Trees are fairly consistent in height with variation depending on crown position in canopy namely whether a tree is dominant, co-dominant, intermediate or suppressed. Stand consists of many ages and tree sizes. Trees originate more or less continuously. Get J-shaped curve when plot TPA versus DBH class. Either a stand is overstocked or understocked .
Most natural stands are understocked.
Note that the spacing of trees is not taken into consideration. It is said that SDI is independent of site quality and stand age. Like SDI this metric is independent of site quality and stand age. Height growth is not affected by stand density but DBH growth is. Value of 1 implies no openings in the stand.