Introduction: Forests are the most complex and valuable terrestrial ecosystems, playing an irreplaceable role in timber production, carbon sequestration, soil conservation, and regulation of hydrological cycles. With increasing demand for wood and over-exploitation of natural forests, developing fast-growing plantations has emerged as an effective strategy for sustainable wood supply. Loblolly pine (Pinus taeda L.), due to its rapid growth, high ecological adaptability, and considerable productive capacity, holds a special position in afforestation programs. In Guilan province, plantations of this species cover more than 3,000 hectares, with an annual production potential exceeding 18 m³ per hectare under appropriate management. However, timely implementation of thinning operations before the age of 30 remains one of the main management challenges for these stands. The present study was conducted to evaluate the growth structure and compare the growth performance of loblolly pine plantations with the species' native range in the United States.
Method: This research was carried out in two 30-year-old stands with an initial spacing of 3×3 m in the educational and research forests of the Faculty of Natural Resources in Sowme'eh Sara and the Haft Daghnan region. In each region, three 0.072-ha permanent sample plots were established using a systematic-random sampling method. All standing trees were measured using a complete census approach. Diameter at breast height (DBH) at 1.3 m above ground level was measured using a caliper (0.1 cm precision), and total height was measured using a Suunto clinometer. Statistical indices including mean, median, standard deviation, skewness, and kurtosis were calculated. Stand structure was analyzed using the structural triangle method, slenderness coefficient (SC), and Hart-Becking index (S%). Dominant height was determined based on the mean height of the 40 largest-diameter trees per hectare. Annual diameter and height growth were calculated. Comparisons with three site index curves from the native range in the USA were performed using an independent samples t-test at the 5% significance level.
Results: Mean diameter was 24.94 cm in the Sowme'eh Sara stand and 25.25 cm in Haft Daghnan; mean height was 12.79 m and 13.52 m, respectively; and mean volume was 0.422 and 0.430 m³ per tree. The t-test showed no significant differences between the two stands for any variable (p > 0.05). 90 percent of trees in the Sowme'eh Sara stand and 88.75 percent of trees in the Haft Daghnan stand were in the 10–30 cm diameter classes. The structural triangle analysis classified the stands in the low-diameter (PB) structural group. Stand density was estimated at 370.4 trees per hectare, showing a considerable decrease from the initial planting density (1111 trees per hectare). Dominant height in Sowme'eh Sara and Haft Daghnan was 14.33 m and 17.11 m, respectively, and the Hart-Becking index was 36.25% and 37.30%, respectively, indicating a relatively suitable spacing. Site index at the base age of 30 was estimated at 18.3–27.5 m (moderate to moderately-poor site quality). Annual diameter and height growth were 0.83–0.84 cm and 0.43–0.45 m, respectively. Comparison with the native range showed that diameter growth was at the moderate site index level (27.5 m) and height growth was at the poor site index level (18.3 m).
Conclusion: The studied stands have relatively even-aged and uniform structures and are classified in the low-diameter (PB) group. The current density (about 370.4 trees per hectare) compared to the initial planting density (1111 trees per hectare) indicates considerable natural self-thinning, and the Hart-Becking index (36.25% and 37.30%) indicates relatively suitable spacing under current conditions. However, the dominance of trees in thin diameter classes indicates that intra-stand competition in the early years of growth (before natural self-thinning occurred) may have contributed to the current diameter growth limitation. Comparison with the native range showed that diameter growth is at a moderate level and height growth is at a poor level. This inconsistency indicates a limitation of height growth relative to diameter growth. Lower diameter growth compared to high-quality sites can be attributed to several factors, including site quality, initial competition, and differences in climatic and soil conditions. Given the current stand condition, implementing selective and planned thinning operations is recommended to gradually improve diameter growth and enhance stand stability.