Functional assembly of tropical montane tree islands in the Atlantic Forest is shaped by stress-tolerance, bamboo-presence and facilitation

Aims: Amidst the Campos de Altitude (Highland Grasslands) in the Brazilian Atlantic Forest, woody communities grow either clustered in tree islands or interspersed within the herbaceous matrix. The functional ecology, diversity and biotic processes shaping these plant communities are largely unstudied. We characterised the functional assembly and diversity of these tropical montane woody communities and investigated how they fit within Grime's CSR (C – competitor, S – stress-tolerant, R – ruderal) scheme, what functional trade-offs they exhibit and how traits and functional diversity vary in response to bamboo presence/absence.

Methods: To characterize the functional composition of the community, we sampled five leaf traits and wood density along transects covering the woody communities both inside tree islands and outside (i.e. isolated woody plants in the grasslands community) . Then, we used Mann Whitney test, t-test and variation partitioning to determine the effects of inside vs outside tree island and bamboo presence on community weighted means, woody species diversity and functional diversity.

Results: We found a general SC/S strategy with drought-related functional trade-offs. Woody plants in tree islands had more acquisitive traits than those within the grasslands. Trait variation was mostly taxonomically than spatially driven, and species composition varied between inside and outside tree islands. Leaf thickness, wood density and foliar water uptake were unrelated to CSR-strategies, suggesting independent trait dimensions and multiple drought-coping strategies within the predominant S-strategy. Islands with bamboo presence showed lower Simpson diversity, lower functional dispersion, lower foliar water uptake and greater leaf thickness than in tree islands without bamboo.

Conclusions: The observed functional assembly hints towards large-scale environmental abiotic filtering shaping stress-tolerant community strategy, and small-scale biotic interactions driving small-scale trait variation. We recommend experimental studies with fire, facilitation treatments, eco-physiological and recruitment traits to elucidate on tree island expansion and communities response to climate change.