DOI: 10.5061/dryad.djh9w0vxm
title: Fast and furious: Early differences in growth rate drive short-term plant dominance and exclusion under eutrophication

description: 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.
description: 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):
 <i>Alopecurus pratensis </i>L. (<i>Al</i>),
 <i>Anthoxanthum odoratum </i>L. (<i>An</i>),
 <i>Arrhenatherum elatius </i>(L.) P. Beauv. ex J. Presl
 & C. Presl (<i>Ar</i>), <i>Festuca rubra
 </i>ssp. <i>commutata </i>Gaud. (= <i>Festuca
 nigrescens </i>Lam.) (<i>F</i>), <i>Holcus lanatus
 </i>L. (<i>H</i>) (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<sup>-2</sup> per plot divided equally among the
 species of each mixture (corrected based on the results of prior
 germination trials). Plants were established in 1
 m<sup>2</sup> 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<sup>-2
 </sup>yr<sup>-1</sup> 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<sup>-2</sup>
 year<sup>-1</sup> during 2007 and two applications of 625 g
 m<sup>-2</sup> 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<sup>th</sup>-20<sup>th</sup> 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.
description: 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