# From inselberg to inselberg: floristic patterns across scales in French Guiana (South America)

Corinne Sarthou, Sandrine Pavoine, Jean-Pierre Gasc, Jean-Christophe de Massary, Jean-François Ponge

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Corinne Sarthou, Sandrine Pavoine, Jean-Pierre Gasc, Jean-Christophe de Massary, Jean-François Ponge. From inselberg to inselberg: floristic patterns across scales in French Guiana (South America). Flora, 2017, 229, pp.147-158. 10.1016/j.flora.2017.02.025. hal-01494703

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1 From inselberg to inselberg: floristic patterns across scales in French

2 Guiana (South America)

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4 Corinne Sarthou¹,*, Sandrine Pavoine², Jean-Pierre Gasc³, Jean-Christophe de Massary⁴, Jean-

5 François Ponge³

6 ¹Muséum National d'Histoire Naturelle, Institut de Systématique, Évolution, Biodiversité,

7 ISYEB UMR 7205 – CNRS, MNHN, UPMC, EPHE, Sorbonne Universités, 57 rue Cuvier, CP

8 39, 75005 Paris, France

9 ²Muséum National d'Histoire Naturelle, Département Écologie et Gestion de la Biodiversité,

10 UMR 7204 – CNRS, UPMC, 55-61 rue Buffon, 75005 Paris, France

11 ³Muséum National d'Histoire Naturelle, Département Écologie et Gestion de la Biodiversité,

12 UMR 7179 – CNRS, MNHN, 4 avenue du Petit Château, 91800 Brunoy, France

13 ⁴Muséum National d'Histoire Naturelle, Service du Patrimoine Naturel, 36 rue Geoffroy

14 Saint-Hilaire, CP 41, 75005 Paris, France.

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16 *Corresponding author

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18 E-mail addresses of the authors:

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20 C. Sarthou: sarthou@mnhn.fr

21 S. Pavoine: pavoine@mnhn.fr

22 J.P. Gasc: gasc@mnhn.fr

23 J.C. de Massary: jean-christophe.demassary@mnhn.fr

24 J.F. Ponge: ponge@mnhn.fr

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ABSTRACT

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Granitic outcrop vegetation was compared in 22 inselbergs of French Guiana, South America, using RLQ and fourth-corner analyses to identify the main relationships between environmental gradients and plant traits. At the scale of the whole territory the distribution of species and species traits was mostly driven by a spatially-structured gradient embracing regional climate (annual rainfall), forest matrix (canopy openness), and inselberg features (altitude, shape, habitats, summit forest, degree of epiphytism, fire events). Biogeographic, environmental and past historical factors contribute to explain the variation observed at coarse scale and two groups of inselbergs are identified. A first group occupies the southern peneplain in a semi-open forest matrix and exhibits a higher representation of suffrutescent species and climbers, a lower representation of upright shrubs, a lower degree of Guiana Shield endemism, and a higher incidence of human use and autochory. All these features suggest an adaptation to more disturbed environments linked to past climate changes and savannization and to human influences. A second group, characterized by opposite plant traits, occupies the northern part of French Guiana and the far south within a closed forest matrix. Within archipelagos (inselbergs at less than 7 km distance), C-score and Mantel tests revealed a random co-occurrence of plant species and an increase of floristic dissimilarity with distance without any concomitant change in plant traits, respectively, suggesting that spatially-structured stochastic factors (limitation by dispersal) were the driving force of vegetation change at fine scale.

**Keywords:** French Guiana; Inselbergs; RLQ and fourth-corner analysis; Endemism; Human use; Savannas
## 1. Introduction

Inselbergs are dome-shaped rocky outcrops rising above surrounding plains (Bornhardt, 1900). They have been reviewed worldwide for their geological origin and for their biotic diversity by Porembski and Barthlott (2000). They vary in size and height, being sometimes limited to only a few meters (Bremer and Sander, 2000). They can be considered as analogous to functional islands (Prance, 1996) in a general matrix such as deserts, savannas or forests. Whatever their dimensions, the crystalline rocky surface of inselbergs is covered by a peculiar vegetation growing in a shallow soil layer, a general property of rocky outcrop vegetation adding an ecological dimension to the geomorphological definition of “inselbergs” (Gröger and Barthlott, 1996).

In northern South America, inselbergs are especially frequent in French Guiana within the Guiana Shield (Fig.1). They consist of Precambrian granites. Detailed accounts on their geomorphology and geology were provided by some authors (Hurault, 1973; Teixeira et al., 1989; Delor et al., 2003). French Guianan inselbergs are isolated or clustered in groups called archipelagos. They are mainly distributed in all the southern landscapes (Fig. 1): especially large inland plains, plateaus and mountain chains (Guitet et al., 2013). They are covered with discontinuous vegetation forming a type of mosaic locally called “rock savanna”. This vegetation is characterized by numerous scattered herbaceous and shrubby patches separated by bare rock (de Granville, 1978; Sarthou, 1992; Sarthou and Villiers, 1998; Sarthou et al., 2003). The “rock savanna” vegetation is kept in a dynamic state by erosion and fire events (Kounda-Kiki et al., 2008; Sarthou et al., 2009). It undergoes extreme local climate variation during the day-night cycle: high and frequent variations in temperature (18-55°C) and relative humidity (20-100%; Sarthou, 1992). Cyanobacteria cover the surface of granitic outcrops (Sarthou et al., 1995) and take part in organic matter production and incipient soil formation (Vaçulik et al., 2004). Soils are generally shallow, acid with low water-holding capacity
(Sarthou and Grimaldi, 1992). So, inselbergs may be viewed edaphically and climatically as xeric habitats, creating xeric habitat islands in a humid forest matrix.

