RGR predicts short-term competitive dominance and
exclusion in a field experiment
Our nitrogen and combined nitrogen and phosphorus addition treatments
created productive conditions and reduced plant diversity while
phosphorus addition alone did not significantly affect either
productivity or diversity. In the third year of our field experiment,
there was a marginally significant interaction between nitrogen and
phosphorus addition on biomass production (F1,20=3.8,
P=0.065) and plant species richness (F1,20 = 3.7, P =
0.069) (Table S4). Nitrogen addition increased biomass production from
an average of 101 ± 11 g 0.25 m-2 (mean ± s.e.m.) in
the control plots to 140 ± 11 g 0.25 m-2 and decreased
species richness from 36 ± 2 species 0.25 m-2 to 22 ±
2 species 0.25 m-2. In contrast, the levels of biomass
production (114 ± 11 g 0.25 m-2) and species richness
(35 ± 2 species 0.25 m-2) under phosphorus addition
were indistinguishable from those observed in the control plots. The
combination of nitrogen and phosphorus addition had a large effect on
productivity, which increased to 198 ± 11 g 0.25 m-2,
while this treatment resulted in a smaller but still significant
decrease in plant species richness than observed with just nitrogen
treatment leading to 28 ± 2 species 0.25 m-2.
Similar to the results of our common garden experiment, rankings of
species growth rates changed with both growing season and nutrient
treatments (Fig. S4). We found that the relationship between early
season relative differences in species growth rates and relative
differences in species biomass varied with both the growing season and
nutrient treatments (Fig. 2A, Fig. S5A). The percentage of variance
explained was maximum at day 150 in the control (R2 =
0.29, F1,169 = 70.1, P <0.001), 146 with
nitrogen addition (R2 = 0.35, F1,169= 89.3, P <0.001), 164
with phosphorus addition (R2 = 0.11,
F1,151 = 18.0, P <0.001) and 146 with nitrogen
and phosphorus addition (R2 = 0.26,
F1,151 = 53.2, P <0.001). When significant,
relationships were always positive (Fig. 2A, Fig. S5A). Results based on
relative differences in species growth rates calculated between the two
first measurements of biomass confirmed that early differences in growth
rates predict competitive dominance at harvest except in the phosphorus
addition treatment (Fig. S6).
We found that the relationship between early season species growth rate
values and the likelihood of loss of a species varied with both the
growing season and nutrient treatments (Fig. 2B, Fig. S5B). The
percentage of variance explained was maximum at day 146 in the control
(R2 = 0.05, F1,118 = 2.9, P = 0.11),
147 with nitrogen addition (R2 = 0.12,
F1, 118 = 11.5, P = 0.003), 177 with phosphorus addition
(R2 = 0.07, F1,112 = 3.6, P = 0.06)
and 172 with nitrogen and phosphorus addition (R2 =
0.11, F1,118 = 11.6, P = 0.001). Short-term competitive
exclusion could only be predicted by early differences in species growth
rate under productive conditions (nitrogen and nitrogen & phosphorus
addition) and, when significant, relationships were always negative
(Fig. 2B, Fig. S5B).
Under
unproductive conditions (control and phosphorus addition),
short-term
competitive exclusion could not be predicted from early differences in
growth rate.