Structure and diversity of the Araucaria forest in southern Brazil: biotic homogenisation hinders the recognition of floristic assemblages related to altitude

Forest structure and species abundance may respond to environmental features triggered by geomorphology, altitude gradients and human impacts. In this study we investigated if data from a large-area inventory would support the segregation of floristic assemblages related to altitude-oriented forest physiognomies established for the Brazilian Araucaria forest, even after decades of intense logging and degradation by canopy opening, burning and cattle grazing/trampling inside forests. We applied cluster analysis and non-metric multidimensional scaling (NMDS) to group 143 systematically distributed 0.4 ha plots, located in the southern Brazilian Araucaria forest, according to their species abundance. Subsequently, forest structure attributes, species richness and diversity were assessed. We found 368 species and an average of ∼34 species per sample plot. Our estimations yielded a mean basal area per sample plot of 24.4 ± 2.1 m^² ha⁻¹, and a mean aboveground biomass of 86.9 ± 7.3 Mg ha⁻¹ (± 95% confidence interval). Besides a subtle differentiation of the sample plots’ species abundance along the altitudinal gradient (514–1 560 m above sea level) revealed by the NMDS, we did not find ecologically meaningful floristic assemblages related to altitude. Possibly, this result is a consequence of floristic and stand structure homogenisation caused by intensive historical (and ongoing) logging, cattle grazing inside stands and general land-use changes. In turn, forest structure attributes, such as total tree height, tree density and basal area, were more evidently related to the altitude gradient, most likely due to lower temperatures, stronger winds, greater cloud cover and frost frequency occurring at high-altitude sites.