Spatial analysis reveals facilitation in young clonal trees and competition in older trees during re-invasion of encroaching trees in an African savanna

Spatial analysis has revealed interesting patterns of woody plant encroachment that were not previously revealed by conventional analyses. We examined the spatial patterns of regeneration 23 years after a long-term tree removal experiment was performed near Magudu (northern KwaZulu-Natal, South Africa) by the Agricultural Research Council of South Africa. We performed spatial analyses of trees using nearest-neighbour analysis from 12 plots differing in degree of clearing as well as point-pattern analyses from a pair of complete-removal plots and a pair of complete controls (unmanipulated). After 23 years, the same dominant encroaching tree species (Dichrostachys cinerea) had re-invaded all the removal plots despite the fact that several additional encroaching species occurred in the vicinity. Nearest-neighbour analyses showed that there was a positive correlation between the sizes of the trees and their nearest-neighbour distances, indicating that competition was acting. The spatial distribution of subordinate to dominant species from point-pattern analysis ranged between random to aggregated. We also found that there appeared to be facilitation of young trees by adult trees in both the control plots and the completely cleared plots based on point-pattern analyses. This is likely an effect of the root-suckering habit of the predominant encroaching species, D. cinerea, and not the statistical definition of facilitation per se, borne out by the clustering of juvenile trees of this species. We conclude that facilitation of young trees is an artefact and that intraspecific competition is the over-riding effect on the distribution of trees.