Reviewer Comments
This manuscript explores possible future scenarios of both increasing woodland cover in the UK from 13% to 17-19% and the impacts that may have on future BVOC emissions under current climate and potential future climate (predictions of climate in 2050) scenarios using the Community Land Model (CLM) (v4.5) with the embedded MEGAN model.
It is my opinion that while the use of regionally appropriate data and realistic woodland planting scenarios for estimating BVOC emissions is commendable and much needed, the manuscript's exclusive focus on modelled BVOC outputs, without examining their implications for air quality or climate, may have limited is scientific impact. As BVOC emissions are only one component of a broader assessment, I suggest that the study does not significantly advance beyond previous research, especially considering the substantial uncertainties associated with how the BVOC emission plant functional types are derived in this or any modelling study, and including the additional uncertainties that underpin the measurement studies that these emission potentials are often based on. It is no surprise here that changing the landcover type to be a higher or lower emitter of isoprene or monoterpene results in a high modelled emission of isoprene or monoterpene for example relative to a substitution of the grass landcover type in the baseline runs in the model. To what extend do these BVOC emissions matter? In addition to these points, the uncertainties in the location of the woodland to be created in the UK, although I acknowledge that the latest informed current and best informed efforts by the authors has been applied to try to overcome this, which is again commendable, it still cannot be overlooked that this is an additional uncertainty in the BVOC emissions. However, incorporating analysis of how these more regionally specific BVOC emissions, created landcover types and the more realistic planting scenarios could potentially impact air quality and/or climate in the UK, as suggested in the manuscript's opening introduction and concluding sentence, would substantially strengthen its contribution as it would enable a better understanding to how the magnitude of BVOC changes due to increased woodland cover, as described in the manuscript, may impact the UK more widely. Overall, I feel the manuscript would benefit from further development and a substantial revision to meet the publication standards of the journal.
I have made additional suggestions for several moderate to minor corrections below, some of which feed into the overall opinion detailed above.
Lines 11- 13 “Woodlands also have the potential to degrade air quality, due to the emission of biogenic 11 volatile organic compounds (BVOCs) which are precursors to major atmospheric pollutants, ozone 12 (O3) and particulate matter (PM)”
Please rephrase this statement as the emphasis is on the woodlands degrading air quality when in fact woodlands may be a benefit on air quality too. I suggest something along the lines of Woodlands emit VOCs….which are know precursors to pollutants and its these which degrade air quality.
Lines 26-27 “ Our study highlights the potential for net-zero aligned afforestation to have substantial impacts on UK BVOC emissions, and therefore air quality, but also demonstrates routes to minimizing these impacts through consideration of the emissions potentials of tree species planted.”
I suggest given that the year for which the simulations were run was a heatwave and subsequent drought year for the UK and the meteorology (temperature/sunshine hours) and soil moisture may reflect this which has an influence on the BVOC emissions I would specify this in the abstract to make clear to the reader the focus of the manuscript by highlighting this specific example relates to the suggested potential for BVOCs in a warmer and drier, potentially likely future climate effects. This is especially true given the emphasis on elevated CO2 levels too.
In addition, just because BVOC emissions have increased significantly does not mean that in future climates that air quality will be significantly or adversely affected. The correlation is more complex given the future climates could see a reduction in other anthropogenic precursor compounds related to air pollutants.
Line 30 – I would argue that the pathways are still at this point “suggested pathways” for the mitigation of climate change and just one of several some of which may or may not involve trees.
Line 46-52 – There have also been other suggested benefits of tree planting in the UK which include a reduction in PM 2.5. To give the manuscript a balanced perspective this point should be added to the list in lines 46-52. Please note the following publications that discuss this:
1. Effects of Vegetation on Urban Air Pollution (2018), Air Quality Expert Group Report, DEFRA. https://uk-air.defra.gov.uk/assets/documents/reports/cat09/1807251306_180509_Effects_of_vegetation_on_urban_air_pollution_v12_final.pdf
2. E. Nemitz et al., “Potential and limitation of air pollution mitigation by vegetation and uncertainties of deposition-based evaluations,” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 378, no. 2183, Oct. 2020.
3. Purser, G., Heal, M. R., Carnell, E. J., Bathgate, S., Drewer, J., Morison, J. I. L., and Vieno, M.: Simulating impacts on UK air quality from net-zero forest planting scenarios, Atmos. Chem. Phys., 23, 13713–13733, https://doi.org/10.5194/acp-23-13713-2023, 2023.
Line 50-52 - “There is also the potential for delivery of trade-offs, such as the degradation of air quality potentially associated with the emission of biogenic volatile organic compounds (BVOCs).”
Please add references for this statement.
Line 59 - Trees’ emissions of BVOCs are controlled largely by temperature and light, but also leaf age, atmospheric CO2 concentrations and soil moisture (Guenther et al., 1993; Potosnak et al., 2014; 60 Sharkey, 1996; Zeng et al., 2023).
I suggest rewording for clarity to “BVOC emissions from trees are controlled….. “
Line 63 – Please add a reference for how plant disease increase may impact quantity and composition change to VOCs.
