High temperature sensitivity of Arctic isoprene emissions explained by sedges

Domain: earth_science
Published: 2024-07-21
Editorial outcome: major_revision

Abstract
It has been widely reported that isoprene emissions from the Arctic ecosystem have a strong temperature response. Here we identify sedges (Carex spp. and Eriophorum spp.) as key contributors to this high sensitivity using plant chamber experiments. We observe that sedges exhibit a markedly stronger temperature response compared to that of other isoprene emitters and predictions by the widely accepted isoprene emission model, the Model of Emissions of Gases and Aerosols from Nature (MEGAN). MEGAN is able to reproduce eddy-covariance flux observations at three high-latitude sites by integrating our findings. Furthermore, the omission of the strong temperature responses of Arctic isoprene emitters causes a 20% underestimation of isoprene emissions for the high-latitude regions of the Northern Hemisphere during 2000-2009 in the Community Land Model with the MEGAN scheme. We also find that the existing model had underestimated the long-term trend of isoprene emissions from 1960 to 2009 by 55% for the high-latitude regions.

Introduction
Rapid climate change in the Arctic is strongly influencing terrestrial ecosystems1,2. The change in both climate and ecosystems could alter the atmospheric chemistry and composition in the Arctic atmosphere through biogenic volatile organic compounds (BVOCs) emitted by plants3–5. Since the Arctic has limited anthropogenic VOC sources, BVOCs have a key role in high-latitude atmospheric chemistry6. Because BVOCs are the main precursors of secondary organic aerosol (SOA)7, changes in BVOC emissions will likely affect the quantity and characteristics of SOA and, thus the climate system in the Arctic8–11. Furthermore, the rise in BVOC levels could decrease the atmospheric oxidation capacity and prolong the lifetime of methane, thereby exacerbating global warming11,12.

Isoprene is the most abundant reactive BVOC emitted globally and in the Arctic3,13,14. Isoprene can help vegetation tolerate abiotic stresses15, and isoprene can act as a signaling compound to stimulate plant defense mechanisms during stress periods16. Isoprene is synthesized from dimethylallyldiphosphate (DMADP) derived from the methyl erythritol 4-phosphate (MEP) pathway through the enzyme isoprene synthase (IspS)17. Isoprene emission is controlled by environmental conditions, especially temperature and solar radiation18. Thus, a rapidly warming climate in the Arctic is favorable for increasing the emission of isoprene19–22. The temperature response curves of isoprene emission, used in the current earth system models (ESMs) and the chemistry transport models (CTMs), are based on measurements of a few temperate plants13,23, and a typical isoprene temperature response curve has a Q10 of about 3, which is thought to be driven by the influence of temperature on substrate supply and the activity of IspS17. However, recent whole-ecosystem measurements suggest that the temperature response of isoprene emissions in high-latitude tundra ecosystems has a Q10 over 8, which is also much higher than that predicted by the widely used BVOC emission model, the Model of Emissions of Gases and Aerosols from Nature (MEGAN)21,24–28,29. In contrast, leaf/branch-level studies showed that the Arctic willow species (Salix pulchra, Salix glauca, and Salix myrsinites), which are one of main isoprene emitters in high-latitude tundra ecosystems, have a short-term temperature response that is similar to temperate plants25,30. Our previous study also confirmed that the hourly temperature response curve of Salix spp. is consistent with that of temperate plants as well as the MEGAN model13,31. Additionally, we found that the isoprene emission factors of willows show a greater-than-expected response to the mean ambient temperature of the previous day31. Nonetheless, we concluded that the temperature response of willows in the Arctic is greater than that predicted by current models but still cannot fully explain the high-temperature sensitivity of isoprene emissions from high-latitude ecosystems31. Consequently, species-specific investigations are necessary to further explore the strong temperature responses observed at the ecosystem level.

In this study, we identified sedges (Carex spp. and Eriophorum spp.) as the key contributors to the pronounced temperature sensitivity of isoprene emissions in the Arctic ecosystems. Sedges exhibit a more significant temperature response than both willows and MEGAN. Additionally, we observed that sedges can adjust their temperature sensitivity and emission capacity, which is represented by the emission factor in this paper, in response to changes in ambient growth temperatures. We integrated these findings into the MEGAN v2.1 model13, enhancing its capability to simulate observed ecosystem-scale flux measurements. Moreover, the updated MEGAN model projects a 20% increase in Arctic isoprene emissions and a 55% rise in the long-term isoprene emission trend. Given the ongoing intensification of global warming, the changes in isoprene emissions have the potential to substantially alter atmospheric chemistry in high-latitude regions.

