The article “Physiological and climate controls on foliar mercury uptake by European tree species” by Wohlgemuth et al. [bg-2021-239] summarizes an important study which examines controls on foliar mercury concentrations across several European tree species. This article is important because the authors synthesize mercury patterns across a relatively large spatial area involving many observations of different tree species and interprete these patterns in the context of tree physiological traits and climate conditions which control stomatal conductance. While the study presents few new finings, the authors do a great job presenting, synthesizing and interpreting their findings in context of a rather broad literature. Also based on their observations, the authors make recommendations for global mercury models to improve simulation of the role of “global vegetation as a mercury pump” and allows for depiction of the effects of changing climate on this important process.

The manuscript is well written and well organized. I only have a few minor suggestions below. I recommend the publication of this paper pending minor revisions.

Specific comments

Line 80.  …deposition to the land surface may …

Lines 96, 176, 243, 304, 329 and 335. The wording should probably be “among” rather than “between”.

Lines 152, 203, 215 and 414.  "in-situ" should be in italics.

Line 179.  … needles and largest average …

Line 199.  … at various dates over the annual cycle, making …

Figure 3. The authors might point out that the range of observations for species with a large number of observations is large compared to species with few observations and the reason for this variation.

Line 345.  Average LMA values was … 

A well written paper on an important subject, based on a large new dataset that is analyzed thoroughly and rigorously. I particularly like the practical conclusions on how foliar uptake of mercury could be practically implemented in mercury fate models. I find little to criticise.

Line 49: Delete comma before “during” and add “the water” before “vapor”.

Line 51: Delete comma before “during”.

Line 52: Delete “corresponding”.

Line 54: This sentence appears somewhat unmotivated in the abstract. Finding a proxy for stomatal conductance during an entire growing season was probably not among the original objectives of this study and is too hypothetical at this stage to merit inclusion in the abstract. It distracts from the main message. It also only merits a single sentence in the entire main body of the paper (line 480). 

Line 65: use “exposure” instead of “exposition”.

Line 110 and line 122: 3515 foliage samples versus 3569 foliage samples. What is the reason for that discrepancy? Does either of these numbers include the outliers identified in line 165?

Line 137 to 138: Brandenburg and Baden-Wuerttemberg are not countries.

Line 200: The text prior to here makes reference to multiple needle year classes (Line 117/141, line 127-130). The sentence “by normalizing foliar Hg concentrations of samples to their respective life period in days from the beginning of the growing season (leaf flushing) to date of harvest” raises the question of how was this done for needles older than 1 season. (Judging from page 18 and figure 7, it appears that older needles were not subjected to the normalisation procedure and were used in an entirely separate analysis.)

Line 216: Make it clear that you refer to water vapour here. This is advisable as there could in principle also be a mercury vapor pressure deficit.

Line 250: Use “example” instead of “exemplary”

Figure S5: Maybe state that current-season needles are displayed.

Section 3.2 The comparison of foliar uptake rates across space suggests that variations in GEM concentrations in the atmosphere (in space and time) are deemed not to be important. That is likely correct, but should still be stated explicitly.

This is also important as some of the parameters explored later (soil dryness, VPD) could have a geographic component. You have to exclude the possibility that some of the observed relationships with these parameters are not artefact caused by a correlation of the parameters with atmospheric GEM concentrations (e.g. lower atmospheric GEM levels in the more water-stressed southern parts of Europe).

Line 345: Figure 4 not 44

Figure 4: Either use empty space at lower right for figure legend or vertically stack all three panels of the figure.

Line 414: “In the future”

Line 417: “gravel” is a soil parameter?

Line 424: When performing 54 linear regressions with a p value of 0.05, you would randomly expect 2 to 3 “significant” ones, even if there aren’t any. Therefore, not too much should be made of the findings described on lines 423 to 427.

Line 463: “foliage takes up Hg(0) over the entire life time”. Should this not be rephrased as “over the entire growing season” as the text earlier admits that mercury uptake during winter is poorly understood?

Line 464: Again, I think it is necessary to state here that normalisation by the prevailing GEM concentration in the atmosphere is required when comparing foliar Hg uptake rates from different sites. That could be relevant when comparing between foliage from different hemispheres, and between foliage from areas with large differences in mercury source strength.

Comments on: Wohlgemuth et al (2021) Physiological and climate controls on foliar mercury uptake by European tree species

The authors of this paper have analyzed how meteorological/climate variables as well as leaf traits affect the uptake rate of mercury (Hg) of leaves/needles in a range of tree species over a substantial part of Europe. The data base is large, which is an important asset in this type of analysis, formed by observations made using the ICP Forest level II plots and in addition data from a dense sampling network in Austria. Important results include the relationship of foliar Hg uptake rate with leaf nitrogen concentration and leaf mass per area (LMA) as well as links with soil moisture, water vapour pressure deficit (VPD) and other meteorological factors, suggesting a close link of Hg uptake rate with stomatal conductance and physiological activity. In general, this is a well-written and valuable piece of work which should be published. However, improvements are possible, and a number of mostly relatively minor changes and amendments should be considered.

