Age trends in genetic parameters and early selection of lodge-pole pine provenance with particular reference to the Lambeth model

This study of time trends in genetic parameters is based on height growth from 50 provenance test plantations involving 140 provenances. Total heights were measured at plantation ages 1,3,6,10,15 and 20. We used correlative heritability (h**2jm) to measure the response of early selection; h**2jm is the product of age-age genetic correlation (rGjm), and square root of provenance heritability at early (h**2j) and target (h**2m) age of selection (h**2jm=hjhmrGjm). We expect h**2jm to be a realistic measure of early selection since it reflects the collective effect of all determinant parameters. We estimated efficiency of early selection (Ejm) as the ratio of h**jm and provenance heritability at target age of selection. We employed the logarithm of early-target age ratio (log(tj/tm)) as predictor to extrapolate Ejm to different target ages of selection beyond age 20 (LAMBETH 1980). Cumulative growth indicated that provenance variation in mean height became stable after age 6 and experienced only minor shifting in rankings after age 10. Heritability remained high throughout the testing period (h**=0.70 to 0.85) and increased with age except for a slight decline from ages 1 to 3. Both age-age phenotypic and genetic correlations increased steadily with age, and coefficients of the latter were higher than those of the former. Genetic correlation reached 0.95 between age 6 and 20 on average. h**2jm showed an age trend similar to that of genetic correlation suggesting the latter drove early selection. Ejm increased exponentially and reached over 80% at age 6 and 90% at age 10 if target age of selection was 20. We found high predictability of log (tj/tm) to project Ejm (r**2=0.92), indicating high robustness of log (tj/tm)as a proxy for age-age correlation as expounded in the Lambeth model. However, we have some reservations about the soundness of gain per unit time as a measure of optimum age of selection. Instead we propose Ejm as an alternative to assess optimum age of selection. We discuss its advantages in both genetic and non-genetic considerations.