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.