Contrasting responses of different functional groups stabilise community responses to a dominant shrub under global change

1. Changes in precipitation regimes and nitrogen deposition levels due to
global change are altering terrestrial ecosystems worldwide. Most attempts
at understanding how biotic interactions affect plant species and
community responses to global change have focused on biotic interactions
measured at the community-level. However, these approaches ignore that
communities include functionally dissimilar species that might respond
differently to changes in the effects of dominant neighbours along
environmental gradients. We test the hypothesis that plant species with
differences in functional traits may exhibit contrasting interactions with
a dominant shrub species within a single community, thereby stabilizing
community-level responses to the effects of global change. 2. We
manipulated water and nitrogen applications in a semi-arid dune community
in northern China, quantified the biomass of herbaceous species occurring
below the dominant shrub, Artemisia ordosica, and in adjacent open patches
(without the shrub), and measured herbaceous species height and leaf dry
matter content. The effects of A. ordosica were quantified at the species,
group, and community levels using the relative interaction index (RII).
Redundancy analysis was performed on species traits and the RII values to
assess the relationships between functional differences in species and
plant-plant interactions. Species were then grouped using a cluster
analysis and the RII values were recalculated at the group level. 3. The
redundancy analysis showed that species height and leaf dry matter content
were significantly correlated to the changes in species’ responses to the
effect of A. ordosica along treatments. The four groups of species
identified by the cluster analysis showed contrasting variations in
competitive or facilitative responses depending on species traits and
environmental treatments. However, the interactions measured at the group
level balanced the community-level responses, as we found no significant
changes in the effects of A. ordosica along treatments for community
biomass, richness, and functional diversity indices. 4. Synthesis. Our
findings indicate that species with different functional strategies within
a community exhibit contrasting responses to a dominant shrub along
environmental gradients. These contrasting changes in plant-plant
interactions of functionally different species may balance the responses
of community-level metrics. This suggests that functional differences
between species groups and the balance of plant-plant interactions
stabilize community responses to global change.