Inselbergs are hills rising abruptly from the surrounding plains where cyanobacteria are the only... more Inselbergs are hills rising abruptly from the surrounding plains where cyanobacteria are the only living organisms under conditions of intense solar radiation. A survival mechanism to prevent UV-damage has been associated with synthesis of the ultraviolet-screening, photostable sheath pigment scytonemin. The organic extract of Scytonema sp., collected on the Mitaraka inselberg, French Guyana, yielded three new pigments, tetramethoxyscytonemin (1), dimethoxyscytonemin (2), and scytonine (3), derived from the scytoneman skeleton of scytonemin. These structures were assigned mainly on the basis of (1)H and (13)C NMR and MS experiments.
The combined effects of isolation within an unfavourable matrix and vegetation development were s... more The combined effects of isolation within an unfavourable matrix and vegetation development were studied in patches of scrub vegetation on a tropical inselberg (Nouragues, French Guiana). Three thickets similar to 10m2 area were sampled, located at similar to 30 m distance from each other, in which the same vegetation types were present at less than 3 m distance: canopies of Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrticeae), two stages of thicket development, and zones of destruction by fire. The shallow organic soil was sampled over similar to 2 dm2 and down to 10 cm. The matrix was comprised of granite without any soil development. Arthropods were classified into morphospecies and their food diets were characterized by optical methods. The effect of isolation (between-thicket variation) on morphospecies composition was negligible, while that of vegetation type and fire (within-thicket variation) was prominent, as ascertained by principal component analysis and tested by pa...
We questioned whether and how plant communities vary in space and time along an inselberg-rainfor... more We questioned whether and how plant communities vary in space and time along an inselberg-rainforest ecotone in relation to present-day warming and whether biotic and non-biotic factors could explain the observed patterns. The study took place on a granitic inselberg in the French Guianan (South America) rainforest (Nouragues Natural Reserve: 4°5′N, 52°41′W). In a diachronic study (1995–2005) embracing a severe El-Niño event in 1997, we analysed vegetation structure and composition along three transects subsuming whole environmental and topographical variations in the transition zone from shrub vegetation at the fringe of open-rock vegetation to tall-tree rainforest. Data were analysed by PCA. Major variations in species and trait distribution were described in the low forest, with two floristic types evidenced by first PCA component and verified by cluster analysis: one with floristic composition reminiscent of open-rock vegetation but with higher and continuous canopy, the other t...
A number of plant communities have been described on tropical inselbergs, known as hot spots of p... more A number of plant communities have been described on tropical inselbergs, known as hot spots of plant and animal biodiversity. However, few studies tried to question what drives seral processes in these harsh environments, submitted to natural hazards (violent storms, intense runoff and lightning strikes) which may destroy the vegetation cover and its accumulated organic matter. We analysed quantitative data from the granitic Nouragues inselberg (French Guiana) in order to discern how successional processes featured their variety. We showed that the transition from herbaceous carpets (bromeliaceous mats and grassy meadows) to woody vegetation (shrub thickets) was not conditioned by slope, but was truly successional. We also showed that there was a cycle of change in shrub thickets, reinitiated by the destruction of scrub vegetation by fire (lightnings), wood-destroying fungi and termites.
The census of vascular plants across a 10-year interval (1995-2005) at the fringe of a neotropica... more The census of vascular plants across a 10-year interval (1995-2005) at the fringe of a neotropical rainforest (Nouragues inselberg, French Guiana, South America) revealed that species richness decreased, both at quadrat scale (2 m2) and at the scale of the inselberg (three transects, embracing the whole variation in community composition). Juvenile stages of all tree and shrub species were most severely affected, without any discrimination between life and growth forms, fruit and dispersion types, or seed sizes. Species turnover in time resulted in a net loss of biodiversity, which was inversely related to species occurrence. The most probable cause of the observed species disappearance is global warming, which severely affected northern South America during the last 50 years (+2° C), with a concomitant increase in the occurrence of aridity.
