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the Creative Commons Attribution 4.0 License.
| 28 May 2025
Global warming alters mercury accumulation in trees
Abstract. Stomatal uptake of mercury (Hg) by trees is the major pathway of atmospheric Hg to the terrestrial environment and most studies have suggested that Hg concentrations in tree rings are determined primarily by gaseous elemental mercury (GEM) concentrations in the atmosphere. Most studies on tree-ring Hg records were aiming to reconstruct historical atmospheric Hg emissions at polluted sites. However, the role of changing climate such as rising temperature and frequent drought events on tree-ring Hg concentrations has been rarely addressed. We show that the overall Hg load in tree rings in oaks and Douglas fir from contaminated and uncontaminated sites in Germany has been strongly determined by local atmospheric Hg emissions, but its long-term evolution has been largely determined by hydroclimate and did not follow the trend of GEM emissions in Europe. Due to different physiological heat adaption strategies Central European oaks show continuously increasing tree-ring Hg concentration with rising temperature and precipitation rates during the past century, whereas coniferous trees show a strongly declining trend in Hg concentration in the same period, which was also observed at other sites within the Northern Hemisphere. Our findings indicate that besides atmospheric Hg concentrations, climate change alters the Hg accumulation in different forest types and most likely the related transfer of atmospheric Hg to the soil.
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AC1: 'Comment on egusphere-2025-2325', Harald Biester, 16 Jun 2025
Dear reviewers and editors,
there is an error in the manuscript at p6, line 183, where the text refers to Fig. S1. Figure S1 does not exist, there is no supplemental information.
H.Biester
Citation: https://doi.org/10.5194/egusphere-2025-2325-AC1 -
RC1: 'Comment on egusphere-2025-2325', Anonymous Referee #1, 18 Jun 2025
The submitted paper by Land et al. is well written, both interesting and controversial study. After reading the whole paper I was not convinced that the oak tree ring Hg records are a valid archive of atmospheric Hg pollution. Also, I believe that drawing a global conclusion about changes in the assimilation of mercury (Hg) by trees due to climate change, as indicated by the title of the paper, requires more than a study of two sites (one contaminated and one background). Many of the presented observations lack sufficient evidence and previous study including two oak species by Gustin et al. (2022) concluded that broadleaved species are not suitable for studying Hg trends in tree rings over time. But let’s discuss this with authors…
First, increasing air mercury (Hg) due to Hg re-emission originating from increasing air temperature due to climate change is definitely plausible. The question is to what extent climatic variables affect Hg assimilation and tree ring Hg. A recent paper by Boonen et al. (2025) that was not included in the list of references assessed that 7% of the variability in the annual tree-ring Hg of Douglas fir could be explained by changes in precipitation during the active growth period. For European larch, 2% of the variability in tree-ring Hg was affected by spring temperatures. Since temperature has increased in the study area over time, a consistent trend of increasing or decreasing Hg in tree rings would correlate with temperature. However, this does not necessarily imply a causal relationship. It is implausible that a temperature increase of about 2°C would cause a tenfold increase in mercury concentrations in oak trees.
I did not fully understand the explanation as to why, at the two studied oak sites, tree ring mercury (Hg) increased over time, while none of the other oak tree ring records (nor those of many other species) from the Northern Hemisphere (except those from permafrost areas) indicated elevated Hg assimilation due to climate change. Other oak tree ring records, including those from Scanlon et al. (2020), Siwik et al. (2010), and Gustin et al. (2022), all from the Northern Hemisphere, indicated either no trend in tree ring Hg concentration or reversed behavior with respect to annual changes in air Hg.
Previous studies have shown that at sites with elevated atmospheric mercury (GEM) concentrations, trees typically have elevated mercury in their assimilatory organs, resulting in elevated concentrations of mercury in their tree rings. Thus, one would expect to see reports of increasing Hg in tree tissues. However, in the German Specimen Bank (https://www.umweltprobenbank.de/de), all sampled assimilatory organs of spruce, beech, and poplar trees indicate decreasing air Hg concentrations, similar to the indications of decreasing GEM concentrations from European air monitoring. This leaves the reader with the question of why increasing Hg in the tree rings of oaks at the two studied sites (one contaminated and one background) would not be coupled with increasing air Hg. As demonstrated by McClenahen et al. (2013) in Pennsylvania, oak assimilatory organs reflect decreasing atmospheric Hg emissions and air Hg levels.
