Evaluation on the Effect of Different Thinning Intensities on Artificial Cypress Plantation in Hilly Areas of Central Sichuan

In order to improve the ecological and economic functions of massive artificial cypress plantations in the hilly areas of central Sichuan, thinning experiments with different intensities were carried out to explore the suitable thinning intensity and stand density, which provided scientific reference for the precise improvement of the quality of low efficiency protective forest and sustainable management in this area. The artificial cypress plantations with canopy density ≥ 0.8, understory shrub coverage ≤ 20%, herb coverage ≤ 30% in 35−40 years were selected as the research objects, and three thinning intensities (10%−15%, 15%−25%, 25%−35%) and control (no thinning) were set up by random block method. 20 sample plots (5 for each thinning intensity) with the size of 20 m × 20 m were set up after 12 years of thinning. Tree stand growth factors, shrub and grass biodiversity, natural regeneration seedlings, litter accumination and water holding-capacity, soil physical and chemical properties were investigated and measured. Finally, the sample plots were evaluated comprehensively based on principal component analysis. The results showed that: (1) The growth of the remaining trees was significantly promoted after thinning. Compared with the control, the current annual increment of tree height, DBH and volume in thinned forest stands increased by 4.48%, 17.91%, 58.21%, 49.06%, 122.64%, 201.89% and 18.21%, 53.30% and 95.24% for thinning intensity of 10%−15%, 16%−25%,26%−35%, respectively. (2) The richness, Simpson index and Shannon-Wiener index of shrubs and herbs in thinned forests were higher than those of the control. Particularly, for thinning intensity of 26%−35%, the richness of shrubs and herbs, Simpson index and Shannon−Wiener index increased by 68.00%、108.00%、131.23%、 78.74%、134.98% and 179.73%, respectively compared with the control. (3) The number of seedlings in each height grade in thinned forests was higher than that of the control. Taking thinning intensity of 26%−35% as an example, the number of natural regeneration seedlings of < 10 cm, 10−30 cm, 30−50 cm and 50−150 cm were 83.78%, 80.00%, 83.33% and 72.09% higher than that of the control respectively. (4) The total litter volume in the forests with different thinning intensity was in the order of 26-35% (3.86 ± 0.10 t. hm⁻²) > 16−25% (2.93 ± 0.07 t.hm⁻²) > 10%−15% (2.06 ± 0.13 t. hm⁻²) > control (1.40 ± 0.06 t. hm⁻²). The maximum water-holding capacity, maximum interception amount and effective interception amount of litter in thinned forest stands were higher than those of the control. The maximum water-holding capacity, maximum interception amount, and effective interception amount in 26%−35% thinned forest was 1.54, 2.75, 2.76 times of the control. (5) The soil bulk density in different thinning intensity was ranked as 26%−35% < 16%−25% < 10%−15% < control. The order of soil capillary porosity was 26−35% > 16−25% > 10%−15% > control. The ranking of soil saturated water holding capacity was 16%−25% > 26%−35% > 10%−15% > control. The pattern of soil total C, total P and total K content in each treatment was consistent, with the order of 16%−25% > 26%−35% > 10%−15% > control. The total N content and organic matter content in different thinning intensities were as follows: 26%−35% > 16%−25% > 10%−15% > control. (6) Based on the principal component analysis, the comprehensive evaluation scores for the four thinning intensities were ranked as 26%−35% > 16%−25% > 10%−15% > control. The suitable thinning intensity of artificial cypress plantations in the hilly areas of central Sichuan was 20%−35%, that is, the number of reserved trees was 1700−2100 indv · hm⁻².