This document discusses addressing forest canopy decoupling on a global scale. It provides background on decoupling, which occurs when there is insufficient mixing of air masses above and below the forest canopy. This can bias carbon flux measurements made above the canopy. The document outlines a global decoupling synthesis study involving over 30 forest sites. Preliminary results show decoupling occurs at all sites and is influenced by atmospheric conditions, canopy properties, and surrounding topography. Topography in particular can impact flow patterns and cause horizontal advection during decoupled periods. In conclusion, complementary below-canopy measurements are recommended to better understand decoupling and its effects on carbon flux estimates.

1. Addressing forest canopy decoupling
on a global scale
(Georg Jocher,
The Czech Academy of Sciences, Global Change Research Institute,
Brno, Czech Republic)

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Schedule:
Background
State of the art
Open questions
Global decoupling initiative and its aims
Preliminary results
Conclusions and outlook

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Background
What is decoupling?
Decoupling: insufficient
mixing of below and above
canopy air masses.
Above canopy EC
measurements are not fully
representative for the whole
ecosystem of interest in
case of decoupling.
CO2 fluxes derived above
canopy might be biased in
case of decoupling and
below-canopy advection,
predominantly due to
missing respiration
components in the above
canopy derived fluxes.
Thomas et al., 2013

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Background
Historical review
adopted from Foken & Meixner, AMS, 2021
Year Progress Authors
1936 Mean temperature profiles Siegel
1983 Sweeps and ejection (quadrant analysis) Shaw et al.
1997 Sweeps and ejection (wavelet analysis) Katul et al.
2007 5 class coupling scheme based on wavelet analysis Thomas and Foken
2012 3 class coupling scheme for reactive trace gases Foken, Meixner et al.
2013 Simplified two-level coupling approach based on σw
above and below canopy
Thomas et al.
2020 3 class coupling scheme based on σw and N Peltola, Lapo and Thomas

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State of the art
Standard treatment of EC fluxes:
• It is not always possible to determine an u*-threshold.
• As comparison studies between u*-filtered data and explicitly for decoupling filtered data
(based on multilevel measurements) have shown, the u*-filtering is rarely enough to ensure
sufficient mixing across the canopy.
But:
• Filtering for quality (stationarity and turbulence development test)
• Filtering for an u* - threshold which shall ensure sufficient turbulent mixing across the canopy

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State of the art
Example: Scots pine forest in Northern Sweden, LAI ~ 3 m2 m-2
Jocher et al., 2018

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How representative are our standard EC measurements above forest canopies for the
ecosystems of interest?
Open questions
More specifically, how does decoupling and its potential biasing influence on
above canopy derived EC fluxes depend on:
• Atmospheric conditions
• Canopy properties
• Tower surrounding topography?

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Current initiative and its aims
Global decoupling synthesis study
Bringing as many as possible forest sites together where decoupling is studied to derive global relations
between decoupling and its influence on above canopy derived EC fluxes on the one side and
• Atmospheric properties
• Canopy characteristics
• Tower surrounding topography
on the other side.
Participants:
Approx. 30 sites from Spain, Belgium, Germany, Austria, Czech Republic, Sweden,
Switzerland, France, Italy, Finland, Israel, Indonesia, USA, Australia, Japan

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Current initiative and its aims
Global decoupling synthesis study: methods
Assessment of atmospheric impacts: turbulence development above canopy (σw)
Assessment of canopy property impacts: canopy height, leaf area index
Assessment of tower surrounding impacts: elevation differences to tower location within circles of 200
m and 1000 m radius around the measurement tower
Assessment of decoupling itself: evaluating the correlation of σw between above and below canopy air
masses. If this correlation is linear, full coupling across the canopy is assumed (Thomas et al., 2013).

