3. Based on the relationship between DBH (diameter at breast height) and crown amplitude, an appropriate nutrient area is identified to determine the number of trees to be retained. The slope tree farm uses the least squares method to develop a regression equation for crown diameter and DBH:
y = 0.9533 + 0.118x + 0.0013x²
In this formula, y represents the crown diameter, while x stands for the breast height diameter.
This quadratic relationship helps in determining the optimal number of trees to retain in larch plantations on shaded slopes, as shown in Table 5-7.
For a 14-year-old larch plantation, the current density is 3,755 trees per hectare, with an optimal nutrient area of 3.14 m² per tree. The ideal number of trees to retain is 3,185, which means 570 trees should be removed. This corresponds to a thinning intensity of 15% based on the number of trees.
Table 5-7: Diameter at Breast Height and Crown Diameter of Larch Plantation
Breast Diameter
Crown Diameter (Theoretical)
Crown Diameter (Actual)
Deviation
Nutrient Area (m²)
Number of Trees per Hectare (Based on Nutrient Area)
7
1.84
1.83
+0.1
2.66
3,759
8
1.97
2.02
-0.5
3.05
3,278
9
2.12
2.10
+0.02
3.53
2,833
10
2.26
2.24
+0.02
4.01
2,494
11
2.41
2.41
0
4.56
2,193
12
2.56
2.59
-0.03
5.13
1,949
13
2.71
2.69
+0.02
5.77
1,733
(IV) Quantitative Thinning Based on Chest Height Area
It is often challenging to accurately measure tree height, and there can be significant errors. Using the angle gauge method to estimate the stand area is more efficient and requires no standard plots or measuring tapes. Based on the objectives of the operation, the developmental stage of the stand is determined to set the optimal chest height area, which then guides the selection of the best trees to retain.
The optimal chest height area is crucial for determining the thinning intensity. As illustrated in Figure 5-1, two horizontal lines represent different levels of thinning. The lower line indicates the minimum acceptable area after each thinning, which plays a key role in managing forest density throughout the rotation period. For example, to promote diameter growth without reducing total yield, the suitable range for retained cross-sectional area is typically between 11 and 40 m²/ha. Coniferous species generally have higher values than broad-leaved trees, and shade-tolerant species tend to have higher thresholds compared to light-demanding ones. For instance, spruce and fir may range from 30 to 40 m²/ha, while less shade-tolerant species may fall between 14 to 30 m²/ha, and broad-leaved trees vary from 10 to 23 m²/ha.
There is no clear consensus on whether the reserved area size is directly related to site conditions. However, it is generally accepted that for the same tree species, more trees are left in areas with better site quality.
Figure 5-2 illustrates how the lower limit of the middle section can be referenced using the growth process table for the species, or by converting the number of trees into cross-sectional area through natural spacing processes. Age, DBH, and cross-sectional area can also be used to establish a regression relationship, or the data can be analyzed graphically based on actual field surveys.
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