Name: leaf area index estimation using three distinct methods in pure deciduous stands
Uploaded: 2019-08-29T13:00:00
Description: Watch this Scientific Journal Video about Leaf Area Index Estimation Using Three Distinct Methods in Pure Deciduous Stands at JoVE.com

We are indebted to the editorial board of the Journal of Forestry Research for encouraging and authorizing us to use the representative results in this protocol from the article published there. We also kindly thank two anonymous reviewers for their valuable comments, which have substantially improved the manuscript. The research was funded by the Ministry of Agriculture of the Czech Republic, institutional support MZE-RO0118 and the National Agency of Agricultural Research (Project No. QK1810126).

1. LAI estimated using litter traps
<ol>
	<li>First, perform a field survey, investigating the site conditions and structure of the studied stands (i.e., inclination and exposition of the slope, forest or vegetation type, forest or vegetation density, homogeneity of the canopy closure, the crown size, and the crown base height).</li>
	<li>Select a suitable litter trap type for positioning below the canopy by choosing the mesh size of the net based on the size of the assimilation apparatus of the studied stands...

Litter traps are deemed as one of the most accurate methods for performing LAI estimation8, but they are more labor-intensive and time-consuming than the indirect methods35,64 which were incorporated into this protocol. Within the entire LAI estimation procedure using litter traps, a precise estimation of the SLA is the most critical point10 because the SLA can vary with plant species65, date...

