Apparent winter CO2 uptake by a boreal forest due to decoupling

Jocher, Georg; Löfvenius, Mikaell Ottosson; Simon, G. De; Hörnlund, Thomas; Linder, Sune; Lundmark, Tomas; Marshall, John D.; Nilsson, Mats; Näsholm, Torgny; Tarvainen, Lasse; Öquist, Mats; Peichl, Matthias

doi:10.1016/j.agrformet.2016.08.002

Cited by 53 publications

(59 citation statements)

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“…Negative buoyancy forcing in combination with a slope in the vicinity of the measurement tower can serve as a clear indicator for potential below‐canopy drainage flow (Staebler & Fitzjarrald, , ). Such drainage flow has been verified for our study site for a winter period (Jocher et al, ). Generally, wind directional shear across the canopy was more pronounced during nighttime than during the day (Figure ).…”

Section: Resultssupporting

confidence: 82%

“…The annual sum of R eco based on unfiltered data must be treated with skepticism as it is influenced by the unrealistically high‐modeled GEE during spring (compare Figures a and d). The unrealistic negative R eco values (Figure d) during winter are caused by the use of gap‐filled

F_{{CO}_{2}}

here as the measured

F_{{CO}_{2}}

revealed sporadic apparent uptake during winter (Jocher et al, ). In summary, the two‐level approach minimized decoupling effects on R eco estimates.…”

Section: Resultsmentioning

confidence: 90%

“…However, 20 years after the pioneering work by Goulden et al (), who first described the potentially biasing influence of decoupling on above‐canopy derived

F_{{CO}_{2}}

, u * filtering based on single‐level flux measurements is still the most common method to ensure sufficient canopy mixing. Recently, several studies have investigated the impact of canopy coupling stages on forest NEE with additional measurements within/below the forest canopy (e.g., Alekseychik et al, ; Feigenwinter et al, ; Jocher et al, ; Misson et al, ; Oliveira et al, ; Thomas et al, ; Vickers et al, ). Each of these studies has highlighted the value of additional within‐/below‐canopy measurements for studying canopy coupling stages and the danger of potentially biased

F_{{CO}_{2}}

and consequently biased NEE estimates in their absence.…”

Section: Resultsmentioning

confidence: 99%

“…Both EC systems sampled raw data at a rate of 20 Hz. The two systems showed no systematic deviations relative to each other (Jocher et al, ).…”

Section: Methodsmentioning

confidence: 99%

“…This question arises because the air masses below and within the forest canopy can be decoupled from the air masses above the canopy due to the canopy roughness and divergence in atmospheric stratification across the canopy. It also arises due to topography surrounding the measurement tower which may induce below-canopy horizontal advection exchanging respiratory CO 2 with areas outside the EC footprint (e.g., Aubinet et al, 2005;Belcher et al, 2012;Butler et al, 2015;Feigenwinter et al, 2008Feigenwinter et al, , 2004Jocher et al, 2017;Kutsch & Kolari, 2015;Staebler & Fitzjarrald, 2004;Wang et al, 2017). During such decoupled periods, an above-canopy EC system measures only a part of the total NEE and might overestimate net CO 2 uptake from the atmosphere because of missing respiration components.…”

Section: Introductionmentioning

confidence: 99%

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Impact of Canopy Decoupling and Subcanopy Advection on the Annual Carbon Balance of a Boreal Scots Pine Forest as Derived From Eddy Covariance

et al. 2018

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Apparent net uptake of carbon dioxide (CO2) during wintertime by an ∼ 90 year old Scots pine stand in northern Sweden led us to conduct canopy decoupling and subcanopy advection investigations over an entire year. Eddy covariance (EC) measurements ran simultaneously above and within the forest canopy for that purpose. We used the correlation of above‐ and below‐canopy standard deviation of vertical wind speed (σw) as decoupling indicator. We identified 0.33 m s−1 and 0.06 m s−1 as site‐specific σw thresholds for above‐ and below‐canopy coupling during nighttime (global radiation <20 W m−2) and 0.23 m s−1 and 0.06 m s−1 as daytime (global radiation >20 W m−2) σw thresholds. Decoupling occurred in 53% of the annual nighttime and 14% of the annual daytime. The annual net ecosystem exchange (NEE), gross ecosystem exchange (GEE), and ecosystem respiration (Reco) derived via two‐level filtered EC data were −357 g C m−2, −1,138 g C m−2, and 781 g C m−2, respectively. In comparison, both single‐level friction velocity (u*) and quality filtering resulted in ~ 22% higher NEE, mainly caused by ~ 16% lower Reco. GEE remained similar among filtering regimes. Accounting for changes of CO2 storage across the canopy in the single‐level filtered data could only marginally decrease these discrepancies. Consequently, advection appears to be responsible for the major part of this divergence. We conclude that the two‐level filter is necessary to adequately address decoupling and subcanopy advection at our site, and we recommend this filter for all forested EC sites.

