# The use of vascular plant densities to estimate the age of undated lava flows in semi-arid areas of Fogo Island
(Cape Verde, Atlantic Ocean)

Sébastien Larrue, Raphael Paris, Samuel Etienne

► To cite this version:

Sébastien Larrue, Raphael Paris, Samuel Etienne. The use of vascular plant densities to estimate the age of undated lava flows in semi-arid areas of Fogo Island (Cape Verde, Atlantic Ocean). Journal of Arid Environments, 2019, 173, pp.104042. 10.1016/j.jaridenv.2019.104042. hal-02367322

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1 Vascular plant densities estimate the age of undated lava flows in semi-arid environment
2 on Fogo Island (Cape Verde, Atlantic Ocean).

3

4 Sébastien Larrue¹, Raphaël Paris², Samuel Etienne³.

5

6 ¹University Clermont Auvergne (UCA), GEOLAB-CNRS F-63057 Clermont-Ferrand,
7 France.

8 ²Laboratoire Magmas & Volcans, Université Blaise Pascal - CNRS - IRD, OPGC, 6 avenue
9 Blaise Pascal, 63178 Aubière, France

10 ³PSL University, EPHE, CNRS, LETG, Dinard, France

11

12 Abstract: The age of some lava flows on the east-north-east coast of Fogo Island (Cape Verde) remains debated or unknown due to the inapplicability of the conventional dating methods, while this information is important for a retrospective approach of volcanic hazard evaluation and prevention. Historical lavas represent an opportunity to study plant succession within a well-constrained chronological framework, and lava flow age was used usually as variable to explain plant densities and species composition. Here, vascular plant densities were also assigned as variables to explain lava ages. We surveyed 44 plots on eleven semi-arid lavas representing a chronosequence of 60- c. 350-yr-old lavas, including two undated lavas. We used Spearman's correlation to test the relationships between plant densities and lava ages. Stepwise regression model was then used to identify which species were more closely related to the age of lavas, and model equation was employed to predict the age of undated lavas. Finally, we confront regression model age-predictions with principal component analysis (PCA) result. Among 27 vascular plants, nine species were significantly linked with lava ages. Regression model showed that *Sarcostemma daltonii* and *Lantana*
1 *camara* densities explained significantly lava ages (adjusted R² = 0.733, P < 0.0001). PCA

2 demonstrated similar plant species densities on lavas were related to similar year-old lavas.

3 Model age-predictions were congruent with PCA result supporting that vascular plant species

4 densities may provide useful information about the relative age of undated lava flows in arid
5 environment.

6

7 Keywords: Fogo Island, semi-arid lava flows, vascular plants, chronosequence, Cape Verde,

8 island biogeography.

9

10 Introduction

11 Fresh mineral material deposited at the surface of the Earth offer the opportunity to study the
12 pattern and the timing of colonization of volcanic substrate by vascular plant species.

13 Volcanic lava flows are especially unique as they are sterile material recovering a preexisting
14 landscape where volcanic eruption acts as a reset factor for plant colonization (e.g. Whittaker
15 1984, Thornton et al. 1992). Hence, plant colonization on lava flows is mainly limited by the
16 suitable conditions for establishment and it is well known that such conditions evolve mainly
17 with time and climatic conditions (e.g. Whittaker et al. 1984, Fenner 1987, Drake 1992, Drake
18 and Dombois 1993, Aplet et al. 1998, Cutler et al. 2008).

19 Primary plant succession in island volcanic environment has been studied by many workers
20 and the increase of plant diversity with the age of lava flow seems quite expected (e.g.

21 Whittaker et al. 1992, Partomihardjo et al. 1992, and Thornton et al. 1992, on Anak Krakatau
22 Island; Aplet et al. 1998 on Hawaii Island; Cutler et al. 2008 on Iceland; Magnússon et al.

23 2014 on Surtsey Island), although the exact shape of the age-diversity might differ (Cutler
24 2010). Furthermore, in volcanic environments where historic effusive products create new
25 sterile land surfaces, lava flows can be used as chronosequences to study plant population
1 development (Drake and Dombois 1993, Cutler et al. 2008, 2010). Indeed, plant succession

2 has been investigated by comparing sites that differ in age but were geologically and

3 climatically similar (e.g. Drury and Nisbet 1973, Miles 1979, Drake and Dombois 1993).

4 Usually, historical lavas represent an opportunity to study plant succession within a well-

5 constrained chronological framework, and lava flow ages were used as variable to explain

6 plant densities and species composition (e.g. Aplet 1998). Conversely, plant densities and

7 composition on lava flows might provide useful information for the distinction of undated

8 lavas. Indeed, where land surface ages is unknown, geo-ecological successions have been

9 used formerly as relative-age dating of glacial foreland (Matthews 1992). Here, plant densities

10 in semi-arid conditions were assigned as variables to explain lava flow ages and such

11 approach is still very poorly documented.

12 On the east-north-east coast of Fogo Island (Cape Verde), the ages of some lava flows remain

13 unknown due notably to the lack of charcoal beneath these lava flows, preventing ¹⁴C dating

14 to be applied (Ribeiro 1960, Machado 1965, Torres et al. 1997, Day et al. 1999, Foeken et al.

15 2009), while this information may be relevant for a retrospective analysis of volcanic hazard

16 evaluation and prevention for local authorities. We argue that vascular plant species densities

17 may be used as indicator to provide information on the relative age of volcanic eruptions, as

18 complementary methodology or in case of non-applicability of the conventional methods such

19 as ¹⁴C. We hypothesized that similar plant species densities in comparable semi-arid

20 environment on lava flows might be related to similar year-old lavas.