The influence of past climatic changes in tropical rainforests, especially in South America, has been widely debated (Colinvaux et al. 2000, Willis and Whittaker 2000). The periodic glaciations of the Quaternary corresponded to drier climatic conditions (Hooghiemstra and Hammen 1998). Climatic changes during the last Pleistocene glaciation (between 22 000 and 10 000 years ago) were characterized by a reduction of rainfall and a drop in temperature by up to 5°C compared to the present (Dynesius and Jansson 2000).
Consequently, species distributions have been strongly affected (Pennington et al. 2000, Bonaccorso et al. 2006). It has been suggested that these drier and colder periods would have led to the expansion of open vegetation types such as savannas (Hooghiemstra and Hammen 1998). However, climate shifts did not have the same effect over the whole French Guiana territory. During the Holocene, rain forests of the northern part of French Guiana probably remained within the range of more than 2000 mm annual rainfall (Hooghiemstra and van der Hammen, 1998), and soil carbon isotope studies showed that they did not undergo savannization (Freycon et al., 2010). Forest refuges were located in the north while savanna-like vegetation extended in the south (de Granville, 1982; Tardy, 1998). Following de Granville (1982), inselbergs may be interpreted as xeric flora refuges during the present wet period.

The present paper makes the first synthetic and comparative overview of the vascular vegetation spread over granitic outcrops of French Guiana. Our aim is to elucidate the patterns of floristic variation in relation to space, local environment, surrounding environment, plant traits, biogeographic affinities and past history. We also postulate, according to Wiens (1989) that factors acting at fine scale (i.e. within archipelagos) differ from those acting at coarse scale (i.e. over the whole territory of French Guiana). Several hypotheses will be tested. First,
we hypothesize that at the scale of French Guiana (coarse scale) differences in floristic composition are mainly explained by environmental factors operating directly and indirectly through the selection of adaptive traits (Hypothesis 1). We also hypothesize that past floristic history (biogeographic affinities, climate changes, human settlements) influenced the present-day composition of inselberg vegetation (Hypothesis 2). At last we hypothesize that at fine scale, i.e. within archipelagos, differences in floristic composition result from dispersal limitation by distance rather than from positive and/or negative species interactions (Hypothesis 3).

## 2. Materials and methods

### 2.1. Study area

As part of the Guiana Shield in northern Amazonia, French Guiana (4°13′N, 52°59′W) covers about 85,000 km² and has a mean altitude of about 140 m above sea level with few mountainous peaks exceeding 800 m. Its basement, approximately 2.2–1.9 G years in age, corresponds to the oldest and most homogeneous part of the Guiana Shield (Delor et al., 2003). Its climate is equatorial and is characterized by 3000 mm mean annual rainfall, distributed along a Northeast to Southwest gradient decreasing from about 4000 to 2000 mm (Fig. 1), with rainfall being concentrated mainly between December and July (Héritier, 2011). Granitic inselbergs were mostly concentrated in southern and eastern parts of the territory (Fig. 1). In the extreme south, groups of elevated dome-shaped inselbergs border the Brazilian Amazonian basin. The peneplain in the southern half part of French Guiana (Fig. 1) is characterised by low elevated and slightly slanting rocky slags in a semi-open forest matrix. The northern half part of French Guiana is covered with a dense rain forest with isolated dome-shaped inselbergs (Gond et al., 2011).
We studied 22 inselbergs (of which 15 are grouped in 4 archipelagos) scattered over the territory of French Guiana (Fig. 1). We carried out field work to analyse the vegetation of 20 granite outcrops from 1988 to 2011: Nouragues, Trinité A, Mont Chauve, Bakra (A, B), Roche Dachine, Arawa, Wanapi (A, B, C, D, E), Haut Marouini (A, B, C, D, F, G), Mont Saint Marcel, Mitaraka Sud (each letter designates an isolated inselberg). The data set was completed for Trinité B and Virginie through an extraction from the Cayenne Herbarium (CAY) database (available at http://publish.plantnet-project.org/project/caypub): the sampling effort devoted to Trinité B and Virginie by the collectors allowed us to consider these additional records as comprehensive and fully reliable (Appendix A). Geographic coordinates of each site (longitude, latitude) were recorded by a GPS device and are expressed in decimal degrees (coordinate system WGS 84; Appendix A). Sampling effort was adapted to area and environmental heterogeneity of each site, and was included in data analysis in the form of three scores: 1 from one to two days, 2 from 10 to 14 days, and 3 for more than 14 days.

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## 2.2. Sampling design

A comprehensive inventory of the vascular flora was undertaken on each rocky outcrop. However, we excluded from the analysis: (1) species growing in the ecotone zone between rock-savanna and the surrounding forested matrix; (2) species growing in summit forests (when present). Consequently, the analysis is restricted to the “rock savanna” facies *sensu stricto* characterized by vegetation patches on bare rock. Furthermore, we also exclude epiphytic species because the epiphyte layer requires a peculiar sampling protocol. We estimated that the sampling of epiphytes was insufficient.

