Dominant increase
In plots where dominance increased, environmental effects on
density-independent growth rates were not always consistent with changes
in species group abundance over time. For example, in the SNW plots,
only the dominant species had a positive effect of added N, which is
consistent with its increase over time (Fig 2, Table S1). However, in
SNW plots, subdominant and moderate species groups also strongly
decline, suggesting that other density-dependent mechanisms are at play.
In SW plots, where we observed the second highest increase in dominance,
temperature and snow depth had counteracting (but weak) effects onDeschampsia growth rates. In contrast, snow addition and ambient
N deposition increased subdominant growth rates while warming decreased
subdominant and (to a lesser extent) moderate species growth rates (Fig
2, Table S1). For moderate species, the negative effects of warming in
SW plots were consistent with their decline over time. However,
counteracting (neutral) environmental effects on dominant and
subdominant species do not explain their strong directional shifts in
abundance.
In NW plots, where we observed the weakest increase in the dominant
species, Deschampsia growth rates were positively influenced by N
addition and strongly negatively influenced by warming (Fig 2, Table
S1), suggesting that counteracting influences of nitrogen and warming
muted the dominant increase over time. In addition, warming had a
positive effect on the growth rates of moderate species, dampening the
weaker negative effect of N addition, which may have reduced their
magnitude of decline in these plots over time compared to other
treatment types. However, subdominant species had no clear effects of
the environment despite their decline in abundance.