Dominant increase
Models revealed several changes in species interactions consistent with
observed declines in evenness. First, competitive effects on the
dominant species declined in all plots where Deschampsiaincreased over time (SNW, SW, and NW) (Fig 3, Table S3). In SNW and NW
plots, this was driven primarily by reduced interspecific competition,
while in SW plots this was driven primarily by reduced intraspecific
competition of the dominant species on itself (Fig 3, Table S3). The
dominant species increased its intraspecific competition in plots with
added N (SNW, NW), consistent with the positive effect of N addition on
its density-independent growth rates (Fig 2). Furthermore, net
competitive effects increased in SNW and SW plots for subdominant and
(to a lesser extent) moderate species, primarily driven by increased
interspecific competition with each other, reflecting higher-order
interactions that benefit the dominant species, and contribute to their
decline in these treatments over time.
In the NW treatment, competitive effects declined for all species
groups (except rare) (Fig 3, Table S3). Reduced competition, in
combination with density-independent patterns observed, help explain the
lower magnitude of moderate and subdominant species declines in NW
compared to SW and SNW treatments. However, this pattern was more
pronounced for moderate than subdominant species, likely due to a strong
reduction in the competitive effect of the dominant on moderate species
(Fig S8c). In addition, predictive steady-state distributions revealed a
non-linear (left-skewed) distribution of subdominant species across the
observed N gradient in NW plots, suggesting that subdominant species
only benefits from competitive release at low N levels, after which the
dominant takes over (Fig 4).