The final data set comprises 318 plant species (Appendix B). Voucher specimens were deposited in the Cayenne Herbarium (CAY) and were registered in the Cayenne Herbarium database. Plant identification was performed according to Funk et al. (2007).
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## 2.3. Environmental data

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To characterize relevant aspects of the environment we used ten parameters: latitude-longitude, mean annual rainfall according to Héritier (2011), maximum altitude, outcrop area, geographical distance between outcrops, openness of the forest matrix surrounding outcrops, dominant savanna facies, habitats, and occurrence of a summit forest and a cliff (Appendix A).

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Mean annual rainfall was coded as 1 from 2200 to 2400 mm, 2 from 2400 to 2600 mm, 3 from 2600 to 2800 mm, 4 from 2800 to 3000 mm, 5 from 3000 to 3200 mm, 6 from 3200 to 3400 mm, 7 from 3400 to 3600 mm, and 8 from 3600 to 3800 mm and was treated as an ordinal variable.

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Maximum altitude was measured during field survey with a GPS device (Appendix A). The area of each outcrop was calculated from aerial photographs and was measured directly in the field for the smallest inselbergs (Appendix A). Geographical distances between inselbergs were calculated using the GIS program Mapinfo® version 9.0.

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Canopy openness of the forest matrix surrounding each inselberg was classified as an ordinal variable with three modalities according to data obtained from remote sensing (Gond et al., 2011): dense and regular canopy, mixed canopy and open canopy (Appendix A), coded 1, 2 and 3, respectively. Two facies of savanna (flat-savanna and steep-savanna) were distinguished, their dominance depending on whether the inselberg was flattened or dome-shaped, respectively (Appendix A). Each of these facies was coded as 1 or 0 according to its presence or absence in a given inselberg (nominal variable). Note that when both facies were co-dominant (in the case of Roche Dachine) these variables could thus not be considered as redundant and were treated separately.
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The list of habitats available on the rock outcrops was based on Porembski et al.

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(2000) after refinement from field observations. Twelve types of habitat were retained and

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treated as nominal variables with two attributes (presence/absence; see Appendix A):

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cryptogamic vegetation of rock surfaces (habitat 1), wet flush vegetation (habitat 2), vascular

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epilithic vegetation (habitat 3), vegetation of horizontal or vertical crevices (habitat 4),

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vegetation of seasonal rock pools in depressions (habitat 5), vegetation on rock debris in

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depressions (habitat 6), herbaceous vegetation of soil-filled depressions (habitat 7), woody

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vegetation of soil-filled depressions (habitat 8), ephemeral flush vegetation (habitat 9),

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monocotyledonous mats on sun-exposed steep rocky slopes (habitat 10), suffrutescent

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vegetation on sun-exposed rocky slopes (habitat 11) and woody vegetation on sun-exposed

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rocky slopes (habitat 12). Note that the first habitat type (cryptogamic vegetation =

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cyanobacteria or lichens in the inselberg context) was present everywhere and thus was not

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accounted for in the analyses. The number of epiphytic species was measured by an ordinal

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variable coded as 1 (< 5 epiphyte species), 2 (5-10 species), or 3 (> 10 species).

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## 2.4. Plant traits and biogeographic affinities

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To understand the response of outcrop vegetation to environmental variability, we

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selected seven plant traits and considered biogeographic affinities of each species (Table 1,

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Appendix B). Growth-form classification was adapted from Ramsay and Oxley (1997). Fruit

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types were grouped in four major categories according to Lorts et al. (2008). Seed dispersal

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syndromes were grouped in four categories according to van der Pijl (1982) and Howe and

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Smallwood (1982). Dispersal mechanisms of species were either recorded in the field or based

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on fruit and seed morphology. Data were also collected in literature (Roosmalen, 1985). Each

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plant species was assigned to one or more of these dispersal syndromes, which were treated as

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distinct dummy variables.
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Considering the high xeric environmental constraints on inselberg vegetation, species presenting a crassulacean acid metabolism (CAM) were identified from literature (Lüttge, 2007; Silvera et al., 2009, 2010; Crayn et al., 2015) and succulent species were also identified (personal observations).

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Human use of plant species in the Guianas was documented from Grenand et al.

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(2004), DeFilipps et al. (2004) and Cadamuro (2000) in order to disclose potential human-mediated effects.

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Data concerning the geographic distribution of taxa were mainly obtained from diverse Neotropical floras, especially Steyermark et al. (1995-2005) and Funk et al. (2007), and from the on-line Brazilian Herbarium (available at http://reflora.jbrj.gov.br/jabot/herbarioVirtual/). Biogeographic classes were first specified according to de Granville (1992) and Kelloff and Funk (2004), then were grouped in three categories according to their affinity to the Guiana Shield, the Neotropics or elsewhere (Table 1, Appendix B). A nominal variable describing the degree of Guiana Shield endemism was created by assigning to each group a score varying from 1 to 3.

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## 2.5. Statistical analyses

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The joint structure of species, species traits and environmental factors at the scale of the whole territory of French Guiana (Hypothesis 1) was analysed by a combination of two methods: RLQ (Legendre et al., 1997) and fourth-corner tests (Dolédec et al., 1996). Both methods are complementary and can be combined in a procedure described by Dray et al. (2014) which allows the graphical representation of species-trait-environment relationships and to test them by a Monte-Carlo simulation method. These calculations were done using the module ade4 package of R (R Core Team, 2016).
Three matrices were created for RLQ: a matrix crossing sites (inselbergs) with environmental variables (matrix R), a matrix crossing sites with species occurrences (matrix L) and a matrix crossing species with species traits (matrix Q). They were simultaneously ordinated, resulting in a factorial hyperspace of reduced dimensions (RLQ factorial axes) in which sites, species, species traits and environmental variables can be simultaneously projected, with permutation procedures to evaluate the significance of their relationships.

