DOI: 10.5061/dryad.rn8pk0p8z
title: Competition components along productivity gradients – revisiting a classic dispute in ecology

description: 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
description: <strong>Experimental setup</strong><br>To
 separate the effects of root and shoot competition the ‘target
 technique' was used (McPhee and Aarssen 2001), where target plants of
 the study species are separated from the surrounding vegetation by the
 following treatments:<br><br>    … growing without roots and
 shoots of neighbours (no competition, NC)<br><br>    …
 interacting with roots, but not with shoots of neighbours (root
 competition, RC)<br><br>    … interacting with shoots, but not
 with roots of neighbours (shoot competition, SC)<br><br>    …
 interacting with roots and shoots of neighbours (full competition,
 FC)<br><br>    … equipped with a weed fleece to suppress
 competitors (reduced competition, RedC)<br><br>In all
 treatments except for the full competition treatment neighbouring plants
 in a radius of approximately 15 cm were cut and a 12.5 × 12.5 cm weed
 fleece was placed around the target plant to prevent other plants from
 germination. To exclude root competition, partitions consisting of plastic
 tubes (diameter 16 cm, depth 18 cm) were established in the soil around
 the target plants. Above-ground partitions made of inverted wire mesh
 funnels (diameter 16 cm at the ground, and 50 cm at the top, height 50 cm)
 were installed to exclude shoot competition; to exclude above- and
 below-ground competition both methods were combined (Fig. 1). The effect
 of the weed fleece was investigated in the fifth treatment. We
 investigated three plants of A. minima per treatment and field, at sites
 with very small populations plant numbers were
 reduced.<br><br>To ensure comparability within the sites,
 plants with initial rosette diameters around 2 cm were selected and
 treatments were randomly assigned to the plants. At each site three
 replicates of the five treatments were established within plots of 5 × 5
 m. The plots were at least 5 m apart along the field edge. The experiment
 was set up at the beginning of April 2019 (sites 1–15: 02–05.04.2019,
 sites 16–19: 10–11.04.2019) and plant performance measurements were
 conducted monthly (early May: 06–09.05.2019; early June: 03–06.06.2019).
 <br><br>Plant height, rosette diameter, number of primary
 shoots, number of flowers, number of seed pods and above-ground biomass
 were measured as proxies of fitness. The results shown in the study,
 however, focus on above-ground biomass, since all other measured plant
 traits followed similar patterns as biomass in the competition treatments.
 <br>The biomass was harvested in early July 2019, shortly before the
 end of the experiment and the harvest of crops; it was dried at 65°C for
 48 h before weighing.<br><br><strong>Study
 sites</strong><br>To test the basic theory of competitive
 interactions using the study species A. minima, we performed an experiment
 on 18 arable fields in the natural subunit Franconian Basin between
 Bamberg and Roth in N-Bavaria, from early April till early July 2019.
 Altitude of the sites was 288–412 m, with higher ones in the south, and
 lower ones in the north. Mean annual temperatures ranged between 9.0 and
 9.6°C and mean annual rainfall was 655–757 mm (1988–2018; Supporting
 information). During the study period in 2019 temperatures deviated from
 these long-term values as follows (average over all sites): April +1.3,
 May −2.7 and June +3.6°C, while precipitation showed the opposite pattern
 being lower in April (−16 mm) and June (−33 mm), and higher in May (+15
 mm).<br><br><strong>Soil
 analysis</strong><br>Soil variables measured for the field
 sites: Sand: 2000-63 µm, coarse sand: 2000-630 µm; medium sand: 630-200µm;
 fine sand: 200-63µm; coarse silt: 63-32µm; Classification of the particle
 size fractions according to the German nomenclature (Blume et al. 2010);
 ions were measured in watery solution using an ion chromatograph (Thermo
 Scientific, Dionex ICS 1600); ph was measured in calcium chloride (0,01 M)
 solution with a pH electrode (WTW SenTix 41); Corg and Norg were analyzed
 using a CN analyzer (elementar vario MAX cube)