Comparing estimates of gross primary production in a mature Norway spruce plantation in hemiboreal Sweden

Rütting, Tobias; Finnander Linderson, Maj-Lena; Weslien, Per; Tarvainen, Lasse; Marshall, John D.

doi:10.5194/egusphere-2026-5424

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https://doi.org/10.5194/egusphere-2026-5424

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16 Sep 2026

| 16 Sep 2026

Comparing estimates of gross primary production in a mature Norway spruce plantation in hemiboreal Sweden

Tobias Rütting, Maj-Lena Finnander Linderson, Per Weslien, Lasse Tarvainen, and John D. Marshall

Abstract. Forest plantations can sequester large amounts of carbon dioxide from the atmosphere while producing commercial wood products. Quantification of carbon fluxes usually depends on the eddy covariance (EC) method, which may be biased, especially in the presence of a dense canopy. We aimed to test estimates of canopy photosynthesis, or gross primary production (GPP), derived from EC against an independent method based on transpiration (xylem sap-flux) and isotopic (tree ring δ¹³C) estimates of photosynthetic water-use efficiency. The addition of tree-ring δ¹³C to existing sap-flux data made it possible to infer GPP in the past, including the drought year of 2018 and the years surrounding it. Previously published EC based estimates at our research site, the Central Forest at Skogaryd Research Catchment in south-western Sweden, had been unable to detect a response of annual GPP to the 2018 drought. A closer look at the daily EC data revealed a decrease of about 13 % over the drought period. However, using the sap-flux and isotopes, we found a decrease in GPP of 31 %, which continued for several weeks after the drought. The loss in GPP occurred primarily because water stress reduced sap flux and it occurred despite an increase in water-use efficiency. Stem biomass growth decreased in 2018, but the ratio of stem biomass growth to GPP (17 %) was within the range of published values. The discrepancies highlight method differences during the severe drought and during the spring of some years, but the long periods of agreement tend to confirm the accuracy of both methods over most of the study period.

Received: 09 Sep 2026 – Discussion started: 16 Sep 2026

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Tobias Rütting, Maj-Lena Finnander Linderson, Per Weslien, Lasse Tarvainen, and John D. Marshall

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-5424', Tamir Klein, 29 Sep 2026
Peer review for the manuscript:
Comparing estimates of gross primary production in a mature Norway spruce plantation in hemiboreal Sweden
By Rutting et al.

The manuscript under consideration presents a five-year study of spruce sap flow (SF) and C uptake, including GPP calculations from EC and SF data. The authors claim general agreement between the two independent methods, with discrepancy during a specific drought event.

This is an important paper relying on a good dataset with rich measurements. The fact that measurements were made in a commercial plantation for wood is another advantage, because we need more information about the carbon sequestration benefits of these landscapes. Finally, the inclusion of the drought event in 2018 adds more interest and implications. The study is nicely presented, however more information and discussion are needed to make the full potential of the data used here.

Specific comments
Considering the inter-method differences in the results, the reader is left wondering about the ground truth? Perhaps a third method is required, e.g., SIF and/or upscaling of leaf/branch chamber measurements?

Looking at Fig. 5, I understand the uniqueness of the 2018 drought event. However, it seems that the EC-SF discrepancy was not too large during this event. Rather, SF-based GPP was consistently higher than EC-based GPP during many measurement days. This might suggest a general calibration issue

Stem biomass growth decreased under drought. This is interesting and agrees with measurements in another forest, showing that growth shifted more belowground under drought (Rog et al. 2024)

Stem biomass to GPP ratio is an important parameter. Your value of 17% seems, however, on the very high side considering earlier studies. See Litton et al. (2007) and Klein and Hoch (2015)

What could be the reasons for method difference during drought and some spring times? Please expand this discussion

For example, it is important to understand forest structure… how about understory plants? Other tree species? You mention this but I couldn’t find the relevant information or analysis

Were there visible, or delayed, drought effects following the 2018 event? This could be important for the interpretation. Looking at 2019 curves, it seems that the 2018 drought effects did not carry on to the next year. Is it?

Can you provide graphical information on the meteorological conditions during 2015-2019? Was 2018 drier in general?

L10 How many trees were monitored for SF? And for d13C? were they the same trees? And were they covered within the fetch area of the flux tower?

L15 What is meant by “a close look”? Was the EC data analysis modified? And if so, how?

L15-16 How id the drought period defined?

L18-19 The phrasing of this sentence is strange. The reader expects something like “Stem biomass growth decreased in 2018, but the ratio of stem biomass growth to GPP maintained at 17%, which was within the range of published values”. Please revise in this way if true, or else if not.

L168 Was day 70 the snowmelt day?

L185 Was iWUE measured at the annual resolution? We know that it changes over the growth season. Is it possible to use the annual growth rings for intra-annual resolution? For example, scaling by stem growth dynamics?