The common development of vegetation and soil is a central issue in plant succession. We hypothes... more The common development of vegetation and soil is a central issue in plant succession. We hypothesized that areas of woody vegetation decay and accumulation on the ground (zones of destruction or 'micro-chablis') played a role in the successional development of vegetation patches on tropical inselbergs and that disturbance events could be inferred from the analysis of the organic matter accumulated along a successional gradient. The study was conducted in French Guiana (South America). Nine humus profiles (each comprised of a varying number of layers) were selected in shrub thickets (similar to 1 acre each) representative of three vegetation types of the rock savanna: canopies of pure Clusia minor (Clusiaceae), C. minor in mixture with Myrcia saxatilis (Myrtaceae) and zones of destruction. Using a dissecting microscope, a count point optical method for small soil volumes was employed to measure the volume ratio of each kind of humus component (107 categories) in the 62 laye...
The aim of our study was to describe succession related changes in humus profiles on moderate slo... more The aim of our study was to describe succession related changes in humus profiles on moderate slopes of a tropical inselberg (Nouragues, French Guiana). Nine humus profiles were collected in a stratified manner under two main communities on well-drained sites: carpets of Pitcairnia geyskesii (Bromeliaceae) and shrub thickets of Clusia minor (Clusiaceae), the latter including two stages of its dynamic development. The 53 sampled layers were analysed by an optical method, the volume ratio of 109 classes of litter/humus components being quantified by a count point method. Correspondence analysis (CA) revealed marked differences among humus forms. Pitcairnia carpets were characterized by the dominance of cyanobacteria which formed crusts with low faunal activity, except when they were colonized by enchytraeid worms. With advancing succession, we observed that leaf litter did not accumulate but rather was incorporated into organo-mineral excrements of macro-invertebrates under C minor. T...
In French Guiana, inselbergs in the form of granite outcrops rise abruptly from the surrounding r... more In French Guiana, inselbergs in the form of granite outcrops rise abruptly from the surrounding rain forest. They constitute isolated islands of a special type of vegetation restricted to this peculiar substrate. Shrub granitic vegetation, organized in thickets on open exposed rocks of inselbergs, are described using the Braun-Blanquet method combined with Correspondence Analysis. This phytosociological study revealed only one particular shrub community on each inselberg, including predominantly evergreen and sclerophyllous shrubs, especially microphanerophytes, belonging to the Clusiaceae, Myrtaceae and Bombacaceae. These outcrop communities exhibit species endemic to the Guianas region and also species rare in French Guiana. Affinities with flora of other inselbergs and vegetation types in South America are examined and discussed. Reasons for observed floristic and structural changes in each community are also discussed.
South American inselbergs constitute singular and fragmented habitats in the tropical rain forest... more South American inselbergs constitute singular and fragmented habitats in the tropical rain forest. Pitcairnia geyskesii is restricted to these habitats and exhibits both sexual and asexual reproduction. The genetic structure of populations on three inselbergs in French Guiana is examined by analysis of ten isozyme loci. All analyzed populations show high levels of genetic variation. On average, 63.3% of loci per population were polymorphic, with a mean number of 2.21 alleles per polymorphic locus, and mean observed and expected heterozygosities of 0.185 and 0.183, respectively. The analyses of genetic variability displayed at different levels (inselbergs, subpopulations, and mats) give different but complementary information. A significant multilocus disequilibrium was detected in one subpopulation, whereas none was observed within the whole populations sampled on the three inselbergs. Tests on spatial genetic structure indicate a patchy distribution of genotypes on two inselbergs. The data give some insights on the reproductive behavior of P. geyskesii. (1) Efficient sexual reproduction leads to seed recruitment at the level of the inselberg. (2) Both clonality and seed recruitment occur within mats. (3) Vegetative spread by fragmentation is involved in the establishment of new mats. There is substantial differentiation (F(ST) = 0.322) and low gene flow among inselbergs (Nm = 0.234). High genetic diversity within inselbergs appears as a consequence of the association of genet longevity induced by clonal replication and recruitment of new genets produced by sexual reproduction.
