leaf area index app guide
Contents
1. W Wines Ba p KW A x ASS LPT oy K A DECAGON DEVICES We measure the world Leaf Area Index LAI is one of the most widely used measurements for describing plant canopy structure LAI is also useful for understanding canopy UBI eai Dece tce MIN O e Co ETT Tee TT T o e alc eNA CocurT et ire leer nae Fol ee de anr ane LA Is oen ae biopnysical variable used in biogeocnemical hydrological and ecological PEGE Swe MSc SOrcOniiOn R E HSE STE E OST STE DHE 0 C COrc ele sm ore 10 and productivity at spatial scales ranging from the plot to the globe In the past measuring LAI was difficult and time consuming However Hearn EVO LEGNINCIOS IST EE li FSCS SEIS lene Tae EH SE Ue LA mucn simpler and more feasible for a wide range of canopies he iolon epe de T Mmlenced 10 CrOvide a Ole HT ee s TTT eT o neon ele MSE TS HT D EI TT IGN T R IST Ia TTT Ta TH HET D EEIT TESTS eT TEH Oae Gle S aI most appropriate method for their research needs LAI Theory and Practice v 1 0 Copyright 2014 Decagon Devices Inc All Rights Reserved Printed in the U S A Table of Contents S SR T L S iter ere tce tne eect e rene eset tree T T I 2 How to Measure B TTT 2 ZAP DIFE CE measurement aa aa E a N T T 2 G Ile RR L EIL 2 2 22f HeMmMSPNehicalapMOlLOSla Piles desacees sen eee mie ce cise tes cen ce eee es 2 2 2 Dol ISI RTE RZ LAST TH RT Le aera a a 3 223 kadiatonrenectance mee erm na r a crate r E2. Seasonally annual and deciduous canopies and croplands can exhibit large variations in LAI For example from seeding to maturity maize LAI can range from O to 6 Obviously LAI is a useful metric for describing both spatial and temporal patterns of canopy growth and productivity Ground Area 1 m Ground Area 1 m Leaf Area 1 m Leaf Area 3 m LAl Leaf Area Ground Area 1 1 1 LAl Leaf Area Ground Area 3 1 3 Figure Conceptual diagram of a plant canopy where a LAI 1 or b LAI 3 2 Measuring LAI There is no one best way to measure LAI Each method has advantages and disadvantages The method you choose will depend largely on your research objectives The researcher who needs a single estimate of LAI might use a different method than the one who is monitoring changes in LAI over time for example and the grassland researcher may prefer a different method than the forestry researcher In this guide we ll discuss the theoretical basis of each of the major methods along with key advantages and limitations 2 1 Direct Measurement Traditionally researchers measured LAI by harvesting all the leaves from a plot and painstakingly measuring the area of each leaf Modern equipment like flatoed scanners have made this process more efficient but it is still labor intensive time consuming and destructive In tall forest canopies it may not even be feasible It does however remain the most accurate method of cal
3. 4 3 Using the LP 80 Ceptomete cccecnssscccennsscccennsescenneeseennnnssesnnns 7 i ed Paes 0 Tale Se a a ene een ae eee eae aaa 9 eE E E E a E E 9 373 2 ClUMPING andispatalsampline Sa a a a a ce ene eee ce ete oe oe eee IO AAO ele le allet Len 3 5 Influence of non photosynthetic elements sese x e x K x K x K x K x K x K x K K K KKK 4 Using the SRS NDVI Sensor ss ss ss ss es s s es es anana anana anana eaa 12 4 1 Developing field based NDVI LAI regression models 000eeeee eens I3 4 2 SRS NDVI Sampling considerations 1 cece x e c ee eee K Kee 14 4 3 Influence of soil background on NDVI measurements sexe x e x e x e x e x K eee ee I5 4 4 Dealing with NDVI saturation in high LAI canopies ceeeeeeeeeee I6 References Sr aT a E etree eae anime E T E E E I8 What is LAI Leaf Area Index LAI quantifies the amount of leaf material in a canopy By definition it is the ratio of one sided leaf area per unit ground area LAI is unitless because it is a ratio of areas For example a canopy with an LAI of 1 has a 1 1 ratio of leaf area to ground area Fig 1a A canopy with an LAI of 3 would have a 3 1 ratio of leaf area to ground area Fig 1b Globally LAI is highly variable Some desert ecosystems have an LAI of less than 1 while the densest tropical forests can have an LAI as high as 9 Mid latitude forests and shrublands typically have LAI values between 3 and 6
