Drought responses of a Norway spruce forest on drained peat soil: combining sap-flow sensors, eddy-covariance, soil and UAV data

Alekseychik, Pavel Konstantinovich; Peltoniemi, Mikko; Mäkipää, Raisa; Tuominen, Ville; Laurila, Tuomas; Jones, Hamlyn; Müller, Mitro; Lopatin, Evgeny; Rautakoski, Helena; Vesala, Timo; Launiainen, Samuli

doi:10.5194/egusphere-2026-568

Preprints

https://doi.org/10.5194/egusphere-2026-568

Preprints

05 Feb 2026

| 05 Feb 2026

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Drought responses of a Norway spruce forest on drained peat soil: combining sap-flow sensors, eddy-covariance, soil and UAV data

Pavel Konstantinovich Alekseychik, Mikko Peltoniemi, Raisa Mäkipää, Ville Tuominen, Tuomas Laurila, Hamlyn Jones, Mitro Müller, Evgeny Lopatin, Helena Rautakoski, Timo Vesala, and Samuli Launiainen

Abstract. The summer of 2021 brought a severe drought to southern Finland. We explored how a drained boreal peatland forest responds to drought by combining a wide range of in situ monitoring and Unmanned Aerial Vehicle (UAV) remote sensing. A dataset combining eddy-covariance (EC) fluxes, sap-flow sensors, UAV mapping, soil and weather data was collected. Spruce stand reaction to drought is examined at sub-daily to seasonal time scales, and its temporal and spatial features are identified separately for a recently thinned Continuous Cover Forestry (CCF) block and a control block. Sap flow data showed that the CCF-harvested block shows greater tree resilience to high vapour pressure deficit (VPD) than the control block, likely due to the higher soil water availability during the rainless and hot period. At the same time, both the control and CCF harvest blocks had notably reduced net ecosystem exchange (NEE) and increased Bowen ratio, particularly on high VPD days. The UAV surveys indicated that trees in the CCF block tend to have higher canopy temperatures and lower Normalized Difference Vegetation Index (NDVI) than in the control block, implying their possible susceptibility to more extreme drought.

Received: 30 Jan 2026 – Discussion started: 05 Feb 2026

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Status: open (until 28 Mar 2026)