21

22 **Materials & Methods**

23 *Study area*

24 The Cape Verde Archipelago includes ten ocean islands stretching in the eastern Atlantic

25 Ocean between 14°51′13″ – 17°03′29″N and 22°41′38″ – 25°10′31″W at c. 580 km from the
1 nearest African mainland and 1,300 km south of the Canary Islands. With a land surface of
2 476 km², Fogo Island is the fourth largest island of the Archipelago and the highest, reaching
3 2,829 m a.s.l. at *Pico do Fogo* volcano (Machado 1965). Fogo is one of the most active
4 volcano of the Atlantic Ocean and the only active volcano of the Cape Verde archipelago for
5 the last centuries (Machado 1965a-b, Torres et al. 1997, Dionis et al. 2015). The island is a
6 shield volcano truncated by an 8 km-wide horseshoe-shaped caldera opened to the east. The
7 caldera is related to a massive collapse of the eastern flank of the island (Day et al. 1999), the
8 latter being mostly covered by historical lava flows (Dionis et al. 2015, Paris et al. 2017).
9 The semi-arid climate of Fogo Island experiences two seasons: a cool dry season (November
to June) with very low rainfall and cool temperatures, and a warmer season with occasional
10 rainfalls (July to October) (Soares 1956, Amaral 1991). Fogo Island has a mean annual
11 temperature of 24.2°C and rainfall of 272 mm year⁻¹ at São Filipe, southwestern coast
12 (https://en.climate-data.org/). However, rainfall experience high variability due to elevation
13 range, influence of the continental Sahel zone, and exposure to the trade winds (Soares 1956,
14 Amaral 1991, Ribeiro 1997, Olehowski et al. 2008). Annual precipitation ranges from c. 150
15 mm at sea level to > 600 mm in higher elevation. Fogo Island has a leeward dry south-west
16 coast and a windward wetter north-east coast exposed to the dominant maritime trade winds
17 (Amaral 1991). Thus, rainfall ranges from 150-200 mm year⁻¹ at sea level on the leeward
18 coast to 200-250 mm year⁻¹ on the windward coast.
19 Since the 15th century, native vegetation cover of Fogo Island has changed widely due to
20 human colonization (Chevalier 1935). However, despite human settlements and activities
21 such as deforestation, agriculture and farmland, overgrazing, and the spread of many
22 naturalized species (Duarte & Moreira 2009, Siegmund et al. 2005), 41 endemic species still
23 observed among a total of 373 vascular plants on Fogo Island (Duarte et al. 2008, Romeiras et
24 al. 2015).
25
1

2 <Figure 1. near here>

3

4 *Lava flows sampled*

5 We selected 11 basaltic lava flows as a chronosequence located in semi-arid climate on the
6 eastern coast of Fogo Island (Fig.1). The year of eruption (AD) was documented by using
7 reports and geological maps for nine lava flows (Torres et al. 1997; revisited by Paris et al.

8 2017), while the date of one eruption was uncertain (proposed as “1664?” in Torres et al.

9 1997, here named Nd.10 (1664?)). The age of dated lava flows ranged from 67 to c. 350 yrs
10 old (defined as “historical” lavas). Two other lava flows locally covered by 1769 lava
11 remained undated (Nd.1 and Nd.9; Nd.1 being younger than Nd.9 as evidenced by the map of
12 Paris et al. 2017).

13 All of the 11 lava flows sampled shared similar field characteristics: they were identified as a'a lavas, i.e. rough lavas with a chaotic surface made of blocks formed during the fragmentation of the cooling crust during lava emplacement. These lavas were also similar in geology as all of the historical eruptions on Fogo Island produced alkali basalts classified as basanites in terms of major-element composition, with clinopyroxene and olivine as dominant phenocrysts (Machado 1965; Gerlach 1988, Doucelance et al. 2003, Cornu 2017). According to field observation, these rough lava flows were not cultivated.

20

21 *Plant count on lava flows*

22 When using a chronosequence, replicate sites of a given age are often unavailable, making
23 statistical analysis difficult (Drake and Dombois, 1993). Here, three replicate plots of 314 m²
24 (each) were delineated in the central part of the 11 lava flow sampled (i.e. not at the flow front
25 or lateral levees where environmental conditions and accessibility to successional plant
species might be different). The distance between the three replicate plots per lava flow

ranged between 5 and 15 m (not shown). A total of 33 314 m² plots were delineated.

In addition, one replicate of 140 m² per lava flow (n=11) was performed using vertical aerial
photographs at ~ 4 m above ground acquired with a DJI Phantom 2 drone (Plot 4, Table 1).

We then identified and counted plant species in laboratory. Note that this additional drone
replicate was done to improve the whole sample and save time during field survey.

All plots were selected in order to maximize homogeneity of environmental parameters such
as geology, elevation, terrain roughness, and climatic conditions. General plot characteristics
are given in Table 1. In these plots, vascular plant species including (i) natives (i.e.

indigenous and endemic species), and (ii) naturalized species (i.e. formerly introduced, since
naturally dispersed in environment) were investigated. We recorded species richness,
abundance (i.e. the total number of plants), and species abundance (i.e. the number of
individuals per species). Fieldwork was led at the rainy season (in September) in the exposure
rainfall zone of 200-300 mm year⁻¹ (Olehowski et al. 2008).

In order to homogenise plot area, we calculated the density of every species per 314 m² and
140 m² plots (i.e. the number of individuals per species divided by plot area; here after species
density per m²). Thus, we obtained (i) the density of every species per plot and (ii) the density
of every species per lava flow (by summing species, divided by total plot area 1-4).

## Statistical analysis

First, we used the simple Sørensen Index to confirm similarity of the three replicate plots on
each lava flow sampled (Plots 1-3, Table 1). Note that Sørensen Index was not calculated for
the 1951 lava flow (lack of vascular species on this lava).

Second, linear regression was used to observe the relationships between plants abundance and
species richness versus age of lava flows, while Spearman's correlation was used to provide
1 an overall picture of the relationships between each species density and the age of lava flows.

2 Note that plots of Nd.1 and Nd.9 lavas were not included in the analysis (undated lavas). In

3 order to limit some effect of intercorrelation between species, all vascular species age-

4 correlated were assigned as variable to explain the age of lava flows in stepwise regression

5 model. We then used stepwise model equation to provide age-prediction of both undated lava

6 flows.

7 Finally, in order to test if similar species densities were related to similar years of eruption,

8 we ran Principal Component Analysis (PCA, multivariate analysis) by using the density of

9 each vascular plant species (species composition) in the 44 plots. This result was presented

10 per lava flow. We observed which lava flows were significantly correlated with undated lavas

11 (Nd.1, Nd.9) and compared PCA result with model age-predictions.

12

13 <Table 1. near here>

14

15 **Results**

16 *Similarity of plot replicates*

17 Sørensen Index ranged between 0.5 to 1 supporting relative similarities of the three plot

18 replicates (Table 1). The mean value of Sørensen Index for the three plot replicates ranged

19 between 0.666 and 0.904 (Table 1).

20

21 *Plant species composition on lava flows*

22 In the 44 plots, we found a total of 27 vascular plant species representing an abundance of

23 3,092 individual plants. Abundance per lava flow ranged from 1 to 663 individuals while

24 species richness ranged from 1 to 19 species. Species richness per plot ranged from 0 to 16

25 while abundance ranged from 0 to 135 (Table 2). We encountered 15 shrubs, 5 trees, 2
1 succulents, 2 herbs, 2 ferns, and 1 vine. We observed more commonly *Sarcostemma daltonii*,

2 *Dryopteris* sp., *Polypodium* sp., *Euphorbia tuckeyana*, *Lavandula rotundifolia*, and

3 *Paronychia illecebroides* (Fig. 2). As expected, old a'a supported more species and

4 abundance than young a'a.

5 Almost 60% of the vascular plants observed in the plots were reported as native while the

6 others were naturalized, including invasive species (Table 2). These latter were mostly found

7 on the oldest and undated lava flows (Fig. 3).

