3.2. Intensity of interspecific competition
Both the type of competitors (old vs. new; CompType) and the nitrogen treatments (NTC) had significant effects on the relative competition index (RCI), and the interactions between PopType and NTC were significant (Table 2). In general, the competition was stronger (i.e., higher values of RCI) when G. quadriradiata was planted with old competitors then planted with the new competitors (Fig. 3), and the RCI value increased with the nitrogen addition (Fig. 3). When under ambient nitrogen, the interspecific competition was significantly stronger for the edge population when compared to the central population (Fig. S2). There was no statistically significant difference in the interspecific competition between edge and central populations when under low and high nitrogen treatments (Fig. S2).