Do large forest trees tend towards high species mingling?

Effects caused by human management and natural disturbances as well as by ecological mechanisms such as those described, for example, by the Janzen-Connell and the herd immunity hypotheses play an important role in maintaining species diversity. These processes are often accompanied by local size-hierarchy effects and as a likely consequence of them we hypothesised that large plants generally have a tendency to show high species mingling, where plants and their nearest neighbours are heterospecific. To carry out a first benchmark study in forests from different parts of Europe, Africa and North America, we selected spatial data from twelve forest ecosystems and analysed the mingling situation based on an index of spatial species mingling. Using stem diameter at breast height and stem diameter differentiation among the nearest neighbours as explanatory variables we then applied logistic regression to explain mingling probability. Overall we found significant support for the expectation that large trees and trees growing at low local densities often (75% of all analysed forest stands) have indeed a tendency towards high species mingling. This supports our expectation that the tendency of larger trees towards high species mingling generally is either a consequence of disturbances/forest management or of the aforementioned ecological processes, although a few forest stands also deviated from the expected pattern. We also found that size differentiation and species mingling are strongly related in local neighbourhoods. Thus the results of our study strongly support the view that local species richness promotes local size hierarchy irrespective of climate zone. This leads to situations where in local neighbourhoods with large size diversity there is also high mingling and vice versa. This allows conservation to focus on maintaining or improving one diversity aspect, e.g. size diversity, whilst obtaining the other, e.g. species mingling, as a by-product.