Response of productivity to stand density in <i>Larix principis-rupprechtii</i> plantations at different age stages.

Taking <i>Larix principis-rupprechtii</i> pure plantations of three age classes (young, middle-aged, and mature) and three stand density levels (high, medium, and low) in the Saihanba Mechanical Forest Farm as objects, we explored the optimal stand density across different developmental stages and identified the key factors influencing stand productivity. The results showed that stand productivity of <i>L. principis-rupprechtii</i> was significantly affected by both stand age and stand density. In young stands, stand productivity increased initially and then declined as decreasing stand density. Productivity in medium- and low-density stands was significantly higher than in high-density stands by 52.1% and 37.2%, respectively. In middle-aged stands, productivity declined consistently with decreasing density, with high-density stands outperforming medium- and low-density stands by 32.2% and 50.0%, respectively. In mature stands, medium-density stands showed significantly higher productivity, being 27.1% and 75.0% higher than high- and low-density stands, respectively. The productivity of forest stands tended to increase first and then decrease with the increases of forest age. The interaction between stand age and stand density had a significant effect on productivity. Available potassium, total nitrogen, cation exchange capacity, organic carbon, and hydroly-zable nitrogen were the key factors influencing stand productivity. The optimal stand density ranges for <i>L. principis-rupprechtii</i> plantations at different age classes in Saihanba Mechanical Forest Farm were 1800-2100 trees·hm^-2 for young stands, 1500-1700 trees·hm^-2 for middle-aged stands, and 700-1000 trees·hm^-2 for mature stands.