Comparisons of observed variance in species diversity and in species richness with expectation assuming a random and independent distribution of species are used to assess the relative importance during succession of niche limitation (limitation of the abundance or occurrence of species by competitive interactions) and nucleation (the development of a community through chance establishment or persistence followed by vegetative expansion from those nuclei). Previous work suggests that a deficit (lower than expected) in variance of richness is consistent with a "niche limitation' process where competitive sorting makes patches more similar in species number than would be expected. Variance that is greater than expected is consistent with and could result from underlying environmental heterogeneity. A deficit in variance of diversity can be interpreted as indicating niche limitation. A deficit of variance in richness could result from limited dispersal and establishment followed by localized spread that causes quadrats to have less compositional overlap than might be expected if species were distributed at random. The authors examine a secondary successional sere with stands representing post-logging and mature forest. The variance of diversity is low in the establishment phase, but not in subsequent phases where the tree canopy is well developed. Significantly low variance of species richness is observed in a young forest just entering the thinning phase. Thus, nucleation may be taking place in the establishment phase of forest development, and is almost certainly important in the early thinning phase. -from Authors