Changes in species assemblages are associated with bounded abiotic conditions (Soberón 2007; Scheele et al. 2017), in which resource constraints should drive species’ distribution (Huston 1994; Wang et al. 2020). Elevational gradients are among the most prominent abiotic gradients affecting species diversity and mirror latitudinal patterns (Rahbek 2005). Alongside elevation, other environmental factors change, such as temperature, atmospheric pressure, UV-B radiation, precipitation, wind speed and seasonality (Körner, 2007). Among these abiotic factors, temperature and humidity are expected to be the most important factors related to plant distributions (Gentry 1988; McCain and Grytnes 2010; Wang et al. 2020).
Elevational gradients are natural ‘laboratories’ for studying processes that cause variation in species distribution patterns (Lomolino 2001; Körner 2007) and thus to understand spatial variation in species composition, or Beta-diversity (Fitzpatrick et al. 2013). Beta-diversity is mainly caused by species turnover, and it should increase with environmental variations, revealing effects of filtering and dispersal limitation (Qian et al. 2009; Fontana et al. 2020). Plant Beta-diversity is expected to be hump-shaped along the elevational gradient (but see Moradi et al. 2020), however, its responses might be positive or negative, considering singular groups or shorter elevational ranges (Fontana et al. 2020). Beta-diversity was depicted into phylogenetic and species beta-diversities, to reveal the role of turnover at different phylogenetic levels. Species turnover should be higher with greater elevational variation, and a stronger effect should be observed to species, than to phylogenetic beta-diversity.
At the community level, environmental changes among different habitats (Levine and HilleRisLambers 2009) are expected to drive the patterns of diversity (Valencia et al. 2004; Laiolo et al. 2018). Four main spatial patterns were used to describe species diversity variation along elevational gradients: (i) monotonic decrease in diversity with elevation, (ii) unimodal pattern with peak at intermediate elevations, (ii) plateau of diversity at low elevations, then decreasing upwards, and (iv) no changes related to elevation (Rahbek 2005; McCain and Grytnes 2010). Species diversity patterns are not consistent among different geographic areas (Hsu et al. 2014; Khine et al. 2019), species groups (Peters et al. 2016) or diversity indices used (Chao et al. 2014; Tucker et al. 2017). For example, diversity patterns can differ between single-taxa, families and the whole community (see Descombes et al. 2017, Kamimura et al. 2017). By assessing how elevation leads to different diversity patterns for species groups (e.g. Vetaas and Grytnes 2002; Kamimura et al. 2017), allows us to elucidate ecological drivers conducting species assemblages (Kreft and Jetz 2007; Peters et al. 2016). However, ecological processes explaining species co-occurrence and distribution in high diversity areas remain controversial (Mittelbach et al. 2007; Condamine et al. 2019).
Both deterministic, such as environmental filtering, and random processes, such as stochastic recruitments, have been found to explain the spatial variation of species diversity in relation to elevational gradients (Rahbek 2005; Swenson 2013; Peters et al. 2016). To understand multiple ecological processes modulating diversity in elevational gradients (see Graham et al. 2014, Laiolo et al. 2018), we assessed diversity-environment relationships using evolutionary and ecological complementary approaches (Pavoine et al. 2011; Graham et al. 2014). Measures of phylogenetic diversity are appropriated to unravel the processes that build species assemblages (Pavoine and Ricotta 2019). Since including basal groups should strongly influence the results of phylogenetic analyzes (Feng et al. 2014; Qian et al. 2019), we disentangled phylogenetic diversity analysis considering the role of one basal and distantly related clade, usually excluded in high-diversity tropical forests studies, the tree ferns (Worthy et al. 2019).
Species-resource relationships and spatial constraints should affect species ranges for the whole community, and also within the richest families (Colwell et al. 2016; Kamimura et al. 2017). Thus, we used a taxonomic complementary approach and analyzed the species diversity-elevation relationships to the five richest botanical families in the community, to shed light on the ecological processes driving local diversity to each family (Chao et al. 2014; Kubota et al. 2018). Conservation priorities can be better planned by mapping biodiversity patterns (Socolar et al. 2016), understanding the processes underlying the variations patterns of beta- and alpha-diversity (Tucker et al. 2017), and assessing the species distributions of high diversity families (Murray-Smith et al. 2009). As species elevational ranges change in response to climate change (Zu et al. 2021), expanding the comprehension of diversity-elevation relationships is critical to guide efforts of biodiversity conservation.
Here, we used a large plot-dataset of preserved communities distributed along an elevational gradient in the Atlantic Forest, through an integrated phylogenetic-taxonomic approach of beta- and alpha-diversity metrics at different species scenarios. We addressed three questions: (i) does beta-diversity change along the elevational gradient? Are phylogenetic and species beta-diversities similar in their response to elevation? (ii) How does phylogenetic diversity and structure change along the elevation gradient? Is this pattern different considering basal group in the analysis? (iii) How does alpha-diversity change along the elevational gradient? Is this pattern similar for the whole community and for the five richest families in the community? We hypothesized that the beta-diversity would show high turnover for both phylogenetic and species beta-diversity, which should be highly correlated to elevational changes (Condit et al. 2002; Elliott and Davies 2019). We expected to find a close covariation between species and phylogenetic diversity (Sandel 2018), and an increase of phylogenetic clustering at higher elevations, due to stronger environmental filtering (Körner 2007; Zhang et al. 2016; Qian et al. 2020). Finally, alpha-diversity should vary unimodally along the elevational gradient for the whole community (Rahbek 2005; Eisenlohr et al. 2013), emerging from patterns of the richest families (see Kamimura et al. 2017, Massante and Gerhold 2020).