Assessing the influence of the minimum measured diameter on forest spatial patterns and nearest neighborhood relationships

Forest structure analysis is important for understanding the properties and development of a forest community, and its outcomes can be influenced by how trees are measured in sampled plots. Although there is a general consensus on the height at which tree diameter should be measured [1.3 m: diameter at breast height (DBH)], the minimum measured diameter (MMD) often varies in different studies. In this study, we assumed that the outcomes of forest structure analysis can be influenced by MMD and, to this end, we applied g(r) function and stand spatial structural parameters (SSSPs) to investigate how different MMDs affect forest spatial structure analysis in two pine-oak mixed forests (30 and 57 years old) in southwest China and one old-growth oak forest (>120 years old) from northwest China. Our results showed that 1) MMD was closely related to the distribution patterns of forest trees. Tree distribution patterns at each observational scale (r = 0–20 m) tended to become random as the MMD increased. The older the community, the earlier this random distribution pattern appeared. 2) As the MMD increased, neighboring trees became more regularly distributed around a reference tree. In most cases, however, nearest neighbors of a reference tree were randomly distributed. 3) Tree species mingling decreased with increasing diameter, but it decreased slowly in older forests. 4) No correlations can be found between individual tree size differentiation and MMD. We recommend that comparisons of spatial structures between communities would be more effective if using a unified MMD criterion.