1. Understanding how species assemble into communities is a central issue in community ecology. So far, most studies have focused on the assembly mechanisms of vascular plant communities, while the role of deterministic (environmental filtering and biotic interactions) and stochastic (e.g., dispersal limitation) processes structuring bryophyte assemblages remains poorly understood.
2. To evaluate how different assembly processes shape bryophyte communities in mountain streams, we examined functional trait patterns across spatial scales and along environmental gradients. To do so, we sampled 754 microhabitat plots (0.25 m²) nested within 165 sites (100-m long stream segments) and 13 sub-basins, located in the northwest and central-west of Portugal. At each spatial scale, observed functional diversity indices (Functional Richness and Rao’s quadratic entropy) were compared to random expectations derived from null models, followed by the analysis of changes in functional trait patterns along environmental gradients by fitting a series of generalized additive mixed models (GAMMs).
3. At local scales (site and microhabitat plot), coexisting species tended to be more functionally similar than expected, suggesting the prevalence of environmental filtering effects. In contrast, no significant deviations from random expectations were detected at the broadest spatial scale (sub-basin), indicating the prevalence of stochastic processes. We found contrasting assembly processes along environmental gradients: environmental filtering prevailed in stressful environments, while competitive interactions were more important in favourable conditions.
4. Synthesis. Our results highlight the role of environmental filtering in bryophyte community assembly at fine spatial scales, emphasizing the importance of measuring environmental conditions at the same spatial scales where biotic interactions take place. In line with the stress-dominance hypothesis, the relative importance of environmental filtering increased with abiotic stress. Thus, analysing functional trait patterns across different spatial scales and environmental gradients may contribute to a better understanding of the mechanisms underlying community assembly.

The study was carried out in streams distributed 13 throughout 13 sub-basins, located in the northwest and central-west of Portugal.

Fieldwork was conducted from May 2003 to October 2008 in 165 sites, selected according to the spatial distribution and density of river networks within each sub-basin. Sites were also chosen to span the range of hydromorphological conditions.  In each site, bryophyte species abundance was registered as percentage cover using sampling plots of 0.25 m² (0.5 m x 0.5 m), placed in distinct microhabitats recognized in the streambed structure, with different levels of submersion and substrate stability. Thus, within each site, each microhabitat plot represents a distinct micro-hydromorphological unit. The number of microhabitat plots in each of the 165 sites was proportional to the diversity of micro-hydromorphological conditions (1–18 plots per site). In total, this survey resulted in 754 microhabitat plots. At the site scale, species abundance was estimated as the mean value of species abundances across microhabitat plots within each 100-m stream segment. Likewise, species abundance at the sub-basin scale was calculated as the mean value of species abundances across microhabitat plots within each sub-basin.