8

9 <Table 2. near here>

10 <Figure 2. near here>

11 <Figure 3. near here>

12

13 *Relationships between plant species and age of lava flows*

14 Overall, total abundance and species richness in the 314 m² plots were significantly related to

15 the age of lava flows (Fig. 4a-b). Among the 27 plant species observed in the plots, nine

16 species were significantly linked with the age of lava flows: *Abrus precatorius*, *Asparagus*

17 sp., *Euphorbia tuckeyana*, *Jatropha curcas*, *Lantana camara*, *Lotus Jacobaeus*, *Lotus*

18 *purpureus*, *Polypodium* sp., and *Sarcostemma daltonii* (Table 3). Strongest relationships, with

19 R² ranging from 0.324 to 0.711, were observed with both native species *S. daltonii*, *E. tuckeyana*, and the naturalized species *J. curcas*, *L. camara* (Table 3). No significant

20 relationship was found with the density of other species and the age of lava flows in the 44

21 plots (Spearman's correlation). Density of the pioneer endemic *S. daltonii* increased from 65

22 to *c.* 230-yr-old lava flows (Fig. 5). *Euphorbia tuckeyana* density increased from *c.* 220 to *c.*

23 250-yr-old lava flows and then decreased, while naturalized species *L. camara* and *J. curcas*

24 density strongly increased from *c.* 250 to *c.* 350-yr-old lava flows (Fig. 5).

25
1 Among the nine vascular plant species age-correlated, stepwise model showed a better fit of

2 species density versus age of lavas by using mostly *S. daltonii* with *L. camara* (Table 4).

3 According to stepwise model equation, Nd.1 and Nd.9 lavas would be ~ 249 and 256 ± 40 yr-

4 old, respectively.

5

6 <Figure 4a-b. near here>

7 <Table 3. near here>

8 <Figure 5. near here>

9 <Table 4. near here>

10

11 *Lava flows comparaison*

12 Significant relationships were found between species densities and similar years of volcanic

13 eruption (PCA, Fig. 6). Spearman's correlation values ranged from 0.497 to 0.778, P ≤ 0.01

14 (Table 5). Lava flows were clearly distinguished with two groups: Nd.10 (1664?)-1785 and

15 1799-1857 (Fig. 6). Species densities of undated Nd.1 and Nd.9 lava flows were linked and

16 fitted with the 17th-18th century lava flows (Table 5 & Fig. 6). Nd.1 lava was linked with 1769

17 and 1785 lavas while Nd.9 lava was linked with Nd.10 (1664?) and 1769 lavas. The lava flow

18 Nd.10 (1664?) was significantly linked with 1785 lava (Table 5).

19 PCA axis F1 explained a large part of the total inertia of the dataset (44.63%, Fig. 6). This

20 axis was controlled mostly by *S. daltonii* (127.8%), *Dryopteris* sp. (11.1%), and *Polypodium*

21 sp. (2.5%) (Table 6). Axis F2 explained 18.5% of the total inertia; *Dryopteris* sp. (23.5%),

22 *Polypodium* sp. (9.2%), and *E. tuckeyana* (3.8%) were the most important explanatory

23 variables for this axis (Table 6).

24

25 <Table 5. near here>
1 <Figure 6. near here>

2 <Table 6. near here>

3

4 Discussion

5 *Trend in species and age of lava flows*

6 On Fogo Island, plant species richness and abundance are related to the age of lava flows.

7 Thus, colonization of the sampled lava flows by vascular plants is characterised by a temporal evolution related to the age of lava flows with successive stages of vegetation development.

9 This result is congruent with many studies. For example, Drake and Dombois (1993) demonstrated, on Hawaii Island, an increase in plants density on a chronosequence of six lava flows aged between 47 and ~3,000-yr-old with changes in dominant species on the oldest lavas. In Iceland, similar result was obtain from Cutler et al. (2008) whose defined, with a 850-yr chronosequence on seven lava flows, the stages of vegetation development as “Pioneer colonization stage”, then “Pioneer expansion stage”, and “Higher plant colonization stage” including additional species growing within the pioneer matrix.

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

On Fogo Island, the increase of plant species number with time appears to be very slow on the semi-arid lava flows sampled. Indeed, we found only five and one species on the 1852 and 1951 lava flows, respectively. So, a period of at least 65 years is required for plant colonization, namely by *S. daltonii*. This result is in agreement with Chevalier (1935) who reported only one species (*S. daltonii*) on the 1852 lava flow, which was 83-yr-old in time of observation. First, this time lapse before pioneer plants start to colonize lava may be explained by changes of lava environment, here with a weak alteration of lava due to poor chemical weathering of basic volcanic rock in such semi-arid conditions (Barros 1998, Karthe and Siegmund 2004) leading to slow plant colonisation. For example, Anak Krakatau Island (where vascular plants were extirpated due to major eruption in 1952) supports an extensive
1 forest along the wet eastern/north-eastern coastline, while the xeric island interior lavas
2 remain sparsely colonized by vascular plants (Partomihardjo et al. 1992). Second, plants
3 colonization on lava is controlled by the time required for random seeds dispersal from adult
4 plants. Indeed, it has long been demonstrated that during primary ecological succession on
5 lava flows, no seed bank exists in the sterile new substrate. Thus, all pioneer plants must be
6 recruited from the seed rain, involving wind dispersed taxa as well as mammal and bird-
7 dispersers availability (e.g. Drake 1992, Whittaker et al. 1992).
8 Our findings show that some vascular plant species led the relationships between species
9 density and the age of lava flows, among which S. daltonii, E. tuckeyana, J. curcas, and L.
10 *camara* are significant species. The endemic species S. daltonii is the most abundant species
11 observed on the lava flows sampled and the dominant pioneer on lavas of this area. It may
12 refer to early "Pioneer colonization stage" on lava flows < 160 yr-old. We then also observed
13 a significant increase density with time for species E. tuckeyana, supporting a "Pioneer
14 expansion stage" on 160-230 yr-old lava flows. Here, we also found naturalized species J.
15 *curcas*, and the well-known island invasive species L. *camara*. Moreover, stepwise model
16 confirms that density of S. daltonii associated with that of L. camara is the best indicator to
17 explain the age of lava flows sampled.
18 Overall, the presence of natives and naturalized species on the same lava might be viewed as a
19 bio-indicator of the oldest lava flows sampled and interpreted as "Higher plant colonization
20 stage" on > c. 230 yr-old lava flows. However, because a chronosequence assumes a
21 substitution of space for time, result provides only a partial assessment of succession (see
22 Drake and Dombois, 1993).
23
1 *Some references to invasive species*

2 On Fogo Island, the invasive species *L. camara* represents a potential threat for native species

3 established on the same lava flows, especially for the native *E. tuckeyana*, *L. jacobaeus* and *L.*

4 *purpureus*. Indeed, Chevalier (1935) reported that *E. tuckeyana*, *L. jacobaeus* and *L.*

5 *purpureus* were mostly found on the oldest lava flows on Fogo Island but *L. camara* was

6 never mentioned by the botanist among these natives. Here, *L. jacobaeus* and *L. purpureus*

7 were barely linked with the age of lava flows, while a strong significant relationship was

8 observed for the invasive *L. camara*. Similar result was found for the naturalized *J. curcas*.

