measurements using ceptometer and licor lai- 2000 · introduction to light interception and leaf...
TRANSCRIPT
Introduction to light interception and leaf area index
Measurements using
ceptometer and LiCor LAI-2000
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
• Ground area = 1 m2
• Leaf Area = 1 m2 • LAI = 1/1 = 1 m2 m-2
• Ground area = 1 m2
• Leaf Area = 3 m2 • LAI = 3/1 = 3 m2 m-2
• LAI: ratio of leaf surface area per unit ground area
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/gldas/GLDASlaigreen.php
LAI and Plant Production
Sinclair and Gardner (1998)
LAI is linked to plant production
Martin and Jokela (2004)
LAI and Transpiration • The energy absorbed by canopies is also a primary
determinant of their transpiration rate.
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.
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.
http://www.fao.org/
Penman-Monteith equation
LAI and Transpiration • The energy absorbed by canopies is also a primary
determinant of their transpiration rate.
Breda and Granier (1996)
Quercus petraea
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.
LICOR LI-3100 Leaf Area Meter
CI-203 Handheld Laser 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 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
http://www.decagon.com
LAI - Indirect Measurement Radiation Reflectance. NDVI: Normalized Difference Vegetation Index
http://www.spacegrant.montana.edu/ NDVI = 𝑁𝑁𝑁 −𝑁𝑅𝑅𝑁𝑁𝑁+𝑁𝑅𝑅
Green
NIR
RED
LAI - Indirect Measurement
www.decagon.com
Radiation Reflectance. NDVI: Normalized difference Vegetation Index
Gamon et al. 1995
Instrument: Spectroradiometer
LAI - Indirect Measurement
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 non-vegetated pixels.
http://www.delta-t.co.uk/
Radiation Transmittance
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
LAI - Measurement
𝑃𝐿𝑃𝑖𝑃𝐿𝑃𝑜
= 1 − 𝑒−𝑘∙𝐿𝐿𝑁
𝐿𝐿𝐿 =−ln (1 − 𝑃𝐿𝑃𝑖
𝑃𝐿𝑃𝑜)
𝑘
Radiation Transmittance Beer-Lambert Law
PARi = PAR transmitted PARo = PAR on top of canopy k = light extinction coefficient
CEPTOMETER
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 Using Ceptometer and LAI measurements to determine k
Dalla-Tea and Jokela 1991
𝐿𝐿𝐿 =−ln (1 − 𝑃𝐿𝑃𝑖
𝑃𝐿𝑃𝑜)
𝑘