Biodiversity is known to depend mainly on abiotic drivers, such as climate and topography (Field et al., 2009). However, the importance and explicit impacts of geodiversity on biodiversity have long been insufficiently researched and partly ignored. Only recently has the topic started to receive more attention (e.g. Gray, 2004; Lawler et al., 2015; Bailey et al., 2017; Alahuhta et al., 2020; Barajas-Barbosa et al., 2020). Geodiversity is in many respects an abiotic equivalent to biodiversity (Gray, 2011) and represents the variability of chemical components, surface structure, and edaphic and hydrological features (Gray, 2004; Bailey et al., 2017). This variability contributes to habitat diversity and thus affects biodiversity patterns via the provisioning of ecological niches (Liu et al., 2013; Gillespie and Roderick, 2014; Bailey et al., 2017). Geological elements provide unique or distinctive habitats for plants and insects, deliver initial growth conditions for vegetation or fungi formation, and are part of nutrient cycling and soil–atmosphere interactions (Tukiainen et al., 2016).

Biodiversity is distributed unevenly throughout the world (Gaston, 2000), with a disproportionately large contribution from oceanic islands due to their high endemic richness (Kier et al., 2009). Substrates that differ in geochemistry and petrography are likely to be relevant for biodiversity on oceanic islands, where most rocks commonly share similar volcanic genesis, resulting in only slight differences in the parent material. Distinct substrates with limited extent, such as individual rock types, may function as a second isolating abiotic filter for populations in addition to the spatial isolation of oceanic islands that are known to be of outstanding importance for speciation at the global scale (Kier et al., 2009). Specific rock habitats, particularly rocks that exhibit petrographic and geochemical substrates such as serpentinites, are known to be rich in habitat-specific endemics (e.g. Harrison et al., 2006; Kazakou et al., 2010). Those species evolved specific adaptations to the unique nutrient contents and soil conditions and the presence of heavy metals that cannot be tolerated by other plant species (Harrison and Rajakaruna, 2011). This phenomenon is known as well on continents, where substrates such as serpentinite and gypsum outcrops host specialised floras and contribute to broad-scale diversity (see, e.g. Chiarucci et al., 1998; Pausas et al., 2003). It underlines the relevance of understanding the importance of geodiversity for insular biodiversity, which is particularly vulnerable to extinction due to highly restricted ranges and small population sizes of insular endemic species (Paulay, 1994). Phonolites are rocks that occur at volcanic intraplate settings in insular and continental contexts worldwide (Garcia et al., 1986; Ackerman et al., 2015; Hagos et al., 2017). They exist in a variety of geologic outcrops formed by volcanic activity. Such outcrops mainly exist on continents, where they are often linked to faults and tectonic activity. Major components of these extrusive igneous rocks (formed from lava with low silica content) are alkaline feldspars together with foid minerals, nepheline, and pyroxene (Abratis et al., 2015; Ackerman et al., 2015) or their conversion products.

On the island of La Palma (Canary Islands, Spain), several phonolitic rock outcrops are embedded into a matrix of basaltic origin (Middlemost, 1972). The dominant rock type found on La Palma is olivine and augite–titanaugite porphyric basalt, resulting from rapidly rising magma from the upper mantle (Middlemost, 1970). In contrast, there were times when a sizeable magmatic chamber below the island enabled differentiation of magma and the removal of silica, thus yielding ultramafic, trachytic, and phonolitic rocks (Middlemost, 1970). Phonolite trachytes (showing the exhalation of gases during eruptions) occur on various volcanic islands such as La Palma, St Helena, Fernando de Noronha, Trinidade and Sal (Cabo Verde). On La Palma, their distribution is focused on the southern (young) part of the island. The major chemistry of phonolites on La Palma is comparable to that of “average phonolites”, as described by Nockolds (1954). The current volcanic activity and lava flow deriving from the Cumbre Vieja volcano (Pankhurst et al., 2021) are a demonstration of how phonolitic rocks became isolated by younger lava solidifying around the peaks of phonolitic rock. This event illustrates that the isolation of the investigated phonolitic habitats (e.g. Roque Teneguía) is far from being a singularity but rather a process that is highly likely to have happened repeatedly on oceanic islands in general.

Volcanic activity with production of tephra and lava flows is a noticeably young phenomenon in the southern part of La Palma, with even present-day eruptions (Pankhurst et al., 2021). Thus, the remnant phonolitic rocks are the tips of a former land surface that are today embedded in a basaltic matrix of noticeably immature age (Garantje et al., 1998). As a consequence, weathering processes on phonolites were active on longer timescales compared to other surrounding rocks. In addition to petrography, differences in weathering between the rock types and resulting nutrient availability also infer different timescales of exposure.

