Introduction to light interception and leaf area index Measurements using ceptometer and LiCor LAI- 2000

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1 Introduction to light interception and leaf area index Measurements using ceptometer and LiCor LAI Plant Ecophysiological Measurement Techniques - BOT 6935 March 10, 2014

2 Leaf Area - Importance Canopies are formed by the crowns of plants (trees). The architecture of a canopy is described by the vertical and horizontal arrangement of foliage through the canopy space. The architecture of a canopy and canopy leaf area determine how much PAR is intercepted by a canopy, and hence the photosynthetic production.

3 Leaf Area - Importance Both agricultural and natural ecosystems collect solar energy over extended periods and store it as chemical energy. The chemical energy is stored in carbohydrates, proteins and lipids, which are about 95% of total plant dry mass. Leaf area is a major determinant of photosynthesis in forests and crops and, hence, the measurement of leaf area is important is assessing growth potential.

4 Leaf Area - Importance In natural and plant production ecosystems, PAR interception and its use to form harvestable plant mass can be described by 3 processes. Daily interception of PAR (dependent on area of leaves). Efficiency to use PAR to fix CO 2 and produce plant materials (quantum yield). Allocation of the plant materials to plants parts important to the ecosystem.

5 Leaf Area and PAR interception The fraction of PAR intercepted by leaf canopy is dependent on the extent of leaf surface area. Canopy leaf area depends on the number and size of leaves (both influenced by environment and plant genetics). Leaf area is expressed as leaf area index (LAI). LAI is an index of canopy density.

6 LAI - Definition Definition : Leaf Area Index (LAI) is the ratio of green leaf surface area per unit ground area. Leaf area per unit horizontal land below. Units for LAI: m 2 m -2

7 LAI has different measures All-sided LAI or total LAI: Based on total outside area of the leaves (surface area), taking leaf shape into account. One-sided LAI: (usually half of the total LAI) Used as represents the gas exchange potential. Projected LAI: The area of horizontal shadow that would be cast beneath a horizontal leaf from a light at infinite distance directly above it. Common in remote sensing applications as represents the maximum leaf area that would be seen by sensors from overhead. Silhouette LAI: Projected area of leaves inclined to the horizontal. Useful for modelling effects of light penetration through a canopy and for remote sensing.

8 LAI - Definition LAI: ratio of leaf surface area per unit ground area Ground area = 1 m 2 Leaf Area = 1 m 2 LAI = 1/1 = 1 m 2 m -2 Ground area = 1 m 2 Leaf Area = 3 m 2 LAI = 3/1 = 3 m 2 m -2 Conceptual diagram of a plant canopy with one-sided LAI=1 and LAI=3

9 as/gldaslaigreen.php LAI - Variation 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 shrub lands typically have LAI values between 3 and 6.

10 LAI and Plant Production LAI is linked to plant production Sinclair and Gardner (1998) Martin and Jokela (2004)

11 LAI and Transpiration The energy absorbed by canopies is also a primary determinant of their transpiration rate. Penman-Monteith equation where R n is the net radiation, G is the soil heat flux, (e s - e a ) represents the vapour pressure deficit of the air, ρ a is the mean air density at constant pressure, c p is the specific heat of the air, represents the slope of the saturation vapour pressure temperature relationship, γ is the psychrometric constant, and r s and r a are the (bulk) surface and aerodynamic resistances. The Leaf Area Index (LAI), a dimensionless quantity, is the leaf area (upper side only) per unit area of soil below it. It is expressed as m 2 leaf area per m 2 ground area. The active LAI is the index of the leaf area that actively contributes to the surface heat and vapor transfer. It is generally the upper, sunlit portion of a dense canopy.

