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Abstract: The effect of genetic improvement on loblolly pine height-dbh relationships and height and dbh distributions was investigated using four loblolly pine genetic varieties, a half-sibling family, a full-sibling family, and two rooted-cutting clones, growing at two different levels of stand density. Height-dbh relationships were modeled using the Korf function and height and dbh distributions were described using sample moments of the distributions. The fixed effect of genetic variety on the height-dbh relationship was tested using likelihood ratio test and root mean square error statistics. Moments of height and dbh distributions were compared using analysis of variance techniques. The effect of genetic variety on the height-dbh relationship depended on stand density with differential effects being observed at higher stand density. Variance of the height-dbh regression line decreased with increased genetic uniformity among the trees in a stand; hence, vertical crown differentiation in a stand may decrease with genetic selection. Higher levels of genetic improvement resulted in an increase in the mean of the height distribution but had no effect on the mean of the dbh distribution, suggesting that genetic selection for faster height growth in loblolly pine may affect diameter and height development disproportionally. The effect of genetic improvement and planting of monoclonal stands on variance of height and dbh distributions depended on stand density with there being no effect at the lower density. However, variance of the distributions for nonclonal stands was greater at the higher density. There was no change in skewness of the distribution with genetic improvement or between clonal and nonclonal stands. Modeling effects of genetic improvement on loblolly pine height-dbh relationships and diameter and height distributions may require knowledge of the genetic mechanisms behind the differential responses of genetic varieties to stand density. FOR. SCI. 59(3): 278-289.
Keywords: Pinus taeda, modeling genetic improvement, clonal loblolly pine
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Height-dbh relationships are useful in forestry for estimating heights of trees when only the dbh is given and subsequently for tree volume or biomass calculation. In growth-and-yield modeling, height-dbh relationships are useful in diameter distribution growth and yield models in which they are used to estimate heights and hence tree volume from the predicted diameter distributions (Clutter et al. 1983, p. 125-131, Avery and Burkhart 2002, p. 363-371)....