A higher nutrient availability enables higher plant abundances and larger plant size. Porder et al. (2004) found comparable conditions at a catena of different rock ages on the Hawaiian Islands. Compared to basaltic lava outcrops, phonolites differ in their chemical composition and additionally in colour, texture, density, weathering, and formation fracturing (von Fragstein et al., 1988). Tafoni-weathering (Formoso et al., 1989) can be observed on phonolitic surfaces, indicating temperature and moisture gradients between the surface and the solid body of rocks (Brandmeier et al., 2011) that appear in combination with wind exposure. Circulating leachate reaches the rock's surface and evaporates, exposing its dissolved mineral content and enabling the development of secondary mineral assemblages (Spürgin et al., 2019). These can contribute to plant nutrient supply, which is also why ground phonolite rock powder is used as an effective fertiliser (Faccini et al., 2015). For phonolites, increased release of nutrients can be mediated by bio-weathering actions and plants receiving this fertiliser showed higher productivity, and increased accumulation of the macronutrient potassium in plants could be detected when applying phonolite rock powder (Tavares et al., 2018; Nogueira et al., 2021). Phonolites and the related larger-grained nepheline syenites contain significantly larger amounts of the essential nutrient potassium (see Table A1 in the Appendix for a literature overview). Even if quantitatively small, such processes are of particular importance at nutrient-poor sites. In contrast, the young basalts in the southern part of La Palma are barely weathered (Carracedo et al., 1999), appearing rough and friable with sharp spikes. We expect these petrographic and geochemical differences of parent material to affect vegetation cover and species occurrence.

Geologic outcrops, such as phonolites, increase microenvironmental heterogeneity, enhancing species richness at a landscape scale (Hjort et al., 2015). Increased speciation rates on isolated outcrops of scarce rocks are thought to lead to a higher percentage of endemic species than the surrounding matrix (Ricketts, 2001). Geodiversity may thus promote both species richness and endemism. However, relatively little is known about the extent to which phonolites promote species diversity in general and particularly endemism. To approach this topic theoretically, phonolitic outcrops could be considered as small habitat islands within a basaltic matrix (Fig. 1b). The established species–area relationship (SAR) and the species–isolation relationship (SIR; MacArthur and Wilson, 1967; Rosenzweig, 1995; Giladi et al., 2014) predict a smaller number of species on these small and isolated phonolitic rocks in comparison with basaltic rock outcrops in their surroundings. From the beginning of these concepts (MacArthur and Wilson, 1967), they were not only meant for real islands but instead took “islands” as examples of isolated habitats (or habitat islands) within a terrestrial landscape matrix. However, the expected higher availability of nutrients would give such habitats more favourable conditions for plant growth. In addition, it is by no means certain that the phonolitic rocks were permanently separated from each other in southern La Palma's geological evolution. Possibly, a historically much larger phonolitic rock is today largely buried by basaltic eruptions (Garantje et al., 1998). Thus, a few phonolite outcrops may serve as refugia for remnant populations (Eriksson, 1996) of species specialised to phonolitic rocks.

La Palma hosts 159 vascular plant species that are endemic to the archipelago and 47 single-island endemics (hereafter SIEs; Beierkuhnlein et al., 2021). The endemic plant species Cheirolophus junonianus (Svent.) Holub, comprising its var. junonianus and var. isoplexiphyllus (Svent.) G. Kunkel (Vitales et al., 2014a, b, Beierkuhnlein et al., 2021), occurs within a range of only 3500 m2, solely on La Palma (Bañares et al., 2004). Within this small range, individuals of this species occur only on a few outcrops, which are almost exclusively phonolitic rocks with a chemical composition different from most of the surrounding substrates. Therefore, the species is very restricted in its range size to just a few small locations (Muer et al., 2016; Atlantis, 2021) and appears to be restricted to phonolites (Fig. 1a). This example species evokes the question of whether or not phonolites are of special importance for endemic species on the Canary Islands.

We aim to investigate plant species richness, abundance and performance on phonolites compared to surrounding basaltic lavas. Therefore, we investigated the occurrences and traits of plant species in a comparative study matching basaltic and phonolitic rock formations on La Palma of comparable size, shape, and extent to test the following hypotheses: i.

Species diversity: we expect plant species richness on phonolitic rocks to be higher than on basaltic rocks because phonolites offer more favourable plant growth conditions.

We recorded 68 species of vascular plants (pteridophytes and spermatophytes) overall. Of these species, nine were SIEs restricted to the island of La Palma, 16 were MIEs co-occurring on other islands in the archipelago, 39 were non-endemic natives, and 4 were introduced. The SIE Cheirolophus junonianus was only found on phonolite, and most individuals of var. junonianus occurred on one isolated outcrop (Roque Teneguía) and individuals of var. isoplexiphyllus on another one (Escarpa del Volcán Teneguía, Fig. 2c).

