Discussion
Our competition experiment in a common garden shows that early-season
differences in species’ growth rates in monoculture are good predictors
of short-term differences in relative abundance in pairwise and five
species mixtures and that predictions were stronger under productive
(light-limited) conditions. The species that grew faster early in the
season (i.e. H. lanatus and A. pratensis) , had the
greatest competitive advantage relative to slower-growing species (i.e.A. odoratum , A. elatius and F. rubra ). Relative
differences in species growth rates became smaller as the growing season
progressed until they eventually became negatively associated with
differences in species biomass.
This
switch corresponds to the time at which faster growing species had
already reached their maximum growth rate and gradually slowed down
while the RGR of slow-growing species was still rising (around day 134
in the year – 13/05/2008). Early differences in species’ growth rate
also governed short-term competitive outcomes in our semi-natural
grassland subjected to nutrient addition, thereby extending the results
of the common garden experiment to a real-world grassland ecosystem.
Together these results indicate that species growing faster during the
early stage of the growing season, and thus reducing light availability
during this early phase of vegetation growth, had a competitive
advantage relative to species that initially grow more slowly.
Addition of nitrogen in our semi-natural grassland ecosystem increased
productivity and reduced plant diversity, allowing us to further assess
whether
differences
in species growth rate predict short-term competitive exclusion due to
nutrient addition. We found that difference in early season RGR predict
short-term competitive exclusion under productive conditions, but not
under unproductive conditions. Under productive conditions, the species
that grew faster early in the season (e.g. Anemonetrullifolia,Gentiana
sino-ornata, andSaussurea
nigrescens) , competitively excluded initially slower growing species
(e.g. Potentillaanserina,
Potentillafragarioides,
Euphorbiaaltotibetica
and Geraniumpylzowianum ).
This result suggests that when nutrient limitation is alleviated and
productivity is increased, the resulting decline in diversity is partly
caused by species that grow fast initially reducing resource
availability and outcompeting species that grow more slowly.
Previous studies have shown that under productive conditions, when
competition is mainly for light, asymmetric competition causes plant
species intercepting more light early in the season to have a
disproportionate advantage, leading to competitive exclusion of
subordinate species (HautierVojtech et
al., 2018, Vojtech et al., 2008,
Vojtech et al., 2007,
Hautier, 2009,
DeMalach et al., 2017). Our study is the
first to our knowledge to reveal the critical time during the growing
season when exclusion mechanisms act. We show that differences in early
season growth rates (day 53 when the growing season starts at
~ day 106 in Zurich and around day 155 when the growing
season starts at ~ day 136 in Gansu) provide an
explanation of competitive outcomes, thereby serving as a predictor and
early signalling of plant competitive abilities. This is because under
productive conditions, asymmetric competition leads to increased
relative size differences between species early in the season. This
early advantage allows fast-growing species to maintain and increase
their initial dominant position throughout the growing season, leading
to the exclusion of initially slower growing species. Our study is in
agreement with earlier studies demonstrating that instantaneous
measurements of light obtained early in the season, at the critical time
when light becomes limiting for plant growth, were the best predictors
of competitive outcomes (Vojtech et al.,
2007, Violle et al., 2007).
Our results from the field experiment are based on a subset of the total
number of species occurring in the community. Growth rates were derived
from the twenty most common species across all treatments, accounting
for 85 ± 10% of the total aboveground biomass. Our results are
therefore most likely conservative because they are restricted to
competitive exclusion amongst the twenty-most common species, thereby
failing to consider the exclusion of the rarest species, which comprise
a large proportion of the total species number and are more susceptible
to human disturbances.
Previous studies have shown that the outcome of competition in pairwise
mixtures could be best predicted by differences in light intercepting
ability in monocultures (I* ) under productive (light-limited)
conditions and by differences in nutrient uptake ability in monocultures
(R* ) under unproductive conditions
(Dybzinski and Tilman, 2007,
Vojtech et al., 2007,
HautierVojtech et al., 2018). However, in
real-world ecosystems that encompass nutrient gradients, both forms of
competition are likely to act at the same time, with light competition
becoming more important as nutrient competition lessens. Our results are
consistent with the resource ratio hypothesis envisaging a trade-off
between competition for light under fertile conditions and for nutrients
under less fertile conditions. Under fertile conditions, species growing
faster early in the season have a competitive advantage over initially
slower-growing species (consistent with them being better competitors
for light). This relationship between RGR and competitive success
weakens under less fertile conditions (compare fertile conditions with
added nitrogen from less fertile conditions without added nitrogen in
Figures 1, 2 S3, S5 and S6). However, we would expect, based on earlier
work (Tilman and Wedin 1991,
Wedin and Tilman 1993), that slow-growing
species with the lowest R * for soil resources would dominate the
community in the long-term (a long-term outcome that we were not able to
assess in our relatively short-term study). This would require that slow
growing species do not entirely disappear from the landscape.
Our study thus suggests that human activities that increase the
availability of nutrients to ecosystems will likely further reduce plant
diversity in the future by benefitting initially fast-growing species.
In contrast, management practices directed towards reducing the growth
of fast-growing species early in the season should help efforts to
protect and restore biodiversity in an increasingly human-dominated
world. For example, parasitic plants such as Rhinanthus species
can restore biodiversity in productive grasslands
(DiGiovanni et al., 2017,
Bardgett et al., 2006,
Pywell et al., 2004,
Bullock and Pywell, 2005). A potential
mechanism is through the reduction of the biomass of competitively
dominant grasses (Davies et al., 1997,
Ameloot et al., 2005), simply because the
parasite reduces host resources leading to a reduction in host growth
rate and future resource uptake (Hautier
et al., 2010). Our results suggest that Rhinanthus species could
be particularly effective because they cancel out the initial advantage
of fast-growing species early in the season thus limiting the exclusion
of slower-growing species. Adjusting the timing and frequency of cutting
could also be used as a restoration tool in nutrient-rich grasslands.
For example, a higher frequency of cutting that alters the structure of
the canopy layer can reduce asymmetric competition for light and the
initial advantage of fast growing species, giving slow growing species
more equal chances to compete for the limiting resources
(HautierVojtech et al., 2018,
Talle et al., 2018). On the other hand,
multiple cuts per season may reduce the number of flowering plant and
seeds that impact pollination, food for plant-feeding insects, seed
recruitment and nesting sites for birds
(Plantureux et al., 2005). Our results
suggest that an early cut combined with a late cut in the season could
constitute a good management strategy. While an early cut reduces
competition for light and the competitive dominance of fast-growing
species, thus promoting diversity, a late cut provides nesting sites and
allows plants to produce flowers and mature seeds. Additionally, cutting
with subsequent haying has the advantage of removing plant biomass and
excess accumulated nutrients in the soils, allowing the subsequent
recovery of diversity (Storkey et al.,
2015). Alternatively, low-diversity stable state could persist even
after decades of cessation of nutrient enrichment if biomass is not
removed and recycled within the system
(Tilman and Isbell, 2015,
Isbell et al., 2013).