Results
Isoprene from sedges is sensitive to temperature changes
Our species-level gas exchange chamber experiments showed that the main isoprene emitters among the species measured at Toolik Field Station (TFS), Alaska, USA, are sedges, a major component of Arctic graminoid plants, and willows, a major component of Arctic woody plants (Supplementary Fig. 1). Other studies have also indicated that the Salix spp., Carex spp. and Eriophorum spp. exhibit significantly higher isoprene emission levels compared to other tundra species, e.g., Betula spp. and Cassiope spp25,30,32–34. and Sphagnum spp35–37. The temperature response of willows in the Arctic cannot explain the high-temperature sensitivity of isoprene emissions from high-latitude ecosystems31.

Arctic sedges studied here show a more pronounced temperature response than other plant species, including Arctic woody willow shrubs and any of the plant responses used to develop the MEGAN model. Our data confirm that the sedges are responsible for the heightened temperature responses of isoprene emissions from high-latitude ecosystems (Fig. 1a and 1b). We calculated the Q10 coefficient for isoprene emissions from sedges (Carex spp. and Eriophorum spp.) and willows between 25 and 35 °C. The Q10 coefficient represents the isoprene emission rate change with a 10 °C rise in the leaf temperature. The Q10 values of Carex spp. (15.6 ± 8.8) and Eriophorum spp. (9.1 ± 7.0) are much higher than the Q10 of the Arctic willows (3.2 ± 1.8), which is close to the Q10 of the MEGAN model (2.91) (Supplementary Fig. 2). We applied the Arrhenius equation to model the exponential temperature response curves of Carex spp. and Eriophorum spp. (refer to the Methods section), where the activation energy (Eq. 5) denotes the temperature sensitivity of isoprene emission. Our findings indicate that the temperature response curves of both Carex spp. and Eriophorum spp. exhibit high-temperature sensitivity (or high activation energy) up to 35 °C (Supplementary Fig. 3). However, the activation energy and R2 decrease beyond 40 °C, suggesting a slower increase rate of isoprene emissions from both species (Supplementary Fig. 3).

Moreover, there is an inverse relationship between the temperature sensitivity and emission capacity (or emission factor) of isoprene in sedges, both of which are acclimated to the temperature history of the previous days (Fig. 1c). We used the 10-day average temperature as an indicator of the recent growing environment. This choice is based on the Pearson correlation coefficient for the temperature sensitivities (activation energy in Eq. 5) and emission factors in relation to the mean temperature of the preceding 1 to 15 days for Eriophorum spp. (Supplementary Fig. 4). The use of a 10-day average temperature also follows the current framework of the MEGAN model but the actual time period influencing isoprene emission is uncertain. With higher 10-day average temperatures, Carex spp. and Eriophorum spp. exhibit lower temperature sensitivity of isoprene emissions, as indicated by a decrease in activation energy, and higher emission capacity, or emission factors, compared to those in colder environments. Isoprene emissions are controlled by the enzyme activity and supply of substrates17. In addition to these two factors, we speculate that the pronounced response of sedges to high temperatures could also be related to enzyme accumulation. The mRNA levels and IspS protein concentrations are likely low in sedges in a cold environment until a warm environment or a specific temperature threshold initiates the gene expression necessary for IspS synthesis. The IspS protein has a half-lifetime of 5.3 days in a 20 °C environment38. Its accumulation, following the onset of gene expression for IspS synthesis, would result in increased isoprene emissions as conditions transition from cold to warm. When the basal level IspS reaches the maximum, isoprene emission would become primarily dependent on enzyme activity and substrate availability17,39. One potential piece of evidence is that the isoprene activation energy of Eriophorum spp. after a warming period is comparable to that of aspen (Populus spp.) at 72.1 kJ mol−1 39 and that of MEGAN at 95 kJ mol−1 13, suggesting that IspS levels are no longer limiting factors, and the emission patterns resemble those of temperate isoprene emitters (Fig. 1c). It has been reported that the isoprene emission of the Arctic sedges varies with the air temperature40,41, and our results provide evidence that the sedges are sensitive to both minutes-hours scale temperature change as well as day-weeks change.

Updated MEGAN model can better explain the flux measurements
The MEGAN v2.1 model, which has 16 plant functional types (PFTs) each with a uniform temperature response curve, cannot fully capture the observed variability in isoprene emissions in Arctic regions. To address this, we updated the temperature response curves in MEGAN version 2.113,42 and evaluated the model using eddy-covariance flux measurements from three high-latitude sites. Supplementary Table 1 shows that sedges are the dominant isoprene emitters at the Abisko-Stordalen and Siikaneva sites, while the Finse site is dominated by both sedges and willows. In this study, the PFT categories of Arctic grass and boreal broadleaf deciduous shrubs are updated based on our measurements.

