Functional Distance Mediates Plant Interactions

Question

The outcome of plant interactions depends on the physiological, morphological, and reproductive traits harbored by the interacting species. The outcomes of plant interactions depend on: (a) trait dissimilarity mechanisms, whereby species with similar traits compete more intensely due to niche overlap, and (b) trait hierarchy mechanisms, whereby species with higher relative fitness outcompete species with less efficient traits under specific environmental conditions. We hypothesized that the functional distance between interacting species affects the outcome of plant interactions simultaneously through both mechanisms.

Location

We collected 10 Mediterranean herb and shrub species in two locations in central Spain.

Methods

We established a manipulative experiment including 10 species growing in pairs. We estimated neighbor effects by measuring the relative change in values of 13 above- and belowground traits of a focal species caused by the presence of a heterospecific (compared to a conspecific) neighbor. We explored (i) which traits were more affected by neighbors, (ii) which species were more affected by an interacting species, either as focal or neighbor and (iii) how the neighbor effect varied with the functional distance between pairs.

Results

Ten out of thirteen traits varied in the presence of a heterospecific (compared to a conspecific) neighbor, six of them increasing (e.g., total photosynthetic area, number of root nodules, root weight) and four decreasing their values (e.g., leaf and root C content). The relative change of trait values in heterospecific pairs significantly increased as functional distance decreased for most plant-performance traits. Only root biomass showed the opposite trend.

Conclusion

Trait hierarchy mechanisms prevailed but trait dissimilarity mechanisms operated on specific root traits, indicating that both simultaneously determine the outcome of interactions. The heterogeneity of neighbor effects responded mainly to differences between above- and belowground traits, reinforcing the need to consider them both for a mechanistic comprehension of community dynamics.