Taxonomic units, growth forms, fruit types, levels of endemism and carbon metabolic types were considered as nominal variables with 4, 9, 4, 3 and 3 modalities each, respectively.

Dispersal types, which are not exclusive, were considered as separate dummy variables. To original spatial variables (longitude as x and latitude as y), which only describe east-west and north-south gradients, respectively, we added polynomial combinations of scaled latitude and longitude ($x^2, y^2, x^3, y^3, x^2y, xy^2$) which could potentially reveal a higher variety of spatial patterns (Borcard et al., 1992). In matrices R and Q, ordinal variables were rank transformed and treated as quantitative. Both matrices were then analysed using Principal Component Analysis (PCA) designed for both nominal and quantitative variables (Hill and Smith, 1976).

Fourth corner tests were first used to test the significance of the relationships between each trait modality and each environmental variable. The high number of combinations did not allow finding significant relationships between traits and environment, once significance levels were adjusted for simultaneous comparisons, despite global significance of trait-environment relationships. Thereby the alternative procedure suggested by Dray et al. (2014) was applied. Cross comparisons between traits and environmental variables were replaced by testing directly the links between RLQ axes and both traits and environmental variables, after adjustment for multiple comparisons (Benjamini and Yekutieli, 2001), using the fourthcorner.rlq function.
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Variance partitioning between space and environment was achieved by the procedure proposed by Borcard et al. (1992), using simple and partial Redundancy Analysis (RDA) with either species or traits as dependent (explained) variables and environmental variables as independent (explanatory) variables. The species matrix was the L matrix used for RLQ. The trait matrix was constructed by assigning to each site (inselberg) the % contribution of each trait modality, which was calculated as the number of species possessing a given trait modality divided by the number of species recorded in the inselberg (Appendix A). Floristic and environmental data were standardized by reweighting and focusing (mean = 0, standard deviation = 1). Calculations were done with XLSTAT® version 2016.

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The influence of past floristic history (Hypothesis 2) was partly tested by the above procedures (RLQ followed by fourth-corner tests). The human use of plant species and their degree of endemism to the Guiana Shield (biogeographic affinity) were considered as traits, which were thus included in the Q matrix and analysed by RLQ and fourth-corner tests. Other factors could not be introduced as variables but a critical examination of a wide corpus of literature allowed discussing these influences in a special chapter.

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Fine-scale patterns of floristic change (Hypothesis 3) were studied by comparing species or trait dissimilarity (Jaccard index and Spearman-rank dissimilarity, respectively) with geographic distance for couples of inselbergs belonging to the same archipelago (Wanapi, Haut Marouini, Trinité, Monts Bakra), using the Mantel test. Monte Carlo simulation (9999 replicates) was performed within each inselberg group (archipelago) before computing a global correlation coefficient according to a procedure implemented in ade4 (R Core Team, 2016).

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Departure from random variation in the co-occurrence of plant species was tested both at coarse-scale (the whole set of inselbergs) and at fine-scale (within archipelagos) using checkerboard C-scores (Stone and Roberts, 1990). This measure of community structure
calculates the number of checkerboard units (specimens in which two taxa are not found together) between all possible taxon pairs in a matrix, and calculates a single score for the entire data set. The C-score is the average for all of the possible pairs in the matrix. This measure is compared with a null distribution of random matrices of the same size. If the observed C-score is larger than the score for the null hypothesis, it suggests significant segregation between taxa, and if the observed C-score is smaller than the score for the null hypothesis, it suggests significant aggregation between taxa. Regression tests were done with XLSTAT® version 2015, and C-scores were calculated using EcoSim Professional®.

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## 3. Results

Permutation tests (9999 replicates) showed that there was a global link between traits and environment (P < 0.01). The distribution of eigenvalues (1.485, 0.069, 0.046, 0.029, and 0.016 for the first five factorial axes) showed an abrupt shift from the first factorial axis (88.4% of total inertia) to the second one (4.1% of total inertia). Thereby only the first factor (called RLQ1) will be taken into consideration in the following, although two-dimensional plots were thought to be visually more comfortable. Results of RLQ analysis and fourth-corner tests (Hypotheses 1 and 2) were represented by figures 2 to 5. For the sake of clarity, sites, species, traits and environmental variables were represented as separate bi-plots but it should be noted that their projection was simultaneous.

The projection of sites (Fig. 2) showed a clear separation between two groups of inselbergs, (i) the southern peneplain (Haut Marouini, Wanapi, Arawa) with negative coordinates, and (ii) a group with positive coordinates comprising the far south (Mitaraka, Mont Saint Marcel) and the northern group (Roche Dachine, Monts Bakra, Mont Chauve, Nouragues, Virginie, Trinité).
For the two groups, dominant families and their species numbers are indicated in Table 2. The group including northern inselbergs, Mitaraka and Mont Saint-Marcel inselbergs, exhibits a greater number of families and species than the peneplain group (69 families versus 55 families; 265 species versus 149 species). In the first group, the best represented families are Orchidaceae and Bromeliaceae, including numerous epilithic and terrestrial herbs, and Myrtaceae, Clusiaceae and Rubiaceae, including numerous upright shrubs growing in dense thickets. All these families are poorly represented in inselbergs of the peneplain group (Table 2). In the two groups, Poaceae, Cyperaceae, Fabaceae and Melastomataceae families are well represented.

