the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jan 2024
Identification of volatile organic compounds emitted by Sitka spruce and determination of their emission pathways and fluxes
Abstract. The biogenic volatile organic compounds (BVOCs) emitted by Sitka spruce (Picea sitchensis) trees, housed in a plant growth chamber, were characterised by a combination of on-line (time-of-flight chemical ionisation mass spectrometry) and off-line (gas chromatography-mass spectrometry) techniques. In total, 74 BVOCs were identified in the Sitka spruce emissions, 52 of which were oxygenated compounds, with piperitone (C10H16O), an oxygenated monoterpene, being the dominant emission. Other prevalent emissions included isoprene and five monoterpenes (myrcene, β-phellandrene, δ-limonene, α-pinene, and camphene). Temperature, photosynthetic photon flux density (PPFD) and stress were all found to alter the emission profiles, with different BVOCs exhibiting different responses. Three different plant growth cycles were used to identify the emission pathways (pooled or biosynthetic) for each BVOC, through determination of the relationships of the emission flux with temperature and with PPFD. The majority of the BVOCs emitted by Sitka spruce were found to originate from biosynthetic and pooled pathways simultaneously, with those from a stressed tree having a much lower contribution from the biosynthetic pathway than a healthy tree. Standardised emission fluxes (temperature 30 °C and PPFD 1000 µmol m-2 s-1) were calculated for all BVOCs using the appropriate standardisation model (pooled, biosynthetic or combined). Annual emission fluxes for all Stika spruce plantations in Ireland were determined for piperitone (8,200 tonne year-1), isoprene (13,000 tonne year-1) and monoterpenes (1,600 tonne year-1). At the current conditions of the Irish climate the annual BVOC flux for isoprene was found to exceed that for piperitone, although this is expected to change in a warming climate.
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RC1: 'Comment on egusphere-2024-154', Anonymous Referee #1, 27 Feb 2024
General comments:
The authors investigated the BVOC emissions from Sitka spruce under laboratory conditions. Sitka spruce is an important tree species and is commonly used as part of afforestation programs. Using a combination of TOF-CIMS and TD-GC-MS techniques, the authors were able to detect many compounds that have not previously been reported in the emissions from Sitka spruce. The authors also investigated the temperature/PPFD response and emission pathways of the different BVOCs to determine if they originated from storage pools, biosynthesis, or both. The authors then extrapolated their results to estimate the annual BVOC emission fluxes for all Sitka spruce plantations in Ireland.
The main weakness of this study is the low number of plant replicates used. Only 3 seedlings were sampled in this study, and one of them (Spruce 3) was unhealthy and its emissions are not reported. Furthermore, there is a lot of variability in the BVOC emission profiles, temperature/PPFD response, and CO2 fluxes of the two remaining seedlings (Spruce 1 and 2). For example, isoprene was the second highest emission from Spruce 1, but was only emitted at trace levels from Spruce 2. The authors also note that Spruce 2 might have experienced mild stress, which may be why its emission behavior was different from Spruce 1. It is hard to obtain meaningful/convincing statistics and to draw generalized conclusions about the emission behavior of Sitka spruce from these results. I would strongly recommend that the authors conduct measurements on additional Sitka spruce seedlings. There should be at least 3 healthy plants to obtain meaningful statistics and to account for intraspecific variability. There is only one healthy plant (Spruce 1) and one mildly-stressed plant (Spruce 2) in the current study. And if the authors wish to compare the emission behavior of healthy vs. stressed Sitka spruces, then ideally, they should include 3 stressed plants in addition to the 3 healthy plants.
Line-by-line comments:
L79: Since these are not mature trees, I would suggest replacing the word “tree” with “seedling”. Please also provide the height of the 3 seedlings used in this study.
L89: Please provide the manufacturer and model of the lamps, if available.
L90: PPFD is usually expressed in units of µmol m-2 s-1. Please correct this.
Figure 1: Recommend replacing “Ball Meter” with “Ball Flowmeter” to make its purpose clearer to the reader.