Granite inselbergs protrude from forest and savanna in the tropics. They are exposed to harsh cli... more Granite inselbergs protrude from forest and savanna in the tropics. They are exposed to harsh climates (alternation of heavy rain and severe drought) and provide little nutrient for plants. Soil animals and humus components were investigated in cyanobacterial crusts close to patches of epilithic vegetation on the surface of the Nouragues inselberg (French Guiana). Three biological crust samples, corresponding to bromeliacean carpets of increasing size (supposed of increasing age), were sampled for faunal and micromorphological studies. Arthropods (mainly mites and insects) were abundant and highly diversified, the more so after enchytraeid worms ate and transformed the cyanobacterial mass. Below the superficial cyanobacterial crust, humus was made of a loose assemblage of enchytraeid faeces where these animals were present, or of a compact assemblage of cyanobacteria and amorphous organic matter where mites were the dominant animal group. Roots abounded in the humified part of the c...
Tropical inselbergs are isolated rock outcrops with a special type of vegetation surrounded by ra... more Tropical inselbergs are isolated rock outcrops with a special type of vegetation surrounded by rain forest. They are exposed to a harsh climate (alternation of heavy rain and severe drought) and provide few nutrients for plant growth. The aim of our study was to investigate a possible correlation between primary plant succession, size and diversity of soil arthropods. The study site was the Nouragues inselberg, in French Guiana (South America). Nine soil samples (three samples in each vegetation type) were taken for the study of soil arthropod communities and their food habits in three habitat types: Pitcairnia geyskesii (Bromeliaceae), Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrtaceae), which represent three stages in a primary plant succession on this inselberg. Soil arthropods were classified into morphospecies under the dissecting microscope and their food habits were characterized by examining their gut contents under the light microscope. A variation in food habits was...
Tropical forests can be described as a mosaic of juxtaposed eco-units corresponding to different ... more Tropical forests can be described as a mosaic of juxtaposed eco-units corresponding to different stages of regeneration after treefals. However, these small-scale regeneration mechanisms alone cannot account for the different patterns 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
Inselbergs are hills rising abruptly from the surrounding plains where cyanobacteria are the only... more Inselbergs are hills rising abruptly from the surrounding plains where cyanobacteria are the only living organisms under conditions of intense solar radiation. A survival mechanism to prevent UV-damage has been associated with synthesis of the ultraviolet-screening, photostable sheath pigment scytonemin. The organic extract of Scytonema sp., collected on the Mitaraka inselberg, French Guyana, yielded three new pigments, tetramethoxyscytonemin (1), dimethoxyscytonemin (2), and scytonine (3), derived from the scytoneman skeleton of scytonemin. These structures were assigned mainly on the basis of (1)H and (13)C NMR and MS experiments.
The combined effects of isolation within an unfavourable matrix and vegetation development were s... more The combined effects of isolation within an unfavourable matrix and vegetation development were studied in patches of scrub vegetation on a tropical inselberg (Nouragues, French Guiana). Three thickets similar to 10m2 area were sampled, located at similar to 30 m distance from each other, in which the same vegetation types were present at less than 3 m distance: canopies of Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrticeae), two stages of thicket development, and zones of destruction by fire. The shallow organic soil was sampled over similar to 2 dm2 and down to 10 cm. The matrix was comprised of granite without any soil development. Arthropods were classified into morphospecies and their food diets were characterized by optical methods. The effect of isolation (between-thicket variation) on morphospecies composition was negligible, while that of vegetation type and fire (within-thicket variation) was prominent, as ascertained by principal component analysis and tested by pa...
We questioned whether and how plant communities vary in space and time along an inselberg-rainfor... more We questioned whether and how plant communities vary in space and time along an inselberg-rainforest ecotone in relation to present-day warming and whether biotic and non-biotic factors could explain the observed patterns. The study took place on a granitic inselberg in the French Guianan (South America) rainforest (Nouragues Natural Reserve: 4°5′N, 52°41′W). In a diachronic study (1995–2005) embracing a severe El-Niño event in 1997, we analysed vegetation structure and composition along three transects subsuming whole environmental and topographical variations in the transition zone from shrub vegetation at the fringe of open-rock vegetation to tall-tree rainforest. Data were analysed by PCA. Major variations in species and trait distribution were described in the low forest, with two floristic types evidenced by first PCA component and verified by cluster analysis: one with floristic composition reminiscent of open-rock vegetation but with higher and continuous canopy, the other t...
