Determining maximum stand density index in mixed species stands for strategic-scale stocking assessments

Stand density index (SDI), although developed for use in even-aged monocultures, has been used for assessing stand density in large-scale forest inventories containing diverse tree species and size distributions. To improve application of SDI in uneven-aged, mixed species stands present in large-scale forest inventories, trends in maximum SDI across diameter classes and species combinations were observed for eight common tree species of the United States. Additionally, the relationship between a stand's mean specific gravity of component trees and maximum SDI was explored. Results indicate that the maximum SDI that any particular species may attain is affected to varying degrees by the species composition of subject stands. A strong relationship was found between the mean specific gravity of all trees in a stand and the 99th percentile of the observed distribution of stand SDI's by classes of mean stand specific gravity. A model is proposed whereby the mean specific gravity of individual trees in a stand may serve as a predictor of a stand's maximum stocking potential, regardless of the stand's diameter distribution and species composition.