Lecture 7

Introduction to light interception
and leaf area index
Measurements using
ceptometer and LiCor LAI2000
Plant Ecophysiological Measurement Techniques - BOT 6935
March 10, 2014
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.
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.
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 CO2 and produce plant
materials (quantum yield).
– Allocation of the plant materials to plants parts important to
the ecosystem.
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.
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: m2 m-2
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.
LAI - Definition
• LAI: ratio of leaf surface area per unit ground area
• Ground area = 1 m2
• Leaf Area = 1 m2
• Ground area = 1 m2
• Leaf Area = 3 m2
• LAI = 1/1 = 1 m2 m-2
• LAI = 3/1 = 3 m2 m-2
Conceptual diagram of a plant canopy with one-sided LAI=1 and LAI=3
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.
http://ldas.gsfc.nasa.gov/gld
as/GLDASlaigreen.php
LAI and Plant Production
LAI is linked to plant production
Sinclair and Gardner (1998)
Martin and Jokela (2004)
LAI and Transpiration
• The energy absorbed by canopies is also a primary
determinant of their transpiration rate.
Penman-Monteith equation
where Rn is the net radiation, G is the soil heat
flux, (es - ea) represents the vapour pressure
deficit of the air, ρa is the mean air density at
constant pressure, cp is the specific heat of the
air, ∆ represents the slope of the saturation
vapour pressure temperature relationship, γ is
the psychrometric constant, and rs and ra are the
(bulk) surface and aerodynamic resistances.
http://www.fao.org/
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 m2 leaf area per m2 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.
LAI and Transpiration
• The energy absorbed by canopies is also a primary
determinant of their transpiration rate.
Quercus petraea
Breda and Granier (1996)
LAI - Phenology
Projected LAI - Soybean
Projected LAI - Maize
All-sided LAI
Pinus elliottii
LAI - Measurement
• Direct.
• Indirect
–
–
–
–
Plant allometry
Hemispherical Photography
Radiation Reflectance
Radiation Transmittance
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
LAI - Indirect Measurement
Using litterfall (Semi-direct method)
– Use litter traps and collect foliage fall periodically (WL, 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, m2 kg-1):
• leaf (needle) area / dry weight
– LA = WL * SLA = kg * m2 kg-1
– Leaf Area Index (LAI):
– LAI = F ∙ ∑ 𝐿𝐿𝑖
(F = expansion factor)
LAI - Indirect Measurement
Plant allometry.
– Using allometric functions to estimate leaf mass (kg)
• WF = a*Db
–
–
–
–
And Specific Leaf Area (SLA, m2 kg-1)
LA = WF * SLA
Leaf Area Index (LAI):
LAI = F ∙ ∑ 𝐿𝐿𝑖
(F = expansion factor)
LAI - Indirect Measurement
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
http://www.decagon.com
Use multiband radiometers or
spectroradiometers
LAI - Indirect Measurement
Radiation Reflectance. NDVI: Normalized Difference Vegetation Index
NIR
RED
Green
NDVI =
𝑁𝑁𝑁 −𝑅𝑅𝑅
𝑁𝑁𝑁+𝑅𝑅𝑅
http://www.spacegrant.montana.edu/
LAI - Indirect Measurement
Radiation Reflectance. NDVI: Normalized difference Vegetation Index
Gamon et al. 1995
Instrument:
Spectroradiometer
www.decagon.com
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.
http://www.delta-t.co.uk/
G(θ) = exp( –K(θ)*LAI )
Rich et al. 1999
G is gap fraction,
K(θ) is the light extinction coefficient at angle θ,
θ is zenith angle.
LAI - Measurement
Radiation Transmittance
Beer-Lambert Law
CEPTOMETER
𝑃𝑃𝑃𝑖
= 1 − 𝑒 −𝑘∙𝐿𝐿𝐿
𝑃𝑃𝑃𝑜
𝐿𝐿𝐿 =
−ln(1 −
𝑘
𝑃𝑃𝑃𝑖
)
𝑃𝑃𝑃𝑜
PARi = PAR transmitted
PARo = PAR on top of canopy
k = light extinction coefficient
k = 0.5
LAI - Measurement
k (light extinction coefficient)
Depends on solar zenith angle and leaf angle distribution
LAI - Measurement
Radiation Transmittance
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.
LAI - Measurement
Radiation Transmittance
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
LAI - Measurement
Comparison of LAI measured with branch allometry and
LAI-2000
Pataki et al. 1998
LAI - Measurement
Using Ceptometer and LAI measurements to determine k
𝐿𝐿𝐿 =
−ln(1 −
𝑘
𝑃𝑃𝑃𝑖
)
𝑃𝑃𝑃𝑜
Dalla-Tea and Jokela 1991
LAI - Measurement
Relationship between LAI-PAR-Productivity
Dalla-Tea and Jokela 1991
LAI - Measurement
Using Ceptometer and single k to determine LAI
Liu et al. 1997
LAI - Measurement
Using Ceptometer and single k to determine LAI
Use k=0.5 (all-sided)
Liu et al. 1997
Gholz et al. 1991