The explanation for the increasing trend of mercury (Hg) in tree rings could be the physiological functioning of oak bole. Previous studies have shown patterns of increasing elemental concentrations in direction from the pith to the bark, as demonstrated by Lévy et al. (1996), Bukata and Kyser (2008), and Jaozandry et al. (2024). Jaozandry et al. (2024) explained increasing nutrient concentrations (and also some pollutants) from pith to bark by translocation of elements between rings. The porous character of oak wood, which is typical of vessels (diffusive-porous wood type), is thought to play a significant role in radial translocation. The authors quickly dismiss the alternative explanation that the temporal mismatch between the tree-ring Hg and Hg emissions in Fig. 4 (which seems rather small to me) is caused by radial translocation. As we know from literature in case of conifers translocation occurred the other way i.e. contrastingly to oaks in direction from bark to the pith. For broadleaved species like oak, it is much more plausible that a substantial portion of Hg (and many other nutrients and other elements) remains in the xylem sap or is re-mobilized and moves outward as the sapwood grows.
It seems that the explanation of the tree-ring Hg increasing trend could be the physiological functioning of oak bole. Previous works presented patterns of increasing elemental concentrations from pith towards bark e.g. Lévy et al. (1996), Bukata et Kyser (2008) and Jaozandry et al. (2024). Jaozandry et al. (2024) explained the increasing nutrient concentrations from pith to bark by nutrient translocation between rings. In the same time the distribution of water within the oak bole has been rather constant in opposed to conifers where sapwood water content is much greater than that in heartwood. Due to porous character of the oak wood typical with vessels (diffusive-porous wood type) suggested to play significant role in the process of radial translocation. The authors quickly dismiss the alternative explanation that the temporal mismatch between the tree-ring Hg and Hg emissions in Fig. 4 (which seems rather small to me) is caused by radial translocation. In broadleaved species like oak, it seems much more plausible that a substantial portion of Hg remains in the xylem sap or is re-mobilized and moves outward as the sapwood grows.
At the same time, the absence of a mercury (Hg) signal in the tree rings at the cinnabar smelting site raises questions about the validity of oak tree rings as an archive of atmospheric pollution. As indicated in the beginning of this assessment, broadleaved species were evaluated as not suitable for evaluation of Hg trends in tree rings over time (Gustin et al. 2022). Although Gustin et al. (2022) paper is in the list of references, no discussion was dedicated to addressing this statement.
Additionally, I don’t believe the distinction between isohydric and anisohydric species adequately explains the significant variations in tree-ring mercury (Hg) levels. In study of previously mentioned Boonen et al. (2025), European larch (anisohydric) produced similar results to Douglas fir (isohydric), which suggests that stomatal behavior may not be a key factor in long-term trends.
Given the above-mentioned reservations, I cannot recommend publication of this paper unless the authors reconsider and reinterpret their observations.
More comments and remarks:
Sampling sites
In a study dealing with climate change I would expect to have a complete information on site elevation, mean annual precipitation and temperature to have a background for further interpretation and discussion. This information is missing.
Paragraph 95
The method of sapwood tree ring estimation is missing… Another issue is the specification of mid-point years 2017, 2012, 2007 and so forth… when were the trees sampled and why is the first midpoint 2017?
Paragraph 105
Here is a major problem, why would you aggregate contaminated site and uncontaminated site tree ring Hg records for further analysis – this does not make sense at all. If the contaminated site has been affected by the local emission or re-emission sources then it should not be used for extrapolation with the regional averages. Why was the further analysis not performed on the record of background site only?
Paragraph 155
Where in paper was the leaf activity used?
Paragraph 185
I could not find Fig. S1 within the submission…
Paragraph 200
The concentration ranges should have units.
Paragraph 215
If such an important strong Hg emission source would not be recorded in a Hg tree ring record I would end up asking myself if the tree ring record records atmospheric Hg levels or not? The possible tree physiology effects such as radial translocation are kind of left behind in this paper. Many previous works (including those in the list of refs) indicated risks of using the sapwood tree rings without caution. Example for all one of the most recent papers Peng et al. 2024.
Paragraph 220
…studies in oak are quite rare. Please include the refs so that readers can refer to these studies.
Paragraph 320-360
This part of the discussion is missing the fact recently published by Peng et al. (2024) indicating that sapwood tree rings should be interpreted with high caution.
Paragraph 345
…soil water lowers… e.g. soil moisture decreases… would be better
Refs
Boonen, K., Shetti, R., Navrátil, T., Nováková, T., Rohovec, J., & Lehejček, J. (2025). Atmospheric mercury pollution recorded in conifer tree rings: Disentangling the effects of tree-ring width, water content, and climate on mercury concentrations. Dendrochronologia, 126370.
Bukata, A. R., & Kyser, T. K. (2008). Tree-ring elemental concentrations in oak do not necessarily passively record changes in bioavailability. Science of the total environment, 390(1), 275-286.
Gustin, M. S., Ingle, B., & Dunham-Cheatham, S. M. (2022). Further investigations into the use of tree rings as archives of atmospheric mercury concentrations. Biogeochemistry, 158(2), 167-180.
