Determining the best form factor formula for Loblolly Pine (Pinus taeda L.) plantations at the age of 18, in Guilan-northern Iran

In order to determine the best form factor formula for Loblolly Pine (Pinus taeda L.) plantations in Talesh (Western Guilan province-Iran), a number of 110 trees were selected based on their distribution in diameter classes, from 12 to 34 cm (in a two-cm diameter interval). First, several quantitative factors including diameter at breast height, diameter at 0.65 m of height, and diameter at stump were measured using a diameter tape, just before the trees being felled. After cutting the trees, the heights and diameter from breast height up to the height where diameter is 5 cm was measured using a diameter tape in a two meter interval. Finally, diameter at 0.1, 0.3, 0.5, 0.7, and 0.9 meter of the total height was measured respectively. As a consequent, each tree's volume was precisely calculated as the real volume. Next, the real form factor (r f) was calculated and its average was statistically compared to the averages of Natural (1. 0 f), Artificial (5. 0 f), and Hohenadl's (h f) form factors using pair sample T-test. Results showed that there is no significant difference between the averages of real and Hohenadl's form factors (at = 0.01 level). Furthermore, the averages of real and artificial form factors were not significantly different. Hence, both artificial and Hohenadl's form factors are capable to replace the real form factor of Loblolly Pine over the study area.