Variation and Genetic Control of Wood Properties in the Juvenile Core of Pinus patula Grown in South Africa.

Abstract

Pinus patula is a closed-cone pine that has a rather narrow, but long, distribution in Mexico. It ranges from approximately 16 to 24 degrees North latitude, is generally restricted to humid, sub tropical to near temperate sites, with deep, fertile clay soils, and an estimated mean annual precipitation of 1000 to 2500 mm. Approximately one million hectares of P. patula plantations have been established in the tropics and sub tropics for saw-timber and paper products. The majority of the P. patula forests are located in southern Africa with South Africa having more than 300,000 hectares under operational management. Since 1986 the CAMCORE Co-operative has sampled 25 provenances and 624 mother trees of P. patula in Mexico. The CAMCORE collections represent the most complete coverage of the species natural distribution to date. During December 1990, a series of five trials of open-pollinated P. patula family/provenance seedlots were established adjacent to each other at Maxwell in KwaZulu-Natal, South Africa. These trials are 10.5-years-old and offered a unique opportunity to sample material from the entire geographic range of the species grown on a single site.

Nine hundred and seventy-two individual trees, representing 12 provenances and 108 half-sib families were sampled non-destructively by removing 12 mm increment cores at breast height. Wood anatomical properties (wood density traits, tracheid length and tracheid cross-sectional properties) were measured using gamma ray densitometry and image analysis. The phenotypic variation in wood properties was characterised at the provenance, family and individual tree level. A strong clinal trend was shown to exist between wood property traits and latitude of collection site, with the highest correlation (r = 0.97) with tracheid radial diameter. Provenance differences were strong and significant for most wood properties, while large individual tree-to-tree variation existed for all wood properties. P. patula var. patula and P. patula var. longipedunculata appear to have markedly different wood.

The additive genetic control of wood properties varied from zero (for tracheid length from ring 8) to moderately strong (h2 = 0.51) for tracheid radial diameter. Heritability estimates for wood density traits were slightly lower than what have been reported by other authors. However, mean earlywood density, a component of overall wood density, had a much higher heritability(h2 = 0.37) than area weighted mean core density (h2 = 0.27). Contrary to most reports in the literature, the findings in this study show that tracheid length and cell wall thickness in P. patula are under very weak or negligible additive genetic control. Given the low level of additive genetic variation, tracheid length and cell wall thickness should not be included as a selection criteria in P. patula breeding programs in South Africa.

Moderate gains, ranging from one to 22 percent, are possible from direct selection on wood property traits. Predicted correlated responses show that, in some cases, indirect selection may lead to larger or equivalent responses to direct selection. The finding that a moderately strong negative genetic correlation (r = -0.46) exists between diameter growth and wood density has important implications for applied breeding programs.