L99: Why is there such a large difference in the flow rates into each enclosure? Large differences in flow rates can result in different humidity levels in each enclosure, which in turn may affect the BVOC emission rates. For future experiments, it would be advisable to try to equalize the flow rates into each enclosure using mass flow controllers (or more cost-effective needle valves).
L110: The relative humidity inside the miniature enclosure may not be the same as the RH in the other enclosures, as the RH would be dependent on the amount of foliage in each enclosure, the flow rate into the enclosure, and the transpiration rate of the enclosed plant.
Section 2.2: Please state the detection limit and accuracy (measurement uncertainty) of the TOF-CIMS and TD-GC-MS.
L155: Just to confirm, are you sure that the heated transfer line was connected to a split/splitless injector, and not directly to the GC column?
L159: The phrase “fire purge” is a bit misleading. I think you mean to say “pre-trap fire purge”, or “pre-desorption purge”.
L162: You listed the dimensions of the GC column as 60 m × 0.3 mm × 1.8 µm. 0.3 mm is not a standard column ID. Please specify whether the column inner diameter (ID) was 0.25 mm or 0.32 mm.
L163: For clarity, please specify if the pressure was 23 psig or 23 psia.
L209-210: These PPFD levels seem quite low. For comparison, can you provide the typical PPFD levels expected in Irish summer conditions?
L229: How were the Tenax tubes sealed? Did you use Swagelok brass caps?
L232: Is it possible that the plant biomass might have changed during the BVOC measurement period?
L243: The “Flow” term is missing from Eq. 1. Please correct this.
L278: Some monoterpenes originate from de novo biosynthesis and show both temperature- and light-dependency. For example, see study by Ghirardo et al. (2010) on Norway spruce and other species:
https://doi.org/10.1111/j.1365-3040.2009.02104.x
Section 3.1: Can you include a figure that shows the relative contributions of individual compounds to the total BVOC emissions from Spruce 1 and Spruce 2? For example, you mention that myrcene was the dominant monoterpene emitted from both spruces, followed by β-phellandrene, but this is not illustrated in any of your figures/tables.
L392: Can you specify which BVOC species showed a pronounced increase in emission upon illumination, and which had a more muted response?
L402: Is it C10H14O or C11H14O?
L404: What is the average lifetime of the post-illumination bursts observed in your study?
L522-524: In the text, the CO2 flux is reported in units of nmol s-1 g-1, but in Figure 9, the units for the CO2 flux are in nmol h-1 g-1. One of these units is wrong, please correct it.
L569-571: Furthermore, the value calculated in this work comes from a single Sitka spruce seedling, and therefore may not be an accurate representation.
L582: Fig. 9 shows the time series of CO2 flux. Please cite the correct figure.
L582: Do you mean Table S6 and S7?
L599-600: Some monoterpenes originate from de novo synthesis and show both temperature- and light-dependency, e.g., see Ghirardo et al. (2010). You have also shown this in your own results.
L686: It might not be appropriate to scale the emission results from a single seedling to determine the BVOC emission fluxes for all Sitka spruce plantations in Ireland. For a more accurate representation, measurements on additional seedlings would be required to account for intraspecific variability.
L705-710: Furthermore, the BVOC emissions of mature trees can differ from those of younger seedlings.
Supplementary Information:
Table S2: What do the different colors (red and yellow) represent?
Figure S7 and S8: Please confirm whether the units for the CO2 fluxes are supposed to be in nmol s-1 g-1 or nmol h-1 g-1.
Table S6 and S7: The units for PPFD and emission flux are wrong. Please correct these.
Technical corrections:
L13: The scientific name (Picea sitchensis) should be italicized.
