B. pinnatum and E. repens hypervolume calculation

1. Trait intraspecific variability determines community dynamics and
species coexistence. In response to competition, plants can display
intraspecific variability to enhance its competitive ability or
stabilize its niche differences with their competitors. This response is
multidimensional because it involves changes along different functional
axes and inevitable trade-offs between traits. Here, we transposed the
recent concept of the multidimensional trait space to the analysis of
intraspecific plant response to competition. We specifically tested the
following: (1) in the absence of competitors, the plant multidimensional
trait space will be packed towards strategies promoting plant
colonisation, and (2) with competitors, the plant multidimensional trait
space will be directed towards competition with its size and shaping
characteristics dependent on competitor species richness.

2. We
studied trait intraspecific variability of two clonal species,
Brachypodium pinnatum (L.) P. Beauv. and Elytrigia repens (L.) Gould, in
response to competition. We analysed plant response in the absence of
competitors and in competition. Competition treatments included
intraspecific and interspecific experimental mixtures with increasing
species richness. For each target species and each treatment, we built
an hypervolume based on six traits involved in the three-dimensional
competition (i.e. ramet and connection traits). We measured these
hypervolumes for their size, similarity and the contribution of traits
in their shaping.

3. In the absence of competitors and for both
species, we demonstrated a multidimensional trait space packing towards a
colonisation strategy. Under competition, the multidimensional trait
spaces of the two target species were the widest at the extremes of the
richness gradient, i.e., intraspecific and interspecific high richness
competition treatments. High intraspecific variability either promoted
niche differentiation for individuals of similar species or reflected
the large range of competitive responses deployed when plants were faced
with many different competitor identities. The multidimensional
response process was based on fine adjustments of various traits
depending on the surrounding neighbourhood composition and more
specifically, on the competitor abundances and functional similarity
with the target species.

4. This study emphasised the
multidimensionality of species competitive response, and also underlined
the so far neglected importance of competitor species richness for
trait intraspecific variability and subsequently community assembly.