4. for any given canopy K only changes as the Sun moves across the sky The LP 80 automatically calculates K each time it measures LAI Once K is calculated and all other variables quantified LAI is calculated as n A LT nt A 1 047f Equation3 where L is LAI and A is leaf absorptivity By default A is set to 0 9 in the LP 80 Leaf absorptivity is a highly consistent property for most healthy green foliage and a value of 0 9 is a good approximation for most situations In extreme cases e g extremely young leaves highly pubescent or waxy leaves senescent leaves A may deviate from 0 9 leading to errors in estimates of LAI If you are using the LP 80 in non typical conditions you may need to manually combine the outputs from the LP 80 with a modified A value to calculate LAI Using the LP 80 Ceptometer 8 3 1 Using the LP 80 in short canopies e g cereal crops grasslands In typical scenarios it is best to hold the ceptometer at a consistent height underneath the canopy while the attached external PAR sensor is held above the canopy Use the attached bubble level to ensure that the light bar and external PAR sensor are held level For row crops or small sample plots researchers often mount the external sensor on a tripod in between rows or above the canopy The LP 80 makes simultaneous above and below canopy PAR measurements each time the button is pressed accounting for any changes in light conditions If the canopy is s
5. leaf off stage o Using the SRS NDVI Sensor In depth technical SRS NDVI specifications and operating details are provided in the manual which can be accessed on the web decagon com support manual spectral reflectance sensor srs 4 Using the SRS NDVI sensor The SRS NDVI sensor measures canopy reflectance in red and NIR wavelengths which allows for calculation of the Normalized Difference Vegetation Index NDVI In turn NDVI can be used to estimate LAI We provide a brief overview of the SRS NDVI operating theory here The SRS NDVI measures canopy reflectance in red and NIR wavelengths and its measurements can be used to calculate or approximate LAI Red and NIR reflectances are used the following equation to calculated NDVI Equation 4 NDVI PNIR Pred Purr Pred where p denotes percent reflectance in NIR and red wavelengths Mathematically NDVI can range from 1 to 1 As LAI increases red reflectance will typically decrease due to increasing canopy chlorophyll content whereas NIR reflectance increases due to expanding mesophyll cells and increasing canopy structural complexity So under typical field conditions NDVI values typically range from somewhere around O to 1 representing low and high LAIs respectively In cases like phenology and stay green phenotyping where absolute values of LAI are not required NDVI values can be used directly as proxies for LAI For example if the objective of a study is to track th
6. 5a In the presence of clouds or haze however some portion of the beam radiation is scattered by water vapor and aerosols in the atmosphere Fig 5b This scattered component is referred to as diffuse radiation f is calculated as the ratio between diffuse and beam radiation The LP 80 automatically calculates fo by comparing measured values of incident PAR to the solar constant which is a known value of light energy from the sun assuming clear sky conditions at any given time and place on earth s surface x leaf angle distribution The leaf angle distribution parameter x describes the projection of leaf area onto a surface Imagine for example a light source directly overhead The Shadow cast by a leaf with a vertical orientation would be much smaller than the shadow cast by a leaf with a horizontal orientation In nature canopies are typically composed of leaves with a mixture of orientations This mixture is often best described by what is known as the spherical leaf distribution with a Y value 1 the default in the LP 80 Canopies with predominately horizontal orientations such as strawberries have x values gt 1 whereas canopies with predominately vertical orientations like some grasses have Y values lt 1 Extensive details about the LP 80 s LAI model are provided in the user s manual www decagon com education lp 80 manual Using the LP 80 Ceptometer Figure 4 Solar zenith angle changes during the day Ob
7. alues collected at different locations or at different times of the year It can also make it difficult to establish a reliable NDVI LAI regression model as discussed in Section 4 1 The Modified Soil Adjusted Vegetation Index MSAVI was developed by Qi et al 1994 as a vegetation index that has little to no soil sensitivity MSAVI is calculated as La 2 _ 2P NIR 1 20 T 1 BLO Prea 2 Equation6 MSAVI Advantages of MSAVI include 1 it requires no soil parameter adjustment and 2 it uses the exact same inputs as NDVI red and NIR reflectances meaning that it can be calculated from the outputs of any NDVI sensor Using the SRS NDVI Sensor NDVI LAI Figure 9 NDVI has limited sensitivity to LAI values greater than 3 4 4 4 Dealing with NDVI saturation in high LAI canopies In addition to soil sensitivity NDVI also suffers from a lack of sensitivity to changes in LAI when LAI is greater than approximately 3 to 4 depending on the canopy Fig 9 Decreased NDVI sensitivity at high LAI is due to the fact that chlorophyll is a highly efficient absorber of red radiation Thus at some point adding more chlorophyll to the canopy e g through the addition of leaf material will not appreciably change red reflectance see Fig 3 Several solutions to NDVI saturation have been developed One of the simplest solutions uses a weighting factor that is applied to the near infrared reflectance in both the numerator and den