RLQ1 was significantly and positively associated with rainfall, latitude, squared and cubed latitude, inselberg area, presence of a summit forest, of steep slopes, epiphytism (all with P < 0.01), altitude and presence of habitat 10 corresponding to monocotyledonous mats (both with P < 0.05). It was significantly and negatively associated with openness of the surrounding forest matrix, longitude, the product of squared latitude with longitude, the presence of flat areas and of habitat 11 corresponding to suffrutescent vegetation (all with P < 0.01). The gradient depicted by RLQ1 associated spatial and environmental factors, opposing small, low elevated and flattened inselbergs of the southern peneplain to large, dome-shaped, elevated inselbergs of the northern group and of the extreme south along a gradient of increasingly rainy climate and associated ecological features (closed forest matrix, summit forest, bromeliacean mats, and shrub thickets with epiphytes).

Among plant traits, RLQ1 was significantly and positively associated with the highest degree of Guianan endemism (P < 0.05), upright shrubs (P < 0.01), and was significantly and negatively associated with human use, autochory, suffrutescence (all with P < 0.01) and climbers (P < 0.05).
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The decomposition of the total variance in environmental, spatial and spatially-structured (shared) variation (Table 3) showed that traits compared to species displayed a higher (x 2) contribution of spatially-structured environment and a lesser contribution of pure space (x 0.74). Both species and traits showed a higher contribution of pure environment than of pure space, the ratio being higher for traits (1.9) than for species (1.5).

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At fine scale (Hypothesis 3), Figure 6 showed a discrepancy between species and traits when their dissimilarity was plotted against geographical distances: dissimilarity between inselbergs included within archipelagos increased with distance when species were considered (Mantel correlation coefficient r<sub>M</sub> = 0.53; P = 0.019), while the correlation between trait and geographical matrices was feeble (r<sub>M</sub> = 0.10) and not significant (P = 0.62) when traits were considered.

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Checkerboard scores (Table 4) showed that plant species co-occurred at random at fine scale (among inselbergs located within the same archipelago) but did not at coarse scale (over the whole set of inselbergs), pointing to stochastic factors explaining changes in floristic composition within archipelagos, while floristic changes between archipelagos are mainly explained by non-stochastic factors.

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# 4. Discussion

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## 4.1. Abiotic parameters and response traits at coarse and fine scale

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RLQ and fourth-corner analyses support our Hypothesis 1 and reflect how at coarse scale, differences in floristic composition are mainly explained by environmental factors operating directly and indirectly through the selection of adaptive traits. Species trait distribution changed according to a composite, spatially-structured gradient, embracing variations in climate (annual rainfall, and associated vegetation features: epiphytism, summit forest, habitats), inselberg shape (flattened versus dome-shaped, low versus high elevation),

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composition of the surrounding forest (semi-open versus closed canopy) and human
influences. Among traits, suffrutescence, climbers, and autochory are drought-adaptive traits
(Monneveux and Belhassen, 1996; Schnitzer, 2005). These traits are better represented at a
lower level of annual rainfall on peneplain inselbergs by *Ernestia* spp. *Spermacoce* spp.,
*Dioclea guianensis*, *Cissus erosa*, *Omphalea diandra* (Fig.3). Conversely, upright shrubs
represented by *Clusia* spp. and *Myrcia* spp. (Fig. 3), which constitute dense thickets on
northern inselbergs (Sarthou et al., 2003; Fonty et al., 2009), and Guianan endemics, adapted
to a more rainy climate than Central Amazonian species (Ter Steege et al., 2000), are better
represented at a higher level of annual rainfall, showing the importance of climate in the
composite gradient depicted by RLQ. A similar situation has been recorded in a study
performed on African inselbergs by Parmentier et al. (2005), showing rainfall as the second
main driver of floristic heterogeneity at regional scale, after altitude.

Among habitats, annual-perennial herbaceous mats, the dominant vegetation type of
inselberg savannas (Porembski et al., 2000) show a southwards shift from epilithic
bromeliacean mats (habitat 10) on slopes with flowing water, dominated by *Pitcairnia
geyskesii* (Fig. 3) to grassy suffrutescent meadows (habitat 11) on flat areas with shallow
mineral soil, dominated by suffrutescent species such as *Stylosanthes guianensis*,
*Chamaecrista desvauxii*, *Ernestia* spp., *Unxia camphorata*, *Sinningia incarnata*, *Sipanea
pratensis*, *Spermacoce* spp. and graminaceous species such as *Ischaemum guianense* and
*Axonopus ramosus* (Fig. 3). Habitat 11 is reminiscent of the dominant inselberg vegetation of
less rainy countries both in the Neotropics (Porembski et al., 1998) and in Africa (Parmentier
et al., 2005) which have in common poikilohydric sedges belonging to the genera *Trilepis* and
*Afrotrilepis*, respectively.

The climate effect, mediated by adaptive traits, does not preclude the existence of
other indirect effects of climate, such as those mediated by surrounding cryptogamic
vegetation (biological crusts), known to affect germination and survival of vascular plants and nutrient availability (Belnap et al., 2001; Sarthou et al., 2009): the lower biomass of cyanobacteria or the replacement of cyanobacteria by lichens along the southwards aridity gradient has probably influenced the floristic composition to some extent.