LAI, defined as half of the total leaf surface area per unit of horizontal ground surface area1, is a key variable used in many bio-geophysical and chemical exchange models focused on carbon and water fluxes2,3,4. LAI is directly proportional to the active surface of leaves where it drives primary production (photosynthesis), transpiration, energy exchange, and other physiological attributes connected with a range of ecosystem processes in plant communities5.
Numerous approaches and instruments for performing LAI estimation have been developed, and they are currently available on the market6,7,8,9. Ground-based methods for performing LAI estimation can be grouped into two main categories: (i) direct, and (ii) indirect methods10,11,12. The first group includes methods measuring leaf area directly, while the indirect methods infer LAI from measurements of more readily measurable parameters, using radiative transfer theory (in terms of time, labour-intensiveness, and technology)13,14.
This protocol deals with the practical use of litter traps and the needle technique, as non-destructive semi-direct methods10; and the optical device plant canopy analyzer as an indirect method6,7 for performing LAI estimation on a chosen sample from temperate deciduous forest stands in Central Europe (see its structural and dendrometric characteristics in Appendix A and Appendix B).
In deciduous forests and crops, it is possible to perform non-destructive semi-direct LAI estimation using litter traps11 distributed below the canopy layer15. Litter traps provide precise LAI values for deciduous species in which LAI reaches a plateau within the growing season. However, for species that can replace leaves during the growing season, such as poplar, the method overestimates LAI11. This method assumes that the content of the traps represents the average amount of leaves that fall during a leaf-fall period in the stand16, especially during the autumn months. Traps are opened boxes or nets (Figure 1) with a predetermined sufficient size (minimum 0.18 m2, but preferably over 0.25 m2)10,17, lateral sides preventing the wind from blowing leaves into/out of the traps, and with a perforated bottom avoiding decomposition of the leaves; which are located below the canopy layer of the studied stand, however, above the ground surface11. The distribution of the traps can be either random18 or systematic in transects19 or a regular spacing grid20. The number and distribution of traps are a crucial methodological step for performing an accurate LAI estimation reflecting the unique stand structure, spatial homogeneity, expected wind speed and direction, especially in the case of sparse stands (or alleys and orchards), and the work capacity for evaluating data. The precision of LAI estimation increases with the rising frequency of traps within studied stands11,21 (see Figure 2).
The recommended frequency of collecting samples of the litter-fall from each trap is at least monthly10 and even twice per week in periods of heavy fall, which may coincide with heavy rainfall. It is necessary to prevent decomposition of the litter in the traps and the leaching of nutrients from the material during rain episodes in the case of chemical analysis. After collecting leaves in a field, a mixed sub-sample is used for estimating the specific leaf area (SLA, cm2 g-1)22, defined as the fresh projected area of leaves to its dry mass weight ratio. The rest of the collected litter is dried to a constant weight and used for calculating the dry mass of the litter as g cm-2 in the lab. Leaf dry mass on each collection date is converted into the leaf area by multiplying the collected biomass by SLA or leaf dry mass per area (LMA, g cm-2) as the inverse parameter to SLA23,24. A fresh projected area of particular leaves can be determined using a planimetric approach. The planimetric method is based on the dependency between the area of a specific leaf and the area covered by the leaf in the horizontal surface. The leaf is horizontally fixed to the scan screen, and its average is measured using a leaf area meter. Then, its area is calculated. Many leaf area meters based on different measurement principles are available on the market. Some of them include, for instance, the LI-3000C Portable Leaf Area Meter, which uses the orthogonal projection method, and the LI-3100C Area Meter, which measures leaf average using a fluorescent light source and a semi-conducted scanning camera. The next device, the CI-202 portable laser leaf area meter, codes a leaf length using a code reader. Besides them, the AM350 and BSLM101 Portable Leaf Area Meters are also commonly used for performing accurate leaf area estimation.
Furthermore, leaf area meters based on systems that analyze video exist. These leaf area meters consist of a video camera, a digitalisation frame, a screen, and a PC, including suitable software for making the data analysis such as WD3 WinDIAS Leaf Image Analysis System11. Currently, conventional scanners connected to a PC can be used for an estimating leaf area. Afterwards, the leaf area is calculated as a multiple of the number of black pixels and its size depends on the selected resolution (dots per inch &#8211; dpi), or the leaf area is measured through specific software, for instance, WinFOLIA. Finally, the total dry mass of leaves collected within a known ground surface area is converted into the LAI by multiplying by SLA and a shrinkage coefficient25 which reflects the changes in the area of fresh and dried leaves. Shrinkage depends on the tree species, water content and leaf softness. The shrinkage of leaves in length and width (what affect the projected area) is usually up to 10%26, for instance, it ranges from 2.6 to 6.8% for oak27. Sorting leaves by species for weighing and establishing the specific leaf area ratio is necessary to determine the contribution of each species to the total LAI28.
LAI determination by the needle technique is an inexpensive method derived from the inclined point quadrat method29,30,31,32. In deciduous stands, it is an alternative for performing LAI estimation without using traps10 based on the assumption that the total leaf number and their area in a tree are equal to what is collected on the soil surface after a complete leaf-fall20. A thin sharp needle is pierced vertically into the litter lying on the ground immediately after the leaf-fall10. After the complete leaf-fall, the leaves are collected from the ground onto a needle of a vertical probe, are related to the contact number and equal the actual LAI value. An intensive sampling (100-300 sampling points per studied stand per field probe) by the needle technique is required to quantify a mean contact number and to derive the LAI value correctly10,20,33.
The plant canopy analyzer (e.g., LAI-2000 or LAI-2200 PCA) is a commonly used portable instrument for performing an indirect LAI estimation by taking a measurement of the light transmission throughout the canopy7 within the filtered blue portion of the light spectrum (320-490 nm)34,35 to minimize the contribution of the light which has passed through the leaves, was scattered by the canopy and is passing through the foliage7,34. In the blue part of the light spectrum, the maximum contrast between the leaf and sky is achieved, and the foliage appears black against the sky34. Therefore, it is based on the canopy gap fraction analysis7. The instrument has been widely used for making eco-physiological studies in plant communities such as crops36, grasslands37, coniferous stands8, and deciduous stands38. The plant canopy analyzer uses a fisheye optical sensor with a FOV of 148&#176; 35 to project a hemispherical image of the canopy onto silicon detectors to arrange them into five concentric rings39 with central zenith angles of 7&#176;, 23&#176;, 38&#176;, 53&#176;, and 68&#176; 9,40,41. Five view caps (i.e., 270&#176;, 180&#176;, 90&#176;, 45&#176;, and 10&#176;) can be used to restrict the azimuth view of the optical sensor27 to avoid shading by obstacles in an open area (for the above-referenced reading) or the operator in the sensor&#8217;s FOV during LAI estimation can adjust the FOV sensor to an open area for above-canopy readings. Measurements using the plant canopy analyzer are taken above (or in a sufficiently extended open area) and below the studied canopy7. The same view caps must be used for both above and below readings to avoid biases of gap fraction estimation34. The LAI-2000 PCA produces an effective leaf area index (LAIe) as introduced by Chen et al.42, or rather an effective plant area index (PAIe) as woody elements are included in the sensor reading value. In deciduous stands with flat leaves, the LAIe is the same as the hemi-surface LAI. In the case of evergreen forest stands, the LAIe is necessary to correct for the clumping effect at the shoot level (SPAR, STAR)43, the clumping index at scales larger than the shoot (&#937;E)44, and the contribution of woody elements including stems and branches (i.e., woody-to-total area ratio),45 which cause a systematic LAI underestimation20. The clumping index on a higher spatial scale than the shoot or leaf could be quantified as an apparent clumping index (ACF), which can be estimated using the plant canopy analyzer when more restrictive view caps are used27. As these authors state that this ACF is deduced from a ratio of LAI values calculated from transmittance by different procedures for homogeneous and non-homogeneous canopies according to Lang46, we presume that this clumping index describes rather canopy homogeneity. Besides the ACF calculation, new diffuser caps that enable a more extensive application of LAI-2200 PCA in respect of weather conditions, a user menu instead of Fct codes, and the possibility to take many more measurements per file session are among the main technological upgrades compared to the former LAI-2000 PCA34,47. Measurements and subsequent internal software calculations are based on four assumptions: (1) light blocking plant elements including leaves, branches, and stems, are randomly distributed in the canopy, (2) foliage is an optically black body that absorbs all the light it receives, (3) all plant elements are the same projection to the horizontal ground surface as a simple geometric convex shape, (4) plant elements are small compared to the area covered by each ring11.

