Remko DuursmaHawkesbury Institute for the EnvironmentUniversity of Western Sydney

Water availability, but not [CO2], drives leaf area dynamics

ACEAS Phenology 2014

EucFACE• Six 'rings' of 25m diameter• 3 at ambient [CO2], 3 at ambient + 150ppm• 'Fully' instrumented• Supersite nearby • Eucalyptus tereticornis

• Importance in scaling up leaf-level processes. For example,

ET (eCO2) ET(aCO2) * Leaf level response * LAI response

• Very little known about LAI response to CO2 in closed-canopy forests

What do we expect?• LAI should go up because eCO2 enhances growth• LAI should go up because of 'water savings'• LAI should go up because the models say so

Why leaf area index?

Indirect measurement of leaf area index

Indirect methods based on light transmittance have a long history (Monsi & Saeki 1953)

Gap fraction and LAI are related via he Lambert-Beer law:

PPFDbelow = PPFDabove * e-kLAI

Where k is an extinction coefficient (lots of theory)

Transmittance (τ) is defined as PPFDbelow/PPFDabove

At EucFACE : - Measurements of PPFD below canopy with Licor PAR sensors (3 per ring), and above canopy in each ring.- Measurements of transmittance with photographic method

0 1 2 3 4 5

0.0

0.2

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1.0

LAIm2m 2

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Measurements of PPFD above and below the canopy

Sunny

Time (hours)

PP

FD

mo

lm2

s1

0 4 8 12 16 20 24

05

00

10

00

15

00

20

00

Cloudy

Time (hours)

PP

FD

mo

lm2

s1

0 4 8 12 16 20 240

20

04

00

60

08

00

Canopy transmittance

Sunny

Time (hours)

Tra

nsm

ittan

ce

PP

FD

belo

wP

PF

Dab

ove

0 4 8 12 16 20 24

0.0

0.2

0.4

0.6

0.8

1.0

Cloudy

Time (hours)

Tra

nsm

ittan

ce

PP

FD

belo

wP

PF

Dab

ove

0 4 8 12 16 20 240

.00

.20

.40

.60

.81

.0

Fraction diffuse radiation (Fdiff) (measured at CUP eddy flux site)

Sunny

Time (hours)

Fra

ctio

nd

iffus

eP

PF

D

-

0 4 8 12 16 20 24

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Cloudy

Time (hours)

Fra

ctio

nd

iffus

eP

PF

D

-

0 4 8 12 16 20 240

.00

.20

.40

.60

.81

.01

.2

We measured canopy diffuse transmittance when Fdiff > 0.98, and averaged daily when at least 3 timesteps available (1.5 hours), and within-day σ < 0.03 for all rings.

'Flat canopy' photos (canopy cover photography)

ca. 30 degrees

• Automated tresholding (blue channel)• Ca. 21 photos per ring, ca. monthly, when

cloudy• R package for image analysis (including

thresholding)

Methods comparison

One day (22 May 2013)Ca. 21 photos per ring; LAI-2200 at same points. 3 PAR sensors.

Diffuse transmittance

Sub-canopy PAR sensorsAll cloudy days (at least 1.5 hours Fdiff > 0.98)These data were then inverted to estimated total area index.

Methods comparison

Woody + infrastructure area index?

Scaling up : the importance of rainfallLAI estimated from MODIS 8day fPAR product.30,000 1km2 pixels across SE Australia

Shiva Khanal

Future work and why phenocams?

• Test prognostic LAI of ecosystem models, in response to variation inwater availability (rainfall, evaporative demand), and elevated CO2.

• Towards a more mechanistic model of leaf area dynamics

Data sources• MODIS has its limitations; a national network that can measure green-up of

vegetation would be very useful• A network of below-canopy diffuse transmittance is more difficult• Upward-looking phenocams?