Stress decreases functional beta diversity and increases the relative stochasticity of taxon-based community assembly in contrasting vegetation types

Background/Question/Methods

Community assembly occurs by both selective filtering of species by their functional traits and by more neutral and/or stochastic factors such as drift and dispersal.   The relative influences of deterministic, selective filtering likely increases with stress and interspecific interaction intensity.  Much of the evidence for this comes from patterns in species beta diversity, whereby increased beta diversity is interpreted as increased stochasticity of community assembly.  We investigated patterns of species and functional beta diversity along soil moisture gradients in two contrasting vegetation types: open grasslands/wetlands and deciduous forest understory.  Sampling employed locating releves of relatively homogeneous vegetation (n = 23 each) and replicate quadrats in each releve (n = 4, 5 respectively).  SLA and other functional traits were measured on each species in each sample instance (n = 600, 913 respectively).  Beta diversity was measured for each releve as the jaccard distance in community composition and as the mean difference in SLA between species in different quadrats.  We identified a new metric: (species beta)/(functional beta) as an indicator of the degree to which communities are relatively stochastic in species composition while being relatively deterministic in trait expression. 

Results/Conclusions

In both vegetation types, the beta ratio increased with moisture stress.  This supports a general conclusion that moisture stress increases the role of deterministic assembly at the level of functional traits in accordance with assembly theory.  That is, species have consistent expression of traits, but the identity of the species may vary.  With increased moisture, constraints on functional assembly are relaxed and so functional beta diversity increased in both vegetation types.  Taxonomic beta was independent of moisture, and was related to light availability and spatial variation in light availability.  Structural equation models showed that the relative influences of light were not consistent across the vegetation types, while the influences of moisture were consistent.  Functional beta diversity and the ratio of taxonomic to functional beta provide important and insightful additional perspectives when developing a general theory of community assembly and functional biodiversity.