Long-term growth and ecophysiological responses of a southeastern Oklahoma loblolly pine plantation to early rotation thinning

Abstract

A thinning levels study was initiated in a 9-year-old loblolly pine ( Pinus taeda L.) plantation containing 26.6 m 2 ha −1 basal area during the spring of 1984 in southeastern Oklahoma. Thinning treatments consisted of (1) three control plots (BA100), (2) three plots thinned to approximately 50% of the original basal area (BA50) and (3) three plots that were thinned to 25% of the original basal area (BA25). In 1987 the BA50 and BA25 plots were both rethinned to a basal area of 12 m 2 ha −1. No other thinnings were done through age 24. The control plots have attained a basal area of 45.3 m 2 ha −1 and basal area is now starting to decline. The BA25 and BA50 plots have basal areas between 34 and 35 m 2 ha −1. Mortality has averaged about 90 trees ha −1 per year from age 10 to age 24 on the control plot, declining from 2078 trees ha −1 at age 10 to 827 trees ha −1 at age 24. Mortality losses in the BA25 and BA50 plots have been only 3.2–7.7 trees ha −1 per year over the entire study period. Cumulative stem biomass lost to mortality was 10.5, 16.0 and 61 Mg ha −1, respectively, for the BA25, BA50 and BA100 treatments. Cumulative standing live biomass at age 24 in the BA100 treatment is 132 Mg ha −1. Cumulative standing live biomass in the BA25 and BA50 treatments at age 24 is 86 and 79%, respectively, of that observed in the BA100 treatment. These results suggest wide ranges of residual stand densities left after an early thinning will produce a high percentage of the potential total maximum standing stem biomass. Diameter distributions at age 24 show only 33% of the trees in the BA100 treatments have the dimensions to be sawtimber (≥30 cm) but 92 and 95% of the trees in the BA25 and BA50, respectively, are sawtimber dimension or larger. Mean annual stem biomass production (MAI) of the BA100 treatment is 7.5 Mg ha −1 per year at age 24. MAI of the thinned treatments is about 5.1 Mg ha −1 per year and is converging to that of the BA100 treatment. The basis for this convergence is not that the live trees in the BA100 treatment are producing live biomass less rapidly than the thinned plots, but that mortality losses in the BA100 plot are much higher. Current annual stemwood production in all treatments is often limited by the severe summer droughts that occur in this region. The wide variations in weather experienced at this site also result in variations in earlywood:latewood ratio and ring specific gravity.