9 Thus, our results suggest that *L. camara* and *J. curcas* seem to occupy habitat colonized by *L.*

10 *jacobaeus* and *L. purpureus* previously. We argue that *J. curcas* should be also viewed as an

11 invasive species on the northeast coast of Fogo Island.

12 Finally, we also observed some adults *A. precatorius*, only on the oldest sampled lava flows.

13 Formerly, this vine was reported as invasive species in numerous tropical islands, e.g. in the

14 Caribbean Islands (Holm et al. 1979), New Caledonia (MacKee 1994), Hawaiian Islands

15 (Wagner et al. 1999), and French Polynesia (Florence et al. 2007). Although not studied here,

16 it is possible that *A. precatorius* also become a dominant alien species on the oldest lava flows

17 of Fogo Island.

18

19 *Species densities fit period of eruption*

20 Plant species densities of the Nd.10 (1664?-)1785 lava flows group are clearly different from

21 the 1799-1857 lava flows group. This finding confirms the hypothesis that similar plant

22 species densities in comparable semi-arid environment on lava flows are related to similar

23 year-old lavas.

24 However, significantly-linked density group encompasses lavas with large age range (several

25 decades), e.g. Nd. 10 (1664?) linked with 1785, or 1799 linked with 1857. This confirms that
species composition and densities might remain similar during at least several decades on Fogo Island. Thus, a range of several decades is necessary to detect a significant gap in plant species densities and composition among semi-arid lava flows.

Species densities of the Nd.10 (1664?) lava flow are significantly correlated with those of 1785 lava flow. There is no doubt that Nd.10 (1664?) lava is older than 1785 lava, as available maps of lava flows show that the Nd.10 lava is locally below the 1785 lava (Torres et al. 1997; Paris et al. 2017). Torres et al. (1997) tentatively linked the Nd.10 lava flow to the 1664 eruption (Ribeiro 1997). Our results support that the Nd.10 (1664?) eruption occurred lately in the 17th century or early in the 18th century. There are indeed several historical accounts of eruptions during this period, including 1664, 1695, and 1721-25 (Ribeiro 1997).

Surprisingly, although species densities on the Nd.10 (1664?) and 1769 lavas were strongly linked with 1785 lava, species densities of the Nd.10 (1664?) and 1769 lavas were not linked.

This might be explained by (i) the distance between Nd.10 (1664?) and 1769 lava (see Fig. 1) leading to possible geographic disjunction in plants accessibility between sampled sites or (ii) related to human disturbances. Indeed, while we assume that lava flows sampled were not cultivated, Fogo Island is inhabited by more than 37,000 people with a mean density of 77 people per m²; hence the most part of the island is cultivated mainly by subsistence rainfed farming (Barros 1998). Thus, human impacts such as goats grazing, fire, or human picking landscape plants on the lava flows sampled might disturb plant densities and colonization.

This may explain some lack of correlation as well as different levels of similarity (Sørensen) observed among replicate plots, especially for *S. daltonii*, *Dryopteris* sp. *Polypodium* sp., and *E. tuckeyana* which are the most important species to discriminate lavas on the east-north-east coast of Fogo Island (PCA).
1 *Undated lavas*

2 Model age-prediction suggest that Nd.9 lava might be related to an eruption around 1761 ± 40

3 yr, and PCA shows that species densities of undated Nd.9 lava are linked with the former

4 1664-1769 lava flows. Accordingly, these findings lead us to classify Nd.9 lava as early 18th

5 century lava flow, possibly related to 1721-25 eruption. Similarly, model age-prediction

6 supports that Nd.1 lava might be related to an eruption around 1768 ± 40 yr. Species densities

7 of Nd.1 lava is linked with the 1769 lava, supporting a similar or slightly older period of
8 eruption in the 18th century.

9

10 **Conclusion**

11 Species densities are significantly related to the age of lava flows on Fogo Island. Moreover,

12 similar plant species densities in comparable semi-arid environment on lava flows were

13 related to similar year-old lavas. However, species densities may provide only an estimate of

14 year-old lavas with a gap of several decades on Fogo Island. Nevertheless, vascular plant

15 densities are helpful to detect the period of eruption. Consequently, when lava flow ages are

16 unknown because of non-applicability of the conventional dating methods, vascular plant

17 species densities may provide useful information on the relative age of volcanic eruptions.

18 Although not extended here, our results support that plant densities and composition might

19 represent another input for volcanic hazard evaluation, establishing a reliable chronological

20 framework of volcanic eruptions for the evaluation of volcanic hazard, especially for
21 probabilistic models.

22

23
1 Acknowledgements

2 Fieldwork on Fogo Island was mostly supported by subsidy from the project PEPS 2015-

3 CNRS, University Clermont Auvergne (UCA), France. This is Laboratory of Excellence

4 ClerVolc contribution n°.

5

6

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19 biogeographical assessment. *Progress in Physical Geography* 8:60–81.

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3 **Figure and caption**

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

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7 Figure 1. Study area with location of the sampling sites (black squares) on eleven lava flows
8 of the east-north-east coast of Fogo (Cape Verde, Atlantic Ocean). Bold numbers on the
9 surveyed lava flows refer to year of eruption (AD). The year of eruption of lava flows named
10 "Nd.10 (1664?)" was uncertain, while "Nd.1" and "Nd.9" were not documented.

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<figure><img src="image_3.png" /><figcaption>Figure 2. Vascular plant species density observed on the overall 11 lava flows (Fogo Island, Cape Verde). Error bars refer to standard deviation.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

Abrus precatorius

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

Abutilon pannosum

<figure><img src="image_7.png" /></figure>

Acacia spirobis

<figure><img src="image_8.png" /></figure>

Asparagus sp.

<figure><img src="image_9.png" /></figure>

Catharanthus roseus

<figure><img src="image_10.png" /></figure>

Coniza varia

<figure><img src="image_11.png" /></figure>

Desmodium tortuosum

<figure><img src="image_12.png" /></figure>

Dichrostachys cinerea

<figure><img src="image_13.png" /></figure>

Diplotaxis irta

<figure><img src="image_14.png" /></figure>

Dryopteris sp.

