Evaluation of Height as an Early Selection Criterion for Volume and Predictor of Site Index Gain in the Western Gulf

Abstract

Data from repeated periodic measures of height, diameter and volume from eleven loblolly pine progeny tests maintained as part of the Western Gulf Forest Tree Improvement Program (WGFTIP) were analyzed to 1) determine the potential of using early height, diameter, or volume as selection criteria for rotation-age volume, and 2) to develop a method of expressing height performance as predicted change in site index. Using family means, few differences in family mean correlations existed between 5-year traits with volume at 15 or 20 years, but they were slightly higher for volume than the other two traits. Gain efficiency estimates for all three traits at age 5 were similar, suggesting that the traits were equally efficient in predicting rotation volume. However, at age ten, per-acre volume was the better predictor of per-acre volume at later ages. Predicted gains in breeding value for height, expressed as percent change in site index (SIBV), were estimated following published WGFTIP methodology for volume. Age specific site index equations and coefficient of genetic prediction (CGP) estimates for height were developed using the 15-year data from the eleven progeny tests. Estimates of CGP for height at ages 5 and 10 with height at age 15 were 0.55 and 0.61, respectively. Correlation between parental breeding values for volume production and site index breeding values were high (r=0.80). Predicted genetic gain for site index provides additional information for decision-making. Uses and limitations of this gain information are demonstrated.