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 - Phenology All-sided LAI Pinus elliottii

Projected LAI - Soybean

Projected LAI - Maize

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

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 Comparison of LAI measured with branch allometry and LAI-2000

Pataki et al. 1998

LAI - Measurement Using Ceptometer and LAI measurements to determine k

Dalla-Tea and Jokela 1991

𝐿𝐿𝐿 =−ln (1 − 𝑃𝐿𝑃𝑖

𝑃𝐿𝑃𝑜)

𝑘

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

Liu et al. 1997

Use k=0.5 (all-sided)

Gholz et al. 1991