Abstract
This study, undertaken in the Riyadh region of Saudi Arabia, was focused on high initial density plantation and the subsequent thinning of Acacia salicina trees, to ensure high individual tree growth suitable for saw timber production. It investigated the effect of high stand density on restricting early branch development. Trees were planted in 1998, at a density of 6400 trees ha(-1) (with spacing of 1.25 m x 1.25 m). After two years, half of the experiment was subjected to annual mechanical thinning up to 2003. From 2004 to 2010, the densities of 400 and 3200 trees ha(-1) were maintained for the thinned and unthinned stands; respectively. In 2010, five trees from each of the two stands were selected randomly, felled, and processed for estimating the biomass of stem, branches, and foliage. Tree height, tree diameter, log diameter along the stem, number of branches, branch diameter and distance of each branch from tree base were measured. Thinning significantly increased the diameter at breast height (dbh) and the total tree height at the age of 12 years, the mean values being 25.2 cm, and 16.9 m for diameter and height, respectively, in thinned population (400 trees ha(-1)), while these were 12.2 cm and 10.7 m, respectively, in unthinned population (3200 stem trees ha(-1)). Though the individual stem volume increased significantly in thinned population, the total stem stand volume per hectare was far greater in the unthinned population. The thinned trees had a lower form factor (or stronger taper) than the unthinned trees. Thinning significantly increased the dry biomass of different plant parts and had a significant effect on the diameter of the thickest branch.