Table 5-4: Comparison of Theoretical Values of Measured DBH and Canopy Area of Larch Plantations
Breast Diameter (cm) | Crown Area (m²)
---|---
6 | 2.516 | 2.5675 | +0.0515
7 | 3.237 | 2.9856 | -0.2514
8 | 3.644 | 3.3257 | -0.338
9 | 4.199 | 3.8077 | -0.312
10 | 4.199 | 4.0918 | -0.0272
11 | 4.227 | 4.4781 | +0.2511
12 | 4.600 | 4.8659 | +0.2659
13 | 5.147 | 5.2559 | +0.1086
14 | 5.147 | 5.684 | +0.501
15 | 5.853 | 6.042 | +0.187
16 | 6.202 | 6.438 | +0.236
17 | 6.514 | 6.836 | +0.322
18 | 7.306 | 7.236 | -0.07
19 | 7.645 | 7.6379 | -0.0071
20 | 8.093 | 8.0425 | -0.0515
21 | 8.973 | 8.4478 | -0.525
Using this empirical equation, the maximum density index of the stand can be calculated for different growth stages of larch plantations. This indicator is naturally greater than the density of standing trees under the condition of stand growth. However, it is advisable to determine the reasonable management and density of larch plantations. Some positioning studies need to be done to obtain the most reliable data. Currently, according to the data from the Hebei Academy of Forestry Science, based on the correlation between the growth of breast diameter and the density of standing trees, it is initially proposed that the operating density indexes of different DBH growth stages of the larch plantations should be referenced in Table 5-5 for tending.
Table 5-5: Management Index of Larch Plantation Density
Breast Diameter (cm) | Maximum Density (1.0) (trees/ha) | Operating Density (0.7–0.8) (trees/ha)
---|---|---
6 | 3940 | 2758–3152
7 | 3392 | 2374–2714
8 | 2980 | 2086–2384
9 | 2670 | 1869–2136
10 | 2404 | 1683–1923
11 | 2195 | 1537–1756
12 | 2020 | 1414–1616
13 | 1873 | 1311–1498
14 | 1746 | 1222–1397
15 | 1636 | 1145–1309
16 | 1540 | 1078–1232
17 | 1456 | 1019–1165
18 | 1380 | 966–1104
19 | 1311 | 901–1050
20 | 1252 | 876–1002
21 | 1198 | 839–958
22 | 1148 | 804–918
23 | 1102 | 771–882
24 | 1060 | 742–848
25 | 1022 | 715–818
For example, in the larch plantation of the Mengzi Forest Management Bureau’s Longtoushan Forest Farm, after investigation, the existing stand density is 2170 plants/ha, with an average DBH of 11.6 cm. Using the management density table, when the average DBH reaches 12 cm, the corresponding stand density is 1616/ha or 1614/ha. However, the existing stand has a density of 2170/ha, which is too dense. Therefore, about 554 to 756 plants per hectare need to be removed, with a thinning intensity of 20% to 30%, to adapt to the normal growth of the stand. After tending, when the diameter of the stand grows to 15–17 cm, the next thinning will be carried out.
When using the management density index, some forests are densely planted at the beginning and may have delayed tending. If the first thinning is conducted by felling a reasonable number of trees, it could lead to short-term instability, such as crown competition. Therefore, when determining the number of reserved trees, it is better to slightly increase the number so that the stand can gradually transition to a reasonable density.
The Longtoushan Forest Farm summarized the experience of thinning larch plantations and established a "reference table for tending and thinning" (Table 5-6), based on the relationship between the DBH, the age of the forest, and the number of reserved trees.
Table 5-6: Criteria for Thinning
Stand Diameter (cm) | Age (years) | Initial Thinning | Reserved Number (trees/ha) | Thinning Intensity (%) | Interval (years)
---|---|---|---|---|---
5–6 | 10–12 | 3500–4000 | 35–40 | 20–25 | 4–5
8–10 | 15–18 | 2000–2500 | 30–35 | 20–25 | 4–5
12–16 | 20–25 | 1200–1600 | 30–35 | 20–25 | 5–6
18–24 | 28–36 | 800–1000 | 25–30 | 20–25 | 5–6
To determine the average diameter of the retained trees after cutting, the Udaot empirical formula is used:
N = 10000 / (0.164 × d²)
Where:
- N = number of reserved trees after harvesting
- d = average diameter of retained trees
- 0.164 = coefficient
This formula is considered applicable in most cases. When using it to determine thinning intensity, the average diameter of the retained trees must be estimated in advance based on the thinning method.
According to the survey and analysis of sixteen-year-old larch plantations, the number of remaining trees was calculated using the Udaot formula before applying lower-layer thinning:
N = 10000 / (0.164 × 9²) = 2258 (where 9 is the average DBH of the retained trees)
This result aligns with other calculation methods, which suggest the appropriate number of preserved trees per hectare ranges from 1750 to 2250.
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