Stronger effect of individual species’ traits than shading on aquatic plant community productivity and interspecific competition

Competition is one of the major factors structuring plant communities.
Species with similar traits generally compete more intensely and have more
similar yield than functionally dissimilar species, which often respond
differently to environmental change. Little is known about how the
interacting species’ traits influence the effect of environmental
change on interspecific competition. However, theory predicts that
environmental change should lead to more asymmetric competition, by
favouring the species best adapted to the particular environmental change.
Here we used a mesocosm experiment with three common aquatic plant species
from the Baltic Sea (Northern Europe), to test how community productivity
and competition asymmetry were affected by functional dissimilarity,
individual species’ traits, and a common stressor: shading. Competition
asymmetry was defined as the absolute difference in reductions in yield
relative to monocultures of two interacting species. Community
productivity decreased and competition asymmetry increased with functional
dissimilarity of the interacting species, possibly explained by the traits
of the superior species, which had higher specific leaf area, maximum
canopy height, and primary production rate than the subordinate species.
Community productivity was not affected by shading, contrary to our
expectation, while competition asymmetry was higher in shaded than ambient
conditions. Individual species yield depended on species identity and
species combination. Only the shortest species was negatively affected by
shading. Thus, by favouring tall-growing species, shading can alter
interspecific competition. Together, these findings suggest that
non-random species loss following environmental change can be caused by
competitive exclusion, in addition to a direct effect of abiotic
filtering.