Section IV: Forestry Survey
In forestry surveys, the first step is the selection of sample plots or standard sites. This method involves choosing one or more representative plots within a forest stand to serve as a benchmark for estimating the overall condition of the entire area. The standard survey technique is widely used in various forestry activities such as secondary forest tending, artificial forest management, afforestation planning, and monitoring forest growth and pest infestations.
The purpose of setting up standard plots is to select a number of representative areas that can reflect the average characteristics of the forest stand. These plots act as a microcosm of the entire forest, enabling the collection of detailed data on tree species, age, height, diameter, canopy density, and site conditions. The results from these standard plots are then extrapolated to estimate the total condition of the forest. Standard plots can be either temporary or permanent. Temporary plots are established for short-term needs and require only a single survey, while permanent plots are used for long-term research and continuous data collection, requiring multiple assessments with strict measurement and design standards.
Principles for Setting Up Standard Plots:
1. A comprehensive survey of the forest area under investigation should be conducted to understand its overall characteristics, including tree species composition, forest age, tree height, diameter, canopy density, spacing, and site conditions.
2. A representative plot should be selected based on the characteristics of the forest stand, ensuring it reflects the overall condition of the stand in terms of density, volume, and site quality.
3. The size of the standard plot should be determined based on the required accuracy and the structural variability of the stand. Typically, the plot should not be smaller than 5% of the class area.
4. If the forest has significant variations in topography or stage of development, additional plots may need to be set up or strip plots along slopes to ensure representativeness.
5. For long-term observation, standard plots should be located away from forest edges, roads, and other areas prone to human disturbance.
Calculating the Size of the Standard Plot:
The formula for determining the number of trees (n) in a standard plot is:
n = (t² × c²) / p²
Where:
- n = number of trees to be included in the plot
- t = reliability index (for 95% confidence level, t = 2)
- c = coefficient of variation of tree diameter
- p = acceptable error margin
In practice, the number of trees in a standard plot should be no less than 250 for near-mature forests and 300 for young stands. Investigators can take a small sample of 400 trees to check the main species and use proportional methods to calculate the appropriate plot size.
Second, Stand Factor Investigation:
This process involves collecting data on key indicators that describe the quantity and quality of the forest, such as site conditions, area, forest type, small class, tree species, origin, species composition, age, canopy density, average height, diameter, density, volume, growth rate, and other relevant factors. Additional details include elevation, slope direction, slope position, soil texture, depth, parent rock, understory vegetation, and ground cover.
Section V: Tending and Thinning Construction Technology
Tending and thinning operations involve several key steps, including planning and preparation (as outlined in Chapter 6: Final Cutting and Replanting). The core of tending and thinning lies in selecting the right trees for removal. Before any work begins, technicians assess the site and determine which trees should be cut to promote healthy growth and eliminate those that hinder development or are defective. Tree grading standards serve as an essential basis for scientifically selecting which trees to remove, supporting both qualitative and quantitative thinning strategies.
The essence of tending and thinning is the selective removal of trees. Therefore, correctly identifying which trees to cut is crucial for achieving the desired outcomes. Due to varying site conditions, growth stages, initial planting density, and management goals, the decision-making process can be complex. Technicians must carefully observe and compare trees before making selections. Generally, the following principles should guide the selection of trees for thinning:
1. Select trees strictly according to the cutting intensity and target species, aiming for even distribution of retained trees.
2. Avoid creating "sun roofs" by improper selection.
3. Prioritize removing damaged, diseased, decayed, compressed, crooked, double-trunked, or poorly developed trees.
4. Delay logging if the cutting object negatively affects the growth of retained trees or the forest cover.
5. Consider wildlife habitat when selecting trees, especially in areas with nests. Unless necessary, preserve non-diseased trees. Undergrowth should not be completely removed unless it hinders the growth of main species, helping maintain soil pH and litter composition.
There are two main approaches to tree selection during thinning. One focuses on individual tree growth, where each retained tree is nurtured until final cutting. In dense stands, high-quality trees are selected early and marked for future harvesting. This approach suits large-diameter timber production and uses low-density, long-interval management. Another approach emphasizes the growth of the entire forest, ensuring all retained trees have sufficient space. This method is suitable for small and medium-diameter timber and involves high-density, short-interval management. It is commonly used in larch plantations.
Over time, the thinning of larch forests has been guided by specific criteria, including approval from higher authorities, thorough understanding of the plan, review of small classes, and careful field investigations. When marking trees, color paint is applied at 1.2–1.5 meters from the base. Two passes are typically made, with the first pass ensuring the correct number of trees is harvested. Afterward, a sample is tested to verify accuracy. If the results fall within ±5% of the design, the operation proceeds; otherwise, adjustments are made for the second pass.
Section VI: Effects and Impacts of Thinning
Thinning plays a vital role in promoting forest health, improving growth conditions, and enhancing the quality of timber. By selectively removing trees, foresters create better growing spaces, reduce competition, and encourage the development of healthier, more productive stands. The long-term benefits include increased biodiversity, improved water retention, and enhanced resilience to pests and diseases. Additionally, proper thinning contributes to sustainable forest management, ensuring that resources are available for future generations.
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