The relative economic importance of stem straightness, branch diameter and stem diameter in P. radiata breeding

In the 1950's it was recognised that Pinus radiata D.Don exhibited genetic variation in many traits. As a result several tree breeding programs were begun to provide genetically improved stock as a basis for future plantations. Worthwhile gains were made during the first breeding generation using such simple techniques as intensive selection in plantations and grafting of 'plus’ trees for clonal seed orchards. If worthwhile gains are to continue in advanced generations the efficiency of selection must be maximised. This will entail the use of selection indices incorporating information about the relative economic importance of traits and estimates of phenotypic and genetic variances and covariances. This thesis describes the development of a method to calculate economic weights for three important traits (stem straightness, branch diameter and stem diameter) for use in selection indices. The suitability of subjective assessments of stem straightness and branch diameter in tree selection is also examined. Finally, an economic evaluation is made of a first generation P.radiata clonal seed orchard. The thesis is based on an examination of the relationships between tree breeders' assessments of the three traits and stem economic value,using data collected from a saw mill study. Sixty-five P.radiata stems from a 34 year old progeny test in the Australian Capital Territory were felled. Stem straightness had previously been assessed subjectively on these stems at ages 16, 25 and 34 (three times by two observers). Branch diameter had been measured at age 13 and assessed subjectively at ages 25 and 34. Stem diameter had been measured at ages 13, 25 and 34. After the trees were felled, detailed measurements were made of sweep and the diameter of a sample of branches on logs cut from the stems. Data were also obtained from an additional 25 stems of an unimproved stock of the same age as the trees in the progeny test. In the mill recovery study, 273 logs from the felled stems were sawn and the quantity and grade of timber recovered from each log determined. The relationships between value recovery from the logs and the accurate measurements of sweep and branch diameter were analysed. The results enabled the loss of value recovery from each log and stem, caused by sweep and large branch diameter, to be analysed. Economic weights for stem straightness were found by regression analyses of the relationships between the estimated loss of value recovery, caused by sweep in each stem, and the subjective assessments of stem straightness at ages 16, 25 and 34. In a similar manner, economic weights for branch diameter were calculated from regression analyses of the relationships between the loss of value recovery caused by branch diameter and assessments of this trait. Economic weights for stem diameter at ages 13, 25 and 34 were calculated using regression relationships between value recovery from whole stems and stem diameter. In the progeny test from which the sample logs were harvested, the economic weights showed stem diameter to be nearly 8 times more important, economically, than stem straightness and 5 or 6 times more important than branch diameter. Smith-Hazel indices were calculated for tree selection in the progeny test, using phenotypic parameters calculated for all trees in the test at age 34, standard genetic parameters for P.radiata listed by Cotterill et al (1988) and the economic weights. The indices were also calculated using equal emphasis weights. The results of the analyses demonstrated that use of incorrect economic weights in index selection can result in serious reduction in economic gains. At all selection ages, the economic weighted index gave higher predicted economic gain than the equal emphasis weighted index. The difference was 82% at age 34, 63% at age 25 but only 2% at age 13. The importance of using accurate economic weights was dependent on the other parameters used in the selection index. When stem straightness (judged subjectively by two observers on separate occasions) was given equal importance to other traits in the selection index, differences in the observers' scores caused large variation in predicted economic gains. At age 34, and placing equal emphasis on stem straightness, branch diameter and stem diameter, one observer's scores for stem straightness resulted in 20% more predicted economic gain than those of the second observer. The difference was less (8%) when the subjectively scored trait, stem straightness, was of low relative importance. A separate study of stem straightness in P.taeda showed that while it is possible to calculate economic weights from subjective scores, a large amount of variation in stem economic value cannot be explained by variation in subjective scores. Stem straightness in 80 stems was scored using precise stem descriptions based on measurements of bends in felled stems. The precision of the relationship between loss of recovery caused by sweep and the subjective scores determined from these precise assessments was nearly double that of the relationship determined using a subjective assessment of stem straightness before the stems were felled.