Floristic diversity, composition and dominance across Amazonian forest types respond differently to latitude

Abstract

Aim The latitudinal biodiversity gradient is considered a first‐order biogeographical pattern for most taxonomic groups. Latitudinal variation in plant diversity is not always consistent, and this could be related to the particular characteristics of different forest types. In this study, we compare latitudinal changes in floristic diversity (alpha diversity), composition (beta diversity) and dominance across different tropical forest types: floodplain, terra firme and submontane forests. Location Western Amazonia (Ecuador, Peru and Bolivia). Taxon Woody plants. Methods We inventoried 1978 species and 31,203 individuals of vascular plants with a diameter at breast height ≥ 2.5 cm in 118 0.1‐ha plots over an 1800 km latitudinal gradient in three different forest types. The relationships between alpha diversity, latitude and forest type were analysed using generalised linear mixed models. Semi‐parametric permutational multivariate analysis of variance was used to investigate the effects of latitude and forest type on beta diversity. Dominant species abundances were correlated with non‐metric multidimensional scaling ordination axes to reflect their contributions in shaping changes in beta diversity. Results Alpha diversity increased towards equatorial latitudes in terra firme and submontane forests but remained relatively constant in floodplains. Beta diversity of all forest types changed with latitude, although less clearly in floodplains. Also, in floodplain forests, there were fewer dominant species contributing to beta diversity and more species homogeneous along the gradient. Main Conclusions Latitudinal diversity patterns are manifested in alpha and beta diversity since latitude summarizes climatic and edaphic changes. However, we found different responses of each forest type. In floodplain forests, inundation regime is a stronger predictor than latitude, limiting floristic diversity and composition. Changes in dominant species abundance over gradients explained species composition, but floodplain forests harboured more homogeneous dominant species than well drained forests. It is key to study environmental trends and habitat characteristics of each forest type to understand their species diversity and dominance patterns.

Understanding different dominance patterns in western Amazonian forests

“…In terra firme forests, the most dominant species tended to be widespread dominants. This pattern could be explained by three reasons: (1) the higher species richness of these forests (ter Steege et al., 2000 ) promotes high levels of local competition, preventing species to attain high local abundances (de Aledo et al., 2023 ); (2) a stronger effect of conspecific negative density dependence has been reported in species with acquisitive ecological strategies (Zang et al., 2021 ), which are more characteristic of terra firme forests than poorer soils environments (Fortunel et al., 2014 ). This phenomenon is likely to reduce recruitment of conspecific individuals at local scales, thereby preventing regionally dominant species from achieving high local abundances.…”

“…Floodplain forests appear to be an intermediary habitat type in terms of harbouring different dominance patterns. Their intrinsic intermediate conditions between terra firme and swamp and white sand forests can explain our findings: (1) their larger area compared to swamp and white sand forests in western Amazonia (ter Steege et al., 2000 ), along with riparian corridors (Wittmann et al., 2011 ), strengthens the connection among locations, promoting high dispersion and permitting species to occur in more locations (Parolin, 2009 ); (2) large area and high connectivity increases local species richness and, consequently, local competition, restricting high local abundances of species, such as in terra firme forests (de Aledo et al., 2023 ); (3) given the flooding variation across zones, some locations may have special environmental conditions that encourage the existence and adaptation of local dominants there (de Aledo et al., 2023 ; Parolin, 2009 ; Wittmann et al., 2006 ).…”

“…This ecological rule also applies in the most diverse ecosystems on Earth, such as Amazonian forests, where 2%–7% of tree species (‘ dominant species ’ from now on) account for 50% of individual trees (following ter Steege et al., 2013 ; but see also Pitman et al., 2001 , 2013 ). Dominant species have a key role in driving large‐scale ecosystem functioning (Fauset et al., 2015 ) and spatial turnover in species composition (de Aledo et al., 2023 ; Draper et al., 2019 ). Therefore, changes in abundance and distribution of these dominant species will determine the response of tropical forests to global change in the coming decades (Avolio et al., 2019 ).…”