Bryophyte assembly rules across scales

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.