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.