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 field experiment was set up in April 2011 and has been described elsewhere (Zhang et al. 2015, Zhou et al. 2017, Zhou et al. 2018). Large herbivores were excluded between March and October by fencing the experimental area. A homogeneous area of meadow covering 230 x 100 m was divided into four parts that were given N, P, their combination or neither. Six plots, each 10 x 20 m, were established within each nutrient area. Fertilization treatments consisted of a factorial combination of N and P addition applied annually to fertilized plots in each of three blocks: N, P and NP.  Nitrogen was supplied at a rate of 15 g N m^-2 y^-1, phosphorus at a rate of 8 g P m^-2 y^-1, and nitrogen and phosphorus at a rate of 10 g N m^-2 y^-1 and 8 g P m^-2 y^-1. While we acknowledge that plots within each nutrient area are not independent, previous studies have shown that there were no significant differences among them in term of plant species diversity, community biomass and community composition at the start of the experiment (Zhou et al. 2018). N was applied as ammonium nitrate (NH₄NO₃) and P as monocalcium phosphate (Ca(H₂PO₄)₂) annually at the end of May. Each plot was subsequently divided into two subplots; one was used to measure aboveground individual biomass through time for twenty common species (Table S1), and the other was used to measure aboveground plant biomass and species composition in early August (see below in Data collection).

Data collection

In 2013, after three years of nutrient addition, in the subplots dedicated to measuring aboveground individual biomass through time, we sampled twenty common species accounting for 85 ± 10% of aboveground biomass (16, 2, 1, and 1 species from Forbs, Grasses, Sedges, and Legumes respectively; Table S1). For each species, we randomly selected, dried at 80°C and weighed 12 individuals on days 146, 157, 167, 177, 197, 207, 238, and 254 in the year of 2013. We stopped sampling species once they were in full flower, resulting in a lower number of species sampled after day 177. In the subplots dedicated to measuring aboveground plant biomass and species composition at peak biomass, the vegetation was clipped in mid-August 2013 at soil level in one randomly selected 0.5 x 0.5 m quadrat, sorted to species, dried at 80°C and weighed.

To evaluate the relative importance of RGR versus other traits likely to influence competitive ability, in 2016 we measured three functional traits for the twenty common species in each treatment: final height, specific leaf area (SLA) and leaf dry mass content (LDMC). These traits generally define species resource utilization strategies in terrestrial ecosystems (Grime 2006). For each species, following the flowering phase, we randomly sampled nine fully developed and undamaged leaves. We weighed and scanned fresh leaves to measure leaf area using ImageJ software (Schneider et al. 2012). We then dried material at 70°C for 48 hr and weighed the dried leaves. We calculated SLA as the ratio of leaf area to dry leaf mass and LDMC as the ratio of dry leaf mass to fresh leaf mass. We also randomly selected 30 flowering individuals of each species to measure the species’ final height in each treatment.

trt: treatments consisting of Control, Nitrogen, Phosphorus, and Nitrogen and Phosphorus

time: date in the year

days: day in the year (Julian days)

species: species names

weight: aboveground biomass

plot: plot number