<figure><img src="image_15.png" /></figure>

Euphorbia tuckeyana

<figure><img src="image_16.png" /></figure>

Ficus thonningii

<figure><img src="image_17.png" /></figure>

Forsskäolea procridifolia

<figure><img src="image_18.png" /></figure>

Furcraea gigantea

<figure><img src="image_19.png" /></figure>

Globularia amygdalifolia

<figure><img src="image_20.png" /></figure>

Grewia villosa

<figure><img src="image_21.png" /></figure>

Jatropha curcas

<figure><img src="image_22.png" /></figure>

Kalanchoe pinnata

<figure><img src="image_23.png" /></figure>

Lantana camara

<figure><img src="image_24.png" /></figure>

Lavandula rotundifolia

<figure><img src="image_25.png" /></figure>

Leucaena leucocephala

<figure><img src="image_26.png" /></figure>

Lotus jacobaeus

<figure><img src="image_27.png" /></figure>

Lotus purpureus

<figure><img src="image_28.png" /></figure>

Paronychia illecebroides

<figure><img src="image_29.png" /></figure>

Polypodium sp.

<figure><img src="image_30.png" /></figure>

Sarcostemma daltonii

<figure><img src="image_31.png" /></figure>

Senna bicapsularis

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Figure 3. Vascular plant species composition (%) on the 11 lava flows (Fogo Island, Cape Verde).

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6
<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /><figcaption>Figure 4a-b. Linear regression. Species richness and number of vascular plants in relation to the age of lava flows in 27 plots (314 m² each). Plots on undated lavas Nd.1 (grey circle) and Nd.9 (black circle) have been added as supplementary information.</figcaption></figure>
<figure><img src="image_34.png" /><figcaption>Figure 5. Mean density of the native species *Sarcostemma daltonii* (grey bar), *Euphorbia tuckeyana* (colorless bar) and the naturalized species *Lantana camara* and *Jatropha curcas* (black bar) on the semi-arid surveyed lavas (Fogo Island, Cape Verde). The undated lavas Nd.1 and Nd.9 have been added as supplementary information. Error bars refer to standard deviation.</figcaption></figure>
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<figure><img src="image_35.png" /><figcaption>Figure 6. Principal Component Analysis (PCA). Relationships between the 11 lava flows based on vascular plant species densities and composition in the 44 plots (Fogo, Cape Verde).</figcaption></figure>

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# 1 Table and caption

2 Table 1. Characteristics of the 44 plots surveyed on 11 lava flows of the east-north-east coast of Fogo Island (Cape Verde, Atlantic Ocean). Sørensen Index (plant similarity) is calculated for the three field plot replicates of 314 m² (plot numbers 1-3). Drone plot area: 140 m² (plot number 4). Numbers in brackets refer to mean value. px: pyroxene; ol: olivine.

<table><thead><tr><th>Lava flows</th><th>Plots &amp; replicates</th><th>North (decimal degrees)</th><th>West</th><th>Elevation (m)</th><th>Exposure</th><th>Slope (°)</th><th>Geology*</th><th>Plant similarity (Sørensen)</th></tr></thead><tbody><tr><td rowspan="4">Nd.1</td><td>1</td><td>14.92231</td><td>24.29604</td><td>370</td><td>ESE</td><td>16</td><td rowspan="4">alkali basalt, px</td><td>0.750</td></tr><tr><td>2</td><td>14.92268</td><td>24.29601</td><td>360</td><td>ESE</td><td>14</td><td>0.714</td></tr><tr><td>3</td><td>14.92251</td><td>24.29647</td><td>371</td><td>ESE</td><td>14</td><td>0.714</td></tr><tr><td>4</td><td>14.92227</td><td>24.29596</td><td>358<br>(364)</td><td>ESE</td><td>14<br>(14.5)</td><td>(0.726)</td></tr><tr><td rowspan="4">Nd.9</td><td>1</td><td>14.92128</td><td>24.29375</td><td>337</td><td>ENE</td><td>14</td><td rowspan="4">alkali basalt, px</td><td>0.875</td></tr><tr><td>2</td><td>14.92137</td><td>24.29434</td><td>333</td><td>ENE</td><td>14</td><td>0.750</td></tr><tr><td>3</td><td>14.92175</td><td>24.29401</td><td>325</td><td>ENE</td><td>14</td><td>0.777</td></tr><tr><td>4</td><td>14.92167</td><td>24.29441</td><td>328<br>(330)</td><td>ENE</td><td>14<br>(14)</td><td>(0.800)</td></tr><tr><td rowspan="4">Nd.10 (1664?)</td><td>1</td><td>15.00661</td><td>24.30733</td><td>200</td><td>NE</td><td>16</td><td rowspan="4">alkali basalt, ol, px</td><td>0.666</td></tr><tr><td>2</td><td>15.00701</td><td>24.30742</td><td>205</td><td>NE</td><td>16</td><td>0.833</td></tr><tr><td>3</td><td>15.00621</td><td>24.30779</td><td>212</td><td>NE</td><td>16</td><td>0.666</td></tr><tr><td>4</td><td>15.00660</td><td>24.30732</td><td>212<br>(208)</td><td>NE</td><td>16<br>(16)</td><td>(0.721)</td></tr><tr><td rowspan="4">1769</td><td>1</td><td>14.93913</td><td>24.29714</td><td>290</td><td>ENE</td><td>19</td><td rowspan="4">alkali basalt, px (ol)</td><td>0.545</td></tr><tr><td>2</td><td>14.93911</td><td>24.29782</td><td>300</td><td>ENE</td><td>19</td><td>0.923</td></tr><tr><td>3</td><td>14.93936</td><td>24.29771</td><td>305</td><td>ENE</td><td>19</td><td>0.666</td></tr><tr><td>4</td><td>14.93912</td><td>24.29714</td><td>295<br>(297)</td><td>ENE</td><td>19<br>(19)</td><td>(0.711)</td></tr><tr><td rowspan="4">1785</td><td>1</td><td>14.98868</td><td>24.29904</td><td>206</td><td>ENE</td><td>15</td><td rowspan="4">alkali basalt, px (ol)</td><td>0.866</td></tr><tr><td>2</td><td>14.98887</td><td>24.29858</td><td>200</td><td>NE</td><td>19</td><td>0.560</td></tr><tr><td>3</td><td>14.98935</td><td>24.29865</td><td>204</td><td>ENE</td><td>16</td><td>0.510</td></tr><tr><td>4</td><td>14.99271</td><td>24.29783</td><td>205<br>(204)</td><td>ENE</td><td>16<br>(16.5)</td><td>(0.645)</td></tr><tr><td rowspan="4">1799</td><td>1</td><td>15.00680</td><td>24.31145</td><td>325</td><td>ENE</td><td>26</td><td rowspan="4">alkali basalt, px (ol)</td><td>0.857</td></tr><tr><td>2</td><td>15.00665</td><td>24.31179</td><td>333</td><td>ENE</td><td>26</td><td>1</td></tr><tr><td>3</td><td>15.00618</td><td>24.31178</td><td>332</td><td>ENE</td><td>26</td><td>0.857</td></tr><tr><td>4</td><td>15.00644</td><td>24.31174</td><td>326<br>(329)</td><td>ENE</td><td>26<br>(26)</td><td>(0.904)</td></tr><tr><td rowspan="4">1816</td><td>1</td><td>14.97621</td><td>24.30168</td><td>372</td><td>ENE</td><td>20</td><td rowspan="4">alkali basalt, px (ol)</td><td>0.875</td></tr><tr><td>2</td><td>14.97560</td><td>24.30101</td><td>365</td><td>ENE</td><td>24</td><td>0.615</td></tr><tr><td>3</td><td>14.97501</td><td>24.30047</td><td>360</td><td>E</td><td>24</td><td>0.615</td></tr><tr><td>4</td><td>14.97619</td><td>24.30165</td><td>361<br>(364)</td><td>E</td><td>24<br>(23)</td><td>(0.701)</td></tr><tr><td rowspan="4">1847</td><td>1</td><td>14.97167</td><td>24.29803</td><td>313</td><td>ENE</td><td>17</td><td rowspan="4">alkali basalt, px (ol)</td><td>0.750</td></tr><tr><td>2</td><td>14.97122</td><td>24.29827</td><td>317</td><td>ENE</td><td>17</td><td>0.571</td></tr><tr><td>3</td><td>14.97157</td><td>24.29841</td><td>320</td><td>ENE</td><td>17</td><td>0.857</td></tr><tr><td>4</td><td>14.97161</td><td>24.29809</td><td>318<br>(317)</td><td>ENE</td><td>17<br>(17)</td><td>(0.726)</td></tr><tr><td rowspan="4">1852</td><td>1</td><td>15.00053</td><td>24.31219</td><td>429</td><td>NE</td><td>22</td><td rowspan="4">alkali basalt, px</td><td>0.857</td></tr><tr><td>2</td><td>15.00033</td><td>24.31219</td><td>430</td><td>NE</td><td>23</td><td>0.857</td></tr><tr><td>3</td><td>15.00009</td><td>24.31228</td><td>433</td><td>NE</td><td>23</td><td>0.750</td></tr><tr><td>4</td><td>15.00022</td><td>24.31254</td><td>428<br>(430)</td><td>NE</td><td>23<br>(22.75)</td><td>(0.821)</td></tr><tr><td rowspan="4">1857</td><td>1</td><td>14.91968</td><td>24.29332</td><td>327</td><td>NE</td><td>15</td><td rowspan="4">alkali basalt, px</td><td>1</td></tr><tr><td>2</td><td>14.91964</td><td>24.29377</td><td>323</td><td>NE</td><td>15</td><td>0.500</td></tr><tr><td>3</td><td>14.92008</td><td>24.29409</td><td>320</td><td>NE</td><td>11</td><td>0.500</td></tr><tr><td>4</td><td>14.91951</td><td>24.29388</td><td>325<br>(323)</td><td>NE</td><td>11<br>(13)</td><td>(0.666)</td></tr><tr><td rowspan="4">1951</td><td>1</td><td>14.91812</td><td>24.29366</td><td>334</td><td>E</td><td>10</td><td rowspan="4">alkali basalt, px</td><td>-</td></tr><tr><td>2</td><td>14.91804</td><td>24.29338</td><td>332</td><td>E</td><td>10</td><td>-</td></tr><tr><td>3</td><td>14.91799</td><td>24.29305</td><td>331</td><td>E</td><td>10</td><td>-</td></tr><tr><td>4</td><td>14.91808</td><td>24.29345</td><td>330<br>(331)</td><td>E</td><td>10<br>(10)</td><td>-</td></tr></tbody></table>

