Jocher, Georg: Addressing forest canopy decoupling on a global scale
Download as PPTX, PDF0 likes35 views
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.
Jocher, Georg: Addressing forest canopy decoupling on a global scale
- 1. Addressing forest canopy decoupling on a global scale (Georg Jocher, The Czech Academy of Sciences, Global Change Research Institute, Brno, Czech Republic)
- 2. Global Change Research Institute CAS Schedule: Background State of the art Open questions Global decoupling initiative and its aims Preliminary results Conclusions and outlook
- 3. Global Change Research Institute CAS 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
- 4. Global Change Research Institute CAS 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
- 5. Global Change Research Institute CAS 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
- 6. Global Change Research Institute CAS State of the art Example: Scots pine forest in Northern Sweden, LAI ~ 3 m2 m-2 Jocher et al., 2018
- 7. Global Change Research Institute CAS 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?
- 8. Global Change Research Institute CAS 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
- 9. Global Change Research Institute CAS 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).
- 10. Global Change Research Institute CAS 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
- 11. Global Change Research Institute CAS 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.
- 12. Global Change Research Institute CAS 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.
- 13. Global Change Research Institute CAS Preliminary results: canopy properties • Harvard Forest, USA • Mixed deciduous • Canopy height ~ 24 m • LAI ~ 3.5 m2 m-2
- 14. Global Change Research Institute CAS Preliminary results: canopy properties Jocher et al., 2020 • Bílý Kříž, Czech Republic • Spruce monoculture • Canopy height ~ 18 m • LAI ~ 9 m2 m-2
- 15. Global Change Research Institute CAS Preliminary results: canopy properties • Mieming, Austria • Pine • Canopy height ~ 12 m • LAI ~ 1.5 m2 m-2
- 16. Global Change Research Institute CAS 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.
- 17. Global Change Research Institute CAS Preliminary results: topography Example: Scots pine forest in Northern Sweden, LAI ~ 3 m2 m-2 Jocher et al., 2018
- 18. Global Change Research Institute CAS 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.
- 19. Global Change Research Institute CAS 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)
- 20. Global Change Research Institute CAS Thanks for your attention!