Family variation in diameter growth and acoustic velocity in three 5-year-old South African-grown <i>Pinus elliottii × Pinus caribaea</i> progeny trials established on three diverse sites

Abstract

This paper discusses family differences in the stiffness of young Pinus elliottii × P. caribaea wood and how these differences relate to differences in growth rate and altitude. Trees for measurement were obtained from three 5-year-old progeny trials established at three diverse sites on the estate of Komatiland Forests (Pty) Ltd. The trials included the following controls: P. elliottii, P. patula, P. taeda, P. patula × P. tecunumanii and P. tecunumanii. Diameter at breast height and sound velocity (which has a direct relationship with wood stiffness) were measured on all healthy trees. Virtually all of the species and hybrids exhibited marked decreases in growth rate and velocity (stiffness) with increasing altitude; the effect on velocity was most likely partly due to the inverse relationship that normally exists between wood density and altitude in most pines. Differences in growth rate explained only a small proportion of the total variation in velocity. The average sound velocities of P. patula, P. patula × P. tecunumanii and P. tecunumanii surpassed the velocity values of most of the P. elliottii × P. caribaea crosses at all three sites. Results of Spearman rank correlations of average treatment velocity between sites suggested only some similarity in the ranking orders among sites. The inverse effect of altitude on wood stiffness suggests that trees grown at increased altitudes are likely to yield wood with higher proportions of wood not meeting the minimum stiffness requirement for structural lumber at final harvest. The wood of some P. elliottii × P. caribaea crosses would be particularly vulnerable. Considering the large variation found, presumably largely genetic, there can be little doubt that a good opportunity exists for the development of genotypes more suitable for planting at higher altitudes.

Keywords: altitude, growth rate, Pinus elliottii × Pinus caribaea, tree breeding, wood stiffness