Laser and Remote Sensing

Paper Review 1 (Terrestrial Laser Scanner)

Authors          : Fanhua Kong, Weijiao Yan, Guang Zheng, Haiwei Yin, Gina Cavan, Wenfeng Zhan, Ning Zhang, Liang Cheng

Journal            : Agricultural and Forest Meteorology

Date                : December 2015

My reason of choosing this paper

This paper provides us about the temperature beneath the tree canopy using several devices or methods such as: Leaf Area Index (LAI), Sky View Factor (SVF) with L_V3DPC and shade. With the conventional device, Researcher always measure the environment as far as time consuming along several days. But, the present study, Terrestrial Laser Scanner (TLS) become popular device to ease us to measure the environment without time limit and weather condition.

Location

The location case of this study is in Nanjing the capital of Jiangsu Province in China. The specific location is conducted in Nanjing Zhongshan botanical garden which has more than 7000 species of plants. There are four woodlands for this observation, three of them are mainly about Cinnamomum camphora (camphor tree), Metasequoia glyptostroboides (dawn redwood), and Magnolia grandiflora (southern magnolia). For the fourth case is mixed forest with broad-leaved species. There are so many trees which were mature and approximately have about 50 years old.

Introduce

The vegetation canopy of trees is a major contributor of cooling effect beneath the trees or called microclimatic environments. Based on the studied before, there are many kinds of trees resulting cooling effect such as tree shape and depth, canopy size, canopy density and leaves shape. Such as maple tree since they have a largest leaves which have power to reduce the amount of radiation than other tree species. Contrary, for the small-leaved tree species are powerful at cooling effect due they maintained lower crown temperature. Based on this issue, many researchers are focus on quantifying and calculating the tree shading using computer simulation of sky view factor (SVF). For this method, it can be calculated by fish-eye photography, GPS signals, LiDAR data, and the use of vector/ raster three-dimensional (3-D) geospatial data. But, in this study we focus on LiDAR device only and other devices to be compared at determining the cooling effect of vegetation. To detecting the cooling effect, in addition to SVF, we should know about Leaf Area Index or called LAI for the other measurement because it has parallel increasing with cooling effects of vegetation, and the cooling effect is respect to temperature reducing. The LAI can be used to describe light interception as well as vegetation respiration, transpiration, and photosynthesis. But, using the LAI device is very difficult to use, since it has a dependent of the day. So, writer use TLS method to introduce the device doesn’t not affected by weather condition. Using three dimensional point cloud (3DPC) data, is better than LAI to measure the vertical distribution of the vegetation canopy by reflecting the leaf area density, crown volume, and discerning between green and non-green materials.

Given the important influence of the vegetation canopy on cooling and the limitations of traditional methods in their ability to precisely quantify the shade and canopy structure, this study was conducted to (1) characterize the diurnal changes of the vegetation cooling effect, (2) employ TLS to describe the vegetation canopy using 3DPC data and propose a new method for calculating the shade, and (3) identify and evaluate the impact of canopy characteristics on the diurnal behavior of the cooling effect.

Method

LAI measurement using LAI-2200 device in the clear sky before sunrise and after sunset and for hemisphere images taken using fish lens by Nikon in the evening at height 1.5 meter the same as TLS measurement but, TLS has no time conditional. Using TLS measurement, tree crown can be described as the point cloud of leaves and the cubic volume described as the sum of points in difference horizontal slices. For the photosynthetic tissue are defined as points of leaves by isolating manually excludes branch and stems, see on figure 1. So, here we classified as photosynthetic and non-photosynthetic component. Using this point cloud data by TLS, we can also determine the shade by tree canopies. Shade was different along the time, so we rotated the tree as parallel as light direction. See on the figure 3.

1

Figure 1. The order of extracting the crown using Cyclone software: (1) original point cloud data (2) point cloud data of limb and ground (3) point cloud data of crown leaves.

But this result will be over-estimate since it is not neglect the gaps in the crown without leaves. For neglecting gap area of leaf, you can see the formulas below. And the detailed method for obtaining the crown volume for individual trees. See on the figure 2.

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Figure 2. Measurement of vegetation crown volume of leaves (L_V3DPC) determined by analyzing slices of the crown (white ellipse) at interval of 1 mm.