Climate influences also the structure of the forest matrix (the rain forest) and thus influences regional species pools from which inselberg floras are filtered (Burke, 2002). A noticeable difference in floristic composition (beta diversity) was reported between the forest located in the northern part and the southern part of French Guiana, in relation to geomorphology (Sabatier and Prévost, 1990), and a general pattern may be described at a regional scale (Gond et al., 2011; Guitet et al., 2015). At present, southern forests, with their lower level of annual rainfall, are characterized by their almost open aspect (discontinuous canopy), especially in the southern peneplain where numerous local clearings are found, while northern forests show a continuous canopy typical for rain forests (Gond et al., 2011).

Elevation may also yield contrasting effects on floristic composition. If present and past climate prove to be important factors, local environmental variables (shape, elevation, presence of summit forest) also contribute strongly to explain inselbergs' floristic composition. As evidence, all dome-shaped and elevated inselbergs with summit forest are quite similar in composition even if far in the (drier) south, like St Marcel and Mitaraka, or in the (wetter) north, like Nouragues. We suppose that this local environmental effect which contributes to the observed floristic variation (Table 3) could be related to micro-climate. In fact it has been shown that at mid-elevation water condensation favours fern diversity (Kluge et al. 2006) and a higher biomass and diversity of epiphytes (Gehrig-Downie et al., 2011, 2013). The frequent mists we observed on the most elevated inselbergs (Mitaraka) in the dry season, together with relatively low temperature (19°C), may explain why inselbergs of the far south, while being in a lower precipitation zone (Fig. 1) share more species and species
394

traits with the northern group than with the southern group. Montane species settled during
395 glacial (drier) periods may have survived here under these conditions, making Mitaraka and
396 Saint Marcel important refuges for Pleistocene floras.

395

396

397

At fine scale, C-scores (Table 4) showed that inselbergs of the same archipelago did
398 not display any checkerboard pattern of floristic composition. There was no segregation of
399 species due to competition (Bowker and Maestre, 2012) and/or priority effects (Fukami,
400 2015), as well as no positive interspecific interactions (López et al., 2016). However, floristic
401 dissimilarity within archipelagos increased with distance, while trait dissimilarity did not
402 change (Fig. 6), supporting our Hypothesis 3 of dispersal limitation by distance in a common
403 environmental context, i.e. when inselbergs share similar environmental conditions and have
404 most probably undergone a common history (Flinn et al., 2010). Parmentier and Hardy (2009)
405 also showed that differences in floristic composition result from dispersal limitation by
406 distance on African inselbergs.

398

399

400

401

402

403

404

405

406

407

408

## 4.2. Influence of past climate changes and human use

409

RLQ and fourth-corner analyses also allowed checking part of our second hypothesis
410 while taking Guiana Shield endemism and human use as proxies for past history.

410

411

Endemism is better represented at a higher level of annual rainfall. Past climate
412 changes during the Pleistocene and the Holocene are generally evoked to explain this. These
413 climatic changes have modified the floristic composition of the matrix, the rain forest being
414 replaced by open forests or even savannas, relative elevation being increased by lowering of
415 the sea level, with mountain taxa coming into lowland forest (Hooghiemstra and Hammen
416 1998, Pennington et al. 2004; Rull, 2005; Pinter et al., 2011). However, in French Guiana, a
417 clear difference appears between northern and southern forests. The northern part revealed a

412

413

414

415

416

417
418 high degree of tolerance during Holocene drier climatic periods (Hooghiemstra and van der

419 Hammen, 1998; Tardy, 1998; Freycon et al., 2010) and constituted rain forest refuges which

420 remained genetically isolated (Granville, 1982; Tardy, 1998; Boisselier-Dubayle et al., 2010).

421 This continuous-in-time isolation may be considered as a reason for a high degree of

422 endemism (Vuilleumier, 1970). Our data (Appendix A) showed that inselbergs of the northern

423 part had on average 30.1±1.4% (mean ± SE) Guianan endemics (Guiana Shield + Guianas +

424 French Guiana) against only 19.7±0.8% for the southern group, the Mitaraka group being

425 intermediate with 23.6±0.4% endemics.”

426 In contrast, the southern part displays much disturbance and fire witnesses (Tardy,

427 1998), suggesting the occurrence of a past savanna-like matrix (and thus numerous exchanges

428 of diaspores) around the numerous inselbergs present today in the peneplain zone. Human

429 use, autochory as characteristic dispersal type, suffrutescence and climbers as characteristic

430 growth types, all these features being associated with the ‘negative’ side of the gradient, are

431 indicators of disturbance of the environment. Autochory, i.e. the absence of dispersal, does

432 not facilitate avoidance in space but generates an abundant soil seed bank, an adaptive

433 strategy used by plant communities when faced to recurrent destruction and regeneration of

434 the vegetation cover (Valbuena and Trabaud, 2001). Suffrutescence and climbers are also

435 associated to such environments, with a high power of post-disturbance regeneration from

436 subterranean organs (Keeley et al., 2006; Schnitzer, 2005). Among causes of disturbance, in

437 addition to seasonal and inter-annual abrupt changes in climate conditions, human activities

438 probably play a non-negligible role, with concomitant increases in trampling, fire, erosion and