A number of plant communities have been described on tropical inselbergs, known as hot spots of p... more A number of plant communities have been described on tropical inselbergs, known as hot spots of plant and animal biodiversity. However, few studies tried to question what drives seral processes in these harsh environments, submitted to natural hazards (violent storms, intense runoff and lightning strikes) which may destroy the vegetation cover and its accumulated organic matter. We analysed quantitative data from the granitic Nouragues inselberg (French Guiana) in order to discern how successional processes featured their variety. We showed that the transition from herbaceous carpets (bromeliaceous mats and grassy meadows) to woody vegetation (shrub thickets) was not conditioned by slope, but was truly successional. We also showed that there was a cycle of change in shrub thickets, reinitiated by the destruction of scrub vegetation by fire (lightnings), wood-destroying fungi and termites.
The census of vascular plants across a 10-year interval (1995-2005) at the fringe of a neotropica... more The census of vascular plants across a 10-year interval (1995-2005) at the fringe of a neotropical rainforest (Nouragues inselberg, French Guiana, South America) revealed that species richness decreased, both at quadrat scale (2 m2) and at the scale of the inselberg (three transects, embracing the whole variation in community composition). Juvenile stages of all tree and shrub species were most severely affected, without any discrimination between life and growth forms, fruit and dispersion types, or seed sizes. Species turnover in time resulted in a net loss of biodiversity, which was inversely related to species occurrence. The most probable cause of the observed species disappearance is global warming, which severely affected northern South America during the last 50 years (+2° C), with a concomitant increase in the occurrence of aridity.
The common development of vegetation and soil is a central issue in plant succession. We hypothes... more The common development of vegetation and soil is a central issue in plant succession. We hypothesized that areas of woody vegetation decay and accumulation on the ground (zones of destruction or 'micro-chablis') played a role in the successional development of vegetation patches on tropical inselbergs and that disturbance events could be inferred from the analysis of the organic matter accumulated along a successional gradient. The study was conducted in French Guiana (South America). Nine humus profiles (each comprised of a varying number of layers) were selected in shrub thickets (similar to 1 acre each) representative of three vegetation types of the rock savanna: canopies of pure Clusia minor (Clusiaceae), C. minor in mixture with Myrcia saxatilis (Myrtaceae) and zones of destruction. Using a dissecting microscope, a count point optical method for small soil volumes was employed to measure the volume ratio of each kind of humus component (107 categories) in the 62 laye...
The aim of our study was to describe succession related changes in humus profiles on moderate slo... more The aim of our study was to describe succession related changes in humus profiles on moderate slopes of a tropical inselberg (Nouragues, French Guiana). Nine humus profiles were collected in a stratified manner under two main communities on well-drained sites: carpets of Pitcairnia geyskesii (Bromeliaceae) and shrub thickets of Clusia minor (Clusiaceae), the latter including two stages of its dynamic development. The 53 sampled layers were analysed by an optical method, the volume ratio of 109 classes of litter/humus components being quantified by a count point method. Correspondence analysis (CA) revealed marked differences among humus forms. Pitcairnia carpets were characterized by the dominance of cyanobacteria which formed crusts with low faunal activity, except when they were colonized by enchytraeid worms. With advancing succession, we observed that leaf litter did not accumulate but rather was incorporated into organo-mineral excrements of macro-invertebrates under C minor. T...