We found higher plant species richness on phonolitic rocks. While 22 species were encountered on both phonolite and basalt, only 11 species were restricted to basalt, and 34 were recorded only on phonolite (Table A4). Endemism groupings showed similar patterns (SIEs – phonolite: 9, basalt: 5; MIEs – phonolite: 15, basalt: 6). Besides the total number of plant species per rock type, we also found higher species richness on phonolite at the plot scale (p=0.0164, Fig. 3a), and higher diversity of SIEs (p=0.00151, Fig. 3b) and MIEs (p=0.00727, Fig. 3d). The percentage of SIEs (p=0.1928, Fig. 3c) and MIEs (p=0.05346, Fig. 3e) relative to total species number did not differ significantly at this scale between phonolitic and basaltic rocks.

On phonolitic rocks, we did not find higher total plant abundance (p=0.169, Fig. 4a). Moreover, there was no significant difference in abundance when only considering the 23 species found in plots on both substrates (p=0.179, Fig. 4b).

For plant species recorded on both rock types, individuals were on average taller and had wider canopies (Fig. 4c–d) on phonolitic than on basaltic rocks (Fig. 4c–d). Plant cover only (excluding lichens) was also significantly higher (p<0.0001, Fig. A2a) on phonolites than on basalt. Lichen cover did not show a significant difference (p=0548, Fig. A2b).

The ordination did not show any difference in the species composition, indicating no aspects of beta-diversity at all (Fig. A1, Table A3). Topographic characteristics of basalt and phonolite plots showed no differences in surface rugosity, aspect (northernness, easternness), or differences in slope inclination (Table A5).

The vegetation on phonolitic rocks differs compared to equivalent neighbouring basaltic rocks by exhibiting higher species richness and higher performance of plant individuals and higher total plant cover. Larger numbers of SIEs and MIEs on phonolites also reflect augmented total species numbers on this rock type.

Phonolite rocks on La Palma and other oceanic islands are arranged as habitat islands in a basaltic matrix (Fig. 1b). On La Palma, the total area of phonolite outcrops is tiny compared to the area of the basaltic matrix. Thus, encountering more species on phonolites than on basalt aligns with our hypotheses but defies the area effect on species richness, which predicts species number to be lower on phonolites. Differences in species numbers might be attributed to lower-than-expected species numbers growing on basalt, higher-than-expected species numbers on phonolites, or both. Our findings are congruent with studies that did not find a species–area relationship or relationships with a less steep slope on habitat islands (Matthews et al., 2016; Deák et al., 2018). The existence of unrealised niches due to unsaturated evolutionary dynamics in a young system could explain lower-than expected species numbers on basalt. A possible extinction debt that might lead to the disappearance of species in the future might explain higher-than expected species numbers on phonolitic outcrops. Only the continuous monitoring of populations on both rock types can help to verify these assumptions. Environmental filters (Liu et al., 2020) enhancing growth conditions on phonolite outcrops may also exist, consistent with our findings that plants are larger on phonolites.

The rock types phonolite and basalt differ in their chemical composition resulting in different nutrient availability, which explains our observation of increased plant performance on phonolites. Phonolites consist of the potassium-rich nepheline, which dissolves much faster than other potassium sources (Manning, 2010). Various studies indicate that phonolites and the related nepheline syenite contain a higher proportion of potassium than basalt (Manning, 2010; Roqueto do Reis, 2021). Therefore, ground phonolite rock powder has traditionally been used as an inorganic fertiliser (von Wilpert and Lukes, 1998; Ramos et al., 2006; Schoen et al., 2016). Basaltic rock powder has also been used as fertiliser but is considered a less important source of potassium than phonolite (Manning, 2010). The usage of ground basalt as fertiliser can also be explained by unclear assignments (potassium-rich trachyte is often assigned to basalt; see Maning, 2010). In addition to geochemical differences, the duration of rock weathering is a decisive factor in providing nutrients. The phonolitic outcrops in southern La Palma are substantially older than the surrounding basalt, which stems from very young volcanic eruptions (Carracedo et al., 1999). The youngest nearby eruption of the Teneguía volcano took place only 50 years ago, in 1971.

