(v) Quantitative Thinning Based on Volume
In Finland, pine forests undergo thinning 2 to 4 times during their rotation period, using a method that controls the intensity based on volume. This technique is referred to as "rushing into thinning," where the thinning age is determined according to a specific growth prediction equation. The advantages include high efficiency and controlled thinning amounts (Table 5-8).
Table 5-8: Sparseness Scale with Age and Superiority
Age | High Advantage (m) | Logging Volume per Hectare (cubic meters) | Thinning Volume Percentage (%)
---|---|---|---
18 | 5 | 8 | 40
33 | 11 | 48 | 33
53 | 17 | 108 | 38
73 | 21.5 | 101 | 32
This high-intensity directed thinning involves the first thinning at 2200 trees per hectare. Subsequent thinnings mainly focus on rods, eventually yielding main construction materials.
Depending on tree species, site conditions, and forest management objectives (such as cultivation period or type of production material), an ideal number of standing trees is designed, and a thinning density management table is developed. For instance, the Longtoushan Forest Farm has utilized the law of tree height change over recent years, taking the height of the upper forest stand as a parameter. It demonstrates how diameter and volume change with stand density, showing the growth rules of tree height, stand density effects, maximum density curve, natural sparseness, and other curves. These curves form the stand growth model and are converted into two easy-to-use tools: "site index curves" (Figure 5-2) and "density management maps" (Figure 5-3).
By checking the site index using the actual height of the stand (measured by selecting the largest trees in a 500–2000 m² sample plot and averaging their height), and then referring to the site index curve, the site index can be determined. Using this index and the corresponding site-type retention sparse density, the number of plants and volume to be retained can be identified from the density management map. Based on this, the thinning intensity can be accurately calculated.
With these charts, it's also possible to design a directional cultivation process based on the predetermined cultivation period and wood species specifications, enabling forest cultivation to reach a level of prediction and orientation.
For example, if a plantation larch has a diameter D = 12 cm and a density N = 2000 trees/ha, the required sparse density after thinning should be no less than 0.8. The yield M, density P, height H, and the number of thinning plants ΔN, along with the volume of logs ΔM and the diameter of cut D1, can be determined by consulting Figure 5-3.
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Figure 5-2: Site Index Curve of Artificial Larch Forest
(Another picture)
Figure 5-3: Density Control Map of Artificial Larch Forest
1. According to the D and N drawn, the volume M = 174 m³/ha was read on the scale of the ordinate, at the intersection of the equidistant line marked with 12 cm and the line with the abscissa 2,000 lines per hectare (there are X marks).
2. Since the intersection point lies between the density lines marked with 0.9, P = 0.94 can be read out in proportion to the increase in the upper layer height.
3. Therefore, the intersection point is marked between the two tree line heights of 14m and 16m. Hence, H = 15m can be read in proportion to the increase in the upper floor height.
4. According to the measured retention density of 0.8, the upper layer height is 15m. When the isodensity line marked with 0.8 intersects with the imagined 15m tree line (there is the point), the retention quantity M1 = 148 m²/ha is determined by the ordinate. The reserved density N1 = 1,400 plants/ha was determined on the abscissa. The diameter D1 = 13.3 m after the thinning was determined on the scale of the adjacent straight line.
5. According to the volume and density after thinning, the number of thinning and the volume of the thinning were determined. The results were as follows:
ΔN = N - N1 = 2000 - 1400 = 600 plants/ha
ΔM = M - M1 = 174 - 148 = 26 m²/ha
In recent years, significant progress has been made in experimental research on quantitative thinning. Mathematical analysis, statistics, and various variables of forest stands have been used (e.g., number of trees per hectare, average height of the stand, average diameter at breast height, changes in canopy diameter, crown height ratio, forest height, and site conditions). Computer-assisted regression and linear equations have been applied to design stand growth models and even sustainable management models, solving problems related to tree species growth forecasting and tending thinning. Researchers continue to explore long-term management and sustainable use methods.
To summarize, in the process of forest cultivation, thinning techniques are comprehensively applied considering the direction of fast-growing and high-yield forests, site conditions, and growth factors of forest trees. This helps mediate the forest growth environment and achieve fast-growing and high-yield goals. In practical applications, the criteria for selecting "harvesting wood" can be flexibly chosen based on the stand's growth status.
Sixth, Survey Design
In the forest stand, a representative area is selected for the layout of standard plots, and different tending methods (lower, upper, integrated, or mechanical) are applied. Referring to the principles of "forest classification" and "three cuts and three stays," the selection of cuttings is carried out and marked. Then, the intensities of thinning, timber production, and estimates of revenues and expenditures are calculated on a standard basis. This section does not describe in detail.
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