Introduction
Anthropogenic inputs of nutrients, including nitrogen (N) and phosphorus
(P), into the biosphere have greatly increased in recent decades and
continue to rise (Sinha et al., 2017).
This environmental eutrophication represents a major threat to
biodiversity in many terrestrial, freshwater and marine ecosystems
worldwide, as it is usually associated with biodiversity loss
(Borer et al., 2014,
Ren et al., 2017). In grasslands,
nutrient enrichment, both deliberate (agricultural fertilization) and
unintentional (atmospheric deposition), has been shown to have profound
impacts on ecosystems (Erisman et al.,
2008). Nutrient input usually increases primary productivity and
reduces plant diversity and community stability
(Midolo et al., 2018,
Soons et al., 2017). This loss of plant
diversity can then impact the functioning of ecosystems and their
associated ecosystem services (Hautier et
al., 2015, HautierIsbell et al., 2018,
Hautier et al., 2014,
Hector et al., 2010,
Isbell et al., 2015). However, we do not
have a complete understanding of the mechanisms by which nutrient inputs
lead to the loss of plant diversity
(Harpole et al., 2017) or the timing
during the growing season when these mechanisms are most important.
In low-fertility grasslands, where soil resources are strongly limiting,
diversity is often high. However, resource competition theory
(R*theory) predicts dominance by the single species that can deplete soil
resources to the lowest level (with the lowest value of R *)
(Tilman, 1982,
Tilman, 1980). We must therefore assume
that low-fertility grasslands are either limited by more than one
belowground resource (Fay et al., 2015,
Hutchinson, 1957), or that additional
mechanisms operate, such as negative soil feedbacks, that introduce
frequency-dependence and hence stabilisation
(Petermann et al., 2008). Coexistence
might be made easier in such systems because competition for belowground
resources is often assumed to be size-symmetric
(Vojtech et al., 2007,
HautierVojtech et al., 2018), thus
leading to relatively small fitness differences between species, which
can be offset by weak niche differentiation
(Chesson, 2000).
Under fertilized conditions, when nutrient limitation is alleviated and
light becomes the limiting resource, resource competition theory
(I * theory) again predicts competitive dominance, this time by
the species that is able to intercept light and reduce it to the lowest
level (Dybzinski and Tilman, 2007,
Vojtech et al., 2007). Because light is a
directionally supplied resource, tall species can intercept and pre-empt
light, making it unavailable to low-growing species. Competition for
light is likely to be highly size-asymmetric and might therefore lead to
very large fitness differences and hence the exclusion of smaller,
slow-growing species (Hautier et al.,
2009, DeMalach et al., 2017,
Borer et al., 2014) even if the same
stabilising niche differences still operate.
While direct measurements of mechanistic plant competition are extremely
difficult, relative growth rate (RGR) is relatively easy to measure, and
many plant species show striking differences in their relative growth
rate, even when grown under similar environmental conditions (Grime and
Hunt 1975). High RGR might confer a strong competitive advantage under
highly fertile conditions, because it enables a species to quickly
capture light and deny it to competitors. But under low-fertility
conditions, we might expect high RGR to be a much poorer predictor of
competitive outcomes, as other traits, reflecting niche differences, may
play a greater role. The timing of growth might also be a key factor in
determining competitive outcomes. For example, a species growing faster
during the early stage of the growing season might reduce light
availability and thus have a disproportionate competitive advantage
relative to species that initially grow more slowly. RGR can be measured
at different time points and thus be used to identify when during the
growing season differences in RGR are particularly important.
We used two studies to test whether early differences in species growth
rates better predict short-term competitive dominance under fertilised
conditions: (1) a common garden experiment where species were grown in
monoculture and in pairwise and five-species mixtures under low and high
soil fertility and (2) an experiment in a natural grassland community
that also included fertilizer treatments. Critically, both studies
provide detailed measurements of aboveground biomass through the growing
season. We focus mainly on competitive outcomes in fertilized
conditions, where we expect competition to be primarily for light, hence
species with high early-season RGR in monoculture should dominate
mixtures. We contrast the fertile situation with less productive
conditions but because the outcome of competition may be slower, the
comparison is limited by the short-term nature of our study.