The age of geological formations influences plant diversity and species compositions (Whittaker et al., 2008; Hulshof and Spasojevic, 2020). As noted in previous studies (Carracedo et al., 1999), the Cumbre Vieja rift on La Palma has evolved throughout several eruptions and therefore contains lava formations from different ages as well as slightly different mineralogical compositions. The current volcanic activity and lava flow at the Cumbre Vieja are a live example of this geological process (Pankhurst et al., 2021). The known phonolite rocks on La Palma are located in the geologically young southern part of the island. As a consequence, the species pool in the surrounding basaltic matrix of these rocks is poor. Under the arid conditions of southern La Palma, only very few early successional species establish on these young basaltic outcrops with not more than initial soil formation (Irl et al., 2019). The few rocky outcrops of phonolite are embedded in this species-poor matrix of young basalt. We observed partly buried phonolites on which the survival of plants or seedlings during volcanic events was improbable (Garantje et al., 1998). Carracedo et al. (1999) showed that the last phonolite formation occurred in 1585, while basaltic eruptions continue until modern times (Pankhurst et al., 2021). 19 plant species, including Cheirolophus junonianus, can solely be encountered on phonolitic rocks (Irl et al., 2015; Muer et al., 2016). This confirms that habitat diversity on islands contributes to their total species richness (Hortal et al., 2009).

Besides petro-chemical characteristics and rock age, the surface structure and colour of phonolites might be suspected to drive plant patterns on such rocks. We observed deeper fractures in phonolitic rocks than in other volcanites on La Palma. In addition, phonolitic rocks show a much smoother surface roughness than their surrounding matrix. Basaltic rocks seem to possess a more dynamic relief, mainly attributed to their origin in congealed lava flows, typically found on oceanic islands. Nevertheless, when testing rock surface rugosity, there were no significant differences between phonolitic and basaltic rocks. Hence, we argue that surface characteristics do not play a role in higher plant growth response, richness, and abundance observed on phonolites. Besides fractures, another visual observation was that phonolites are of lighter colour than their surrounding basaltic matrix. We expected that phonolites possess a higher albedo than surrounding rocks and therefore expected them to have a reduced surface temperature compared to volcanic outcrops with darker colouring, such as basalt. However, in an experiment with differently coloured bricks, Hall et al. (2005) showed that the albedo of white surfaces only leads to significantly lower temperature of the material when the surface temperature falls below air temperature. With monthly temperatures between 17 and 25 ∘C within large areas of oceanic islands (Harter et al., 2015), no major temperature differences between basalt and phonolite surfaces can be expected. We therefore consider that this effect has no major impact on plants' habitat suitability.

While a diversity of rocks with different chemical characteristics and at different ages supports species richness on volcanic islands, such rock characteristics do not necessarily contribute to higher percentages of endemic plants or distinct compositional vegetation on individual rocky outcrops. The overall percentage of SIEs on phonolites was not significantly enhanced, refuting our expectations. Thus, the differential geology of phonolites itself does not result in a specialised flora. Obviously, the small outcrops of phonolite on La Palma do not suffice to evolve and maintain a substantial set of endemic species, which contrasts with general assumptions that patterns caused by differing topography or discontinuous parent material can be explained by island biogeographic theory (e.g. Kruckerberg 1991). Consequently, lessons learnt from other outcrops (Kruckerberg, 1991) cannot be adapted to the phonolitic rocks on La Palma, and the functioning of phonolites as islands of speciation within a matrix of basalt does not seem to apply.

Despite the limited spatial extent of phonolites on La Palma, they contribute to insular habitat heterogeneity, which translates into increased species richness and abundance as well as higher plant performance. These phenomena are facilitated by specific characteristics of phonolite rock, like high nutrient availability fortified by longer geological timeframes for erosion and nutrient release compared to basalt. We are not aware of other studies conducted in locations where phonolites can be encountered that explore their potential role as exceptional plant habitat islands, even though phonolites can be found all over the world (Garcia et al., 1986; Ackerman et al., 2015; Hagos et al., 2017). Therefore, further studies are needed to investigate whether the patterns encountered on La Palma may also be found on comparable phonolitic rocks in other areas of the world. Their benefits for biodiversity found in this study need to be recognised and valued. Especially for isolated areas such as islands, phonolites can contribute to small-scale biodiversity hotspots and our findings suggest that they should be conserved.

Phonolites provide unique habitat conditions for plants on oceanic islands compared with surrounding areas. Higher species numbers and abundances as well as higher plant performance underline the importance of these rocks for the vegetation on oceanic islands. Despite the small total area covered by phonolites, they play a significant role in enhancing plant biodiversity on the island of La Palma. Our results suggest that exceptional rock outcrops like phonolites contribute to a better understanding of the formation of plant diversity on volcanic islands. As oceanic islands have always been formed through volcanic activity on the oceanic crusts, the combination of basaltic and phonolitic rocks is highly likely a regular pattern in Earth history.