439 deletion of species (Pinter et al., 2011). The low elevated and flattened inselbergs of the

440 peneplain are of easy access and Hurault (1989) indicates a high density of Indian tribes in

441 this southern zone.
Otherwise, the ancient and long-time occurrence of Indian populations in the
peneplain was considered. Convergent data integrate the southern plain of French Guiana
within a large continuous savanna zone spreading from the Venezuelan Llanos, including
Roraima, Rupununi, Sipaliwini savannas (Fig. 1) to the upper and middle parts of the northern
tributaries of the Amazon River during drier periods of the Holocene (Mayle and Power,
2008). This zone also played the role of a corridor for human dispersal (Plew, 2005; Piperno,
2011). In this context, men probably lived in open areas and used slash-and-burn technics
only in the nearby forest, exploiting ecotone plant diversity (Plew, 2005). The fact that wild
manihot (Manihot esculenta ssp. flabellifolia) found on southern inselbergs displays a striking
genetic homogeneity (Duputié et al., 2009) is a probable legacy of ancient human
management by fire and cultivation in a savanna-like context. This created repeated contacts
between inselbergs and temporary colonization of open space in the plain by wild stems of
several cultivated plants. Floristic affinities between rock savannas of the peneplain and
savannas also suggest the occurrence of an ancient connection between them. Thus, we
evaluate that the peneplain inselbergs share 30% of species with Sipaliwini savanna (van
Donselaar, 1968; Oldenburger et al., 1973).

458

459

5. Conclusion

Our study provides evidence that regional climate, surrounding forest matrix and
inselberg features (altitude, shape, habitats, summit forest, epiphytism, fire events) are
associated to shifts in the distribution of species and functional traits and determine floristic
patterns on French Guianan inselbergs. The study also improves our understanding on how
past climatic changes leading to savannization events in the South and how humans using
outcrop plants can affect the species composition of inselbergs in the southern peneplain. The
high degree of Guiana Shield endemism of inselbergs located within the northern rainforest,
467

which were seemingly neither affected by past climatic changes nor by human uses points to

468

the need to protect them.

469

470

**Acknowledgements**

471

This research paper is the end-product of several projects and was mainly supported by

472

a grant from Feder n°1348 (Contrat Plan Etat Région with Ecofor/Gis Silvolab-Guyane, IRD,

473

MNHN as partners). We are grateful to Jean-Jacques de Granville for his field contribution,

474

help in species identification and fruitful discussions. We also thank two anonymous

475

reviewers for their constructive comments on the manuscript. We acknowledge the curator of

476

the Cayenne Herbarium (CAY), Sophie Gonzalez for providing database information. We

477

also thank G. Cremers, F. Crozier, L. Aliker, P. Gaucher for their assistance in field work. M.

478

Boudrie, T. Deroin, P. Grenand, V. Guérin, P. Héritier, P. Joubert and B. Lefeuvre have also

479

provided interesting data in their respective scientific domains.

480

481

**Appendix A. Spatial and environmental data**

482

**Appendix B. Floristic data**

483

Supplementary data associated with this article can be found, in the online version, at

484

http://dx.doi.org/xxxx.

485

486
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707 **Table 1.** Traits and biogeographic affinities of plant species used in RLQ and fourth-corner tests.

708 Numbers between brackets are the scores used for the ordinal variable describing the degree of Guiana

709 Shield endemism

<table><tr><td><strong>Taxonomic unit</strong></td><td>Fern<br>Magnoliid<br>Monocot<br>Eudicot</td><td><strong>Fruit type</strong></td><td>Fleshy fruit<br>Dry dehiscent fruit<br>Dry indehiscent fruit<br>Sporangium</td></tr><tr><td rowspan="10"><strong>Growth form</strong></td><td>Erect herb</td><td rowspan="5"><strong>Seed dispersal syndrome</strong></td><td>Anemochory<br>Zoochory<br>Autochory<br>Hydrochory</td></tr><tr><td>Rosette</td><td>C3</td></tr><tr><td>Cespitose</td><td>C4</td></tr><tr><td>Suffrutex</td><td>CAM</td></tr><tr><td>Upright shrub</td><td>Pantropical or cosmopolite (1)</td></tr><tr><td>Trailing shrub</td><td>Endemic of the Neotropics (2)</td></tr><tr><td>Parasitic shrub</td><td>Endemic of the Guiana Shield (3)</td></tr><tr><td>Small tree</td><td></td></tr><tr><td>Climber</td><td></td></tr><tr><td>Succulent</td><td></td></tr></table>

710

711
<table><caption>**Table 2.** Number of species in dominant plant families over all inselbergs and in each of the two inselberg groups</caption><thead><tr><th></th><th>All inselbergs</th><th>Northern/Mitaraka/Saint Marcel group</th><th>Peneplain group</th></tr><tr><th></th><th>74 families<br>318 species</th><th>69 families<br>265 species</th><th>55 families<br>149 species</th></tr></thead><tbody><tr><td>Bromeliaceae</td><td>15</td><td>12</td><td>6</td></tr><tr><td>Clusiaceae</td><td>10</td><td>11</td><td>4</td></tr><tr><td>Cyperaceae</td><td>19</td><td>18</td><td>10</td></tr><tr><td>Euphorbiaceae</td><td>10</td><td>9</td><td>5</td></tr><tr><td>Fabaceae</td><td>14</td><td>11</td><td>8</td></tr><tr><td>Melastomataceae</td><td>20</td><td>17</td><td>11</td></tr><tr><td>Myrtaceae</td><td>15</td><td>13</td><td>2</td></tr><tr><td>Orchidaceae</td><td>20</td><td>19</td><td>6</td></tr><tr><td>Poaceae</td><td>21</td><td>16</td><td>14</td></tr><tr><td>Rubiaceae</td><td>18</td><td>15</td><td>7</td></tr></tbody></table>

