Fast and furious: Early differences in growth rate drive short-term plant dominance and exclusion under eutrophication

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.