The measurement of leaf area index is different from the measurement of chlorophyll, because chlorophyll is located inside the leaf and must be extracted before it can be measured. Therefore, the chlorophyll meter is a necessary instrument and equipment, and the leaf area index only needs to know its calculation method. Even if there is no leaf area detector, it is very convenient to carry out measurement. Let's look at how to measure plant leaf area index.
Leaf Area Index Determination Method and Equipment Leaf area index refers to the ratio of total area of ​​plant leaf area to land area per unit area of ​​land. It is an important parameter for evaluating plant canopy function and quantitatively analyzing the energy exchange characteristics of the earth's ecosystem. It can reflect the number of plant leaves, the dynamics and balance of canopy crevices, vitality of plant communities, and environmental effects. It has been widely used in promoting the measurement of regional greenhouse gas emissions and ecosystem carbon sinks, and promoting the exchange of ecological energy in the earth. The leaf area index has been proposed for more than 50 years, and the related theories and techniques of its measurement methods have been continuously improved. It has undergone a process ranging from simple measurement to multiple algorithms and various types of instruments. However, the effective improvement of measurement accuracy is still this. The main research content of the field. The author summarizes the existing LAI measuring methods and measuring instruments, providing convenience for LAI research.
How to directly measure the leaf area index of a plant leaf The direct measurement method refers to collecting plant leaf, directly measuring its area or measuring the parameters such as the quality, shape, length and width of the leaf, and converting it into an area. This is a classic, mature LAI measurement method with the highest measurement accuracy. The measured value is often referred to as the true leaf area index and is an important method of indirect measurement. The method includes two steps of blade collection and measurement. The methods of leaf collection include the deciduous box method, the representative plant method, and the regional sampling method. The measurement methods include grid point method, grid method, trace weight method, and scanning and photography instrumental measurement methods. Because the direct measurement has certain damage to the plant itself, and it is necessary to manually collect the leaf sample, it takes time and labor, the sampling is not necessarily representative, and it is generally used only for scientific research. Therefore, indirect measurement is often used in actual measurement and research. Indirect measurement method Compared with direct measurement method, indirect measurement method can measure LAI faster and more extensively, and it does not cause harm to plants, so it is rapidly developed and widely used. However, the LAI obtained by this method is often low, and the range of different types of canopy underestimation is usually 25% to 50%. Although there are currently many solutions that can overcome the inconveniences caused by indirect measurements, there is no fundamental solution to such problems. This method is mainly divided into two types: space measurement method and ground measurement method. Spatial measurement method. The spatial measurement method is also called remote sensing measurement method. The measurement of reflectance spectrum information between vegetation and cover is completed through satellites, which provides an effective way for large-scale study of forest and terrestrial vegetation LAI. Spatial measurement methods are classified into statistical model method, physical model method, and neural network model method. As a new LAI spatial measurement method, the neural network model method has attracted the attention of many researchers. Studies have shown that for LAI <3 vegetation area, the feedback accuracy of this model method is relatively reliable. Chen Jian et al. used the BP neural network improved model to measure the internal reeds in Nan Dagang and found that the model has a strong nonlinear fitting ability with a maximum relative error of 20.71%, effectively improving the accuracy of LAI inversion. Ground measurement method. The ground measurement method is applicable to the measurement of single forest groups and crops and is divided into point contact method (inclined point embedding method), empirical formula method, extinction coefficient method, and the like. Wilson proposed a point contact method, which uses fine needles to reach the bottom from the canopy at different heights and angles. The LAI is calculated based on the number of blades that the tip touches.
LAI=n/G(θ) (1)
In the formula, n is the number of probes touching the blade; G(θ) is the projection function; θ is the zenith angle.
The advantage of the point contact method is that the measurement factor is easy to obtain and has no damage to the plant; the disadvantages are the tree-specificity and dependence on tree size, canopy structure, tree population density, seasonal climate, etc. The confidence is enough when the number of samples is large enough, and the disadvantages are: High canopy and forest implementation are more difficult. Bonhomme et al. used this method to obtain better results for small crops. Wilson studies showed that when the square tilt angle is 32.5°, the measured LAI value is accurate. Ren Hai et al. measured Dinghushan forest community and found that the maximum measurement error was 33.1% compared with the direct measurement method of stratified harvesting.
The empirical formula method, also known as the easy-measuring factor method, can be calculated based on the empirical formula between the easily measured parameters such as DBH, tree height, sapwood area, and crown width, and the leaf area index. Wang Lei et al. showed that there is a high correlation between the DBH and leaf area in the same tree population. Chang Xuexiang et al. further demonstrated that there is an exponential correlation between the LAI and the DBH and the tree height product. Take the measurement of the area index of poplar leaf as an example, the calculation equation is:
LAI=4{ ln[( DBH)2×0.01H]}8.07×10^-8 (2)
In the formula, DBH is the tree breast diameter (cm); H is the height of the tree (cm).