8. ampling under clear sky conditions Garrigues et al 2008 This is because there is a larger proportion of radiation coming from a single angle the beam radiation directly from the sun Under these conditions it is important to correctly model how leaf angle and beam penetration angle interact So when sampling under clear sky conditions make sure that you are using an appropriate Y value ae va 1 F E f p 4 HISSI ITN UI l 1 1 1HE i E iii Ml Wi 1 a eres aS 3 5 Influence of non photosynthetic elements In forests shrublands and other areas where woody species are present LP 8O measurements will be influenced by elements other than leaves For example tree boles branches and stems will intercept some radiation and thus have an effect on estimates of LAI obtained with the PAR inversion technique In fact some researchers refer to the measurement obtained from the LP 80 and similar instruments to Plant Area Index PAI rather than LAI in order to acknowledge the contribution of non leaf material to the measurement It should come as no surprise that PAI will be higher than LAI in any given ecosystem However values of PAI and LAI are often not too different because leaf area is generally much larger than branch area and the majority of branches are shaded by leaves Kucharik et al 1998 In deciduous ecosystems the contribution of woody material can be accounted for by acquiring measurements during the
9. culating LAI because each individual leaf is physically measured Litter traps are another way to directly measure LAI but they don t work well in evergreen canopies and can only capture information from leaves that have senesced and abscised from the plant 2 2 Indirect Measurement Several decades ago canopy researchers began to look for new ways to measure LAI both to save time and to avoid destroying the ecosystems they were trying to measure These indirect methods infer LAl from measurements of related variables such as the amount of light that is transmitted through or reflected by a canopy Figure 2 Hemispherical photograph acquired from a mixed deciduous forest using a digital camera fisheye lens 2 2 1 Hemispherical Photography Hemisphere photography was one of the first methods used to indirectly estimate LAI Researchers would photograph the canopy from the ground using a fisheye lens Fig 2 Photographs were originally analyzed by researchers themselves Now most researchers use specialized software to analyze images and differentiate between vegetated and non vegetated pixels Advantages Hemispherical photography has decided advantages First it delivers more than just LAl measurements It can also provide canopy measurements such as gap fraction sunfleck timing and duration and other canopy architecture metrics Second the canopy images can be archived for later use or for reanalysis as methods change and
10. e temporal patterns of canopy growth and senescence Fig 6 then it may be adequate to simply use NDVI as the metric If your research objectives require estimates of actual LAI it is possible to establish a canopy specific model that will allow NDVI to be converted to LAI This method is described in the next section Using the SRS NDVI Sensor Leaf Area Index NDVI 2000 2001 2002 2003 2004 2005 2006 2007 Year Figure 6 NDVI closely tracks the year to year seasonal dynamics of LAI in a mixed deciduous forest 4 1 Developing field based NDVI LAI regression models To directly estimate LAI using NDVI values you need to develop a site specific or crop specific correlative relationship The best way is to take co located measurements of NDVI and LAI e g using a LP 80 ceptometer For example co located measurements of LAI and NDVI were acquired during a period of rapid canopy growth Least squares regression was used to fit a linear model to the data Fig 7 With this model we can use NDVI to predict LAI without making independent measurements Developing a robust empirical model involves some effort but once the model is complete you can continuously monitor changes in LAI with a SRS NDVI sensor deployed over a plot or canopy long term This method can ultimately save significant effort and time in the long run Al N D VI e 5 74 l 4 Figure 7 Relationship between NDVI and LAI The fitted linear regression mode
