the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Mar 2025
Mature riparian alder forest acts as a strong and consistent carbon sink
Abstract. Alder forests are widely spread across Northern Hemisphere, frequently occupying riparian buffer zones and playing a key role in enhancing soil fertility through symbiosis with nitrogen-fixing bacteria. Despite their ecological significance, studies on carbon (C) and water (H2O) exchange in alder forests remain scarce, particularly in the context of hydroclimatic variability and extreme weather events. In this study, we used eddy-covariance flux measurements from three contrasting years to assess the C balance and H2O exchange of a mature riparian grey alder forest in the hemiboreal zone in Estonia. The site was a strong and consistent carbon sink with annual net ecosystem exchange (NEE) ranging from -496 to -663 g C m⁻² y⁻¹, gross primary production (GPP) from -1258 to -1420 g C m⁻² y⁻¹ and ecosystem respiration (ER) from 595 to 923 g C m⁻² y⁻¹. Evapotranspiration (ET) varied from 194 to 342 kg H2O m⁻² y⁻¹ and ecosystem water use efficiency (EWUE) was 4.2 – 6.5 g C kg H2O-1. The drought and heatwave year (2018) featured the highest net carbon uptake, driven by an increase in GPP during spring and a reduction in ER during late summer and autumn. A minor impact of drought on GPP combined with a 35 % reduction in ET in 2018 lead to peak values of EWUE in response to H2O limitation. In 2019, we found no evidence of a short-term drought legacy effect, as carbon exchange components recovered to the 2017 levels and ET was the highest out of years. Given that this forest is beyond the typical harvestable age, its strong and consistent carbon sequestration, combined with high short-term resilience, provides valuable insights for sustainable forest management. These findings highlight the potential of riparian grey alder forests to maintain productivity under hydroclimatic variability, reinforcing their role in regional carbon cycling as a part of natural climate mitigation solutions.
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Status: open (until 21 May 2025)
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RC1: 'Comment on egusphere-2025-1280', Anonymous Referee #1, 25 Apr 2025
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Dear Authors and Editor,
In general, I find the manuscript interesting. The methodology is sound, and I see merit in the study. However, I am concerned that the manuscript is, at times, overly lengthy and difficult to follow, which makes it hard to read overall. In several sections, critical information is either difficult to locate or entirely absent.
I believe the authors could present more concisely what I see as the key result of this study: why and how evapotranspiration (ET) and gross primary production (GPP) decouple under anomalously dry conditions.
Moreover, the Discussion section often repeats similar sentence structures (e.g., “These findings/results highlight that…”) and reiterates basic, well-established principles of ecosystem functioning. This repetition detracts from the overall readability and does not add new insights.
The authors conduct numerous comparisons with other broadleaved forest ecosystems, which, I assume, are mostly not riparian systems. They attribute differences in net ecosystem exchange (NEE) or other variables to various factors such as soil nutrient availability or vaguely defined climatic variability. These comparisons sometimes feel overly detailed and only loosely connected to their own results. I recommend refining this section to focus on comparisons that directly support their findings.
Although I understand that specific data on other GHG fluxes (e.g., CH₄ and N₂O) are not available for the site under study, I believe the authors should at least qualitatively discuss these fluxes. Making educated assumptions about their potential roles in riparian ecosystems would strengthen the manuscript’s conclusion that riparian alder forests could contribute meaningfully to climate mitigation through carbon sequestration.
Despite these issues, I find the paper interesting and within the scope of Biogeosciences. It should be considered for publication after thorough revisions. I have included specific comments below, but I would like to emphasize that the authors should carefully revise the arguments, logic, and structure, particularly in the Results and Discussion sections, to improve readability and clarity in the next version.
Specific comments:Introduction:
The conclusions of Paragraphs 1 (l.26) and 2 (l.36) could be more clearly distinguished. As written, both highlight the need for monitoring with respect to drought response and carbon sink capacity, but without clearly separating their specific focuses (i.e., short-term drought effects vs. long-term C-sink function).
Line 50: You state that "few studies" exist, but provide no citations. Does this imply that no studies have been published yet? Please clarify or provide supporting references.Line 54: It would be helpful to briefly explain what is meant by "the traditional C budgeting method" to provide context for readers unfamiliar with the term.
Line 53: This paragraph is somewhat unclear. You suggest that climatic factors outweigh stand age in importance, yet both old and young stands are described as carbon sources. Does this imply differing climatic conditions between the sites? Please clarify. If the implication is that the sites differ in climate, that should be explicitly stated. Furthermore, without detailed knowledge of the study by Uri et al. (2019), one could infer that its findings, based on “nutrient-rich former agricultural land” (l.319), may not be broadly applicable to typical riparian alder forests, which are unlikely to share these conditions. While this may not be the case for your study, the question arises whether your results are representative or overly site-specific. Consider expanding this paragraph or the relevant discussion section to clearly position your site within the broader context of existing research, particularly when findings from other studies appear to diverge.
Lines 60f: If you cite your previous research as a foundation here, please briefly summarize its key findings. This paragraph is currently difficult to follow. It doesn’t explain how forests responded to a heatwave, nor why that is relevant to the current manuscript—aside from the mention that water fluxes were not considered previously. Consider including a paragraph that outlines previous findings and highlights the open questions your study aims to address. Then, consider reformulating your objectives for increased precision. In particular, Objective 2 (“different soil moisture regimes and WUE”) may already be included in Objective 1 (“quantify… water exchange… under varying hydroclimatic conditions”).
