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 supportedFunding provided by: Bayerische KulturLandStiftung*Crossref Funder Registry ID: Award Number: Funding provided by: Bayerischer Naturschutzfonds*Crossref Funder Registry ID: Award Number: Funding provided by: Landwirtschaftliche RentenbankCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100018686Award Number:Experimental setupTo 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: … growing without roots and shoots of neighbours (no competition, NC) … interacting with roots, but not with shoots of neighbours (root competition, RC) … interacting with shoots, but not with roots of neighbours (shoot competition, SC) … interacting with roots and shoots of neighbours (full competition, FC) … equipped with a weed fleece to suppress competitors (reduced competition, RedC)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.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). 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. 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.Study sitesTo 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).Soil analysisSoil 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