11. er of sensors 80 Overall length 102 cm 40 25 in Microcontroller dimensions 15 8 x 9 5 x 3 3 cm 6 2 x 3 75 x 1 3 in PAR range O to gt 2 500 umol m s Resolution 1 umol m st Minimum spatial resolution 1 cm Data storage capacity 1MB RAM 9000 readings Unattended logging interval User selectable between 1 and 60 minutes Instrument weight 1 22 kg 2 7 Ibs Data retrieval Direct via RS 232 cable Power 4 AA Alkaline cells External PAR sensor connector Locking 3 pin circular connector 2 m cable Extension cable option 6 m 25 ft SRS NDVI raae Campbell G S Norman J M 1998 An Introduction to Environmental Biophysics 2nd Edition Springer Verlag New York NY U S A Garrigues S Shabanov N V Swanson K Morisette J T Baret F Myneni R B 2008 Intercomparison and sensitivity analysis of Leaf Area Index retrievals from LAI 2000 AccuPAR and digital hemispherical photography over croplands Agricultural and Forest Meteorology 148 1193 1209 Gitelson A A 2004 Wide dynamic range vegetation index for remote quantification of biophysical characteristics of vegetation Journal of Plant Physiology 161 165 173 Hyer E J Goetz S J 2004 Comparison and sensitivity analysis of instruments and radiometric methods for LAI estimation assessments from a boreal forest site Agricultural and Forest Meteorology 122 157 174 Jiang Z Huete A R Didan K Miura T 2008 Developmen
12. getation has a very distinct spectrum Fig 3 In fact some scientists have proposed finding potentially habitable planets outside our solar system by looking for this unique spectral signal A typical vegetation reflectance spectrum has very low reflectance in the visible portion of the electromagnetic spectrum 400 700 nm which is also the PAR region However in the near infrared NIR region gt 700 nm reflectance can be as high as 50 The exact amount of reflectance at each wavelength depends on the concentration of various foliar pigments like chlorophyll and canopy structure e g arrangement and number of leaf layers Advantages Early attempts to use spectral reflectance data to quantify canopy properties found that the ratio of red and NIR reflectance could be used to estimate the percent canopy cover for a given area Later efforts have produced a number of different wavelength combinations that relate to various canopy properties These wavelength combinations or spectral vegetation indices are now routinely used as proxies for LAI or through empirical modeling are used to directly estimate LAI Until recently one of the only ways to collect reflectance data was with a handheld spectrometer an expensive delicate instrument designed for the lab not the field But sensor options have expanded with the development of lightweight multiband radiometers that measure a specific vegetation index These little sensors are inexpens
13. hort enough an even easier approach is to use the ceptometer to acquire both above and below canopy measurements Simply hold the LP 80 above the canopy to acquire an incident PAR measurement Update the above canopy measurement every few minutes or as sky conditions change e g due to variable clouds In either case all the other variables discussed in Section 3 are measured and calculated automatically and LAI is updated with each below canopy measurement 3 2 Using the LP 8O in tall canopies e g forests riparian areas In tall canopies it is often not practical to measure above and below canopy PAR with one instrument When using the LP 8O in tall canopies there are a couple of options available for making above and below canopy measurements of PAR One option is to mount a PAR sensor above the canopy or in a wide clearing with an unobstructed view of the sky This method requires some additional post processing of the data but can give good results The PAR sensor needs to be attached to its own data logger which should be configured to acquire measurements at regular intervals e g every 1 5 minutes so that any variation in ambient light levels will be captured You should collect below canopy measurements with the ceptometer as you normally would then combine the data in post processing using the timestamps to pair each above and below canopy measurement Calculate t with each pair which can then be used as an input to Equatio
