Methods of assessing crown form of Pinus radiata

Crown form is a visual characteristic having many component traits (branch thickness, branch angle, ramicorns, forks and uninodal/multinodal branch habit). Alternative methods of assessing each trait are evaluated in terms of their variance components and heritabilities in a progeny trial of Pinus radiata. A comparison is made between using one overall score for crown form and use of selection indices to combine traits. The efficiency of each alternative is compared in terms of the calculated selection response for each trait. For assessing branch diameter and branch angle a six point score was preferred to the use of a three or nine poInt score. The six point score resulted in large additive genetic variance and a higher heritability, giving good discrimination of genotypes without being confusing to the assessors. For ramicorns and forks, counting the actual numbers present resulted in higher heritabilities than scoring for their incidence. Calculated responses from selecting one tree in 100 on either a single crown form score or multitrait indices indicated that response in each component of crown form is greater for the indices.