References cited

Rog, I., Hilman, B., Fox, H., Yalin, D., Qubaja, R., & Klein, T. (2024). Increased belowground tree carbon allocation in a mature mixed forest in a dry versus a wet year. Global Change Biology, 30(2), e17172.

Litton, C. M., Raich, J. W., & Ryan, M. G. (2007). Carbon allocation in forest ecosystems. Global Change Biology, 13(10), 2089-2109.

Klein, T., & Hoch, G. (2015). Tree carbon allocation dynamics determined using a carbon mass balance approach. New Phytologist, 205(1), 147-159.
Citation: https://doi.org/10.5194/egusphere-2026-5424-RC1
RC2:
'Comment on egusphere-2026-5424', Anonymous Referee #2, 29 Oct 2026
This is a valuable manuscript with a rare dataset comparing long-term sapflux data + deltaC13 annual ring (IsoSF) vs EC in the same stand. These sorts of comparisons open up questions and may lead to a better understanding of the real effects of droughts on forests and the reliability of EC estimates for modelling potential climate change effects. Methodologically, this paper shows novelty in the inclusion of the particular drought event of 2018 and the use of whole-ring deltaC13 estimates combined with continuous sapflux measurements. I consider the manuscript publishable after some revisions. Before accepting the manuscript, I would suggest:
To incorporate into the introduction and discussion sections that both methods have their own biases and may not validate each other.

To elaborate more on the fact that the daily estimate of IsoSF, also in 2018, seems highly driven by sapflow estimates rather than by the isotope

To review and provide more details on scaling from individual tree to stand estimates, in particular as scaling should consider sapwood area (conducting tissue).

To review some units and numbers throughout the manuscript.

Specific comments (including the major key points above)
L20-21 Long-term agreement of both methods is good, and they may agree, but that does not prove that they are not biased. Maybe you can elaborate more cautiously on this necessary but not sufficient condition. Alternatively, you could consider including another independent measurement to contrast (stem growth/NPP).
The daily IsoSF is not isotopic and the signal depends on the deltaC13 of an annual ring (a single value); this means that the daily variation of GPP by IsoSF depends exclusively on sapflow and VPD (Eqn 2). I suggest being cautious about calling it a daily isotopic signal, and also about the interpretation of the 2018 drought effect, which is a product of daily sapflow and a fixed iWUE. This may tend to overestimate the GPP decline under drought and therefore the 31% magnitude estimated by IsoSF.
Scaling tree to stand. Averaging the GPP of 12 trees, if these trees are larger than the stand average, may also cause some overestimation (the non-significant t-test (L159) is based on a small sample with low power). Also, multiplying by stand density may not take into account the sapwood/basal area distribution. Sapflow is usually scaled using sapwood area and the relationship is not always linear. Comparing the sapwood area distribution of the stand vs the plots would be a great way to examine this. Using diameter classes may also help improve scaling considering tree sizes.
Gap-filling with data from the lowest-flow year may introduce some bias. An analysis changing the year for gap-filling would allow this to be tested.
I wonder if there are continuous soil moisture measurements, as these may provide a better connection in the manuscript in terms of site water limitation under drought and may relate better to the dynamics of both methods.
L201-205 The statement that the IsoSF is “more credible” needs to be adjusted, given that both methods show a decline with potential biases. I think that interpreting the magnitude of the drought response as being strongly reflected by IsoSF would be a more solid basis to build on.
L126-127 and other parts of the manuscript, check the VPD limit. In some parts it is given as <0.002 kPa, in others as <0.002 hPa. Please correct.
L159 and Table 1. Check the standard deviation of sapflow trees.
Table 1. Check the N values for the different measurements. Diameter growth of 16 trees? I am not sure how that matches the plot measurements. Also, please clarify how stemwood production was estimated, as the SD is very high.
Check the units of sapflux throughout the manuscript: kg/tree/day (Fig 3) vs L/tree/day (during drought, L270-275); it is better to use the same units.
L265-270 I wonder whether the authors should have explored filtering their results and providing a value for how that may affect the differences between EC and IsoSF in that case. It would be good to have that as an estimate in the discussion.

References: check in-text citations against the reference section
Etzold et al. 2010 in references vs 2019 in text
Farquhar & Richards 1084?
Citation: https://doi.org/10.5194/egusphere-2026-5424-RC2

Tobias Rütting, Maj-Lena Finnander Linderson, Per Weslien, Lasse Tarvainen, and John D. Marshall

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https://doi.org/10.5194/egusphere-2026-5424-supplement

Tobias Rütting, Maj-Lena Finnander Linderson, Per Weslien, Lasse Tarvainen, and John D. Marshall

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Short summary

We tested estimates of gross primary production (GPP) for a forest plantation derived from eddy covariance (EC) against an independent physiological method, measuring transpiration and isotopic estimates of water-use efficiency (WUE). Using tree-ring isotope data and existing sap-flux data, we found a decrease in GPP of 31% during the severe drought in 2018, which was not detected by EC. The loss in GPP occurred because water stress reduced transpiration, and despite an increase in WUE.


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