3. Based on the relationship between DBH (diameter at breast height) and crown amplitude, an appropriate nutrient area is determined to establish the number of trees to be retained. The slope tree farm uses the least squares method to develop a regression equation for crown diameter in relation to DBH:
y = 0.9533 + 0.118x + 0.0013x²
In this formula, y represents the crown diameter, while x represents the breast height diameter.
This quadratic relationship helps determine the number of trees to retain in shaded larch plantations based on nutrient areas (see Table 5-7).
For a 14-year-old larch plantation in a small plot, the current density is 3,755 trees per hectare. The optimal nutrient area per tree is 3.14 m², and the ideal number of trees per hectare should be 3,185. Therefore, 570 trees need to be thinned out, resulting in 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 measure tree height accurately, and errors can be significant. Using the angle gauge method allows for easy determination of the forest branch area without setting up standard plots or using a measuring stick. Based on the objective of the operation, the developmental stage of the stand is identified to determine the optimal chest height area for the stand, which then helps define the optimal thinning strategy.
The lower horizontal line in Figure 5-1 indicates the minimum area that should be retained after each thinning, which plays a crucial role in managing forest density throughout the rotation period. For instance, to promote the growth of tree diameter without reducing total yield, the suitable chest height area range is typically between 11 and 40 m²/ha. Coniferous species generally require higher values than broad-leaved trees, and shade-tolerant species tend to have higher thresholds than light-demanding ones. For example, spruce and fir may fall within 30–40 m²/ha, while less shade-tolerant species might be around 14–30 m²/ha, and broad-leaved trees vary from 10–23 m²/ha depending on their type.
There is no clear consensus on whether the reserved area size correlates with site conditions. However, it is commonly believed that for the same tree species, more area should be left uncut in sites with better growing conditions.
Figure 5-2 illustrates how the lower limit of the horizontal line can be referenced using the growth process table of the species under management, or by converting the number of trees into cross-sectional area through natural thinning processes. Age, diameter at breast height, and chest height area can also be used to establish a regression relationship, or the data can be analyzed graphically based on field surveys.
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