<table>
	<tbody>
		<tr>
			<td>Area Meter</td>
			<td>LI-COR Biosciences Inc., NE, USA</td>
			<td>LI-3100C</td>
			<td>https://www.licor.com/env/products/leaf_area/LI-3100C/</td>
		</tr>
		<tr>
			<td>Computer Image Analysis System</td>
			<td>Regent Instruments Inc., CA</td>
			<td>WinFOLIA</td>
			<td>http://www.regentinstruments.com/assets/images_winfolia2/WinFOLIA2018-s.pdf</td>
		</tr>
		<tr>
			<td>File Viewer</td>
			<td>LI-COR Biosciences Inc., NE, USA</td>
			<td>FV2200C Software</td>
			<td>https://www.licor.com/env/products/leaf_area/LAI-2200C/software.html</td>
		</tr>
		<tr>
			<td>Laboratory oven</td>
			<td>Amerex Instruments Inc., CA, USA</td>
			<td>CV150</td>
			<td>https://www.labcompare.com/4-Drying-Ovens/2887-IncuMax-Convection-Oven-250L/?pda=4|2887_2_0|||</td>
		</tr>
		<tr>
			<td>Leaf Image Analysis System</td>
			<td>Delta-T Devices, UK</td>
			<td>WD3 WinDIAS</td>
			<td>https://www.delta-t.co.uk/product/wd3/</td>
		</tr>
		<tr>
			<td>Litter traps</td>
			<td>Any</td>
			<td>NA</td>
			<td>See Fig. 2</td>
		</tr>
		<tr>
			<td>Needle</td>
			<td>Any</td>
			<td>NA</td>
			<td>Maximum diameter of 2 mm</td>
		</tr>
		<tr>
			<td>Plant Canopy Analyser</td>
			<td>LI-COR Biosciences Inc., NE, USA</td>
			<td>LAI-2000 PCA</td>
			<td>LAI-2200 PCA or LAI-2200C as improved versions of LAI-2000 PCA can be used, see: https://www.licor.com/env/products/leaf_area/LAI-2200C/</td>
		</tr>
		<tr>
			<td>Portable Laser Leaf Area Meter</td>
			<td>CID Bio-Science, WA, USA</td>
			<td>CI-202</td>
			<td>https://cid-inc.com/plant-science-tools/leaf-area-measurement/ci-202-portable-laser-leaf-area-meter/</td>
		</tr>
		<tr>
			<td>Portable Leaf Area Meter</td>
			<td>ADC, BioScientic Ltd., UK</td>
			<td>AM350</td>
			<td>https://www.adc.co.uk/products/am350-portable-leaf-area-meter/</td>
		</tr>
		<tr>
			<td>Portable Leaf Area Meter</td>
			<td>Bionics Scientific Technogies (P). Ltd., India</td>
			<td>BSLM101</td>
			<td>http://www.bionicsscientific.com/measuring-meters/leaf-area-index-meter.html</td>
		</tr>
		<tr>
			<td>Portable Leaf Area Meter</td>
			<td>LI-COR Biosciences Inc., NE, USA</td>
			<td>LI-3000C</td>
			<td>https://www.licor.com/env/products/leaf_area/LI-3000C/</td>
		</tr>
	</tbody>
</table>

