Top height definition and its effect on site index determination in thinned and unthinned loblolly pine plantations

Abstract

Seven definitions of stand (top) height: (1) mean height of dominant and codominant trees at the time of plot measurement, (2) mean height of dominant and codominant trees that have always been dominant or codominant over the life of the stand, (3) Lorey mean height, (4) average height of the 100 thickest trees per hectare at the time of measurement, (5) average height of the 100 thickest trees per hectare at the time of last measurement, (6) average height of the 20% thickest trees at the time of plot measurement, and (7) average height of the 20% thickest trees at each measurement that also survived to the last measurement were evaluated for use in site index (SI) determination. Data collected from permanent plots over a 15-year period in thinned and unthinned loblolly pine plantations were used to make comparisons. Except for a few cases at certain measurements, all seven definitions of top height were significantly different from each other. Site index was predicted based on these definitions to determine if one definition is superior. Magnitudes of the maximum range of residuals and the fit statistics indicated that SI estimated using mean height of dominant and codominant trees that have always been dominant or codominant over the life of the stand is more precise than site indices estimated using other definitions. Using this definition of top height, a new SI equation for these stands was developed.