Nouragues, during in the Guianan soils have been the consequences of recent studies undertaken along a small river with a intensive slash-and-bum agricultural activities. But the 8 kmz catchment area [4]-The lower terrace (1 m high) presence of charcoal layers older than 2 O00 years old sits on a bed of clay and sand containing charcoal frag- (without agricultural evidence) show that natural pro- ments, seeds and dead , dated 530 -+ 70 BP (Beta cesses alone may very well cause rain forest fires. 89824). The examination of the seeds reveals a great Additionally, in order to explain the sedimentary abundance of pioneer species: Henriettea spp, Miconia deposits, one must consider openings large enough to spp (Melastomataceae), Cecropia obtusa, C. sciado- entail intensive soil erosion. Even today, areas under , phy22a (Cecropiaceae), glabra (), slash-and-burn agriculture in French Guiana do not etc. which suggests a period of important disturbance of particularly suffer from erosion, and in pre-Colombian the forest cover during this time. The second terrace times, when stone axes must have limited the size of (3 m high), situated higher up and older, has not been the cleared areas [17], the impact of agriculture on soil dated for the time being. erosion must have been even less than today. Many

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charcoal fragments are found along with the coarse 3.1.5. Concluding remarks on past events sediments, but their ages vary, and it is possible that the sediments were reorganized wfien the deposits The study of the charcoal layers in the soils, the were formed [49]. traces of ancient erosion and alluvions, and the pioneer plant explosions, suggest that climatic perturbations In , the water deficit is most pro- with severe droughts must have favoured the occur- nounced during the dry season, which is cloudless and rence of large-scale forest fires, long after the last very SUMY. This is when trees may suffer important glacial period and in a region now classified as hyper- water losses and become inflammable if the water humid. The scale excludes a solely human origin, table is too low. Forest fires in Guiana would thus indi- particularly before 2 O00 BP, hence, a climatic origin cate the repetition ,of abnormally long dry seasons over appears more likely. Data from other areas of the a number of years, with a consequent severe lowering Amazonian region indicate that this was no mere local of the water table. phenomenon, but that it ranged over the entire conti- nent. In Carajas, in less humid forests (6" S), the char- The vegetation since 3 O00 years ago coal layers in the soils have been dated between 2 880 3.1.4. and 2 080 BP, and layers of minute charcoal remains Identification of pollen [ 191 and seeds from peatbog have been found in lake sediments, peaking between cores [4], as well as charcoal fragments [49], have 7000 and 4000BP and between 900 and 400BP given insights into what the past vegetation must have [48]. In the. Roraima, beyond the northern boundary looked like. The peatbog cores go back 3 O00 years BP of the rain forest, pollen analyses indicate that the for pollen and 1 400 years BP for seeds, and the char- environment changed from semi- forests to coal fragments can be dated over a period of the actual mosaic of dry forests and savannas around O00 years. Overall, between 3 and 2 BP, 2 O00 BP [l]. Lastly, in the hyperhumid Andean 10 O00 O00 foothills of Pery, the geomorpholyists Dumont and the forest appears to have been more humid than Foumier [6] concluded that river flows declined today, with a greater taxonomic diversity. Between around 1900-1 500 BP and again around 900- 1 800 and 1 200 BP, the swamp must have somewhat 600 BP. dried up, and large areas, favourable to pioneer species (Cecropia spp, Solanum spp, Piper spp), must have In the course of the last decades, particularly dry opened up with a peak around 1600-1 300 BP. years have been recorded in Amazonia, in correlation with increases in the surface temperature 'of the Between 1200 and 900BP, the forest progressively Eastem Pacifrc, off the coasts of Peru ('El Niño' phe- . regained ground, and between 900 and 600BP, new nomenon [29, 511). By analogy with these recent cli- disturbances arose, allowing the development of pio- matic events, some authors have suggested that the dry neer plant assemblages (in particular the family periods of the Middle and late Holocene could be Melastomataceae). These subsided from 600 to explained by very intense and frequent 'El Niño' 300BP, and today's pollen and seed assemblages events and/or by the prevalence of high temperatures appeared around 300 BP. in the waters of the Eastern Pacific [22, 531. The apparitions of pioneer species during several consecutive centuries suggest