Data from: Phylogenetic and functional mechanisms of direct and indirect interactions among alien and native plants

Disentangling direct and indirect interactions among alien and native
plants is essential to understanding the success of alien plants in
multi-species communities, but studies have only focused on direct
pairwise interactions. Moreover, it is also essential to explore
phylogenetic and functional mechanisms driving these interactions. In a
greenhouse experiment, we selected nine groups of alien and native plant
species from the herbaceous flora of Germany to disentangle their direct
and indirect interactions. Each group had an alien (A) that is common or
rare in Germany (i.e. non-native range), two natives that are
phylogenetically closely related (Nclose) and distantly related (Ndist) to
A respectively, and a distantly related “target” native (T). We grew the
four species of each group alone, and in two-species and three-species
combinations. Specifically, we tested whether competition is greater
between A and Nclose than between A and Ndist, whether presence of Nclose
rather than Ndist indirectly alleviates competition of A on T, and whether
these interaction patterns depend on commonness of A. Moreover, we tested
how intensity of these interactions is explained by phylogenetic distance,
functional traits (height, seed mass, SLA, leaf size, specific root
length, leaf area ratio, root length ratio (root length/plant mass), shoot
weight ratio) and traits-based functional distance. We found A had
stronger competition on Nclose than on Ndist. In turn, A was more
suppressed by Nclose than by Ndist, but this was only true for rare rather
than common A. The presence of Ndist rather than Nclose indirectly reduced
competition of A on T. The intensity of the interactions was not explained
by phylogenetic or functional distance, but by some of the functional
traits. Specifically, a plant experienced stronger competition when it was
shorter and had smaller leaves and lower shoot weight ratio, and when its
neighbors were taller, had greater SLA, leaf area ratio and shoot weight
ratio, and had a lower root length ratio. Synthesis. Functional traits can
help explain competitive interactions. While direct competition tended to
be stronger between more closely related alien and native plants, this did
not indirectly facilitate other co-occurring native plants.