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Global decoupling synthesis study: decoupling evaluation
Current initiative and its aims
• Different regimes for coupled and decoupled
conditions occur.
• Full coupling can be assumed when the
correlation of σw between above and below
canopy air masses is linear.
• Thresholds can be determined by
segmented linear regression and change
point detection.
• Above canopy derived EC fluxes can be
filtered with these two thresholds
afterwards.
Jocher et al., 2017

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Preliminary results: atmospheric properties
Harvard Forest, 42.5 N, 72.2 W, mixed deciduous, LAI 3.5 m2 m-2, canopy height 24 m, strongest winds in winter.
The strong winds in winter foster the maxima in σw during winter, both above and below canopy.

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Preliminary results: atmospheric properties
Rosinedal, 64.1 N, 19.8 E, conifer, LAI 2.2 m2 m-2, canopy height 19 m, strongest winds in winter.
Though winds are strongest in winter, the high radiative input yields σw maxima during summer.

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Preliminary results: canopy properties
• Harvard Forest, USA
• Mixed deciduous
• Canopy height ~ 24 m
• LAI ~ 3.5 m2 m-2

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Preliminary results: canopy properties
Jocher et al., 2020
• Bílý Kříž, Czech Republic
• Spruce monoculture
• Canopy height ~ 18 m
• LAI ~ 9 m2 m-2

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Preliminary results: canopy properties
• Mieming, Austria
• Pine
• Canopy height ~ 12 m
• LAI ~ 1.5 m2 m-2

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Preliminary results: topography
Median: 10 m
Mean: 16 m
Median: 70 m
Mean: 78 m
Decoupling occurs at each forest site. If it coincides with sloping terrain, above canopy derived
EC results can get biased due to horizontal advection below canopy during decoupled periods.

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Preliminary results: topography
Example: Scots pine forest in Northern Sweden, LAI ~ 3 m2 m-2
Jocher et al., 2018

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Conclusions and outlook
• Decoupling occurs at each forest site.
• Topography may have an impact on flow patterns at almost each EC site.
• The potentially biasing influence of decoupling and advective flows on above canopy derived carbon
fluxes may be relevant at the majority of forest EC sites.
=> Complementary 3-D sonic measurements below canopy can address these issues sufficiently.
Recommendation for each forest EC site, and for measurement networks like ICOS:
Complementary 3-D sonic measurements below canopy as a standard. Easy to set up, the benefits
outweigh the costs and efforts by far.
Benefit: more accurate forest carbon exchange estimates. Consequently, a better picture of
the current state of our forests and a more reliable data base for model input and predicting
the state of our forests in the future.

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References
• G. Jocher, M. Ottosson Löfvenius, G. De Simon, T. Hörnlund, S. Linder, T. Lundmark, J. Marshall, M.B.
Nilsson, T. Näsholm, L. Tarvainen, M. Öquist, M. Peichl. Apparent winter CO2 uptake by a boreal
forest due to decoupling. Agric. For. Meteorol., 232 (2017)
• G. Jocher, J. Marshall, M.B. Nilsson, S. Linder, G. De Simon, T. Hörnlund, T. Lundmark, T. Näsholm, M.
Ottosson Löfvenius, L. Tarvainen, G. Wallin, M. Peichl. Impact of canopy decoupling and
subcanopy advection on the annual carbon balance of a boreal Scots pine forest as derived
from eddy covariance. J. Geophys. Res. Biogeosci., 123 (2018)
• C. Thomas, J.G. Martin, B.E. Law, K. Davis. Toward biologically meaningful net carbon exchange
estimates for tall, dense canopies: Multi-level eddy covariance observations and canopy
coupling regimes in a mature Douglas-fir forest in Oregon. Agric. For. Meteorol., 173 (2013)
• G. Jocher, M. Fischer, L. Šigut, M. Pavelka, P. Sedlák, G. Katul. Assessing decoupling of above and
below canopy air masses at a Norway spruce stand in complex terrain. Agric. For. Meteorol., 294
(2020)

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Thanks for your attention!