Effect of Planting Density on Betula alnoides Plantations in the Dry-hot Valley Region

Planting density is a fundamental silvicultural practice in timber plantation management, critically influencing stand establishment, mid- to late-rotation tending, yield, and timber quality. This study examined growth dynamics in Betula alnoides Buch.-Ham. ex D. Don density trial plantations located in the dry-hot valley region of southern Guizhou Province, China. Growth performance was dynamically analyzed at two developmental stages: the juvenile plantations (5 years) and the mid-age plantations (12 years). The objectives were to identify optimal planting densities for the region and to develop corresponding silvicultural strategies. B. alnoides exhibited strong adaptability, rapid growth, early canopy closure, and high productivity in the study area. The relationship between key growth indicators and planting density differed markedly between growth stages. High initial density promoted vigorous early growth and accelerated natural thinning during the juvenile plantations, but significantly suppressed growth at mid-aged plantations. In contrast, the growth responses of mid--aged plantations to planting density were more stable, rendering this stage more suitable for evaluating long-term density effects. In 12-year-old stands, the natural thinning rate was positively correlated with initial planting density, increasing from approximately 30% at 625 stems·hm^-2 to about 50% at 2500 stems·hm^-2. Diameter at breast height (DBH), tree height, and individual stem volume all increased with decreasing density, attaining maximum values at 625 stems·hm^-2 (21.0 cm, 18.5 m, and 0.3 m³, respectively). Stand volume peaked at 833 stems·hm^-2 (146.9 m³·ha^-1), followed by 625 stems·hm^-2 and 1111 stems·hm^-2, with no statistically significant differences among these three densities. At mid-age plantations, individual stem volume was significantly negatively correlated with density, whereas stand volume exhibited a non-significant negative relationship with density and a non-significant positive relationship with individual stem volume. Based on a comprehensive analysis of stand growth dynamics, productivity, and timber quality objectives, the recommended initial planting density for large-diameter timber production in this region would be 1111 stems·hm^-2 under conventional management and 833 stems·hm^-2 under intensive management. Additionally, a thinning intervention could be advised during the juvenile (5-7 years) and mid-age (12-15 years) plantations respectively to achieve the final target tree density.