14. ive and don t require a lot of power making them perfect for field monitoring This is good news for anyone who wants to monitor changes in LAI over time including researchers interested in phenology canopy growth detecting canopy stress and decline or detecting diseased plants Vegetation indices offer another advantage many earth observing satellites like Quickbird Landsat and MODIS measure reflectance that can be used to calculate vegetation indices Since these satellites observe large areas they may serve as a way of scaling observations made at the local scale to much broader areas Conversely measurements made at the local scale with a multiband radiometer can be a useful source of ground truth data for satellite derived vegetation indices 0 05 0 45 0 4 0 39 0 03 0 25 0 2 Reflectance 0 15 400 500 600 700 800 900 1 000 Wavelenstn nm Figure 3 Reflectance Spectra obtained at different stages of canopy development Note the distinct difference between visible and near infrared NIR reflectance that develops as LAI increases Multiband radiometers also offer a top down option for extremely short canopies like shortgrass prairie and forbs It s difficult if not impossible to use most LAI estimation methods with these canopies because the equipment is too big to fully fit beneath the canopy Vegetation indices are measured using sensors that view the canopy from the top down making them a grea
15. l solid line can be used to predict LAI from NDVI measurements 0 3 0 4 0 5 0 6 0 7 0 8 O08 NDVI Using the SRS NDVI Sensor S E 2 hy AN A N HSI a 2 fs Ale 9 ing 4 2 SRS NDVI sampling considerations The SRS NDVI is designed to be used as a dual view sensor This means that one sensor having a hemispherical field of view should be mounted facing toward the sky The other sensor having a 36 field of view 18 half angle should be mounted facing downward at the canopy Down and up looking measurements collected from each sensor are used to calculate percent reflectance in the red and NIR bands Percent reflectances are used as inputs to the NDVI equation Eq 4 The up looking sensor must be placed above any obstructions that will block the sensor s view of the sky The down looking sensor should be directed at the region of the canopy to be measured The size of the area measured by the down looking sensor is dependent on the sensor s height above the canopy The spot diameter of the down looking sensor is calculated as Equation 5 Spot Diameter tan y xh x1 where y is the half angle of the field of view 18 for the SRS NDVI and h is the height of the sensor above the canopy Equation 5 is valid for measuring spot diameter when the down looking sensor is pointed straight down i e nadir view angle In cases where the down looking sensor is pointing off nadir the spot will be oblique and wil
16. l be larger than that calculated by Equation 5 To quantify spatial variability in LAl several down looking sensors can be set up to monitor different portions of the canopy For example several sensors were mounted above the canopy in a deciduous forest to monitor differences in spring phenology of several trees Measurements of NDVI revealed differences in the timing and magnitude of leaf growth among the trees that were measured Fig 8 A similar approach could be used to monitor the response of plants in individual plots subject to experimental manipulation or to monitor growth patterns across different agricultural units Using the SRS NDVI Sensor O95 sr 0 90 D ASPEN 2 0 85 RED OAK A 0 80 77 RED OAK 2 2 0 75 amp BIRCH 4 BIRCH 2 0 70 O RED PINE 0 65 140 142 144 146 148 I50 152 154 Day of the Year Figure 8 Spatial variability of NDVI during spring green up The variability is driven by differences in the timing of leaf development among individual trees and tree species 4 3 Influence of soil background on NDVI measurements Considerable error in NDVI measurements can occur when soil is in the field of view of the SRS NDVI sensor or in situations where the amount of soil in the field of view changes due to canopy growth e g from early to late growing season Qi et al 1994 showed that NDVI is sensitive to both soil texture and soil moisture This soil sensitivity can make it difficult to compare NDVI v
17. n 3 amp Using the LP 80 Ceptometer The second option is useful to use when it is not feasible to place a PAR sensor above the canopy or when a PAR sensor or data logger is not available If this is the case then you can use the LP 80 to measure incident PAR in a location outside the canopy with an unobstructed view of the sky In measurement mode you can choose whether you are measuring incident or transmitted radiation When using the LP 80O itself to take above and below canopy readings you should take the variability of sky conditions into account On a clear sky day it is easiest to acquire samples toward the middle of the day since the light levels won t change much over the span of 20 to 30 minutes When sky conditions are uniformly overcast PAR conditions can remain sEquation 6 for longer periods of time giving you a longer measurement window before needing to reacquire an above canopy measurement If sky conditions are highly variable however we do not recommend this method unless you can constantly update the incident PAR measurement The LP 80 automatically calculates LAI with each below canopy measurement using the stored incident PAR measurement Reacquire an incident PAR measurement any time light conditions change e g when cloud obstructs the solar disk or after 20 30 minutes have passed to prevent error in the LAI calculation 3 3 Clumping and spatial sampling In most canopies LAI is variable across space F
