Forest loss and treeless matrices cause the functional impoverishment of sapling communities in old-growth forest patches across tropical regions.
Landscape-level disturbances, such as forest loss, can profoundly alter the functional composition and diversity of biotic assemblages. In fact, the landscape-moderated functional trait selection (LMFTS) hypothesis states that landscape-level disturbances may act as environmental filters that select a set of species with disturbance-adapted attributes while causing the loss of species with disturbance-sensitive attributes, ultimately compromising ecosystem functioning. However, the impact of landscape patterns on the functional composition and diversity of tropical regenerating trees (saplings) is unknown. Using a multiscale approach to identify the best spatial scale (i.e. the scale of effect), we tested the effect of forest cover, matrix openness and forest patch density (fragmentation) on functional composition and functional diversity of tree saplings in old-growth forest patches (n = 59) in three Mexican rainforest regions with different degree of deforestation. For 368 species and ~23,000 individuals, we compiled information from global and national databases on six functional traits related to seed dispersal and plant establishment and calculated their community abundance-weighted mean (CWM) and three complementary functional diversity indices. Forest loss and matrix openness reduced functional richness and evenness, but only in the two most deforested regions. Overall, fragmentation had contrasting effects on functional diversity and composition, but correlated negatively with some functional traits in the most deforested region. Importantly, in the regions with high-to-intermediate degree of deforestation, functional composition experienced major changes: maximum height, seed mass, fruit size and wood density decreased, and SLA increased, in forest patches surrounded by open matrices in highly deforested and fragmented landscapes. This caused a shift of community traits towards more disturbed-adapted attributes. Synthesis and applications: In agreement with the LMFTS hypothesis, our results confirm that landscape modifications in regions undergoing high and long-lasting deforestation greatly impoverish the functional composition and diversity of sapling communities. The shift from communities composed mainly by conservative attributes towards communities with a higher prevalence of disturbance-adapted attributes disrupts the future community structure and jeopardizes critical ecosystem functions. Management practices focused on preventing deforestation, increasing forest cover and promoting treed matrices are necessary to preserve the functionality of these species rich but increasingly threatened rainforests.