In addition, Gower et al.'s study on coniferous and broad-leaved forests found that sapwood area has the highest correlation with leaf area. Bartelin's research on beech shows that there is a high correlation between stem area and leaf area of ​​trees. Chen Xia et al. demonstrated the correlation between the canopy openness and the LAI index through the study of three types of forest communities in the warm temperate zone. The empirical formula method has the advantages of easy measurement and high efficiency. However, it has specificity and is too strong for tree species, tree size, canopy structure, tree population density, and seasonal climate. It is not suitable for any tree species. There are some limitations. The point contact method and the empirical formula method are all contact measurements. There are human disturbances in the measurement process and it is difficult to provide plant space and short-term change information. Therefore, since the 1990s, the non-contact measurement-extinction coefficient method has developed rapidly. The extinction coefficient method uses the light transmission information based on the canopy to complete the measurement task.
LAI = ln( Q1/ Q2) /k (3)
In the formula, Q1, Q2 are the upper and lower light radiation of the blade; k is the extinction coefficient of the specific plant canopy.
The extinction coefficient method has the advantages of high speed and versatility, and can be used as a remote sensing ground calibration method, but it is affected by the type of leaf cluster and the non-blade unit, and has a higher requirement for the measurement environment. The choice of extinction coefficient has a great influence on the measurement result. The LAI measured using the extinction coefficient method is often smaller than that measured directly, and the stronger the agglomeration effect of the blade, the greater the difference. In recent years, the leaf area detectors related to the leaf area index have emerged. The leaf area detectors are mainly used. According to the measurement principle, these instruments are divided into two categories, one based on radiation measurement methods; the other is based on image measurement methods. The radiation-based measurement method calculates the leaf area index by measuring the radiation transmittance. The basic principle is that the solar radiation incident on the top of the canopy (called incident radiation) is absorbed and reflected by the vegetation leaves, and when it reaches the bottom (called transmitted radiation), it will have attenuation, the rate of attenuation and the leaf area index, and the canopy structure. There is a quantitative relationship through which the LAI is reversed. Such instruments consist mainly of radiation sensors and microprocessors. Lang et al. used Demon to measure LAI, and the average error was 11% relative to direct measurement. Wilhelm et al. found that the measured values ​​of SunScan, AccuPAR, and LAI-2000 were lower than direct measurements when measuring corn canopy LAI. In terms of measurement accuracy, measurement methods based on image methods are more advantageous.
Based on the image measurement method, the leaf area index is calculated by analyzing the hemispherical digital image of the plant canopy. The basic principle is to obtain the canopy image, and use software to analyze it, calculate the solar radiation transmission coefficient, canopy gap, gap rate parameters, etc., and then calculate LAI. These image analysis systems typically consist of a fisheye lens, a digital camera, canopy image analysis software, and a data processor. Liu Lixin et al. used the Winscanopy2006a Canopy Analyzer to measure the leaf area index of Maoershan natural secondary forest. The accuracy of the LAI measurement results was 25% to 40% lower than that of the direct weighing measurement. Leblanc et al. conducted long-term measurements in arid ecosystems in comparison with LAI-2000, TRAC, HCP, and AccuPAR, and considered HCP to be the most accurate and effective. Chen et al. also recommended the use of HCP for the measurement of effective LAI in coniferous forests. The instrument comparison based on radiation measurement and image measurement is shown in Table 1.
In recent years, a series of instruments used to detect LAI have been applied abroad. However, there is still a lack of self-developed LAI instruments in China. At present, the domestic measurement instruments mainly include LAI-2000, TRAC, CI-110, etc., but most of them lack the direct measurement method of inspection and calibration. The internationally popular methods for measuring LAI in fisheye lenses are used less frequently in China. The measurement of LAI is influenced by its definition, sampling methods, data analysis and instrumental errors, and lacks an accurate and effective universal method.
There is a lack of comparative research between different methods. Although the direct measurement method is destructive to plants, the efficiency is not high, but the result is reliable. The indirect measurement method is an important measure of LAI. It has the advantages of high speed, wide application range and non-destructive, but the measurement accuracy is not high. Among them, the leaf area index measured by remote sensing technology will be more widely used in the future. At present, the instrument for determining the leaf area index is mainly based on foreign instruments. The accuracy of various measuring instruments is high, but because of other factors, the measurement accuracy of LAI is only 20% to 40%. Therefore, exploring new and more accurate LAI measurement techniques has significant fundamental research value.
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