<footnote>* field observations</footnote>
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<table><caption>Table 2. Vascular plant species observed in the 44 plots surveyed with species richness and total number of plants (Fogo Island, Cape Verde). Biogeographical status has been added with (I) indigenous (i.e. native but not endemic), (E) endemic, and (Ntz) naturalized species.</caption><thead><tr><th rowspan="2">Vascular plants (n=27)</th><th rowspan="2">Nd.1</th><th rowspan="2">Nd.9</th><th rowspan="2">Nd.10<br>(1664?)</th><th rowspan="2">1769</th><th rowspan="2">1785</th><th rowspan="2">1799</th><th rowspan="2">1816</th><th rowspan="2">1847</th><th rowspan="2">1852</th><th rowspan="2">1857</th><th rowspan="2">1951</th><th colspan="2">Biogeographic</th></tr><tr><th>Life-form</th><th>Status</th></tr></thead><tbody><tr><td>Abrus precatorius L.</td><td></td><td></td><td>x</td><td></td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Vine</td><td>Ntz</td></tr><tr><td>Abutilon pannosum (G. Forst.) Schltdl.</td><td>x</td><td>x</td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>I</td></tr><tr><td>Acacia spiribos Labill.</td><td>x</td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Tree</td><td>Ntz</td></tr><tr><td>Asparagus sp.</td><td></td><td></td><td></td><td>x</td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub/vine</td><td>Ntz</td></tr><tr><td>Catharanthus roseus (L.) G.Don</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td></td><td>Herb</td><td>Ntz</td></tr><tr><td>Coniza varia (Webb) Wild</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Desmodium tortuosum (Sw.) DC.SG</td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td>x</td><td></td><td></td><td></td><td>Shrub</td><td>Ntz</td></tr><tr><td>Dichrostachys cinerea (L.) Wight & Arn.</td><td></td><td></td><td>x</td><td></td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Tree</td><td>I</td></tr><tr><td>Diplotaxis irta (A.Chev.) Rustan & L.Borgen</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td>Herb</td><td>E</td></tr><tr><td>Dryopteris sp.</td><td>x</td><td>x</td><td></td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>Fern</td><td>I?</td></tr><tr><td>Euphorbia tuckeyana Steud. ex Webb</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Ficus thonningii Blume</td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td>x</td><td></td><td></td><td></td><td>Tree</td><td>I</td></tr><tr><td>Forsskaolea procridifolia Webb</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Furcraea gigantea L. (Law)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td></td><td>Succulent</td><td>Ntz</td></tr><tr><td>Globularia amygdalifolia Webb</td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Grewia villosa Willd</td><td>x</td><td></td><td></td><td></td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>I</td></tr><tr><td>Paronychia illecebroides Webb</td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td></td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Jatropha curcas L.</td><td></td><td></td><td>x</td><td>x</td><td>x</td><td></td><td>x</td><td></td><td></td><td></td><td></td><td>Tree</td><td>Ntz</td></tr><tr><td>Kalanchoe pinnata (Lam.) Pers.</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>x</td><td></td><td>Succulent</td><td>Ntz</td></tr><tr><td>Lantana camara L.</td><td></td><td>x</td><td>x</td><td></td><td>x</td><td></td><td>x</td><td></td><td>x</td><td></td><td></td><td>Shrub/vine</td><td>Ntz</td></tr><tr><td>Lavandula rotundifolia Benth</td><td>x</td><td></td><td></td><td>x</td><td>x</td><td>x</td><td>x</td><td></td><td></td><td></td><td>x</td><td>Shrub</td><td>E</td></tr><tr><td>Leucaena leucocephala (Lam.) de Wit</td><td></td><td></td><td></td><td></td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Tree</td><td>Ntz</td></tr><tr><td>Lotus jacobaeus L.</td><td>x</td><td>x</td><td></td><td>x</td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Lotus pupureus L.</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>E</td></tr><tr><td>Polypodium sp.</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td></td><td>x</td><td>x</td><td>x</td><td></td><td></td><td>Fern</td><td>I?</td></tr><tr><td>Sarcostemma daltonii Decne. ex Webb</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td></td><td>Shrub/vine</td><td>E</td></tr><tr><td>Senna bicapsularis (L.) Roxb.</td><td></td><td></td><td>x</td><td></td><td>x</td><td></td><td></td><td></td><td></td><td></td><td></td><td>Shrub</td><td>Ntz</td></tr><tr><td>Species richness per lava flow</td><td>10 (6-8)</td><td>11 (7-9)</td><td>9 (6-6)</td><td>8 (5-7)</td><td>19 (11-15)</td><td>4 (3-4)</td><td>10 (5-8)</td><td>5 (3-4)</td><td>5 (3-4)</td><td>3 (2-2)</td><td>1 (0-1)</td><td></td><td></td></tr><tr><td>Number of plants per lava flow</td><td>393<br>(77-130)</td><td>663<br>(178-240)</td><td>359<br>(87-132)</td><td>356<br>(88-109)</td><td>380<br>(105-119)</td><td>188<br>(31-88)</td><td>455<br>(80-133)</td><td>175<br>(25-87)</td><td>58<br>(10-30)</td><td>64<br>(15-32)</td><td>1<br>(0-1)</td><td></td><td></td></tr></tbody></table>