18. ominator of Equation 4 The resulting index is called the Wide Dynamic Range Vegetation Index WDRVI Gitelson 2004 The weighting factor can be any number between O and 1 As the weighting factor approaches O the linearity of the WDRVI LAI correlation tends to increase at the cost of reducing sensitivity to LAI changes in sparse canopies The Enhanced Vegetation Index EVI is another vegetation index that has higher sensitivity to high LAI compared to NDVI EVI was originally designed to be measured from satellites and included a blue band as an input to alleviate problems associated with looking through the atmosphere to earth s surface from orbit Recently a new formulation of EVI has been developed that does not require a blue band This modified version of EVI is referred to as EVI2 Jiang et al 2008 Similar to the MSAVI index described in Section 4 3 EVI2 uses the exact same inputs as NDVI red and NIR reflectances and is calculated as DR Pred Susie Woa S iadi Ps OAD RS Another advantage of EVI2 also is that it has less soil sensitivity compared to NDVI Thus EVI2 is a good all around vegetation index for estimating LAI since it has low sensitivity to soil and has a linear relationship with LAI pase CC TARR elie Oi iS ec ETS Te TS H E we e ay Mets Soela de do 31T deel wise 9S eee e ERI G LH ol eneesiis wus Pn OST NEER 5 THH G H IG COTE TETE ISTR iia combination with the information provided above wnen making decision
19. or example in row crops LAI can range from O to 2 3 within a distance of 1 meter Even in forests and other natural canopies variable tree spacing branching characteristics and leaf arrangement on stems causes clumping This means that point based measurements of LAI can be highly biased Lang and Yueqin 1986 found that averaging several measurements along a horizontal transect helped alleviate biases associated with clumping at fine spatial scales The LP 80 uses a similar approach averaging light measurements across eight groups of ten sensors situated along an 80 cm long probe Although this approach reduces errors at the local scale it may not account for variability in LAl at the canopy scale Researchers must consider spatial variability in canopy LAI when developing a sampling scheme In general more heterogeneous canopies will require more LAI measurements across space in order to obtain a LAI value that is representative of the entire canopy Using the LP 80 Ceptometer 3 4 Atmospheric conditions The LP 80 is capable of accurately measuring LAI in both clear sky and overcast conditions This is because the LAI model used by the LP 80 accounts for changes in diffuse and beam radiation S solar zenith angle 0 and because incident and transmitted radiation are measured simultaneously when using an above canopy PAR sensor Errors associated with incorrectly specifying the leaf angle distribution x are most pronounced when s
20. orbed or reflected it thus t will be close to 1 As the amount of leaf material in the canopy increases there is a proportional increase in the amount of light absorbed and a decreasing proportion of light will be transmitted to the ground surface The LP 80 consists of a light bar which has 80 linearly spaced PAR sensors and an external PAR sensor In typical scenarios the light bar is used to measure PAR under the canopy whereas the external sensor is meant to quantify incident PAR either above the canopy or ina clearing Additional measurement scenarios are covered in Sections 3 1 and 3 2 0 solar zenith angle 0 is the angular elevation of the sun in the sky with respect to the zenith or the point directly over your head at any given time date and geographical location Fig 4 The solar zenith angle is used to describe the path length of photons through the canopy e g ina closed canopy the path length increases as the sun approaches the horizon and for determining the interaction between beam radiation and leaf orientation discussed below 0 is automatically calculated by the LP 80 using inputs of local time date latitude and longitude Therefore it is critical to make sure that these are correctly set in the LP 80 configuration menu 86 5 cm f beam fraction In an outdoor environment the ultimate source of shortwave radiation is the sun When the sky is clear most radiation comes as a beam directly from the sun Fig
