Silvicultural Thinning Intensity Effects on Increasing the Growth of Planted Loblolly Pine (Pinus taeda L.) Stands in Northern Iran

Loblolly pine (Pinus taeda L.) is 1 of the main exotic species in commercial plantations in northern Iran. These plantations are situated in the Caspian forests of Iran, which border the southern coast of the Caspian Sea and cover an area of 1.9×10^6 ha. For the present experiment, unthinned, and lightly and moderately thinned stands were selected as 3 populations. Data were collected from a total of 180 trees among the different stand densities. For basal area analysis, 93 plots were measured. Variables such as diameter at breast height (dbh), total height, tree volume, bark thickness, basal area per hectare, crown length, and crown diameter were measured. The growth rate ratios of dbh in the lightly and moderately thinned stands were 1.09 and 1.18, while the mean annual increments of basal area growth rates in the 2 thinned populations were 1.186 and 1.20, respectively. Moderate thinning increased the volume of stands by about 20.60% of the stand volume. This analysis indicated that moderate thinning drastically affected the basal area and the growth rate of volume. After thinning, dimensions of live crowns increased and more branches were evident. Percentages of live crown ratios in the 3 study populations were calculated to be 41.23, 43.87, and 44.52%. Growth rate interpretations were supported by statistical multivariate analysis of variance using Wilk's (Λ) and Hotelling's T(superscript 2 subscript 2) tests. Wilk's test was carried out on a mean vector of 6 variables in the 3 study populations. The null-hypothesis (F(subscript c)=14.435＞F(subscript t)=4.37) was rejected at the 0.01 level of significance. Thus, the results indicated that moderate thinning significantly influenced increased growth rates of planted loblolly pine stands.