that, during each of 3.2. Atypical vegetation types these periods, brief disturbances must have occurred every 10-30 years, impeding the establishment of Rock savannas mature forest species and maintaining a widespread 3.2.1. secondary vegetation. The open vegetation on rocky outcrops, locally traces of herbs such Poaceae and Cyperaceae called rock savannas, are mostly distributed in the No as southem part of French Guiana, 100-300 km inland have been found in the pollen profile, which rules out (but some occur as little as 20 km from the coast), and altogether open vegetation types such as savannas or are generally found on inselbergs. De Granville [lo] woodland-savannas. Moreover, the analyses of the and Sarthou [42] showed that some rock savanna plant charcoal remains have yielded some forest species O00 species were also common to coastal savannas, but ever since 10 BP, although diversity is much lower that others were only found in these particular habi- during the two older periods (8000-7000BP and 0004 tats. Overall, during the current humid period, the 6 O00 BP) than from 2 O00 BP onwards [49]. forest is slowly expanding and gaining over the rock From the sole examination of pollen and seeds, it savannas on its borders. This expansion is sometimes appears that the forest must have undergone two series held back on the steepest slopes where surface water of severe perturbations during the last 3 O00 years: the flow is strong. Rock savannas have their own regener- frrst between 1 800 and 1 200 BP, and the second, less ation mechanisms: the bare rock is first colonized by important, between 900 and 600 BP. nitrogen-fixing cyanobacteria [a],initiating the for-

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mation of a soil substrate [43] and the establishment 3.2.3. Zoochory and autochory of a herbaceous xerophytic vegetation cover, finally invaded by shrubby thickets Zoochory is widespread in French Guiana, and this [42]. dispersal mechanism using -eating vertebrates It is difficult to estimate the age of rock savannas, allows quick recolonization, Today’s plant assemblage but it is likely that they have been around for a long is mainly made up of quick-spreading species, sug- time since they harbour specific plant species. They gesting frequent small-scale forest perturbations. probably expanded and receded over time, with max- A small number of slow-spreading autochorous trees , imum extensions during the glacial periods, when exists nonetheless, and these frequently follow a very forest soils were drastically eroded down. More recent patchy distribution pattern, which cannot be explained erosion phases initiated by droughts and fires must by today’s forest dynamics. A current hypothesis is have allowed rock savannas to expand periodically and that these patches are relics from ancient distributions maintain themselves over time since the last glacial reorganized by the latest palaeofires, and in expansion event. Since then. Tree mapping in Les Nouragues (87 ha) has outlined possible small-scale refuge patches 3.2.2. Low forests around swamps and springs. More detailed studies on Epei-ua falcafa (Caesalpiniaceae) show that the older The low forest, so-called ’transiti6n forest’ is an individuals are situated in the center of the patches, original low thickety forest vegetation generally while the younger are distributed mainly on the found on the outside borders of rock savannas. They periphery, as if the population was currently spreading. are established on porous soils where water stress is from the swampy lowlands onto the plateau [38]. The species was present on the plateau before, as is attested often more important than in the soils of high moist [49]. forest [18]. Today, they tend to gain over the rock by charcoal remains savanna vegetation with the help of sed-dispersing [18,50]. This tendency may be locally reversed in favour of rock Savanna species when the ecotone is 4. DISCUSSION AND CONCLUSION disturbed. High forest similarly gains on the low Exceptional events occurring at several centuries forest, on the outside, at a very slow pace. Low for- ests intervals should be taken into account as well as all the have an original flora: in Les Nouragues, almost common constraints organisms have to face. Their 200 species are found only in this type of vegetation. exceptional nature explains that they have frequently This indicates that #$is so-called low Eorest is not a been ignored in functional ecology models, which mere ecotone between rock savannas and high forest, generally use climatic parameter averages. Connell but actually a distinct vegetation entity. These forests [SJ,however, considered that perturbations occurring are well adapteho xeric conditions, and may have at a moderate rate could be favourable to