21. rovides the theoretical basis for this relationship For the purposes of environmental biophysics Beer s law is formulated as Equation PAR PAR exp kz where PAR is transmitted photosynthetically active radiation PAR measured near the ground surface PAR is PAR that is incident at the top of the canopy z is the path length of photons through some attenuating medium and k is the extinction coefficient In the case of vegetation canopies z accounts for LAI since leaves are the medium through which photons are attenuated You can see that if we know k and measure PAR and PAR it may be possible to invert Eq 1 to calculate z as an estimate of LAI This approach is commonly referred to as the PAR inversion technique The real world is slightly more complex but as you will see in Section 3 Beer s law is the foundation for estimating LAI using measurements of incident and transmitted PAR Advantages The PAR inversion technique is non destructive one obvious but major advantage that allows a canopy to be sampled extensively and repeatedly through time The PAR inversion technique is also attractive because it has a solid foundation in radiative transfer theory and biophysics and is applicable in a wide variety of canopy types For these reasons the PAR inversion technique is currently a standard and well accepted procedure In addition to handheld instruments like Decagon s LP 80 ceptometer standard PAR sensors a k a quant
22. s about WIG TE SHS Me SS Steven Garrity Ph D Canopy Scientist amp Product Manager QUICK LA Method Comparison Cnart Method Relative Temporal Suitability Suitability Spatial Ease of Vertical Cost Sampling for Tall for Short Scaling Collecting Profiling Single or Continuous Canopies Canopies Samples Samples Measurements Destructive H Single L H L VL Yes Harvest Litter Traps M Single H L L M M NIS Hemispherical V Single H L M M NIS Photography PAR Inversion M Both H H M H Yes LP 80 Vegetation L VH Continuous NM VH M H VH No Index a ae with LP 80 een access to Intensive Continuous with top of canopy or large subcanopy PAR open area sensors Requires access to top of canopy Key VL very low L low M moderate H high VH very high SRS Multiband Radiometer Accuracy 10 or better for spectral irradiance and radiance values Dimensions 43 x 40 x 27 mm Calibration NIST traceable calibration to known spectral irradiance and radiance Measurement Time lt 300 ms Connector Type 3 5 mm stereo plug or stripped and tinned wires Communication SDI 12 digital sensor Data logger compatibility not exclusive Decagon Em50 series Campbell Scientific NDVI bands Centered at 630 nm and 800 nm with 50 nm and 40 nm Full Width Half Maximum FWHM respectively LP 80 Ceptometer LP 80 Ceptometer Operating environment O to 5 C O to 100 relative humidity Probe length 86 5 cm Numb
23. server is facing the equator Left Figure 5 Beam fraction under a Sunny and b overcast sky conditions Right In general x describes how much light will be absorbed by the leaves in a canopy at different times of day as the sun moves across the sky The estimation of LAI with the PAR inversion technique is not overly sensitive to the x value especially when sampling under uniformly diffuse sky conditions Garrigues et al 2008 The x value is most important when working with canopies displaying extremely vertical or horizontal characteristics and when working under clear sky conditions where fb is less than approximately 0 4 For additional information about leaf angle distribution the reader is referred to Campbell and Norman 1998 K extinction coefficient The canopy extinction coefficient K describes how much radiation is absorbed by the canopy at a given solar zenith angle and canopy leaf angle distribution The concept of an extinction coefficient comes from Beer s law Eq 1 A detailed explanation of the extinction coefficient can quickly become complicated For LAI estimation it is sufficient to know that the angle of solar beam penetration interacts with leaf angle distribution to determine the probability that a photon will be intercepted by a leaf For purposes of estimating LAI K is calculated as N x tan Equation 2 K X 1 744 x 1 182 From this equation it should be obvious that
24. software programs improve Limitations Hemispherical photography has drawbacks however In spite of the fact that the images are now digitally processed user subjectivity remains a significant issue Users must select image brightness thresholds that distinguish sky pixels from vegetation pixels causing LAI values to vary from user to user or when using different image analysis algorithms Hemispherical photography also remains time consuming It takes time to acquire good quality images in the field and more time to analyze the images in the lab Also sky conditions must be uniformly overcast when the pictures are taken Hemispherical photography does not work well for short canopies like wheat and corn since the