Duursma ACEAS Phenocams 2014

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This document discusses methods for indirectly measuring leaf area index (LAI) at the EucFACE research site using measurements of photosynthetically active photon flux density (PPFD) above and below the forest canopy. On cloudy days when diffuse light dominates, canopy transmittance measurements can be inverted to estimate total LAI. Comparing different indirect LAI measurement methods shows agreement. Analysis of long-term MODIS data across southeast Australia indicates that water availability, not elevated CO2, is the main driver of LAI dynamics at large spatial scales. Future work aims to test ecosystem models of LAI response to rainfall and CO2 using phenocam imagery for continuous monitoring of vegetation green-up.

Education

Remko Duursma
Hawkesbury Institute for the Environment
University 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?

$Indirect measurement of leaf area index Indirect methods based on light transmittance have a long history (Monsi & Saeki 1953) Gap fraction and LAI are related via he Lambert-Beer law: PPFDbelow = PPFDabove * e-kLAI Where k is an extinction coefficient (lots of theory) Transmittance (τ) is defined as PPFDbelow/PPFDabove At EucFACE : - Measurements of PPFD below canopy with Licor PAR sensors (3 per ring), and above canopy in each ring. - Measurements of transmittance with photographic method 0 1 2 3 4 5 0.0 0.2 0.4 0.6 0.8 1.0 LAI m 2 m 2 -$

Measurements of PPFD above and below the canopy
Sunny
Time (hours)
PPFDmolm
2
s
1
0 4 8 12 16 20 24
0500100015002000
Cloudy
Time (hours)
PPFDmolm
2
s
1
0 4 8 12 16 20 240200400600800

Canopy transmittance
Sunny
Time (hours)
TransmittancePPFDbelowPPFDabove
0 4 8 12 16 20 24
0.00.20.40.60.81.0
Cloudy
Time (hours)
TransmittancePPFDbelowPPFDabove
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)
FractiondiffusePPFD-
0 4 8 12 16 20 24
0.00.20.40.60.81.01.2
Cloudy
Time (hours)
FractiondiffusePPFD-
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 sensors
All cloudy days (at least 1.5 hours Fdiff > 0.98)
These data were then inverted to estimated total area index.

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

Future work and why phenocams?
• Test prognostic LAI of ecosystem models, in response to variation in
water 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?

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Greg Smestad, Leonardo Micheli, Thomas Germer, and Eduardo Fernández presented research on characterizing the optical effects of soiling on PV glass and modules. They measured the transmission of glass coupons exposed outdoors at multiple locations over 8 weeks and found soiling reduced transmission more at shorter wavelengths. Particle area coverage on the coupons correlated linearly with reduced hemispherical transmittance. Angular measurements showed soiling impacts transmission more for direct light than hemispherical. The research aims to better understand how soiling impacts PV performance globally.

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Nist talk greg_smestad_120617

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