Comparison of the significance and measurement method of leaf area index

The leaf area index, also known as the leaf area coefficient, refers to the multiple of the total leaf area of ​​the plant on the unit land area. Namely: leaf area index = total leaf area / land area. In field trials, the leaf area index (LAI) is an important indicator of the growth of plant populations, and its size is directly related to the final yield.

The commonly used leaf area index (LAI) was obtained from the following formula: Leaf area The leaf length (Lij) and maximum leaf width (Bij) of each leaf were measured with a ruler.

Leaf area index effect and significance

Leaf area index is a good dynamic indicator reflecting the size of crop population. The leaf area index can be reflected in a certain range, and the crop yield increases with the increase of the leaf area index. When the leaf area index increases to a certain limit, the field is closed, the light is insufficient, the photosynthetic efficiency is weakened, and the yield is decreased. The maximum leaf area index of apple orchards generally does not exceed 5, and it is ideal to maintain 3 to 4. In the red Fuji apple orchard in the fruiting period, the amount of acres in the growing season is maintained between 10 and 120,000, and the leaf area index can basically reach a more suitable index.

Nitrogen increased the leaf area index, photosynthetic potential, chlorophyll content and growth rate, while the net assimilation rate decreased with the increase of nitrogen application. Nitrogen application had no significant effect on the photosynthetic rate of soybean. The positive effect of increasing leaf area index with increasing nitrogen application can offset the negative effect of the decrease of net assimilation rate, and finally obtain a higher growth rate. Therefore, high-yield cultivation should first consider obtaining a suitably large leaf area index.

In ecology, the leaf area index is an important structural parameter of the ecosystem, which is used to reflect the number of plant foliage, canopy structure changes, plant community vitality and its environmental effects, and is a description of plant canopy surface material and energy exchange. Provide structured quantitative information and play an important role in ecosystem carbon accumulation, vegetation productivity and energy balance between soil, plant and atmosphere interaction, and vegetation remote sensing.

The main method of leaf area index determination

Leaf area index direct method

Direct measurement is a traditional, destructive method.

1. Determination of leaf area, traditional grid method and grid method.

2. Drawing and weighing method. On a specific coordinate paper, draw the outline of the blade to be tested with a pencil and cut the coordinate paper according to the leaf shape, weigh the weight of the leaf-shaped coordinate paper, and calculate the leaf area according to the formula.

3 , instrument measurement method. Leaf area analyzer can be divided into two types, respectively, by scanning and capturing images to obtain leaf area. Scanning leaf area meter, mainly composed of scanner (scanning camera), data processor, processing software, etc. Data such as blade area, length, width, perimeter, blade length ratio and form factor, and cumulative blade area can be obtained. In addition, there is a method of measuring with a desktop scanner and professional image analysis software. The image processing type leaf area meter is composed of a digital camera, a data processor, a processing analysis software, and a computer, and can acquire data such as blade area and shape.

Leaf area index indirect method

The indirect method is to obtain the leaf area index by using some measurement parameters or using an optical instrument. The measurement is convenient and quick, but it still needs to be corrected by the direct method.

1 , point contact method

The point contact method uses a thin probe to penetrate the canopy at different elevation and azimuth angles, and then records the number of blades that the tip of the probe contacts during the process from the top of the canopy to the bottom. The point contact method is derived from the method of measuring the coverage of the community. It is more accurate in the measurement of LA I in small crops, but it is more difficult to apply in forests, mainly due to the tallness of forest plant trees and the high density of needles in coniferous species. The leaves affected the assay.

2 , extinction coefficient method

The method calculates the leaf area index by measuring the upper and lower canopy radiation and the parameters associated with the extinction coefficient, provided that the leaves are assumed. The random distribution and the leaf dip angle are elliptical. The extinction coefficient k is related to the plant species, zenith angle, blade dip angle and non-leaf biomass. It is often obtained according to the empirical formula when determining. For example, when Guan Dexin studied the Changbai Mountain coniferous and broad-leaved mixed forest. The observation result is used to inversely push the extinction coefficient k value. If the extinction coefficient can be accurately measured in this method, the obtained leaf area index is also accurate.

3 , empirical formula method

The empirical formula method uses the correlation between the easily measured parameters of the plant's DBH, tree height, sapwood area, crown width and leaf area or leaf area index to establish an empirical formula to calculate. The study shows that the leaf area index and the DBH square height and tree height are calculated. The product has a significant exponential correlation. The sapwood area has a high correlation with the leaf area. The canopy openness has a good exponential relationship with the leaf area index. The advantage of the empirical formula method is that the measurement parameters are easy to obtain and the plants are destroyed. Smallness and high efficiency, however, the empirical formula is specific and not suitable for any tree species, so the application of this method has certain limitations.

4 , remote sensing methods

The satellite remote sensing method provides an effective way to study LA I in a wide range. There are two kinds of remote sensing methods that can be used to estimate the leaf area index. One is the statistical model method, which mainly uses remote sensing image data such as the normalized vegetation index NDV I. The model is based on the vegetation index RV I and the vertical vegetation index PV I and the measured LA I. This method has a single input parameter and does not require complicated calculations. Therefore, it is a common method for remote sensing estimation of LA I. However, LA I and vegetation of different vegetation types. The functional relationship of the index will be different, and it needs to be re-adjusted and fitted in use. The other is the optical model method, which is based on the bidirectional reflectance distribution function of vegetation, which is a model based on the radiation transmission model. Using LA I as an input variable, an iterative method is used to estimate LA I. The advantage of this method is that it has a physical model basis and is not affected by the type of vegetation. However, because the model is too complex, the inversion is very time consuming and the inversion is estimated. Some functions in the LA I process do not always converge.

5 , optical instrument method

The optical instrument method is divided into a radiation measurement based method and an image based measurement method according to the measurement principle.

• Radiation measurement method. The method is to calculate the leaf area index by measuring the radiation transmittance. The main instruments are Sunscan, etc. These instruments are mainly composed of radiation sensors and microprocessors, which obtain solar radiation transmittance through radiation sensors. The leaf area index was calculated by parameters such as canopy void ratio, canopy gap size or canopy gap size distribution. The first five instruments all assumed uniform canopy, random distribution of leaves and elliptical leaf angle distribution, which were measured when measuring leaf clusters. Difficulty. TRAC can effectively solve the problem of agglomeration effect by measuring the agglomeration index, so that the leaf area index calculation can reduce the calculation error between the effective leaf area index and the actual leaf area index without assuming that the leaves are randomly distributed in space. The advantage of based on radiation measuring instruments is that the measurement is simple and fast, but it is easily affected by the weather and often needs to work under sunny conditions.
• Image-based measurement method. This method calculates the leaf area index by acquiring and analyzing the hemispherical digital image of the plant canopy. The instruments mainly include W INSCANOPY, HemiView, etc. These image analysis systems usually consist of fisheye lens, digital camera, and canopy. Image analysis software and data processor. The principle is to obtain canopy images through fisheye lens and digital camera, use software to analyze canopy images, calculate solar radiation transmission coefficient, canopy gap size, gap rate parameters, etc. Furthermore, the effective leaf area index is derived. The instruments and methods based on image measurement have higher measurement accuracy, and the speed is slower than that based on radiation measurement, and often requires post-processing of the image. In addition, a uniform light environment such as dawn is required for measurement. At dusk, cloudy, etc., sunny days can make the fisheye lens underestimate or overestimate solar radiation or scattering.

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