Methods:
Figure 1: The overview figure is very informative. As there is free horizontal space, consider including an additional ground-level photo, perhaps of the instrumentation setup or the canopy. This would help everyone unfamiliar with the ecosystem visualize the site.
Line 79: Minor detail—consider removing the term “total” since you’re only showing the 80% footprint. Out of curiosity: do the remaining 15% correspond to the river and the water bodies?
Lines 79f: It would be useful to include information on variation in stand height, stem diameter, etc. From the image in Figure 1, the stand appears relatively uniform. Quantifying this would strengthen your argument.
Lines 105f: This statement is surprising, as the manuscript has not yet introduced the discrepancy between soil and EC fluxes. Please clarify or provide context earlier.
Lines 122f: Consider briefly explaining your rationale for using the nighttime flux partitioning method (e.g. over the daytime method).
Lines 129f: This paragraph feels a bit too short. A rationale for the analyses would be helpful. If it’s too lengthy for the Methods section, it could be placed earlier near the objectives.
Around Equation 1: It appears there’s an issue: the equation references ERday in the text, but this term doesn’t appear in the equation itself. Additionally, please explain how canopy light use efficiency was calculated and which variables were used. Lastly, please clarify what modifications were made to the model and cite its original source, not just studies that have used it.
Lines 143f: How exactly are start and end of the growing season defined - using a relative or absolute GPP threshold? Consider also explaining why growing season length is relevant to your analysis. Later (e.g., l.176), you note that growing season lengths do not differ significantly between years, but it’s unclear whether or how statistical tests were applied here. Please clarify. If the differences are statistically insignificant and not central to your main conclusions, you might consider shortening this section.Results:
Figure 2 (and all other figures): It looks like standard color palettes were used. Please ensure the color schemes are accessible to readers with color vision deficiencies. If needed, use color-blind-friendly palettes or add alternative line styles. Also, clarify whether panel d) represents net radiation or incoming radiation. Minor suggestion: you might consider omitting the year legend repetition across all panels as readers can refer back easily once it's introduced.
Line 144: Consider using a different abbreviation for the correlation coefficient than "rs," as it could easily be mistaken for stomatal conductance commonly used term in flux studies.
Lines 251f.: This paragraph is difficult to follow. It relies heavily on the brief mention of partial correlation analysis back in line 144, which readers are unlikely to remember without very close reading. Please reintroduce the purpose, method, and results of this analysis in a self-contained way here. A visual representation such as a scatterplot of the residuals (just one of many possibilities) might help as well. Currently, readers will likely jump to Figure 5 and find your interpretation hard to align with what’s shown. In this context, consider moving key results from the appendix table 2 into the main text, possibly in a visually more appealing way.
Line 264: The statement that “SWC was the leading driver…” is a bit surprising given that Fig. 5c2 doesn’t clearly support this. Is this conclusion mainly due to radiation?
Table 2: As mentioned earlier, please clarify the analysis conducted here. Also, since the table isn’t referenced in the main text, its purpose and contribution are unclear—please address this.
Line 269: You state that GPPmax was “slightly lower” in 2019, but Table 2 shows a difference of ~0.07, which is negligible. Please clarify this wording or interpretation.
Discussion
Line 315: Introducing NEP here is slightly confusing. Consider converting their values to NEE for consistency and improved readability.
Line 330: You suggest that rapid SWC fluctuations might reduce annual ER, shouldn’t this be testable based on your dataset, or is something missing?
Table 3: In the entry for the Swedish spruce forest (Lindroth et al., 2020), the GPP value is missing a minus sign. Overall, the formatting in Table 3 is inconsistent. Some entries include ± values, others list single values, and some show a range (“…”). There are also inconsistencies in spacing and in the use of decimal places versus integers. A general reformatting would help improve clarity. As currently formatted, it’s unclear whether you’re showing interannual variability or uncertainty, or whether “…” denotes a range. Please clarify.
Line 337: Small note: The current sentence structure implies a direct connection between your results and the cited studies. Consider rephrasing for example: “Similar to other studies (e.g., Xy et al., Yz et al.), we observed that seasonal ET patterns were shaped by…” or omit the references if they are not directly aligning with your results.
Line 338: This statement is confusing. Your growing-season data (e.g., Fig. 3) shows ET is reduced during the drought year while GPP remains largely stable.
Line 340: The sentence starting with “The mid-season decline in ET…” feels awkward, shouldn’t your analysis directly address and explain this pattern?
Line 350: A reported precipitation surplus in this riparian setting is surprising, given typically high evaporation. This is quite interesting, consider elaborating further.
Line 373: Just a curiosity in this context: how significant is soil water depletion between spring and summer in riparian systems? A brief discussion could be insightful.
Line 377: You note that both GPP and ER decreased by 300 g C—is this the same amount for each, and does that mean NEE?
Lines 394f: The discussion around the lack of a legacy effect and its occurrence in other ecosystems feels somewhat lengthy and secondary to your core findings—but I may be missing the relevance. You demonstrate that physiological stress was present but relatively moderate, and that the ecosystem adapted and recovered quickly. This might be the key takeaway here. For readers less familiar with Nordic ecosystems, it might help to contextualize the severity of the drought in climatological terms. For example., was it a 10-year drought, 50-year event, etc.?
Citation: https://doi.org/10.5194/egusphere-2025-1280-RC1
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