camera body lens and tripod may not physically fit under the canopy For some users instruments that measure PAR offer a shortcut Some models use LAI values to estimate PAR In this case the PAR instrument can be used to directly estimate below canopy levels of PAR improving the accuracy of the model 2 2 2 Radiation Transmittance Several commercially available instruments including Decagon s LP 80 ceptometer offer an alternative to hemispherical photography They estimate LAI using the amount of light energy transmitted by a plant Canopy The idea is fairly simple a very dense canopy will absorb more light than a sparse canopy This means there must be some relationship between LAI and light interception Beer s law p
25. t alternative in cases like these Limitations One of the biggest limitations of vegetation indices is that they are unitless values and when used alone do not provide an absolute measure of LAI If you don t need absolute LAI values the vegetation index value can be used as a proxy for LAI If you need absolute values of LAI however you will need to use another method for measuring LAI in conjunction with the vegetation index until enough co located data has been gathered to produce an empirical model This method can also be limited due to the location of sensors By nature reflectance must be measured from the top of a plant canopy which may not be feasible in some tall canopies 3 Using the LP 80 ceptometer Decagon s LP 80 ceptometer uses the PAR inversion technique for calculating LAI The LP 80 uses a modified version of the canopy light transmission and scattering model developed by Norman and Jarvis 1975 Five key variables used as inputs are discussed below t ratio of transmitted and incident PAR The most influential factor for determining LAI with any PAR inversion model is the ratio of transmitted to incident PAR This ratio t is calculated using measurements of transmitted PAR near the ground surface and incident PAR above the canopy t is a relatively intuitive variable to understand When LAI is low most incident radiation is transmitted through the canopy rather than being abs
26. t of a two band enhanced vegetation index without a blue band Remote Sensing of Environment 112 3833 3845 Kucharik C J Norman J M Gower S T 1998 Measurements of branch area and adjusting leaf area index indirect measurements Agricultural and Forest Meteorology 91 69 88 Lang A R G Yueqin X 1986 Estimation of leaf area index from transmission of direct sunlight in discontinuous canopies Agricultural and Forest Meteorology 37 229 243 Norman J M Jarvis PG 1974 Photosynthesis in Sitka spruce Picea sitchensis Bong Carr Ill Measurements of canopy structure and interception of radiation Journal of Applied Ecology 12 839 878 Rouse J W Haas R H Schell J A Deering D W 1973 Monitoring vegetation systems in the Great Plains with ERTS Third ERTS Symposium NASA SP 351 309 317 Qi J Chehbouni A Huete A R Kerr Y H Sorooshian S 1994 A modified soil adjusted vegetation index Remote Sensing of Environment 48 119 126 Leaf Area Index DISTRIBUTED BY 2365 NE Hopkins Ct Pullman WA 99163 email instruments decagon com phone 1 509 332 2756 fax 509 332 5158 web www decagon com DECAGON DEVICES We measure the world
27. um sensors can also be used to measure transmitted radiation for a PAR inversion model The advantage to using PAR sensors as opposed to a purpose built handheld LAI instrument is that PAR sensors can be left in the field to continuously measure changes in PAR transmittance This may be useful when studying rapid changes in canopy LAI or when it is not feasible to visit a field site frequently enough to capture temporal variability in LAI with a handheld instrument Limitations The PAR inversion technique has a few limitations It requires measurements of both transmitted below canopy and incident above canopy PAR under identical or very similar light conditions This can be challenging in very tall forest canopies although incident PAR measurements can be made in large canopy gaps or clearings Also in extremely dense canopies PAR absorption may be nearly complete leaving little transmitted light to be measured at the bottom of a canopy This makes it difficult to distinguish changes or differences in LAI when LAI is very high Finally estimates of LAI obtained from measurements of transmitted PAR can be affected by foliage clumping Errors in LAI estimation associated with clumping can usually be alleviated by collecting numerous spatially distributed samples of transmitted PAR 2 2 3 Radiation Reflectance Another method for estimating LAI uses reflected rather than transmitted light Radiation that has been reflected from green healthy ve
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