L25: Replace “Stika spruce” with “Sitka spruce”
L62: Replace “Stika spruce” with “Sitka spruce”
L83: Replace “For identification purposed the trees were named;” with “For identification purposes the trees were named:”
L110: Replace “Viasala” with “Vaisala”
L155: Replace “Agilent 5977B MDS” with “Agilent 5977B MSD”
L297: Replace “Fig 2 and S2” with “Fig 2 and Table S2”
L402: Replace “six BOCs” with “six BVOCs”
L446: Replace “measure data” with “measured data”
L490: Replace “measure data” with “measured data”
Figure 8 caption: Replace “measure data” with “measured data”
L589: Replace “Stika spruce” with “Sitka spruce”
L600: Replace “Stika spruce” with “Sitka spruce”
L729: Replace “Stika spruce” with “Sitka spruce”
L747-751: The units for PPFD are incorrect.
Citation: https://doi.org/10.5194/egusphere-2024-154-RC1 -
RC2: 'Comment on egusphere-2024-154', Anonymous Referee #2, 05 Apr 2024
The manuscript quantifies the BVOC emission rates, potentials and blends of Sitka spruce, a species that may be a locally important BVOC source where it is used in plantation forestry. The manuscript highlights the potential of new gas analysers for producing a fuller understanding of total BVOC emissions by including traditionally harder-to-measure oxygenated compounds. With these data, the manuscript aims to produce an estimate for total BVOC emissions from Sitka spruce in Ireland. The question of quantifying BVOC emissions by a tree species that is used in plantation forestry is important, and using two analytical methods for detecting the BVOCs is a valuable contribution. Classifying the compounds by their emission pathway (pool emissions, de novo emissions or combination of both) is also interesting and in line with current research.
The major problem with the methods and interpretation of the results is the small number of study trees. Three to start with is not a lot, and throughout the manuscript, the three trees are reduced to only one. This is a normal problem in biological sciences, but the severe limitations in data should be considered carefully when analysing and using it to form conclusions. Here, the calculations on tree carbon balance and BVOC total emissions are only based on data from one seedling, which is not sufficient to be useful considering that it does not even allow calculating uncertainties in the estimates. Furthermore, the authors themselves also show that there is a large variation in BVOC emission rates between the three trees, so choosing only one of the trees to represent all the trees of the plantations seems weakly justified. Another related problem is the discussion around plant stress. A potential unrecognised stress was used to explain the differences between trees 1 and 2, and based on all the results shown it indeed seems that tree 2 was not performing at the same level as tree 1. Yet, these results are not sufficient to make claims on how stress affects BVOC emissions from Sitka because 1) the stress was unknown and not controlled and 2) again, there are only two trees that are compared to each other, and we cannot know which is “the normal”. In other cases, stress can also increase BVOC emissions from pools.
The manuscript is for the most part very easy to follow and contains most of the pertinent information on methods and calculations (for a couple of further questions on the statistical testing and model selection, see the specific comments below). In particular, the experimental setup was well described which I greatly appreciated. The manuscript results and discussion part is lengthy, especially relative to the small scope of the results. The manuscript could be balanced by condensing and focusing the results and discussion section on one or two questions that can be answered with the given data, and fully removing the parts on carbon balance and BVOC emissions upscaling.
Specific comments:
Abstract:
- The abstract is lacking a bit of the motivation for why the study was done, what the overarching objective or question was, or the context of the study
Introduction:
- I like that the introduction is not too long, but concise and to the point. It also introduces the important concepts for the further manuscript.
- Line 48: age and stress are not environmental conditions or parameters, rather a status or condition of the plant.
- Lines 51-60: these could be combined into one paragraph? And the part on standardisation of the BVOC emission rates could be shortened to a simple phrase, e.g. “ because of the strong dependence of BVOC emission fluxes on the temperature and PPFD, BVOC emission fluxes are often reported at standard conditions of 30 °C and 1000 μmol m-2 s-1 to facilitate comparison between studies (Guenther et al. 1993).”
- Lines 39-47, 74: You could make it even more clear to the reader why it is important to know the emission pathway for each compound. I suspect your motivation here is that the emission pathway is important so that the emission fluxes can be better upscaled using T, or T and PAR?
