1. The reduction of plant diversity following eutrophication threatens many ecosystems worldwide. Yet, the mechanisms by which species are lost following nutrient enrichment are still not completely understood, nor are the details of when such mechanisms act during the growing season, which hampers understanding and the development of mitigation strategies.

2. Using a common garden competition experiment, we found that early-season differences in growth rates among five perennial grass species measured in monoculture predicted short-term competitive dominance in pairwise combinations and that the proportion of variance explained was particularly greater under a fertilisation treatment.

3. We also examined the role of early-season growth rate in determining the outcome of competition along an experimental nutrient gradient in an alpine meadow. Early differences in growth rate between species predicted short-term competitive dominance under both ambient and fertilized conditions and competitive exclusion under fertilized conditions.

4. The results of these two studies suggests that plant species growing faster during the early stage of the growing season gain a competitive advantage over species that initially grow more slowly, and that this advantage is magnified under fertilisation. This finding is consistent with the theory of asymmetric competition for light in which fast-growing species can intercept incident light and hence outcompete and exclude slower-growing (and hence shorter) species. We predict that the current chronic nutrient inputs into many terrestrial ecosystems worldwide will reduce plant diversity and maintain a low biodiversity state by continuously favouring fast-growing species. Biodiversity management strategies should focus on controlling nutrient inputs and reducing the growth of fast-growing species early in the season.

Experimental design

The common garden experiment has been described at greater length elsewhere (Vojtech et al. 2007, Vojtech et al. 2008, Hautier et al. 2018b). Briefly, we established monocultures (n=5), all pairwise mixtures (n=10) and the full five-species mixtures (n=1) of five perennial grass species (Poaceae): Alopecurus pratensis L. (Al), Anthoxanthum odoratum L. (An), Arrhenatherum elatius (L.) P. Beauv. ex J. Presl & C. Presl (Ar), Festuca rubra ssp. commutata Gaud. (= Festuca nigrescens Lam.) (F), Holcus lanatus L. (H) (Lauber and Wagner 2001). Each species combination was replicated five times for a total of 80 plots. Species were sown from seeds at a total target density of 1000 seeds m^-2 per plot divided equally among the species of each mixture (corrected based on the results of prior germination trials). Plants were established in 1 m² plots on highly fertile soil (Garden humus, Ricoter, Aarberg, Switzerland). The experiment ran from April 2004 to June 2008. Plots were watered daily and regularly weeded throughout the duration of the experiment. During 2005 and 2006, plants were regularly fertilized with an NPK fertilizer corresponding to 15 g m^-2 yr^-1 of nitrogen to create highly productive conditions. In 2007, we divided the plots into four subplots of 50 x 50 cm (Fig. S1). In half of these subplots, we maintained the initial highly productive conditions by continuously adding the NPK fertilizer. In the other half of the subplots we reduced soil fertility by a combination of the cessation of fertilization and the addition of sucrose (in five applications of 500 g m^-2 year^-1 during 2007 and two applications of 625 g m^-2 in 2008). Addition of a carbon source limits nutrient availability to plants and reduces productivity due to the immobilisation of nitrogen by soil micro-organisms (Killham 1994) and increases competition between micro-organisms and plants for nitrate and ammonium (Schmidt et al. 1997, Bardgett et al. 2003). Additionally, we crossed the productivity treatments with regular cutting of the canopy structure to create disturbed conditions (Hautier et al. 2018b). Calculating daily RGR per species throughout the growing season for the plots that were disturbed was not possible because of the limited number of samples between each cutting event. Here, we therefore analyse only the undisturbed productive and unproductive conditions.

Data collection

On 10^th-20^th June 2008 (Days 161-171), after two years of treatment, aboveground plant biomass in the inner 30 x 30 cm of each subplot was harvested at soil level, sorted to species, dried at 80°C and weighed (hereafter biomass at harvest). To estimate daily RGR of each species in monoculture, aboveground plant biomass was harvested at soil level within 10 x 10 cm quadrats in the outer 10 cm surrounding the inner 30 x 30 cm of each subplot during sequential harvests on days 53, 67, 88, 109, 116, 123, 130, 145, 152, 162, and 171 in the year 2008. Day 171 was the peak standing biomass. Each time different randomly chosen quadrats were measured. One 10 x 10 cm quadrat was sampled from each subplot on each of the days listed. Each time a different randomly chosen quadrat was clipped in monoculture (Fig. S1). Harvested biomass samples were dried at 80°C and weighed. Soil cores were collected regularly during the growth season in 2008 and analysed for nitrate and ammonium concentrations (Labor für Boden- und Umweltanalytik, Thun, Switzerland). One soil core was collected randomly from one replicate of each monoculture, one replicate of each two-species mixtures and three replicates of the five-species mixtures. We measured plot level light interception ability in monoculture for each species and each nutrient treatment before the harvest in end-April 2008 as the percentage of transmitted photosynthetically active radiation (PAR) reaching the soil surface in the inner 30 x 30 cm of each subplot.

harvest.date : date of the harvest

days: day in the year (Julian days)

Plot: plot number

species: name of the species abbreviated as follow: Alopecurus pratensis L. (Al), Anthoxanthum odoratum L. (An), Arrhenatherum elatius (Ar), Festuca rubra (F), Holcus lanatus L. (H)

abv.mass: aobeground biomass in 10x10 cm quadrats