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<table><caption>**Table 3.** Decomposition of the total variance of species and species traits according to environmental and spatial factors, using simple and partial RDA (Borcard et al., 1992)</caption><thead><tr><th></th><th>Species</th><th>Traits</th></tr></thead><tbody><tr><td>Pure environment</td><td>50.4%</td><td>45.4%</td></tr><tr><td>Pure spatial</td><td>33.3%</td><td>24.5%</td></tr><tr><td>Spatially-structured environment</td><td>10.0%</td><td>19.9%</td></tr><tr><td>Total explained variation</td><td>93.7%</td><td>89.8%</td></tr><tr><td>Unexplained variation</td><td>6.3%</td><td>10.2%</td></tr></tbody></table>
<table><caption>**Table 4.** Checkerboard scores (C-scores) for co-occurrence of plant species on Guianan inselbergs. Numbers of inselbergs for all inselbergs and for Wanapi and Haut Marouini archipelagos are indicated between parentheses</caption><thead><tr><th></th><th>Observed index</th><th>Simulated index</th><th>P (obs &lt; sim)</th><th>P (obs &gt; sim)</th><th>Standardized effect size</th><th>Null hypothesis</th></tr></thead><tbody><tr><td>All inselbergs (22)</td><td>7.71</td><td>7.11</td><td>1</td><td>0</td><td>31.12</td><td>Rejected</td></tr><tr><td>Wanapi (5)</td><td>0.040</td><td>0</td><td>0.54</td><td>0.47</td><td>-0.057</td><td>Accepted</td></tr><tr><td>Haut Marouini (6)</td><td>0.097</td><td>0.096</td><td>0.85</td><td>0.15</td><td>0.94</td><td>Accepted</td></tr></tbody></table>

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719 **Figure captions**

720

721 **Fig. 1.** Map of French Guiana, showing the 22 studied inselbergs (red triangles; each letter designates an isolated inselberg), among the whole set of inselbergs (black dots), the water system and rainfall isohyets. Inserts at the top of the figure indicate the position of French Guiana within South America (left) and within the Guiana Shield (right), with country limits. Country names are indicated by their capitalized initial: B: Brazil; C: Colombia; FG: French Guiana; G: Guyana; S: Suriname; V: Venezuela. Savanna-like vegetation zones are colored. 1: Roraima; 2: Rupununi; 3: Sipaliwini; 4: Coastal savannas; 5: Llanos.

729 **Fig. 2.** RLQ multivariate analysis. Projection of sites (inselbergs) in the plane of the first two main components RLQ1 and RLQ2. Insert: distribution of eigenvalues in order of decreasing importance. WAN A to WAN E = Wanapi archipelago; HMA to HMG = Haut Marouini archipelago; ARA = Arawa; DAC = Roche Dachine; MIT = Mitaraka; SMA = Mont saint Marcel; NOU = Nouragues; VIR = Virginie; CHAU = Mont Chauve; TRI A and TRI B = Trinité archipelago; BAK A and BAK B = Monts Bakra archipelago. See Fig. 1 for their localization.

736 **Fig. 3.** RLQ multivariate analysis. Projection of plant species in the plane of the first two main components RLQ1 and RLQ2. The number of plant species is much higher than the number of dots indicated on the bi-plot, because many of them are superposed. Some important species have been stamped (for abbreviations see Appendix B).

740 **Fig. 4.** RLQ multivariate analysis and fourth-corner tests. Projection of species traits in the plane of the first two main components RLQ1 and RLQ2. Variables with significant relationships with RLQ1 (fourth-corner tests) are indicated with arrows. Each trait name is centered on the corresponding point.
744

**Fig. 5.** RLQ multivariate analysis and fourth-corner tests. Projection of environmental

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variables in the plane of the first two main components RLQ1 and RLQ2. Variables

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with significant relationships with RLQ1 (fourth-corner tests) are indicated with

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arrows. Each trait name is centered on the corresponding point.

748

**Fig. 6.** Relationship between floristic dissimilarity (Jaccard index for species, Spearman rank

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dissimilarity index for species traits) and geographical distance for inselbergs located

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within archipelagos (less than 7 km distance). y = floristic dissimilarity; x =

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geographic distance (km); r<sub>M</sub> = Mantel coefficient of correlation; * = P < 0.05; NS =

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not significant.

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<figure><img src="image_1.png" /><figcaption>Fig. 1</figcaption></figure>

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<figure><img src="image_2.png" /></figure>

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<caption>Fig. 2</caption>

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<figure><img src="image_3.png" /><figcaption>Fig. 3</figcaption></figure>

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<figure><img src="image_4.png" /></figure>

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785 Fig. 4

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<figure><img src="image_5.png" /><figcaption>Fig. 5</figcaption></figure>

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<figure><img src="image_6.png" /></figure>

$$y = 0.014x + 0.16$$

$$r_M = 0.53*$$

$$r_M = 0.10 \text{ NS}$$

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791 Fig. 6

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