12 LAI and Transpiration The energy absorbed by canopies is also a primary determinant of their transpiration rate. Quercus petraea Breda and Granier (1996)

13 LAI - Phenology Projected LAI - Soybean All-sided LAI Pinus elliottii Projected LAI - Maize

14 LAI - Measurement Direct. Indirect Plant allometry Hemispherical Photography Radiation Reflectance Radiation Transmittance

15 LAI Direct Measurement Harvesting all the leaves from a plot and measuring the area of each leaf. CI-203 Handheld Laser Leaf Area Meter LICOR LI-3100 Leaf Area Meter CI-202L Portable Laser Leaf Area Meter

16 LAI - Indirect Measurement Using litterfall (Semi-direct method) Use litter traps and collect foliage fall periodically (W L, kg) For deciduous species : The leaf area that they carry during their vegetation period is equal to the area of the leaf litter they loose in a year (phenological year: March to February) For evergreen species: Have to account for foliage retention (e.g. loblolly pine: 2 years) Determine Specific Leaf Area (SLA, m 2 kg -1 ): leaf (needle) area / dry weight LA = W L * SLA = kg * m 2 kg -1 Leaf Area Index (LAI): LAI = F LL i (F = expansion factor)

17 Plant allometry. LAI - Indirect Measurement Using allometric functions to estimate leaf mass (kg) W F = a*d b And Specific Leaf Area (SLA, m 2 kg -1 ) LA = W F * SLA Leaf Area Index (LAI): LAI = F LL i (F = expansion factor)

18 LAI - Indirect Measurement

19 LAI - Indirect Measurement Radiation Reflectance Radiation that has been reflected from green, healthy vegetation has a very distinct spectrum. High reflectance in NIR Low reflectance in PAR Determine spectral vegetation indices: NDVI: Normalized Difference Vegetation Index RVI: Simple Ratio Vegetation Index TSAVI: Transformed soil-adjusted vegetation index PVI: Perpendicular Vegetation Index Use multiband radiometers or spectroradiometers

20 LAI - Indirect Measurement Radiation Reflectance. NDVI: Normalized Difference Vegetation Index NIR Green RED NDVI = NNN RRR NNN+RRR

21 LAI - Indirect Measurement Radiation Reflectance. NDVI: Normalized difference Vegetation Index Gamon et al Instrument: Spectroradiometer

22 LAI - Indirect Measurement Radiation Transmittance Hemispherical Photography Taking photographs with fish-eye lens. Compute gap fraction as function of sky direction, and compute desired canopy geometry and/or solar radiation indices Use specialized software to analyze images and differentiate between vegetated and nonvegetated pixels. G(θ) = exp( K(θ)*LAI ) G is gap fraction, K(θ) is the light extinction coefficient at angle θ, θ is zenith angle. Rich et al. 1999

23 LAI - Measurement Radiation Transmittance Beer-Lambert Law CEPTOMETER PPP i PPP o = 1 e k LLL LLL = ln (1 PPP i PPP o ) k k = 0.5 PAR i = PAR transmitted PAR o = PAR on top of canopy k = light extinction coefficient

24 LAI - Measurement k (light extinction coefficient) Depends on solar zenith angle and leaf angle distribution

25 Radiation Transmittance LAI - Measurement LAI-2000 (new version: LAI-2200C) Measures de attenuation of diffuse sky radiation at 5 zenith angles simultaneously. Foliage orientation is determined with measuring attenuation at several angles from the zenith. Fisheye lens with hemispheric field-of-view. Five silicon detectors arranged in concentric rings. Measures diffuse radiation in five distinct angular bands about the zenith. A reference reading is made above the canopy, followed by one or more below canopy readings.

26 Radiation Transmittance LAI - Measurement LAI-2000 (new version: LAI-2200C) The light sensor includes a filter to limit the spectrum of received radiation to <490 nm, minimizing the effect of light scattered by foliage. Use of this device generally requires the sun to be obscured, since directly illuminated foliage will scatter more light in the canopy than can be accounted for by the above-canopy reference reading, thus reducing apparent LAI values by 10-50%. Assumptions: The foliage is black (do not include reflection or transmission) The foliage is randomly distributed The foliage elements are small compared to the area of view (distance from the sensor to the nearest leaf should be at least 4 times the leaf width) The foliage is azimuthally randomly distributed

27 LAI - Measurement Comparison of LAI measured with branch allometry and LAI-2000 Pataki et al. 1998

28 LAI - Measurement Using Ceptometer and LAI measurements to determine k LLL = ln (1 PPP i PPP o ) k Dalla-Tea and Jokela 1991

29 LAI - Measurement Relationship between LAI-PAR-Productivity Dalla-Tea and Jokela 1991

30 LAI - Measurement Using Ceptometer and single k to determine LAI Liu et al. 1997

31 LAI - Measurement Using Ceptometer and single k to determine LAI Use k=0.5 (all-sided) Liu et al Gholz et al. 1991