The Arctic grass adopted the temperature curve of sedges (Eq. 5 in the Methods) with a dynamic activation energy (Eq. 6 in the Methods) and emission factor (Eq. 7 in the Methods) derived from Eriophorum spp. measurements in this study (Supplementary Fig. 5). The boreal broadleaf deciduous shrub used the temperature response curve of MEGAN v2.1 (Eqs. 2, 3, and 4 in the Methods) but included the response of emission factors to the previous 1-day average temperature for Salix spp. (Eq. 8 in the Methods)31. The leaf-level emission factor was 12.0 nmol m−2 s−1 for the Arctic grass and 6.5 nmol m−2 s−1 for the boreal shrub (including willows), respectively, which was based on our glass chamber measurements in this study and Wang, et al.31. Besides temperature response curves, isoprene emissions at the field sites were also affected by the abundance of sedge and willow. To eliminate uncertainties associated with vegetation fraction input, we used the least squares fitting method to adjust vegetation proportions, thereby optimizing the performance of the model in comparison with flux measurements.

The updated model demonstrates an enhanced ability to capture the variability of isoprene flux compared to the default temperature response curve of the MEGAN v2.1 model (see Fig. 2 and Supplementary Table 2). Across the three sites, the updated model consistently shows an increase in R² and a decrease in Root Mean Square Error (RMSE). The updated model can capture both the low values during cold periods and the high values in warm periods more accurately than the default model. In addition, for the Siikaneva site, the model accurately captures the elevated values following heatwaves, after accounting for the effects of warming on temperature response and emission factors. We also compared the fitted fractions for Arctic grass and boreal shrub from the updated model with the observed land cover/vegetation fractions at the Finse and Siikaneva sites (see Table. 1). Our findings indicate that the fitted fractions for Arctic grass and boreal shrub closely match the observations or estimates at Finse and Siikaneva sites.

Estimation of high-latitude isoprene emission
We reported a pronounced temperature response curve for sedges in this study. In addition, our previous research indicated that Arctic willows (Salix spp.) can significantly increase their isoprene emission capacity in response to an increase in the average temperature of the preceding day31. The different isoprene emission patterns of the two types of high-latitude plants (Arctic grass and boreal shrub) both point to an increase in isoprene emissions due to Arctic warming. To estimate isoprene emissions in the high latitude regions (north of 60°N), we updated MEGAN v2.1 in the Community Land Model version 5 (CLM5)43 with the temperature curves from this study for the Arctic grass and boreal broadleaf deciduous shrub PFTs. We used the emission factors reported by Guenther, et al.13. for the CLM simulations. The averaged isoprene emissions estimated by the updated model are 20% higher than the original MEGAN estimate for 2000-2009, increasing from 2.71 to 3.25 Tg yr−1, which suggests that the isoprene emissions from the high-latitude regions in the Northern Hemisphere are underestimated in current ESMs. By updating the temperature response, the CLM simulation shows a decrease in isoprene emissions from Arctic grass-dominated tundra, while observing an increase from high-latitude deciduous shrubs (Fig. 3 and Supplementary Fig. 6). In the updated model, Arctic grass isoprene emissions respond less to low temperatures typical of Arctic climates but increases significantly with rising temperatures compared to the default model (Fig. 1 and Fig. 3). The simulations predict a notable increase in isoprene emissions in the Russian Siberian regions dominated by boreal deciduous shrubs. The model results presented by Stavrakou, et al.44. suggested that the interannual variability of Ozone Monitoring Instrument (OMI) formaldehyde (HCHO) measurements in Siberia could be explained by biogenic isoprene emissions. However, more in-situ measurements are crucial to validate the model and understand the biogenic isoprene emissions in Siberia.

Additionally, we calculated the long-term trend of isoprene emissions and found that the updated temperature response resulted in a 55% increase in the trend of isoprene emissions from 1960 to 2009 in the high-latitude region compared to the default model (Fig. 4). The default model framework could not capture the rapid change in high-latitude isoprene emissions and their feedback on atmospheric chemistry and the climate system. Furthermore, the results showed that an Arctic heatwave could create a significant isoprene burst event. For example, abnormally warm Arctic weather in 1991 and 2001 was predicted to have an ~40% increase in isoprene emissions in high-latitude regions in the Northern Hemisphere (Supplementary Fig. 7).