the mainte- known greater extensions in the past. Nowadays, they nance of a high species diversity. Sporadic catastro- could represent relict patches, like the enclosed the phes could give a new impulse to the dynamics of savanaas found near borders of the large forest forest ecosystems, maintaining biotopes and species blocks. which would otherwise tend to disappear. Two other types of vegetation, the liana. forests and Forest fires seem to have been the only type of cat- the bamboo thickets (‘cambrouzes’), materialize a astrophic event to have affected French Guiana in the stagnation in the regeneration process [2] and may course of the last milleniums. The region is located also constitute relict patches. Data are hot Suffkient as away from the cyclone routes and does not exhibit vol- yet to conclude on this point, and a human origin canic or seismic activity. Fires in tropical hyperhumid cannot be ruled out, in particular for the bamboo forests can ody develop beyond a certain water stress [8 thickets J. In French Guiana, the mammalian faunas, threshold, and have probably been triggered by cli- especially bats, often display sibling species which matic fluctuations which intensified the effects of the only differ by minute cranial or dental characters. Typ- dry season. ically, one of the species lives in high forest and the Studies undertaken on the Guianan forest eco- other jn marginal areas such as coastal savanna and system, which have focused on functional aspects as swamp borders or certain types of low forests [3]. well as on spatial organization at species or vegetation These ‘marginal habitats’ must have existed for a long type levels, show that present environmental condi- ’ time to have allowed speciation to take place. The tions by themselves cannot fully explain the situation origin of these formation types is probably unrelated prevailing today. This is particularly noticeable in the to the palaeofires which have occurred during the last case of species ,,and vegetation types which exhibit milleniums. However, their surface area may have long turnover rates or slow colonization rates. Under- been affected by these events. standing the mechanisms of forest processes calls for

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Forest perturbations in French Guiana 301 the integration of several scales of time and space, and, [5] Connell J.H., Diversity in tropical rainforest and cora1 reefs, in particular, scales adapted to slow-evolving biolog- Science 199 (1978) 1302-1310. O00 ical systems (100 ta 10 years, and'local to regional [6] Dumont J.F., Foumier M., Apport des étndes morphostmc- scales). The collection and carbon-dadng of sediments turales à la connaissance des mosaïques forestieres des grandes (in particular charcoal fragments) can constitute a plaines alluviales (Ucayali inférieur, Perou), Résumé symp. Dynamique à long terme des écosystèmes forestiers intemopi- sound basis for this type of approach, providing caw, OrstodCNRS, 1998, pp. 151-158. invaluable information towards the validation of [7] Durrieu De Madron L., Moaaliti, chablis et rôla des tr0uée.s hypotheses. dans la sylviculture avant et après exploitation sur le dispositif Comparisons made between Amazonia and western d'étude de Paracou - Guyane Française, thèse de doctorat, Equatorial Africa underline a fundamental difference [E]Engref, 1993,203 p. between these two regions: in the dry Gasc JP., Sastre C., Les formations ouvertes à graminées bam- Amazania, busiformes (cambrouzes} sont-elles en Guyane fiansaise le season is cloudless and imposes important water 10" stresses on the vegetatim [ 141, whiIe in AGica (in par- signe d'un ancien peuplement humain? Congrès national ticular southern Cameroon, Gabon and the Congo), the des sociétés savantes, Sciences 1 (1978) 98-103. [9] Grand atlas de l'archeologie, Larousse, The Time, 1990. dry season is overcast and misty, although rainless, [IO] De Granville J.J., Rain forest and xeric refuges in French and are less exposed to water stress 463. flora of [21, Guiana, 41: Prance G.T. (Ed.), Biological diversification in the This characteristic the west African dry season is tropics, Columbia Univ. Press., N.Y., 1982, pp. 159-181. thought to be caused by the upwelling of cold polar of 1111 De Granville J.J., Un cas de distribution particulier :les espèces water in the Golf Guinea. In contrast with the abun- forestieres'peri-amazoniennes, C.R. Soc. Biogéogr. 68 (1992) dant &mod fragments found in Amazonian and Gui- 1-33. anan sails, remains of forest fires are rare in west [I21 Grenand P., Histoire des Amerindiens, in: Atlas des D.0.M - Africa and are generally assaciated settle- la Guyane, 1979, pl. pp. 3-4. with human C., 17, ments [45, pers. comm]. 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