Specific comments:

Line 41: “simulated start of the growing season” – some more details about how the simulation was made should be included.

Line 98: consider removing “balanced”, it does not become clear what this refers to in this context, and it is unnecessary to include this word.

Lines 112-118. Information about which needle age classes were harvested should be added as well as which needle age classes were used in different analyses.

Line 117: “typically”? Please be more specific.

Line 173: typo, “factors” should be “factor”.

Line 204 “leaf unfolding” seems not to be the appropriate expression here!? Rather: “emergence of the new flush of needles”? A similar comment applies to the text under 3.1 of the supplementary information. Here it is stated (line 5 and line 22 on page 3) that it is the beginning of the “growing season” that is determined from the PEP725 database. But isn´t it again the emergence of the new flush of needles which is treated here? The growing season of the older needles and thus “the beginning of the growing season of coniferous tree species” (line 5) – with stomatal gas exchange - starts much earlier.

Lines 275-278 and other places: there is a strong focus on current year needles in the study, although not completely. The authors should explain why this is the case. Most conifers retain their needles ~4-10 years and Hg accumulation will continue over several years. In the years after the first, evergreen conifer needles will have a longer period of physiological activity and uptake of Hg, starting earlier in spring and ending later in autumn, compared to broadleaved trees. Thus, even if the uptake rate of Hg per day (mostly used in the paper) is smaller for conifers, this will partly be offset by the longer duration over the year of gas exchange in needles older than current. This is significant when analyzing biogeochemical fluxes on a (multi)annual scale. To focus only on the uptake rate per day obscures the importance of the variability in the duration of Hg uptake over the year in different types of trees.

Lines 287-288: as the authors state, the otherwise very informative paper Zhou et al (2021) does not distinguish between different needle age classes, which makes it hard to compare with needle age specific Hg concentration data. Some other reference should be used to support the statement.

Line 338: should be “Leaf Mass per Area”.

Line 345: should be “Figure 4” (not 44).

Figure 4: This figure contains very interesting information, especially the relationship between daily Hg uptake rate and foliar N concentration (representing physiological activity). Such a relationship could become very useful for large-scale modelling of Hg fluxes if supported also by further data. However, it is confusing that the daily Hg uptake rates, especially of the conifers, in Figure 4 are substantially higher than those reported for the same species in Table 1 based on a larger number of observations. Also, the N concentration values differ between Figure 4 and Table 1 for the same species. The authors should discuss these discrepancies, their causes and implications.

Figure 5 (this applies also to figures in the supplementary): While the relationship for pine has a reasonable distribution of data, permitting the authors to derive a quite clear linear relationship, this is not the case for spruce. For the latter species there are two clusters of data separated by a large empty space with respect to the x-variable, which contains no data. Linear regression is not to recommend for such a data set. An option would be to compare and test the difference between the two clusters with a t-test.

Line 371: It is not fully explained why the specific water VPD thresholds were used for different species. From where were these thresholds taken?

Line 389: replace “had” by “has”.

Lines 393-394: “… the high degree of stomatal closure under drought stress of spruce”. I do not understand this statement in relation to the data presented. The average daily Hg uptake rate, suggested by authors to be a possible proxy for stomatal conductance, does not differ very much between high and low VPD (Figure 5b), indicating a small response in stomatal conductance by high water VPD in spruce!?

Figure 7: Although there is a general trend for higher Hg concentrations in older needles in Figure 7, also seen in other studies, the pattern in the figure is not completely clear. This may partly be caused by the heterogeneity of the data, with a strongly varying number of observations for the different needle age classes. It may also be the case that the larger number of needle age classes included in the figure compared to most other studies, indicate a levelling off in the rate of annual increase in Hg concentration of the oldest needles. The authors should discuss the data presented in Figure 7 in further depth including the possible consequences of the heterogeneity of the data. The quite strong levelling off in annual Hg concentration increase for y4-y6 in Figure 7 is not in complete agreement with the statement on lines 432-436.

Lines 450-453 on stomatal conductance modelling: the phrase “model in a multiplicative way” is not appropriate and hard to understand for people not strongly involved in stomatal conductance modelling. Also, there exist other types (e.g., photosynthesis based) stomatal conductance models than multiplicative which could be considered. It would be appropriate to use a more recent reference than Emberson et al (2000), since a lot of development has taken place in stomatal conductance modelling over the last two decades. The authors may consider Emberson et al (2018) Ozone effects on crops and consideration in crop models. European Journal of Agronomy 100, 19–34, although it is on crops.

Line 480-481: The suggestion by the authors to assume that foliar Hg can represent the stomatal conductance (integrated over longer time scales) is interesting and thought-provoking. This statement must be based on the assumption that the atmospheric concentration of Hg is essentially constant, from year to year and from place to place. To what extent is this assumption valid? This should be discussed.

In the Conclusion part it would be appropriate to discuss the relationship between the net accumulation in leaves/needles over time periods of weeks to years, which is the focus in the paper, and the dynamic bi-directional fluxes of Hg to/from vegetation observed in many studies using highly time resolved measurements.

Essentially, this is a very interesting and valuable paper.