Measurement of plant-plant interactions
The response of herbaceous species to the effect of the shrub species,A. ordosica , was quantified separately for each treatment combination (three precipitation levels × three N levels) using the relative interaction index (RII ; Armas et al. , 2004):
\(\text{RII}=\frac{P_{u}-P_{i}}{P_{u}+P_{i}}\)(1)
where \(P_{U}\) represents the biomass of herbaceous species in the shrub patches, and \(P_{I}\) represents the biomass of herbaceous species away from the shrub patches. RII is symmetrical around zero (a value of zero indicates no significant interaction) and is limited to between −1 and +1. Positive values of RII indicate the occurrence of facilitation and negative values indicate competition.