Materials and methods:
- Line 79-84: So do I understand correctly, these trees were 4-yr old when they were used in the BVOC measurements? Did they have similar or different genetic backgrounds?
- Line 90: The PPFD of 250 µmol-2 s-1 feels low. Was this a limitation of the chamber or chosen based on the mean conditions in Ireland?
- Line 110: “Viasala” -> “Vaisala”
- The growth chamber set-up is very clearly explained, thank you!
- Line 121: You could consider moving the section 2.3 Experimental procedures here (after section 2.1), before giving the details on the gas analysers and auxiliary measurements. For me, it would make most sense to first read how the sampling of BVOCs was done, and then read on how they were analysed.
- Line 136: Isoprene was not calibrated as isoprene?
- Line 147: Were there any large peaks (potentially large emissions) that could no be determined, and could thus bias the results? Or were they mostly small peaks?
- Line 162: Capital T for “the”
- Line 202: during the adaptation time, was the BVOC enclosure also already installed?
- Line 241: The equation is missing the “flow” (through the enclosure)
- Lines 246-249: Did you test this separately for each tree or all trees together? And you used all half-hourly points of tree enclosure and empty enclosure measurements? Was the t-test paired (one tree enclosure measurement corresponded to the empty enclosure measurement closest in time)?
- Line 250: In figure S1 tree 3 does seem quite dead but still it is surprising that you did not get any significant BVOC signal. Do you have a guess why it did not show any significant signal? Did you measure chlorophyll fluorescence and CO2 flux from that tree?
- Line 258: For comparing with other emission values in literature, it would be good to also calculate emissions per needle mass or area (as most other studies tend to do). Adding the branch will make a big impact on the calculated emission rate as branch wood is likely much heavier than the needles.
- Line 275: You could give also here the value you used as the maximum enzyme activity temperature
- Line 290: This paragraph is not quite clear to me. By coefficients, do you mean the coefficients for Epooled and Ebiosynthesis as: Ecombined= a*Epooled + b*Ebiosynthesis? Or coefficients within the Epooled and Ebiosynthesis functions? Were the coefficient values you used in this study the exact same ones as Schuh et al 1997, or fitted for your data?
Results:
- Line 302 or line 315: Somewhere around here, you could add a sentence on how well the two gas analysers captured the same compounds (shown in Table S2).
- Line 308: For which species these examples are? It would be most fruitful to only compare to the same species, or at least the same genus.
- Line 319: There could be a figure or table where the emission rates or contributions of the most common compounds per tree would be shown, also in the main text. Table or a pie chart or bar chart, for example.
- Line 334: Was the resin running or solid? Was it within the enclosure? Exposed fresh resin can be a huge emission source of monoterpenes and dominate the BVOC emission measurements. If this is the case, the measured values should not necessarily be used for further calculations, because they are strongly biased by the exposed resin. Resin on stem surface is a normal occurrence but quite annoying when trying to measure BVOCs from shoots.
- Lines 347-359: Here and elsewhere in the results/discussion, it would be helpful to the reader if you referred to the tables / figures in the manuscript when describing your results. Here for example, you could refer to Table S2?
- Line 362: “six of the detected BVOC..” rather than six of the emissions
- Section 3.2. The figures could be condensed in this section. Figures could cover shorter time spans, for example, 3-4 days - or overlay the days as in Figure 6. The texts also could be condensed a little: avoid describing too many details that are easy to see in the figures but concentrate on the most important general tendencies.
- Line 394: could you add a few other compounds in Figure 3 to show the different responses by different compounds
- Line 402: Figure 4 y-axis says C11H14O
- Line 449: refer to the figure 6
- Line 455: this could be shown in a figure?
- Line 463: You say that the emissions were more strongly influenced by temperature, but is it not rather that they were less influenced by light? I.e., the temperature impact may still be the same.
