DOI: 10.5061/dryad.5mkkwh7b5
title: Bryophyte assembly rules across scales

description: 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. 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). 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.
 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.
description: 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 m<sup>2</sup> (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.