Competition components along productivity gradients – revisiting a classic dispute in ecology

Competition is ubiquitous in plant communities with various effects on
plant fitness and community structure. A long-standing debate about
different approaches to explain competition is the controversy between
David Tilman and Philip Grime. Grime stated that the importance of
competition relative to the impact of the environment increases along a
productivity gradient, while Tilman argued that the intensity of
competition is independent of productivity. To revisit this controversy,
we assumed that the effects of plant-plant interactions are additive and
applied the new competition indices by Diaz-Sierra et al. (2017) in a
field experiment along a productivity gradient in S-Germany, using the
rare arable plant Arnoseris minima as a study species. The ‘target
technique’ was applied, to separate the effects of root and shoot
competition. The study plants were exposed to five competition treatments
with three replicates in 18 sites, respectively. We investigated the
expectation that root competition is more intense in unproductive sites
than shoot competition. Additionally, we predicted survival to be less
affected by competition than growth-related plant parameters. Using the
biomass of individuals without competition as a proxy for site
productivity there was a positive relationship with competition importance
but no relationship with competition intensity when plants experienced
full competition. Survival of the target plants was unaffected by
competition. Root competition was the main mechanism determining the
performance of the target plants, whereas the effect of shoot competition
was relatively low albeit increasing with productivity. We conclude that
when considering plant-plant interactions additive both Grime’s and
Tilman’s theories can be supported