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Section: Resultssupporting

confidence: 82%

F_{{CO}_{2}}

here as the measured

F_{{CO}_{2}}

revealed sporadic apparent uptake during winter (Jocher et al, ). In summary, the two‐level approach minimized decoupling effects on R eco estimates.…”

Section: Resultsmentioning

confidence: 90%

“…However, 20 years after the pioneering work by Goulden et al (), who first described the potentially biasing influence of decoupling on above‐canopy derived

F_{{CO}_{2}}

F_{{CO}_{2}}

and consequently biased NEE estimates in their absence.…”

Section: Resultsmentioning

confidence: 99%

“…Both EC systems sampled raw data at a rate of 20 Hz. The two systems showed no systematic deviations relative to each other (Jocher et al, ).…”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Impact of Canopy Decoupling and Subcanopy Advection on the Annual Carbon Balance of a Boreal Scots Pine Forest as Derived From Eddy Covariance

et al. 2018

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Thirty years of forest productivity in a mountainous landscape: The Yin and Yang of topography

Perry,

Oetter

2024

Ecosphere

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We measured light‐related patterns of primary productivity within a topographically complex Oregon watershed over a 30‐year period. Second‐growth conifer densities were experimentally altered in 1981. Plots receiving at least 3434 MJ m−2 over a 6‐month growing season averaged 40% greater aboveground net primary productivity (ANPP) than those receiving less light (p = 0.000). Unthinned stands potentially built enough LAI to compensate for low light, but risked mortality that exceeded resilience. The two light levels acted as basins of attraction for other physiological and ecological processes, including size–density relationships and limiting foliar nutrients. Initial (1981) LAI and the irradiation step (above or below 3434 MJ m−2) explained 60% of variation in a 30‐year ANPP. Irradiation within each light group did not affect ANPP. At high irradiation, foliar N/Ca and slope steepness (both negative) explained 58% of the variation in residuals from the initial models, while at low irradiation on north, east, and west aspects, 83% of residual variation was explained by foliar Mg (+), understory cover (+), and 30‐year mortality (−). Light use efficiency (LUE) of fully stocked stands correlated with LAI and foliar N/K. Results suggest that understory influence on tree foliar N (+ or −) enhances ANPP by regulating critical nutrient ratios. Mortality reduced or eliminated differences among thinning levels, which did not vary at low light and only between unthinned and heavily thinned at high light. Values associated with relatively open forests (biodiversity, resilience) may be attained without large sacrifice of long‐term carbon sinks. In our study, light interacts with topography to produce nonlinear dynamics in which small changes in irradiation can have large consequences. Reduced sunlight has been suggested as a geoengineering option to combat global warming. Ecological changes out of proportion to lowered irradiation are a distinct possibility, including sharp reductions in terrestrial carbon sinks.

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Reconciling carbon‐cycle processes from ecosystem to global scales

Ballantyne

Liu

Anderegg

et al. 2021

Frontiers in Ecol & Environ

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C arbon (C) is the building block of life. Global photosynthesis generates approximately 100 terawatts (TW) of energy each year by converting solar radiation into stored chemical energy (Barber 2009). Photosynthesis also represents the largest global annual C flux, of ~125 petagrams (Pg; where 1 Pg equals 10 15 grams [g] and 1 Pg C is roughly equivalent to 0.47 parts per million [ppm] of CO 2), with the second greatest flux consisting of the subsequent release of CO 2 via respiration (~122 Pg C/year). Both of these fluxes are an order of magnitude greater than fossil-fuel emissions (Ballantyne et al. 2015). The atmospheric CO 2 that is fixed during photosynthesis is subsequently stored and transferred as chemical energy, which in turn fuels the metabolic reactions of most autotrophs and heterotrophs. Although C is the most common element in the terrestrial biosphere, representing approximately 50 parts per hundred (%) of all organic matter, CO 2 represents only a very small fraction of the atmosphere and is therefore measured in ppm (~415 ppm in 2020). Given the abundance of C in the terrestrial biosphere and the massive fluxes of C occurring between the biosphere and the atmosphere, it is no surprise that scientists have developed a myriad of innovative ways for measuring and simulating C-cycle processes across a range of scales in time and space. For example, chloroplast CO 2 fluxes are estimated over millimeters per second, whereas biome CO 2 fluxes may be estimated over thousands of kilometers per year. There have been many advances in C-cycle science over the past 60 years at leaf, plant, ecosystem, and global scales, but both challenges to and opportunities for scientific advancement remain. Progress is necessary, however, especially at the macrosystem scale, where human management and ecological processes are often at odds and create interesting interactions of C dynamics. One of the greatest impediments to accurate predictions of future climate is the uncertain response of the terrestrial C cycle to impending changes in temperature, precipitation, and atmospheric CO 2 concentrations (Friedlingstein et al. 2013). Even though land-surface models have become increasingly realistic in their mechanistic representation of C-cycle processes by including nutrient limitation (Thornton et al. 2007),

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Apparent winter CO2 uptake by a boreal forest due to decoupling

Cited by 53 publications

References 62 publications

Impact of Canopy Decoupling and Subcanopy Advection on the Annual Carbon Balance of a Boreal Scots Pine Forest as Derived From Eddy Covariance

Impact of Canopy Decoupling and Subcanopy Advection on the Annual Carbon Balance of a Boreal Scots Pine Forest as Derived From Eddy Covariance

Thirty years of forest productivity in a mountainous landscape: The Yin and Yang of topography

Reconciling carbon‐cycle processes from ecosystem to global scales

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