leaf area index estimation using three distinct methods in pure deciduous stands

Accurate estimations of leaf area index (LAI), defined as half of the total leaf surface area per unit of horizontal ground surface area, are crucial for describing the vegetation structure in the fields of ecology, forestry, and agriculture. Therefore, procedures of three commercially used methods (litter traps, needle technique, and a plant canopy analyzer) for performing LAI estimation were presented step-by-step. Specific methodological approaches were compared, and their current advantages, controversies, challenges, and future perspectives were discussed in this protocol. Litter traps are usually deemed as the reference level. Both the needle technique and the plant canopy analyzer (e.g., LAI-2000) frequently underestimate LAI values in comparison with the reference. The needle technique is easy to use in deciduous stands where the litter completely decomposes each year (e.g., oak and beech stands). However, calibration based on litter traps or direct destructive methods is necessary. The plant canopy analyzer is a commonly used device for performing LAI estimation in ecology, forestry, and agriculture, but is subject to potential error due to foliage clumping and the contribution of woody elements in the field of view (FOV) of the sensor. Eliminating these potential error sources was discussed. The plant canopy analyzer is a very suitable device for performing LAI estimations at the high spatial level, observing a seasonal LAI dynamic, and for long-term monitoring of LAI.

Average LAI values at the stand level of all studied stands in the 2013 growing season are presented in Figure 8. On all plots except A, the highest values were measured by litter traps, which serve as the reference level. Contrarily, the highest mean LAI value was estimated through the needle technique on plot A. All differences between LAI values estimated using litter traps and a plant canopy analyzer were not significant (p &#62; 0.05; Figure 8, left). On pl...

Watch this Scientific Journal Video about Leaf Area Index Estimation Using Three Distinct Methods in Pure Deciduous Stands at JoVE.com

Leaf Area Index Estimation Using Three Distinct Methods in Pure Deciduous Stands

An accurate estimation of leaf area index (LAI) is crucial for many models of material and energy fluxes within plant ecosystems and between an ecosystem and the atmospheric boundary layer. Therefore, three methods (litter traps, needle technique, and PCA) for taking precise LAI measurements were in the presented protocol.

Accurate estimations of leaf area index (LAI), defined as half of the total leaf surface area per unit of horizontal ground surface area, are crucial for ...

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