(1) = Range per plot

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<table><caption>Table 3. Relationships between species density and age of lava flows in the plots (Spearman's coefficient correlation).</caption><thead><tr><th>Vascular species</th><th>Sample (n)</th><th>R<sup>2</sup></th><th>Intercorrelation</th></tr></thead><tbody><tr><td>1. Abrus precatorius</td><td>10</td><td>0.114*</td><td>2.3.4.5.6.7.8.9</td></tr><tr><td>2. Asparagus sp.</td><td>14</td><td>0.136*</td><td>1.3.4.8.9</td></tr><tr><td>3. Lotus jacobaeus</td><td>15</td><td>0.160*</td><td>1.2.4.6.8.9</td></tr><tr><td>4. Lotus pupureus</td><td>16</td><td>0.265*</td><td>1.2.3.6.7.8.9</td></tr><tr><td>5. Polypodium sp.</td><td>94</td><td>0.161*</td><td>1.6.7.8</td></tr><tr><td>6. Lantana camara</td><td>24</td><td>0.334**</td><td>1.3.4.5.7.8.9</td></tr><tr><td>7. Jatropha curcas</td><td>20</td><td>0.323**</td><td>1.4.5.6.8.9</td></tr><tr><td>8. Euphorbia tuckeyana</td><td>112</td><td>0.485***</td><td>1.2.3.4.5.6.7.9</td></tr><tr><td>9. Sarcostemma daltonii</td><td>1103</td><td>0.710***</td><td>1.2.3.4.6.7.8</td></tr></tbody></table>

P*** < 0.0001; 0.001** < P < 0.01; *P < 0.05
1

2

3

4

5

6

7

<table><caption>Table 4. Plant species densities versus age of lava flows sampled (Stepwise regression model). Equation model is provided with the table.</caption><thead><tr><th>Rank of variable</th><th>Vascular plant species</th><th>Value</th><th>Standard deviation</th><th>t</th><th>Pr > |t|</th><th>Lower bound (95%)</th><th>Upper bound (95%)</th><th>Adjusted R<sup>2</sup></th></tr></thead><tbody><tr><td>1</td><td>Sarcostemma daltonii</td><td>396.45</td><td>75.71</td><td>5.23</td><td>&lt; 0.0001</td><td>242.41</td><td>550.49</td><td></td></tr><tr><td>2</td><td>Lantana camara</td><td>7337.75</td><td>1493.93</td><td>4.91</td><td>&lt; 0.0001</td><td>0.28</td><td>0.68</td><td></td></tr><tr><td>Model</td><td>Constant</td><td>136.264</td><td>10.087</td><td>13.51</td><td>&lt; 0.0001</td><td>115.743</td><td>156.785</td><td>0.733</td></tr></tbody></table>

P-value to enter < 0.05; p-value to remove > 0.10

Age of lava flows = (136.263974515594 + 7337.7542282911 x L. camara) + (396.447385251673 x S. daltonii)

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<table><caption>Table 5. Relationships between the 11 lava flows based on 27 vascular plant species densities (Spearman's correlation with PCA).</caption><thead><tr><th>Lava flows (n=11)</th><th>Nd.1</th><th>Nd.9</th><th>Nd.10<br>(1664?)</th><th>1769</th><th>1785</th><th>1799</th><th>1816</th><th>1847</th><th>1852</th><th>1857</th><th>1951</th></tr></thead><tbody><tr><td>Nd.1</td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Nd.9</td><td><strong>0.688</strong></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Nd.10 (1664?)</td><td>ns</td><td><strong>0.668</strong></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1769</td><td><strong>0.778</strong></td><td><strong>0.558</strong></td><td>ns</td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1785</td><td><strong>0.528</strong></td><td>ns</td><td><strong>0.589</strong></td><td><strong>0.676</strong></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1799</td><td>ns</td><td>ns</td><td>ns</td><td><strong>0.492</strong></td><td>ns</td><td>1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1816</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>1</td><td></td><td></td><td></td><td></td></tr><tr><td>1847</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>1</td><td></td><td></td><td></td></tr><tr><td>1852</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td><strong>0.497</strong></td><td><strong>0.617</strong></td><td>1</td><td></td><td></td></tr><tr><td>1857</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td><strong>0.541</strong></td><td>ns</td><td><strong>0.500</strong></td><td><strong>0.500</strong></td><td>1</td><td></td></tr><tr><td>1951</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>1</td></tr></tbody></table>