Line 73 – “The emission of monoterpenes is also affected by temperature and light, but the storage of monoterpenes within plants causes distinct patterns in emissions compared to other BVOCs. The emission of BVOCs varies through the day, with a peak in the daytime when incoming solar radiation is at its highest. However, the storage of monoterpenes means their emission is less variable with light.”
I feel this section could benefit from some references and a little more clarity around “de novo” synthesis and release of monoterpenes and those which are emitted from storage pools.
Only some monoterpene emissions have been correlated to both changes in light and temperature and this could be more clearly explained. In addition, I would suggest it is worth making a note of how these daytime releases may vary in light of future climates (heat stress and drought impacted) given this is the focus of the manuscript too.
A suggested publication : Byron, J., Kreuzwieser, J., Purser, G. et al. Chiral monoterpenes reveal forest emission mechanisms and drought responses. Nature 609, 307–312 (2022). https://doi.org/10.1038/s41586-022-05020-5
Line 117 – suggest adding the word “tree” before species in “cover with mixed species afforestation”.
Line 159 – “here is inferred from the UKCEH” suggest adding “…….landcover map (1km x 1km resolution)” to the caption of figure 1.
Line 162-164 – please add a reference(s) for this statement.
” The exact locations for afforestation in the UK are undetermined, though the individual four nations (England, Northern Ireland, Scotland, Wales) have their own ambitions relating to the net-zero aligned planting recommendations.”
Line 172. Please add a reference to this line for the basis of this information highlighted in table 1.
Line 260-269 – In this section the emphasis is on categorising emission scenarios based on low or high emission of isoprene relative to each other and it indicates tree species that these are representative of as a single tree species. However, in the introduction a lot of the emphasis was based on this study focusing on mixed woodland types with emissions potentials for the woodland that was more representative for UK woodland types. Given this generalisation of high/low isoprene/monoterpene scenarios how is this work an advancement on the previous work of Purser et al. (2023) which used a similar high/low isoprene/monoterpene scenarios although based on a single tree species? Is the overall effect that it produces not a similar in this regard? Could it be that the emission potentials used in this study for future woodland increases still be as uncertain as any emission potentials derived from a single specie?
I also wonder if a UK averaged tree species mix is useful in this method, given that the distribution of tree species is likely more regional. For example the tree species mix appropriate for the climate of north west Scotland may be vastly different in comparison to south east England. How can we be sure that this tree species mix is suitable UK wide to understand the changes in increase or decrease in BVOC emissions brought about by future increases in woodland? Is there likely to be some uncertainty here if tree species suitability is not taken into account?
Lines 387-389 “The greatest increase in isoprene was observed in the experiment Afforested_BL_lowMono_highIso, where individual grid cells experienced up to a 250% increase in emissions, and total emissions increased by 1231%.”
Firstly, is this increase relative to the current BVOC emissions or the 2003 with future CO2 500ppm concentration BVOC emissions? I wonder if perhaps this result is no surprise given the scenario is a high isoprene emission potential scenario model run? I wonder if these result are as useful given that we know that the model is designed to replace the grassland land cover type and so either isoprene or monoterpene scenarios? Increases or decreases may result in changes to overall isoprene or monoterpene emissions. Maybe a little more explanation of the impact or significance is needed.
Line 420 – “All experiments result in an increase in monoterpenes.”
Perhaps an explanation could be added after this sentence to reiterate why this might be the case?
Line 458 - “Two of our experiments (Afforested_NL_highMono_lowIso and Afforested_BL_highMono_lowIso) suggest that a reduction in isoprene emissions is possible with an increase in woodland cover of around 50% (or 6% absolute additional woodland cover), when the increase is achieved through planting of either broadleaf or needleleaf species.”
Could you please give an explanation/suggestion as to why this might be the case here. |
This manuscript presents the modelling results of the changes in BVOC emissions from UK afforestation. It is an interesting research topic considering the roles of BVOCs in atmospheric composition and net-zero emission actions. My main struggle with this paper is about the method design.
First, the authors chose to estimate BVOC emissions from CLM with the embedded MEGAN model. To fit into the CLM or climate data resolution, the model ran at very coarse spatial resolution; many higher resolution inputs (emission factor, leaf area index, etc) have to be averaged, or species distribution data have to be lumped, which can bring large uncertainties to the emission estimations. There are a lot of higher resolutions of climate data, which are sufficient to run the MEGAN model alone to get much higher-resolution estimations of emissions. I did not get the idea of choosing the CLM model, and there is no clear description of what variables from CLM (if any) were fed into the MEGAN. I don’t think making decisions based on these coarse resolution maps is so informative. With the coarse resolutions and grouped species, it is still unknown where to grow what species (due to grouped species) to increase or decrease certain BVOC emissions, and the resolution is way too coarse to estimate any air quality impacts.
Then, when converting grass to trees, the model should consider the removed emissions from grass as well.
Lastly, it is unclear why the authors chose to run for one future year, i.e., 2050 and even used the meteorological conditions from 2003. The authors' argument for selecting the meteorological condition from 2003 was based on comparing the maximum 1.5 m air temperature. This does not seem correct to me, considering that temperature is not the only factor influencing BVOC emissions. The year-to-year meteorological variations will undoubtedly affect your current estimations; it does not make sense to only look at 1-year outputs.