Increased heatwave frequency45 and general warming could intensify high-latitude isoprene impacts, with significant shifts in isoprene emissions potentially altering local atmospheric chemistry and climate dynamics. The isoprene emitters, including sedges and willows31, would respond to both short-term, intense heatwaves and long-term warming from minutes to weeks scales by increasing their isoprene emissions (Fig. 5). The lower atmospheric oxidative capacity caused by rising isoprene emissions could further increase the lifetime of methane and then exacerbate warming. Boy, et al.12. suggest that a 6 °C warming could increase the lifetime of methane by 11.4% at a boreal site dominated by monoterpene emissions. The pronounced response of isoprene to warming suggested by this study will exacerbate the BVOC–OH–CH4 feedback. Additionally, the increased isoprene emissions could also disturb aerosol46 and ozone formation, as well as aerosol-cloud interactions10,11. BVOCs, including isoprene, act as sinks for tropospheric ozone in the Arctic47, and the increase of isoprene could further diminish tropospheric ozone. However, the increased frequency and intensity of wildfire and anthropogenic emissions from more ship activities48–50, the atmospheric transport of NOx (NO+NO2) and peroxyacetyl nitrate could alter the chemical regime and change the tropospheric ozone and aerosol formation.

Methods
BEARS-oNS campaign and Glass Chamber Experiments
The Biogenic Emissions and Aerosol Response on the North Slope (BEAR-oNS) project aimed to explore the impact of climate change on the interactions among BVOC, aerosol, and climate in high-latitude regions. The BEAR-oNS campaign was conducted during July and early August of 2022 and 2023 to characterize the BVOC emission and investigate the source of aerosols in the high-latitude tundra ecosystem near the Toolik Field Station (TFS) in Alaska (USA, 68.65N, 149.58W). July mean air temperatures at TFS were 9.6 °C in 2022 and 13.1 °C in 2023, with accumulated precipitation at 220 mm and 105 mm for each year respectively.

The temperature response curve experiments investigated the impact of temperature on isoprene emission. The vegetation specimens analyzed in this study are listed in Supplementary Table 3 and Supplementary Table 4. The vegetation samples were collected from the tussock tundra near the Toolik Field Station. The plant samples were detached from soil with the main root system or cut from the main branch and submerged in water in glass bottles. We chose plants that were in good condition without galls or visible damage.

The temperature curve experiments were conducted with a leaf chamber system (see details in the next section). One chamber blank (background) VOC sample from an empty chamber was collected at the beginning of every experiment before putting the plant into the glass chamber. After placing a leaf, branch, or glass dish with moss into the glass chamber, VOC sampling was initiated after the photosynthesis rate stabilized. For Carex 1 and Eriophorum 1 samples, there were only two temperature steps 20 °C and 30 °C, collected after the photosynthesis rate was relatively stable. For other sedge samples, the leaf temperature was ramped up from 15 or 20 °C to 35 or 40 °C in steps of 5 °C. Each temperature step lasted 1 hour, and samples were collected with sorbent cartridges (see next section) during the last 10 to 15 minutes of the hour with a flow rate of 200 cc min−1 for 5 min resulting in a 1 L VOC sample.

Leaf chamber
The leaf-level BVOC measurements were conducted using a custom-made, field-portable glass chamber with environmental controls. The chamber has an internal volume of 0.62 L and is mounted on a thermoelectric cooler assembly (Custom Thermoelectric, MD, USA), which allows for precise control of the leaf temperature. A miniature fan was installed to stir the air inside the chamber. A white LED source provided artificial illumination with a photosynthetically active radiation output of ~1000 μmol m-2 s-1. The ambient air was pushed into the chamber using a diaphragm pump at rates of 0.9–1.0 L min−1, and the VOCs in the inlet airflow were removed by an activated carbon filter. Part of the effluent air from the chamber was sampled onto sorbent cartridges (Tenax TA and Carbograph 5TD; Markes International, UK) for BVOC analysis. Additionally, an infrared gas analyzer (LI-850; LI-COR Biosciences, NE, USA) was used to measure the CO2 and H2O mixing ratios in the influent (background) and effluent airflows; the analyzer was switched between the chamber’s inlet and outlet every 30 s.