Result

For the result, I will explain first the characteristic of the leaf of the tree. Cinnamomum camphora has 21 meter of high; small and thin characteristic of leaf size; and smooth with the light green of the color. Metasequoia glyptostroboides has 22 meter of high; needle, thin and rough of the characteristic of the leaf; and the leaf has light green of color. Magnolia grandiflora has 12 meter of high; large, thick, and smooth of the characteristic of leaf; and has a dark green of color. For board-leaved mixed forest has a characteristic of middle/ small of size, thin shape, and smooth/rough surface of leaf, and the average color of this area are dark/green. The other location is at the open area defined as reference point. We measure the meteorological data including air temperature (Ta), global solar radiation (SR), relative humidity (RH), wind speed (WS) and wind direction (WD) for each locations recorded by HOBO meteorological station at 1.5m of high.

The microclimatic area, under the crown of the trees, along the morning until afternoon indicate that there are significant difference between reference point and woodland area and between its self (difference type of woodland). The record of all the day, indicate that the average maximum radiation SR was 409.4 W/m2 in the reference point and 64.2 W/m2 at underneath the tree canopy. The reduction was calculated that significantly decrease about 86.5 %. And we found the greatest reduction is occurred at Magnolia grandiflora area. For the Ta, Rh, and WS has the average values 31.2oC, 75.7%, and 1.3 m/s respectively. See on figure 3.

Table 1. The average value of the reduction temperature.

Cinnamomum camphora Metasequoia glyptostroboides Magnolia grandiflora board-leaved mixed forest Open area
Temperature reduction at night 0.9 oC 0.9  oC 0.8  oC 0.3  oC reference
Temperature reduction daytime 2.6 oC 2.8 oC 2.5 oC 1.5 oC reference

During a day, the cooling effect was found at the difference site. The average temperature reduction range from 0.9 (for the broad-leaved mixed forest) to 1.9oC (for the M. glyptostroboides woodland) See on table 1. The greatest area of cooling effect reduction was in M. glyptostroboides 4.6oC. But the difference result of cooling effect obviously seen between daytime and night time. For daytime, the cooling effect is larger than the night. The average reduction in temperature during the daytime was 2.4oC but instead when the nighttime the temperature reduction just was 0.7oC. It was assessed that heat flux in the ground and long-wave radiation in the canopy stored all day, and released it at night so that increase air temperature. Instead, the temperature at the woodland area was higher than reference point (open area) during night time. For the clear explanation, see on the figure 4.

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Figure 3. The average variations of microclimate components at the studied sites.

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Figure 4. The average diurnal variation of temperature reduction within the four woodlands.

Based on the data above, we found the correlation between cooling effect and vegetation canopy structure. That are:

L_V3DPC, Shade, SVF, and LAI on the cooling effect

Based on the measurement all the day, we got the result on cooling effect using several device/method. These measurements indicate that the most affecting on cooling effect was caused by the shading, shade created by leaves may have become main factor and important element influencing the cooling effect. But for evapotranspiration is not important contributor to, so RH and SR were removed from the model because they were not significant. From these measurements, L_V3DPC has more significant accurate relationship with residual temperature compared with SVF and LAI has a good relationship with decreased temperature. For the Wind Speed, it is may affected by tree high and crown high -it was a little strange on cooling effect- when WS was increasing the cooling effect was decreasing, because the exchange with the ground surface and air flow beneath the vegetation. But

Conclusions

In this study, woodland has significant influent on cooling effect rather than open site. But for woodland it-self has a difference strength of temperature reduction. The greatest vegetation on cooling effect are M. glyptostroboides woodland, C. camphora woodland, the M. grandiflora woodland, and the broad-leaved mixed forest woodland respectively. This result indicates that the smallest leaf structure tend to be more effective on cooling effect. Based on the measurements, a distinct advantage of the ground-based LiDAR method over conventional LAI measurements and SVF methods is that the light environment does not matter. The results of this study show that L_V3DPC and shade provide a better indication of the impact of the tree canopy and cooling effect than LAI and SVF. TLS data can be used to describe the tree canopy structure and the volume of leaves (L_V3DPC) and shade and then to analyze the cooling effect of trees. The results show that shading by trees is of prime importance in mitigating the thermal environment. The L_V3DPC, which is strongly related to the vegetation evapotranspiration, is also a major component. Small-leaved species tend to be more effective at cooling than large-leaved species.