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Forest floor fluxes drive differences in the carbon balance of contrasting boreal forest stands

Chi, Jinshu ; Zhao, Peng ; Klosterhalfen, Anne ; Jocher, Georg ; Kljun, Natascha LU orcid ; Nilsson, Mats B. and Peichl, Matthias (2021) In Agricultural and Forest Meteorology 306.
Abstract

The forest floor provides an important interface of soil-atmosphere CO2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO2 exchange (NFFE) and the entire net ecosystem CO2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their... (More)

The forest floor provides an important interface of soil-atmosphere CO2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO2 exchange (NFFE) and the entire net ecosystem CO2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO2 fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (Rff) was higher and photosynthesis (GPPff) was lower, leading to a 1.4 times stronger net CO2 source compared to the pine stand. We further found that drought enhanced Rff more than GPPff, leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (Reco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting Reco, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO2 fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.

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author
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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Boreal forest, CO fluxes, Eddy covariance, Forest floor, Understory vegetation
in
Agricultural and Forest Meteorology
volume
306
article number
108454
pages
13 pages
publisher
Elsevier
external identifiers
  • scopus:85110459687
ISSN
0168-1923
DOI
10.1016/j.agrformet.2021.108454
language
English
LU publication?
yes
additional info
Funding Information: The study was funded by the Knut and Alice Wallenberg Foundation (grant no. 2015.0047). The study sites Svartberget and Rosinedalsheden are part of the Swedish Infrastructure for Ecosystem Science (SITES). Svartberget is also part of the Swedish Integrated Carbon Observation System (ICOS-Sweden) research infrastructure. Financial support from the Swedish Research Council and contributing research institutes to both SITES and ICOS-Sweden are acknowledged. The lead author J.C. gratefully acknowledges the financial support from the Kempe Foundation (grant no. SMK-1743). We would like to thank Per Marklund, Paul Smith, and Giuseppe De Simon from the Svartberget Field Research Station (SLU Unit for Field-based Forest Research)(for technical and logistic support. Funding Information: The study was funded by the Knut and Alice Wallenberg Foundation (grant no. 2015.0047). The study sites Svartberget and Rosinedalsheden are part of the Swedish Infrastructure for Ecosystem Science (SITES). Svartberget is also part of the Swedish Integrated Carbon Observation System (ICOS-Sweden) research infrastructure. Financial support from the Swedish Research Council and contributing research institutes to both SITES and ICOS-Sweden are acknowledged. The lead author J.C. gratefully acknowledges the financial support from the Kempe Foundation (grant no. SMK-1743). We would like to thank Per Marklund, Paul Smith, and Giuseppe De Simon from the Svartberget Field Research Station (SLU Unit for Field-based Forest Research)( for technical and logistic support. Publisher Copyright: © 2021 Copyright: Copyright 2021 Elsevier B.V., All rights reserved.
id
019bea7a-9131-4782-bc12-d594eb5303a0
date added to LUP
2021-08-12 11:10:49
date last changed
2022-04-27 03:04:01
@article{019bea7a-9131-4782-bc12-d594eb5303a0,
  abstract     = {{<p>The forest floor provides an important interface of soil-atmosphere CO<sub>2</sub> exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO<sub>2</sub> exchange (NFFE) and the entire net ecosystem CO<sub>2</sub> exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO<sub>2</sub> fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (R<sub>ff</sub>) was higher and photosynthesis (GPP<sub>ff</sub>) was lower, leading to a 1.4 times stronger net CO<sub>2</sub> source compared to the pine stand. We further found that drought enhanced R<sub>ff</sub> more than GPP<sub>ff</sub>, leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (R<sub>eco</sub>) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting R<sub>eco</sub>, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO<sub>2</sub> fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.</p>}},
  author       = {{Chi, Jinshu and Zhao, Peng and Klosterhalfen, Anne and Jocher, Georg and Kljun, Natascha and Nilsson, Mats B. and Peichl, Matthias}},
  issn         = {{0168-1923}},
  keywords     = {{Boreal forest; CO fluxes; Eddy covariance; Forest floor; Understory vegetation}},
  language     = {{eng}},
  month        = {{08}},
  publisher    = {{Elsevier}},
  series       = {{Agricultural and Forest Meteorology}},
  title        = {{Forest floor fluxes drive differences in the carbon balance of contrasting boreal forest stands}},
  url          = {{http://dx.doi.org/10.1016/j.agrformet.2021.108454}},
  doi          = {{10.1016/j.agrformet.2021.108454}},
  volume       = {{306}},
  year         = {{2021}},
}