In French Guiana, inselbergs in the form of granite outcrops rise abruptly from the surrounding r... more In French Guiana, inselbergs in the form of granite outcrops rise abruptly from the surrounding rain forest. They constitute isolated islands of a special type of vegetation restricted to this peculiar substrate. Shrub granitic vegetation, organized in thickets on open exposed rocks of inselbergs, are described using the Braun-Blanquet method combined with Correspondence Analysis. This phytosociological study revealed only one particular shrub community on each inselberg, including predominantly evergreen and sclerophyllous shrubs, especially microphanerophytes, belonging to the Clusiaceae, Myrtaceae and Bombacaceae. These outcrop communities exhibit species endemic to the Guianas region and also species rare in French Guiana. Affinities with flora of other inselbergs and vegetation types in South America are examined and discussed. Reasons for observed floristic and structural changes in each community are also discussed.
South American inselbergs constitute singular and fragmented habitats in the tropical rain forest... more South American inselbergs constitute singular and fragmented habitats in the tropical rain forest. Pitcairnia geyskesii is restricted to these habitats and exhibits both sexual and asexual reproduction. The genetic structure of populations on three inselbergs in French Guiana is examined by analysis of ten isozyme loci. All analyzed populations show high levels of genetic variation. On average, 63.3% of loci per population were polymorphic, with a mean number of 2.21 alleles per polymorphic locus, and mean observed and expected heterozygosities of 0.185 and 0.183, respectively. The analyses of genetic variability displayed at different levels (inselbergs, subpopulations, and mats) give different but complementary information. A significant multilocus disequilibrium was detected in one subpopulation, whereas none was observed within the whole populations sampled on the three inselbergs. Tests on spatial genetic structure indicate a patchy distribution of genotypes on two inselbergs. The data give some insights on the reproductive behavior of P. geyskesii. (1) Efficient sexual reproduction leads to seed recruitment at the level of the inselberg. (2) Both clonality and seed recruitment occur within mats. (3) Vegetative spread by fragmentation is involved in the establishment of new mats. There is substantial differentiation (F(ST) = 0.322) and low gene flow among inselbergs (Nm = 0.234). High genetic diversity within inselbergs appears as a consequence of the association of genet longevity induced by clonal replication and recruitment of new genets produced by sexual reproduction.
Granite inselbergs protrude from forest and savanna in the tropics. They are exposed to harsh cli... more Granite inselbergs protrude from forest and savanna in the tropics. They are exposed to harsh climates (alternation of heavy rain and severe drought) and provide little nutrient for plants. Soil animals and humus components were investigated in cyanobacterial crusts close to patches of epilithic vegetation on the surface of the Nouragues inselberg (French Guiana). Three biological crust samples, corresponding to bromeliacean carpets of increasing size (supposed of increasing age), were sampled for faunal and micromorphological studies. Arthropods (mainly mites and insects) were abundant and highly diversified, the more so after enchytraeid worms ate and transformed the cyanobacterial mass. Below the superficial cyanobacterial crust, humus was made of a loose assemblage of enchytraeid faeces where these animals were present, or of a compact assemblage of cyanobacteria and amorphous organic matter where mites were the dominant animal group. Roots abounded in the humified part of the c...
Tropical inselbergs are isolated rock outcrops with a special type of vegetation surrounded by ra... more Tropical inselbergs are isolated rock outcrops with a special type of vegetation surrounded by rain forest. They are exposed to a harsh climate (alternation of heavy rain and severe drought) and provide few nutrients for plant growth. The aim of our study was to investigate a possible correlation between primary plant succession, size and diversity of soil arthropods. The study site was the Nouragues inselberg, in French Guiana (South America). Nine soil samples (three samples in each vegetation type) were taken for the study of soil arthropod communities and their food habits in three habitat types: Pitcairnia geyskesii (Bromeliaceae), Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrtaceae), which represent three stages in a primary plant succession on this inselberg. Soil arthropods were classified into morphospecies under the dissecting microscope and their food habits were characterized by examining their gut contents under the light microscope. A variation in food habits was...
Tropical forests can be described as a mosaic of juxtaposed eco-units corresponding to different ... more Tropical forests can be described as a mosaic of juxtaposed eco-units corresponding to different stages of regeneration after treefals. However, these small-scale regeneration mechanisms alone cannot account for the different patterns 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
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