How to describe the crowding degree of trees based on the relationship of neighboring trees.

Whether the forest is dense or not is an important attribute of spatial forest structure. It reflects not only the crowding degree of trees and their neighbors with competition information, but also the condition of whether or not the forest canopy layer covers the woodland continuously. This study proposes the crowding index, a new neighborhood-based parameter, by which we may analyze the crowding degree of a neighborhood unit according to the overlapping of the crown in spatial micro-environment. Crowding can have five possible values ranging from 0 to 1, which clearly define the crowding degree for a reference tree and its four nearest neighbors, and to some degree the competition pressure on it as well. Greater values generally indicate more intense competition around the reference tree. For a stand, the greater the cumulative value of crowding, the higher the stand density, and the more continuous the coverage of the canopy is. Based on observations in field plots with mapped trees, values of crowding were sensitive to various simulated spatial tree patterns with similar stand density, to field data of planted forests with different tree densities, and to mixed and uneven aged natural forests as well. Thus, the type of tree distribution pattern should be considered while calculating the crowding of a forest stand.