Phylogenetic and trait structure along an environmental and canopy gradient in understory communities of a southern European wetland forest

Background/Question/Methods

A pervasive ecological pattern is that local communities consist of species that are subsets of those existing in larger regional species pools. What factors determine which subsets exist in which sites?  There is a growing consensus that functional traits and evolutionary relationships play important roles in structuring ecological communities, but that the assembly process may be also dependent on the types of environment in which local communities occur.  We examined patterns of plant understory community trait and phylogenetic structure of a wetland forest in Southern Spain and how these patterns relate to a gradient of soil moisture and canopy dominance. We identified and georreferenced all canopy trees which were spatially distributed across a compositional gradient varying from pure ash (Fraxinus angustifolia) to pure willow (Salix atrocinerea).  Understory vegetation and environment (light, physico-chemical soil properties) were sampled in 50 2x2m georreferenced subplots randomly distributed across this compositional gradient. We built a phylogenetic supertree and calculated a suite of life history and physiological traits for all plants. We described phylogenetic relatedness using the number of nodes that separate taxa on the supertree and calculated phylogenetic distances. We then related patterns of phylogenetic and trait assembly structure to the compositional and environmental gradients. 

Results/Conclusions

A total of 40 vascular plants were found in the understory (12 monocots, 28 dicots). Floristic patterns showed a clear association with the canopy compositional gradient. Species presented a diverse range of life history traits, including the three extremes of the Grime’s C-S-R model. Competitor syndromes were found to be associated to the ash-dominated extreme (drier) of the gradient whereas ruderal and stress-tolerant syndromes were associated to the willow-dominated extreme (waterlogging, flooding). We found that the mean phylogenetic distance (mean pairwise distance and mean nearest taxon distance) was significantly greater in communities dominated by willow. Phylogenetic distance was positively related with soil waterlogging, fine textures, organic carbon and nitrogen, and negatively related with pH and bases concentration. Under the most stressful environmental conditions (prolonged waterlogging and flooding), wetland forests understory assemblages tended to be dispersed with respect to phylogenetic characteristics, including both ruderal and stress-tolerant species.  Our results also suggest that trait overdispersion, indicative of limiting similarity, is a common pattern in communities found in these wet environments. These results contrast with some previous studies suggesting that stressful conditions act as a major environmental filter that leads to species adaptation through phylogenetic conservatism or alternatively via trait convergence.