Post a comment Subscribe to comment alert

RC1: 'Comment on egusphere-2026-568', Anonymous Referee #1, 11 Mar 2026 reply

General comments
Overall, the paper is suitable for publication in Biogeosciences, as it provides an important overview of the efficacy of various methodological approaches for investigating the responses of boreal forests to drought. However, given the many concepts and datasets presented in this paper, the overall structure needs improvement to provide more clarity for the reader. Some of the ideas presented are not well connected, or require clarification to contextualize them within the paper's premise and the scale of the results. Moreover, because the figures can be very data-heavy, captions and/or legends need to be improved so they can stand alone without requiring embedded supporting text in the manuscript. Nonetheless, the conclusions are clearly drawn and can support further research on the usability of the proposed methods to understand the onset and effects of drought on boreal forest productivity.
Specific comments
The introduction needs some light reorganization to improve the flow of ideas. For example, in lines 51 to 56, the introduction of the Standardized Precipitation Evaporation Index (SPEI) feels disconnected from the ideas presented in the previous paragraph. I understand that this index is used later in the paper to explore regional drought characterization, but it is not well integrated here, nor does it support a strong segue to the next paragraph. A suggestion is to combine the paragraphs from lines 45 to 56 into one and make the connection between Norway spruce responses to VPD clearer in preparation for the next paragraph.
In the introduction, the paper's main goal is first presented: to investigate the drought responses of two adjacent stands in a boreal forest in Finland (lines 98 and 99). However, in line 104, the statement about the difficulty of detecting drought responses in boreal forests serves as the main premise. Given the importance of this premise in the introduction and abstract, I suggest reframing the goals to clarify the paper's main question. If the paper aims to identify the utility of different methodologies to characterize drought and then explain how two adjacent stands behaved differently under drought, a reframing could benefit the reader.
In the methodology section, specifically in “meteorological and soil environmental conditions” (Section 2.2.2), there is mention of datasets that were collected but not incorporated into the discussion. I suggest removing mention of any datasets that are not presented in the results and discussion.
I suggest renaming section 2.2.3, “meteorological drought assessment”, to better represent both the atmospheric and soil drought definitions. Moreover, a clearer distinction between these two definitions is needed, as their use in the results and discussion sections was often confusing (see other comments below). Moreover, this section does not seem to belong in the “field setup” description and would be more appropriate in another section describing derived indices and drought characterization. The same would be more appropriate for Section 2.2.4 on drought proxies. In this section, acronyms need to be more clearly defined and used consistently throughout the paper, and providing more context on why these proxies were chosen would probably help the reader.
The novel approach to measuring tree stomatal conductance (section 2.2.7) also does not appear to be well suited to the “field setup” section. Moreover, this seems like a pivotal contribution of this paper to the literature. It may be best to include it in another section of the methods for physiological stress characterization, for example, or within section 2.2.6 (e.g., 2.2.6.1).
In the results section, I am unclear what is the goal of the monthly SPEI distribution for the entire country (Figure 2c). It is tied to national rainfall patterns, but this is not investigated enough to provide sufficient contextualization for the study site-specific results. I suggest this be reframed for greater clarity or removed entirely. Moreover, Figure 2b needs more clarity: the soil drought points correspond to both atmospheric and soil drought conditions (line 395). The figure should be comprehensible on its own, and the current caption and legend are not sufficiently self-explanatory. Please include a legend for Figure 2a. I assumed that the colors and line patterns corresponded to different blocks and years, as in other figures, but that should be clearer here. Lastly, I understand that Figure 2b is representative of the harvest block, likely due to the placement of the eddy-covariance tower. That is not well described in section 2.2.2, nor in the caption or the text. Make this clearer to the reader in one of these places.
Results for the Bowen Ratio in lines 425-428 are unclear because there is no frame of reference for the Bowen Ratio in the methods section. Please include contextualization for what it means when the Bowen ratio increases or decreases. Contextualization may also be needed for the Canopy Water Stress Index, since the averaged CWSI during drought periods is not presented relative to other values.
In Figure 6a-6d, it is unclear why and how the VPD values were binned. What was the criterion for binning the values? Could a line have displayed that with either shaded standard deviations or a shaded confidence interval? Moreover, the legend is small and hard to read. Please increase the point size of the legend.
