Functional characteristics rather than co‐occurrences determine the outcome of interactions between neighbouring plants in sub‐Antarctic ponds: Consequences for macrophyte community biomass

Douce, Pauline; Saiz, Hugo; Benot, Marie‐Lise; Mermillod‐Blondin, Florian; Simon, Laurent; Renault, David; Vallier, Felix; Oury, Yoann; Fontaine, Matthieu; Bittebiere, Anne‐Kristel (2023). Functional characteristics rather than co‐occurrences determine the outcome of interactions between neighbouring plants in sub‐Antarctic ponds: Consequences for macrophyte community biomass. Freshwater Biology, 68(4), pp. 561-576. Wiley 10.1111/fwb.14047

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Understanding of the mechanisms underlying species coexistence within plant communities is crucial to predict their fate given the current context of biodiversity loss. Freshwater ecosystems are among the most abiotically constrained habitats because they harbour clonal macrophyte communities. Strong habitat filtering in these ecosystems influences the functional composition and diversity of macrophyte communities, determining the outcome of interactions between neighbours and ultimately affecting local spatial arrangement between neighbours (LSA). This influence may be modulated by environmental conditions in abiotically constrained habitats, such as freshwater ecosystems.
We focused on macrophyte communities living in ponds in the Iles Kerguelen, in the sub-Antarctic. These freshwater ecosystems are especially abiotically constrained (cold climate), and their plant communities are remarkably species-poor, simplifying the study of interactions between neighbours. We measured several abiotic variables in the ponds, species LSA and interspecific interactions (using the log response ratio metric), and the functional composition of the community using aerial, root and clonal traits. We also determined the biomass of the whole macrophyte community.
Our results showed that LSA does not effectively assess interactions between neighbours at very small scales, neither at the community (one species vs. all neighbouring species) nor the species level (between pairs of species). Secondly, aerial (leaf and stem) and root traits related to resource acquisition played a more important role in interactions between neighbours than clonal traits (i.e. internode length and specific internode mass related to space acquisition and resource storage, respectively).
Depending on the target species, (1) interactions responded positively or negatively to mean trait and functional diversity of the community; and (2) different traits of neighbours (aerial or root traits) triggered an interaction response. Lastly, abiotic variables, in particular water temperature and light intensity, influenced macrophyte community biomass and plant community structure (i.e. species richness, functional diversity and LSA), either directly or through the modulation of other abiotic variables.
Our results lead to the following conclusions: (1) interactions between neighbours do not reflect their local spatial arrangements but are strongly associated with abiotic variables and neighbourhood functional traits (depending on species); (2) joint integration of biotic and abiotic variables in multivariate analyses enables better inference of biodiversity responses to environment, and the subsequent consequences for ecosystem function, especially in the context of climate change.

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