Objective This paper aims to further improve the research system of natural regeneration obstacles of Larix principis-rupprechtii, reveal or reflect the influencing factors of natural regeneration of L. principis-rupprechtii plantations from the aspect of seedling regeneration characteristics.
Method From 2019 to 2020, the natural regenerations of the 27, 31and 47 years old L. principis-rupprechtii pure plantations under different intensities of thinning operation in June, 2010 were investigated.
Result (1) There were different degrees of natural regeneration under L. principis-rupprechtii plantations in Saihanba area, Hebei Province of northern China after 35% extremely severe thinning, 30% severe thinning, 20% moderate thinning and 15% light thinning and natural thinning for 9−10 years, and the regeneration seedlings were all saplings and there were no larch treelets. The 1-year-old (1a) seedlings were the dominant, and the age of perennial seedlings was mostly 2-year-old (2a). In the performance of seedling regeneration frequency, the regeneration ability was inversely proportional to the stand age. The best performance was 27 years old L. principis-rupprechtii plantations, and the worst was 47 years old plantations. (2) In the distribution of seedling age, 1a seedlings accounted for a higher proportion, higher seedling age accounted for a small proportion, the loss of 1-year-old and 2-year-old seedlings was greater, and there were renewal faults during the transition to perennial seedlings. (3) The differences between seedling ground diameter and seedling height in different stages and stands were small (P > 0.05). After a growing season, the ground diameter of seedlings decreased significantly (P < 0.05), while the height of seedlings did not increase significantly (P > 0.05). (4) The observation results of natural regeneration grade of L. principis-rupprechtii plantations in Saihanba area for two consecutive years were poor regeneration. Stand age and stand density had a great influence on regeneration density (P < 0.05), the overall regeneration density of 31 years old L. principis-rupprechtii plantations was the largest in 2 years (P < 0.05), and there was a positive correlation between regeneration density and stand density (P < 0.05). Compared with that in 2019, the regeneration density of most sample plots in 2020 decreased to a certain extent (P > 0.05). (5) During the growing season, the dynamic changes of seedling density in the three plantations were relatively consistent in two consecutive years. The germination peak was at the end of July, and the seedling loss mainly occurred in winter. (6) The difference of survival rate in growing season and overwintering was small among plantations (P > 0.05), and the survival rate in growing season in the 27 years old L. principis-rupprechtii plantations in 2019 was significantly higher than that in 2020 (P < 0.05), while the overwintering survival rate was significantly lower than that in growing season (P < 0.05).
Conclusion Natural regeneration of L. principis-rupprechtii plantations of three age classes in Saihanba area occurred in different degrees in 9−10 years after thinning under four intensities. Forest age and stand density mainly affect the natural regeneration by affecting the regeneration frequency and regeneration density of seedlings under the forest, but have little effect on the regeneration characteristics such as age distribution, growth status, regeneration dynamics and survival rate of seedlings under the forest. That is to say, forest age and stand density mainly affect the regeneration state under the forest by affecting the number of seedlings. Relatively speaking, for young and middle-aged forests with stronger seed setting ability, thinning can promote the growth of mother trees, and improve the overall seed yield of the stand. It is possible to increase the thinning intensity and shorten the rotation period to enhance the positive impact of thinning, improve the overall seedling quality, improve the environmental factors under the forest and extend the growing season of seedlings in Saihanba area.