- Line 474: Figure 7 shows that also temperature varied during the Light Cycle. Is this because increasing the light level also increased temperature although you intended the temperature to remain at 18 ᵒC? With the increase in temperature during the light cycle it is hard to say how much the increases or decreases in emission rate are due to light level changes and how much due to the consequent temperature variations.
- Line 495: Change the reference format to match the rest of the manuscript
- In order to streamline the manuscript, the CO2 fluxes could be covered in a much shorter format. You could only show the difference in CO2 flux rates between tree 1 and tree 2 to support your hypothesis on the lower level of functioning (stress) of tree 2 (lines 546-553). You could move this part to the beginning of the results section – it would be interesting for the reader to know before looking at the BVOC emission rates that the tree 2 is photosynthesising at a lower level than tree 1. With that, you could also mention that the chlorophyll fluorescence did not differ between the trees, which is surprising. No need to discuss or show the diurnal patterns of CO2 flux, because they are as expected based on respiration and photosynthesis (positive flux in the dark, negative flux in the light).
- Line 521: plant growth produces CO2 because of mitochondrial respiration used as energy source (contributes to positive CO2 flux)
- Section 3.4.2 This is interesting in the sense that you were able to add many more BVOCs in calculating the carbon balance than normally is possible (with instrumentation that does not capture all compounds you could). However, these results are based on only one tree in lab conditions, measured over a short time period, so the usefulness of the results is very limited. The calculation also includes the bias that some of the BVOC emissions included here do come from pools, so the carbon released in their emissions is carbon that has been captured days or months beforehand.
- Sections 3.5.1: I think these tests for the emission pathway are quite interesting, but the discussion could be shortened. You could consider focusing on what were the proportions of total BVOC emissions that were pool emissions, de novo emissions, or both, and how this differed between your two trees. As you anyway don’t show the emission data from the measurement cycles and models for all compounds, you do not need to discuss each of them in a lot of detail. One option could also be to add plots or tables for all compounds in the supporting materials (or actually tables S6 and S7 would already be enough) and guide the reader there in case they are interested in the emission pathway for a specific compound or compound group.
- Lines 578-582: This should be added to the methods. In addition, how did you determine which method best reproduced the emission profile? Visually based on the figure or with some goodness-of-fit metrics?
- Line 615-620: This should be in the methods
- Line 629: Did you calculate a correlation or is this based on the visual assessment of the figure?
- Line 635: for comparisons, you could pull out other studies on conifers (with monoterpene pools in needles), for example see Ghirardo et al. 2010 (https://doi.org/10.1111/j.1365-3040.2009.02104.x)
- Section 3.5.2: I understand the wish to try and upscale the BVOC emissions to see potential total emissions from the Sitka spruce plantations. However, with the emission data that is only based on one seedling, you cannot even really get an estimate for the uncertainty in the calculation. I would propose adding the comparisons from Table 2 at the end of the previous section and removing the emission upscaling part of the manuscript.
Figures:
- Figure 1 is really nice, clear and helpful for understanding the measurement system!
- Figure 2: is this showing results from Tof-CIMS and TD-GS-MS or one of them? Clarify that in the legend. In this figure, the downward columns make me think of negative emissions (deposition), although of course that is not what the figure wants to show. Consider dividing the figure into two subfigures, one per tree, that are stacked one on top of the other and that have the y-axis going from 0 to 17 from bottom up. So, flipping tree 2 around. This would help to avoid misreading the figure, which otherwise is nice and a good idea on how to show the emission spectra.
- Figures 3-5, 9: these don’t need to show the whole time series, and I’d recommend also doing the same as you did in Figures 6 and 7 – overlay all days in one figure
- Tables S6 and S7: it would be interesting if these two tables were combined, it would allow better comparison between the compounds
Citation: https://doi.org/10.5194/egusphere-2024-154-RC2
Data sets
Sitka Spruce code and data Hayley Furnell https://doi.org/10.5281/zenodo.10514476
Model code and software
Sitka Spruce code and data Hayley Furnell https://doi.org/10.5281/zenodo.10514476
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