Beta diversity, geographical and climatic distances, and habitats
We found a positive relationship between geographical and climatic distances and turnover at the intermediate and small scales, with a stronger effect occurring at the intermediate scale (i.e. within island, independently of the habitat type). Multiple studies have shown that increasing distance reduces community similarity (Freestone & Inouye, 2006; Nekola & White, 1999), which is often explained by the fact that increasing geographical distance tends to increase environmental dissimilarity. However, this is not the case in our study that points towards other factors such as limitations to dispersal as potential drivers of turnover (Freestone & Inouye, 2006; König et al., 2016; Soininen et al., 2007). Other distance metrics, such as the distance to the oldest island or stepping-stone distance might be also important when interpreting beta diversity patterns, as suggested for species richness (Carvalho et al., 2015; Weigelt & Kreft, 2013).
Similarly, climatic distance is not related with turnover at the largest scale. Turnover has been reported to be more associated with environmental variables than with geographical distance on islands and island-like systems (Freestone & Inouye, 2006; König et al., 2016). Indeed, previous studies on islands have suggested that higher environmental heterogeneity was associated with higher turnover, highlighting the role of elevation, area and temperature among other factors on species distribution patterns (Cabral et al., 2014; König et al., 2016). However, the Azorean climate is very homogeneous, with high precipitation and mild temperatures throughout the year (Borges et al., 2019). At small scales it perhaps is too homogeneous, to the point of not influencing turnover patterns. At the intermediate scale, Terceira, São Jorge and São Miguel were the only islands in which climatic distance showed a significant relationship with beta diversity. These three islands have a complex topography that may create heterogeneous climatic conditions at small and intermediate scales, that in turn can be affecting turnover. In addition, we have only addressed communities in the remaining patches of native plants across the archipelago, which may be also increasing community similarity between islands.
The absence of effect of habitat composition on turnover may be related to the low number of units we have considered, but also that the categories used are broad enough to have more variation within habitat than between them. For example, seminaturalized vegetation includes a wide variety of shrublands (Schaeffer, 2002) that present very different species compositions. For example, Erica azorica dominated shrublands harbour different species compared with Calluna vulgaris ones (Elias et al., 2016). Also, because the habitats are spatially structured (native forest mainly at high elevations, semi-natural pastures at intermediate elevations) and climatically different, we cannot exclude that the effect of habitat composition is somehow captured by geographical and climatic distances.
There are also other factors, such as soil heterogeneity or human influence, that we have not considered in this study that might be limiting species distributions, and thus affecting turnover patterns. At small scales, soil characteristics are often considered an important driver of beta diversity (Barton et al., 2013). In the Azores, there are three types of andosoles, but one of them occurs mostly in areas occupied by agriculture and thus excluded from this study (Schaeffer, 2002). The other two soil types (typical andosoles and ferruginous soils) are both very fertile and not very different from each other within this archipelago, that combined with the fact that we have restricted our analysis to the few well-conserved areas in the archipelago (Borges et al., 2006, 2019; Picanço et al., 2017), indicates that the patterns found are most probably not affected by soil heterogeneity. Regarding human influence, the Azores is a highly disturbed archipelago whose original vegetation is actually restricted to small areas located above 500 m of altitude (Borges et al., 2019). Native vegetation forms patches with different degrees of connectivity between them, and thus landscape discontinuity could also be affecting our results. However, by selecting only the remaining least affected areas and just the native species of the Azorean flora, we believe the patterns found in this study are solid and are not particularly conditioned by human influence.
In summary, our results show that plant beta diversity patterns in the Azores change depending on the scale considered. The main drivers of turnover change also across scales, with geographical and climatic distance being only important at the intermediate and small scales and in certain islands. Dispersal syndromes play a minor role in structuring beta diversity patterns at all scales. This finding is in contradiction with the significant effect of geographical distance we reported at intermediate and small scales,suggesting that dispersal syndromes are not a good proxy of dispersal ability, at least in this study system and scales considered. Environmental and spatial factors are two main groups of factors driving beta diversity patterns in any ecosystem, but understanding the preeminence of one group over the other is still a work in progress (Soininen et al., 2007). More studies on beta diversity in oceanic archipelagos are necessary to help discriminate if the pattern we have found is specific of the Azores or is comparable in other islands, and thus improve our knowledge of how communities assembly across scales.