P ≤ 0.01; ns = not significant
<table><caption>Table 6. Principal Component Analysis (PCA). Contribution of the species per axis (%) to the total inertia of the dataset.</caption><thead><tr><th>Vascular plants (n=27)</th><th>F1</th><th>F2</th><th>F3</th><th>F4</th><th>F5</th><th>F6</th><th>F7</th><th>F8</th><th>F9</th><th>F10</th></tr></thead><tbody><tr><td>Abrus precatorius</td><td>0.65</td><td>0.03</td><td>0.06</td><td>0.04</td><td>0.02</td><td>0.25</td><td>0.03</td><td>0.04</td><td>0.10</td><td>0.00</td></tr><tr><td>Abutilon pannosum</td><td>0.55</td><td>0.06</td><td>0.00</td><td>0.36</td><td>0.00</td><td>0.17</td><td>0.02</td><td>0.03</td><td>0.30</td><td>0.07</td></tr><tr><td>Acacia spirobis</td><td>0.73</td><td>0.00</td><td>0.03</td><td>0.13</td><td>0.00</td><td>0.03</td><td>0.02</td><td>0.04</td><td>0.08</td><td>0.26</td></tr><tr><td>Asparagus sp.</td><td>0.64</td><td>0.03</td><td>0.09</td><td>0.04</td><td>0.02</td><td>0.28</td><td>0.05</td><td>0.06</td><td>0.05</td><td>0.00</td></tr><tr><td>Catharanthus roseus</td><td>0.71</td><td>0.00</td><td>0.02</td><td>0.02</td><td>0.00</td><td>0.00</td><td>0.07</td><td>0.01</td><td>0.07</td><td>0.34</td></tr><tr><td>Coniza varia</td><td>0.72</td><td>0.00</td><td>0.01</td><td>0.18</td><td>0.00</td><td>0.08</td><td>0.01</td><td>0.00</td><td>0.06</td><td>0.29</td></tr><tr><td>Desmodium tortuosum</td><td>0.70</td><td>0.01</td><td>0.05</td><td>0.06</td><td>0.01</td><td>0.11</td><td>0.03</td><td>0.04</td><td>0.08</td><td>0.06</td></tr><tr><td>Dichrostachys cinerea</td><td>0.64</td><td>0.04</td><td>0.03</td><td>0.04</td><td>0.04</td><td>0.18</td><td>0.01</td><td>0.01</td><td>0.14</td><td>0.10</td></tr><tr><td>Diplotaxis irta</td><td>0.70</td><td>0.00</td><td>0.01</td><td>0.21</td><td>0.01</td><td>0.12</td><td>0.01</td><td>0.00</td><td>0.05</td><td>0.26</td></tr><tr><td>Dryopteris sp.</td><td>11.17</td><td>24.78</td><td>33.55</td><td>6.76</td><td>57.42</td><td>24.77</td><td>0.34</td><td>8.21</td><td>1.06</td><td>1.31</td></tr><tr><td>Euphorbia tuckeyana</td><td>0.23</td><td>3.37</td><td>0.27</td><td>1.11</td><td>0.09</td><td>2.00</td><td>0.46</td><td>0.02</td><td>24.82</td><td>2.33</td></tr><tr><td>Ficus thonningii</td><td>0.71</td><td>0.00</td><td>0.03</td><td>0.04</td><td>0.00</td><td>0.03</td><td>0.05</td><td>0.02</td><td>0.07</td><td>0.23</td></tr><tr><td>Forsskaolea procidifolia</td><td>0.69</td><td>0.00</td><td>0.00</td><td>0.13</td><td>0.00</td><td>0.05</td><td>0.01</td><td>0.26</td><td>0.08</td><td>0.46</td></tr><tr><td>Furcraea gigantea</td><td>0.23</td><td>0.44</td><td>0.10</td><td>5.42</td><td>1.47</td><td>1.42</td><td>3.42</td><td>0.97</td><td>0.12</td><td>0.12</td></tr><tr><td>Globularia amygdalifolia</td><td>0.75</td><td>0.00</td><td>0.03</td><td>0.12</td><td>0.00</td><td>0.04</td><td>0.02</td><td>0.03</td><td>0.07</td><td>0.30</td></tr><tr><td>Grewia villosa</td><td>0.61</td><td>0.02</td><td>0.10</td><td>0.14</td><td>0.00</td><td>0.05</td><td>0.05</td><td>0.06</td><td>0.68</td><td>0.08</td></tr><tr><td>Paronychia illecebroides</td><td>0.02</td><td>2.27</td><td>5.26</td><td>0.21</td><td>2.74</td><td>1.80</td><td>37.91</td><td>0.01</td><td>0.01</td><td>5.18</td></tr><tr><td>Jatropha curcas</td><td>0.56</td><td>0.08</td><td>0.01</td><td>0.02</td><td>0.12</td><td>0.04</td><td>0.01</td><td>0.04</td><td>0.24</td><td>0.85</td></tr><tr><td>Kalanchoe pinnata</td><td>0.61</td><td>0.02</td><td>0.03</td><td>0.32</td><td>0.40</td><td>0.02</td><td>0.45</td><td>0.00</td><td>0.11</td><td>0.25</td></tr><tr><td>Lantana camara</td><td>0.46</td><td>0.11</td><td>0.05</td><td>0.03</td><td>0.16</td><td>0.06</td><td>0.02</td><td>0.01</td><td>0.17</td><td>0.68</td></tr><tr><td>Lavandula rotundifolia</td><td>0.04</td><td>0.65</td><td>0.16</td><td>6.50</td><td>0.92</td><td>1.42</td><td>0.71</td><td>0.93</td><td>0.00</td><td>0.03</td></tr><tr><td>Leucaena leucocephala</td><td>0.75</td><td>0.00</td><td>0.03</td><td>0.12</td><td>0.00</td><td>0.04</td><td>0.02</td><td>0.03</td><td>0.07</td><td>0.30</td></tr><tr><td>Lotus jacobaeus</td><td>0.52</td><td>0.07</td><td>0.11</td><td>0.09</td><td>0.00</td><td>0.11</td><td>0.07</td><td>0.07</td><td>0.02</td><td>0.01</td></tr><tr><td>Lotus purpureus</td><td>0.37</td><td>0.18</td><td>0.01</td><td>0.29</td><td>0.00</td><td>0.07</td><td>0.03</td><td>0.01</td><td>0.01</td><td>0.02</td></tr><tr><td>Polypodium sp.</td><td>2.53</td><td>8.26</td><td>9.60</td><td>6.43</td><td>0.18</td><td>30.75</td><td>0.38</td><td>3.11</td><td>33.61</td><td>54.05</td></tr><tr><td>Sarcostemma daltonii</td><td>127.89</td><td>0.37</td><td>10.61</td><td>5.27</td><td>49.47</td><td>3.22</td><td>0.69</td><td>32.22</td><td>47.80</td><td>5.53</td></tr><tr><td>Senna bicapsularis</td><td>0.64</td><td>0.03</td><td>0.01</td><td>0.20</td><td>0.00</td><td>0.00</td><td>0.02</td><td>0.04</td><td>0.03</td><td>0.00</td></tr></tbody></table>