Jaozandry, C. C., Leban, J. M., Legout, A., van der Heijden, G., Santenoise, P., Nourrisson, G., & Saint-André, L. (2024). Advances in assessing Ca, K, and Mn translocation in oak tree stems (Quercus spp.). Heliyon, 10(13).
Jaozandry, Caroline Christina, et al. "Advances in assessing Ca, K, and Mn translocation in oak tree stems (Quercus spp.)." Heliyon 10.13 (2024).
Lévy, G., Bréchet, C., & Becker, M. (1996). Element analysis of tree rings in pedunculate oak heartwood: an indicator of historical trends in the soil chemistry, related to atmospheric deposition. In Annales des sciences forestières (Vol. 53, No. 2-3, pp. 685-696). EDP Sciences.
McClenahen, J. R., Hutnik, R. J., & Davis, D. D. (2013). Spatial and temporal patterns of bioindicator mercury in Pennsylvania oak forest. Journal of Environmental Quality, 42(2), 305-311.
Peng, H., Zhang, X., Bishop, K., Marshall, J., Nilsson, M. B., Li, C., ... & Zhu, W. (2024). Tree Rings Mercury Controlled by Atmospheric Gaseous Elemental Mercury and Tree Physiology. Environmental Science & Technology, 58(38), 16833-16842.
Scanlon, T. M., Riscassi, A. L., Demers, J. D., Camper, T. D., Lee, T. R., & Druckenbrod, D. L. (2020). Mercury accumulation in tree rings: observed trends in quantity and isotopic composition in Shenandoah National Park, Virginia. Journal of Geophysical Research: Biogeosciences, 125(2), e2019JG005445.
Siwik, E. I., Campbell, L. M., & Mierle, G. (2010). Distribution and trends of mercury in deciduous tree cores. Environmental Pollution, 158(6), 2067-2073.
Citation: https://doi.org/10.5194/egusphere-2025-2325-RC1 -
RC2: 'Comment on egusphere-2025-2325', Anonymous Referee #2, 13 Aug 2025
The study by Land et al. explores how climate change affects mercury (Hg) accumulation in Quercus petraea and Pseudotsuga menziesii. Trees absorb mercury primarily through their stomata from the atmosphere, which is then stored in their annual rings. Previous studies focused on reconstructing historical mercury emissions on polluted sites, but the impact of climate change has been largely overlooked.
Land et al. objectives were to investigate mercury concentrations in the annual rings of oak and Douglas fir trees in Germany over the past 150 years and to analyse (literature study) the influence of temperature and precipitation and the evolution of atmospheric Hg emissions on mercury accumulation in the Northern Hemisphere.
Oak trees showed increasing Hg concentrations with rising temperatures and precipitation while Douglas fir trees showed decreasing Hg concentrations with rising temperatures. At a contaminated site, Hg concentrations in oak tree rings were 10 to 25 times higher than at uncontaminated sites. While Hg concentrations in oak trees have steadily increased over the past century, Douglas fir trees showed a peak in Hg concentrations during the 1950s–1970s, followed by a decline.
The authors speculate that Oak trees keep their stomata open during drought, facilitating mercury uptake (anisohydric) and that Douglas fir close their stomata during droughts, reducing mercury uptake. In sum, climate change and future forest tree species composition (e.g. more deciduous tree species in German forests) may have a strong influence on the Hg sink strength.
Overall, this study provides valuable insights for addressing Hg in forest ecosystems under changing climate conditions. However, I have some major concerns regarding the study design. The authors took only tree ring samples from 5 Oak trees from one uncontaminated and from one contaminated site (10 in total) and from 5 Douglas fir trees from another site. What kind of design is that? Were the trees randomly selected? I assume the 5 trees per site are always pseudoreplicates. Hence, the whole story remains very speculative. Additionally, I don’t understand why the title is about “Global Hg concentrations” when the authors only investigated three sites with only few pseudoreplicates in Germany and the literature-study part is about the Northern hemisphere.
There is no additional information about the forest sites. Was it always the same soil? What was the stocking density, age class distribution etc..? If discussing/speculating about the physiological strategies of forest tree species at different sites, one need more information about soils (soil texture, soil depth, rooting depth etc.). I guess the oak tree species were growing in more stagnic (and much more clay) environments in contrast to the coniferous tree species. How does that effects Hg accumulation in tree stems?
The study is quite interesting but there is so much speculation caused by the limited sampling design. Measurements of more Oak and Douglas fir trees with comparable site conditions and/or comparing different site conditions would provide more evidence.
The authors should make sure not to repeat results in the discussion section and/or only discuss other literature. (e.g. The sectio4.3 The influence of climate on Hg accumulation in oak and Douglas fir tree rings” Line 282 to Line 291 is not a discussion of own results but a nice description of other literature (belongs to Introduction) and Line 292 to 299 is repetition of results.)
Citation: https://doi.org/10.5194/egusphere-2025-2325-RC2
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