Neighbourhood analysis of competition between two Namaqualand ephemeral plant species

By using attributes of neighbours, such as number, size and distance, investigators have developed neighbourhood competition models that are able to predict plant performance of an individual in a plant population. A non-linear neighbourhood model proposed by Silander & Pacala (1985) was applied to performance data (i.e. total above-ground dry mass) of Dimorphotheca sinuata and Ursinia cakilefolia, two Namaqualand ephemeral species. The best neighbourhood size was sought by varying the neighbourhood size and plotting an interference index against dry mass. The non-linear model was then fitted to the data and the r(2) values determined for each neighbourhood. Correlations between performance (total above-ground dry mass) and different interference indices were much higher in mixtures than in monocultures for both species. Number of neighbours seemed to be the best predictor of performance for both D. sinuata and U. cakilefolia in mixtures. In monoculture, the best correlations were obtained when the distance of the neighbours was also taken into account. Even so, these correlations accounted for less than 50% of the variation in plant mass. Although these neighbourhood models may be useful in describing the inter-individual effects of interference on Namaqualand ephemeral species, it may be difficult to apply these results in the field.