Please consider reorganizing section 3.2 for greater clarity. This is a very data-heavy section and can be confusing for the reader. It might be beneficial to discuss ecosystem-level data first, as currently presented in Figures 4 and 6, and then use the block distinctions in Figure 6 to discuss differences between blocks, including Figure 5. The main issue identified here is the discussion of NEE that opens and closes this section, which leads to some repetition; perhaps bringing the two together could facilitate the reader’s comprehension of the data. Moreover, a reorganization may help with the weather contextualization of this section (e.g., drought is often associated with more sunlight, weather conditions for EC measurements, etc.).
Throughout the results section, please consider being clearer about the atmospheric and soil drought. Currently, the term “drought” is used in both contexts with much explanation, which has hindered easier comprehension of the results. Perhaps some context can be introduced in the relevant methods section and reinforced here.
In Figure 7, similar to Figure 6a-6d, it is unclear how the VPD values were binned. Please provide context for the specific criteria used to bin these values.
In lines 511-512, it is noted that the harvest-block trees show higher sap flow than the control trees. Is that the result on average? Is it per range of tree diameter? How are these values contextualized?
In the UAV survey results (section 3.4), inconsistent use of abbreviations and symbols may hinder the reader’s ability to understand the results. Please consider using abbreviations and symbols more consistently, such as when discussing thermal stomatal resistance and stomatal conductance.
The paragraph in lines 585-595 compares the median of the distributions from the tree stomatal conduction to the eddy covariance values, which is not immediately clear from the figure. An indication of the tree medians could be included to facilitate comparison with the EC lines. Moreover, rewording this paragraph may benefit the reader; lines 593-595 were confusing.
In Figure 12, tree height and NDVI values were binned. As with previous figures showing binned VPD values, it is unclear here why or how these variables were binned. Please provide some context.
Again, in the discussion section, there’s confusion about which type of drought the argument concerns (atmospheric or soil). In the first paragraph of this section, for example, it is not immediately clear that changes in vegetation functioning with drought refer to the atmospheric drought. Improving clarity would greatly benefit the dissemination of this paper.
The ideas in the paragraph on lines 654-657 should be distributed across the previous paragraph and the following paragraphs for greater clarity.
In lines 661-665, it is unclear why bring up a previous study finding greater reductions in boreal forest productivity under low soil moisture contents if the last sentence is about the effects of atmospheric drought in the absence of soil drought.
In the results section, it states that soil moisture was measured at 20 cm in addition to 5 cm. Why is that not shown in the paper, and why was it not used to explore additional stress from soil drought further? Moreover, how might the water table depth mediate the soil response to drought stress? How can the decoupling between soil moisture and water table depth help explain the effects of soil drought on boreal forest productivity?
The discussion section follows a strong structure, from ecosystem-level responses to drought and block effects (thinning) to individual tree responses. It ends with a comparison of the EC data to the UAV approach, which is incredibly valuable to the community.
Technical corrections
Line 234: include BR abbreviation in parentheses or brackets (consistent with other abbreviations).
Line 239: include symbol and units for aerodynamic resistance to water vapour transport in parentheses or brackets (consistent with other symbols).
Line 253: include symbol and units for surface conductance in parentheses or brackets (consistent with other symbols).
Line 254: include Canopy Water Stress Index abbreviation in parentheses or brackets (consistent with other abbreviations).
Line 263: include WUE abbreviation in parentheses or brackets (consistent with other abbreviations).
Line 268: include LUE abbreviation in parentheses or brackets (consistent with other abbreviations).
Line 435: Figure 4 needs a title before the description of the specific panels.
Line 467: add a comma after PAR for greater clarity.
Line 474: unsure whether there’s a typo or if the sentence needs more clarification. Unsure what “at no drought on the c shows” means.
Line 481: Figure 6 needs a title before the description of the specific panels.
Line 529: Figure 9 needs a title before the description of the specific panels.
Line 578: Figure 10 needs a title before the description of the specific panels.
Line 669: correct Mirabel et al. (2023) citation.
Line 709: Is Figure 12 the correct figure here? Shouldn’t it be figure 9?

Reply

Citation: https://doi.org/10.5194/egusphere-2026-568-RC1

Viewed

Total article views: 219 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	BibTeX	EndNote
138	65	16	219	38	40

HTML: 138
PDF: 65
XML: 16
Total: 219
BibTeX: 38
EndNote: 40

Views and downloads (calculated since 05 Feb 2026)

Month	HTML	PDF	XML	Total
Feb 2026	76	50	11	137
Mar 2026	62	15	5	82

Cumulative views and downloads (calculated since 05 Feb 2026)

Month	HTML	PDF	XML	Total
Feb 2026	76	50	11	137
Mar 2026	62	15	5	82

Viewed (geographical distribution)

Total article views: 249 (including HTML, PDF, and XML) Thereof 249 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 18 Mar 2026

Short summary

As the Boreal climate shifts towards warmer temperatures and more unpredictable precipitation patterns, forest management needs to become more efficient and climate-friendly. We monitored a managed Finnish spruce forest on peat soil for two summer seasons characterized with severe drought episodes. A wide range of data was used, including CO₂ exchange, evapotranspiration, tree growth, meteorology and UAV maps. Clear, but complex responses to drought on different spatial scales were revealed.


Total:	0
HTML:	0
PDF:	0
XML:	0