In this study, we revealed significant differences in plant community composition, structure, and diversity (taxonomic and phylogenetic) along a local-scale edaphic and altitudinal gradient. Our results showed that significant effects of altitude and edaphic-related factors on species richness and communities phylogenetic diversity highlight the importance of fine-scale environmental heterogeneity for the maintenance of high biodiversity levels in tepuis (Vegas-Vilarrúbia et al. 2012; Safont et al. 2014; 2016). We predicted that harsh edaphic conditions and altitude would impose a strong environmental filter on vegetation, resulting in phylogenetic clustering. However, we presume that other environmental filters, stochastic factors, and biotic interactions also may be responsible for the community assembly of Tepequém table mountain, a tepui with high ecological importance for the Amazon region.
The community composition changes considerably among phytophysiognomies along the altitudinal gradient. According to Schaefer et al. (2016), soil depth, drainage, and landscape stability and evolution have a key role in variations of rupestrian grassland complex phytophysiognomies. Physical environment influences not only the species distributions but also community composition (Nunes et al. 2015; Fernandes 2016). ORG was strongly associated with members of the following families: Velloziaceae (Pandales); Orchidaceae (Asparagales); and Poaceae, Cyperaceae, and Xyridaceae (Poales). In open physiognomies of rupestrian ecosystems, these families are among the most important by dominating the herbaceous monocot stratum (Porembski 2007; Le Stradic 2015; Silveira et al. 2016; Silva et al. 2019). Its high representativeness is due to the ability of the species to colonise harsh environments under severe stress conditions, with a broad array of adaptations (Porembski 2007). Similarly, Fabaceae, Annonaceae, and Lauraceae, with a high representativeness of lineages in SRG and FOR, are essential in the woody strata structuration of rupestrian ecosystems. For example, taxa of the Fabaceae family are important for the dynamics of poor-nutrient ecosystems due to morphological adaptations, such as nodules in the roots (Oliveira et al. 2012).
Variation in environmental conditions across phytophysiognomies along the altitudinal gradient such as lower temperatures as well as shallow soils with low water-holding capacity also seems to be the main factor leading to variation of species richness, PD, MPD, and MNTD in plant communities in the Tepequém table mountain. Harsh climatic and edaphic conditions in high altitudes represent a filter to plant species’ establishment and growth (Zhang et al. 2014; Cordeiro and Neri 2018), and consequently influence the richness and evolutionary history of communities. Between ORG and FOR, there is an altitudinal difference of 650 m, and a corresponding decrease of ca. 3.90°C in temperature. Such a correlation has been observed in other tepuis, which show an adiabatic temperature decrease of 0.60°C/100 m elevation (Huber 1995; Vilarrúbia et al. 2012), and in several tropical mountain ecosystems, which show values ranging between 0.50°C and 0.70°C at each 100 m (Safford 1999; McCain and Grytnes 2010). Along the altitudinal gradient, other harsh climate conditions related to high altitudes, such as high solar radiation, high evapotranspiration, and wind exposure may prevent the establishment of a large number of species (Fernandes et al. 2016; Zhang et al. 2014) and can account for the results obtained.
The ses.MPD of the ORG and FOR was close to zero, suggesting a random phylogenetic structure when deep (old) phylogenetic nodes are considered. ORG showed a similar pattern when only shallow (recent; ses.MNTD) nodes are considered. This pattern may suggest that neutral processes, such as dispersal limitation and random speciation (Hubbel, 2001), can shape local communities (e.g. Chun and Lee 2018; Liu et al. 2019a) or that environmental filtering and competitive exclusion act simultaneously in different phytophysiognomies in the Tepequém table mountain (Mayfield and Levine 2010; Liu et al. 2019a). Conversely, ses.MNTD in FOR resulted in a clustered pattern. Our findings indicate that although the forest phytophysiognomy is characterised by a phylogenetically random tree community for deep phylogenetic nodes (i.e. order and family level), this same community has species more closely related than expected by chance when only terminal nodes are analysed (i.e. intra-familial and/or intra-generic level).
The phylogenetic clustering pattern found in SRG may be related to the higher competitive abilities of some species in this phytophysiognomy. Clustered phylogenetic patterns can also result from density-dependent biotic interactions (i.e. competition), once closely related species tend to have similar competitive abilities (Mayfield and Levine 2010). This occurs when competitive exclusion takes place among distantly related taxa (Mayfield and Levine 2010; Goberna et al. 2014). For example, SRG showed a high density of Byrsonima crassifolia (L.) Kunth and Byrsonima crispa A. Juss. individuals. Both species concentrate approximately 53% of the total individual numbers for this phytophysiognomy. The genus is known for the high production of secondary metabolites, mainly in leaves, with high phytotoxic and cytotoxic activity (Amâncio et al. 2019). However, we highlight that phylogenetic patterns discussed here should be improved by exact measurements of ecological traits for each species in the communities, followed by specific testing for the presence of a strong phylogenetic signal in relation to these traits (Cavender-Bares et al. 2009).
In addition, we cannot discard that further explanations are possible for clustering pattern in the shrubby community assembly (e.g. abiotic constraints; Cavender-Bares et al. 2009). Multiple drivers may generate similar phylogenetic patterns (Cadotte et al. 2017), with environmental filters being the most common in rupestrian habitats (Miazaki et al. 2015; Zappi et al. 2017, 2019). Given the tendency of predominance towards niche conservatism in the rupestrian grasslands (Zappi et al. 2017), we expected that environmental filters related to local-scale edaphic and altitudinal gradient would influence phylogenetic clustering in the phytophysiognomies in the Tepequém table mountain. However, we observed that the standardised metrics were not explained by the variability of soil texture, fertility properties, or altitude. However, we suggest that other abiotic filters not evaluated in this study area could shape the phytophysiognomies in the phylogenetic clustering pattern, such as the temperature and soil water status (Ferrari et al. 2016) and rockiness (Pontara et al. 2018).
Nevertheless, differences in the species richness, PD, and MNTD among phytophysiognomies are related to local-scale edaphic gradients. In rupestrian ecosystems, the soil is an important driver at the local scale on plants’ taxonomic (Nunes et al. 2015; Mota et al. 2018) and phylogenetic diversity (Miazaki et al. 2015; Zappi et al. 2017; Pontara et al. 2018; Campos et al. 2021). Probably the highest species richness, PD, and lowest MNTD found in the FOR are related to the highest finer soil particle contents (i.e. clay, silt, and fine sand) (Supplementary Material, Table A1). Finer soil particles account for greater water retention and availability (Schaefer et al. 2016), as well as enabling greater physical support for roots (Assis et al. 2011), having a strong influence on the forested vegetation of the rupestrian complex (Negreiros et al. 2014; Ferrari et al. 2016). In addition, our results showed that these metrics were influenced by soil fertility. The greater species richness and PD found in FOR may be related to lower nutrient availability (i.e. Ca2+, Mg2+, remaining phosphorus, Mn, Zn, organic matter). The highest values observed in PD and species richness are expected in environments with lower nutrient availability (Huston 1993; Nadeau and Sullivan 2015) since soils with lower fertility normally have reduced interspecific competition (Gastauer and Meira-Neto 2014); this is because efficient competitors may lack resources to outcompete inferior competitors, thereby causing higher tree species number (Tilman 1985).