Gas chromatography system
The sampled sorbent cartridges were transported to our laboratory at the University of California, Irvine, where they were thermally desorbed using a TD autosampler (Ultra-xr; Markes International). The desorbed VOCs were injected into a gas chromatograph (GC) (7890B; Agilent Technologies, CA, USA) equipped with a 60 m Rxi-624Sil MS capillary column (Restek, PA, USA). The column eluate was channeled to an electron impact ionization time-of-flight mass spectrometer (BenchTOF-Select; Markes International) and a flame ionization detector (FID, Agilent) for compound identification and quantification. A detailed explanation of the GC methodology including the GC oven temperature program, calibration protocols, and measurement uncertainties, is provided elsewhere51.

MEGAN model and temperature response curve
MEGAN is a flexible model framework for calculating biogenic volatile organic compound emissions from terrestrial ecosystems13,23. The isoprene flux in MEGAN v2.1 is calculated as:

F = ε · LAI · Cce · γT · γP · γA · γC · γSM (1)

where ε, Cce, and LAI represent the canopy-level standard emission factor (nmol m−2 s−1), canopy factor (=0.3), leaf area index (LAI, m2 m−2). γT, γP, γA, γC and γSM denote the emission activity factors of isoprene emissions including temperature, solar radiation, leaf age, CO2 inhibition and water stress, respectively. MEGAN considers the BVOC responses to the long-term temperature, defined as the temperature of the past one day or longer, and the current temperature, reflecting changes on a minute-to-hour scale. The default short-term temperature response curve, γT, for isoprene in MEGAN is:

γT = Eopt · CT2 · eCT1 · (1/Topt - 1/T) / (CT2 · (1 + eCT1 · (1/Topt - 1/T)) · (1 + eCT2 · (1/Topt - 1/T))) (2)

T (K) is the leaf temperature. R (=0.008314 kJ mol−1), CT1 (=95 kJ mol−1) and CT2 (=230 kJ mol−1) are the gas constant and the activation and deactivation energies, respectively. Eopt and Topt are:

Topt = 313 + 0.6 · (T24 - 297.15) (3)

Eopt = 2 · e0.05 · (T24 - 297.15) · e0.05 · (T240 - 297.15) (4)

where T24 (K) and T240 (K) denote the mean air temperature of the previous 24 hours and 240 hours, respectively. Equations (3) and (4) for Topt (K) and Eopt represent the long-term impact of temperature on the optimal temperature and the shape of the temperature response curve.

The updated short-term temperature response curve for sedges in MEGAN v2.1 is expressed as:

γTsg = Eoptsg · eCsg/R · (1/303.15 - 1/T) (5)

Csg is the activation energy for the isoprene temperature response of sedges and changes with T240 as:

Csg = 95 + 9.5 · e0.53 · (288.15 - T240) (6)

The impact of T240 on isoprene emission factor of sedge is as:

Eoptsg = e0.12 · (T240 - 288.15) (7)

The PFT of boreal broadleaf deciduous shrub in the updated model adopted the temperature response curve described in the Eqs. (2)–(4) with the parameters in Wang et al. (2023), and the long-term temperature response for boreal broadleaf deciduous shrub in MEGAN v2.1 is updated as:

Eoptwillow = 7.9 · e0.22 · (T24 - 297.15) (8)

To investigate the capacity of different temperature response curves to explain eddy covariance measurements, we wrote Eq. (1) as:

Fflux = Σ (Ei · LAI/LAImax · γT_i · γothers) (9)

where γothers represents the product of Cce, γP, γA, γC and γSM. The impact of water stress is neglected in this study (γSM = 1). γT_i and Ei denote the temperature response and the total emission capacity of the vegetation type i, respectively. In this study, γT_i represented the temperature response curves of two types of vegetation, Arctic grass and boreal deciduous shrub, as described in Eqs. (2)-(8). The LAI is normalized by the maximum value of LAI to eliminate the uncertainties of the absolute values of LAI, and the ratio between LAI and LAImax depicts the relative change of leaf biomass. The Ei represents the total emission capacity of the vegetation i in the canopy and is as:

Ei = ε*i · CFi (10)

ε*i and CFi represent the canopy-level emission factors and the cover fraction of vegetation i, respectively. Canopy-level emission factors were scaled up from the leaf-level emission factors to the scenario with LAI=5 in MEGAN v2.113,26. The leaf-level emission factor for sedges is from the observations for Eriophorum spp. in this study at TFS. The leaf-level emission factor for willows is based on the measurements by Wang, et al.31. We fitted the model to get the optimal Ei for the default model and CFi for the updated model using the least-squares method to get rid of the uncertainties associated with vegetation fraction input.

Isoprene flux measurements
Isoprene flux measurements from three high-latitude sites were used to validate the models. The flux data used in this study were measured in Abisko (Sweden, 68.36°N, 19.05°E) in 2018, Finse (Norway, 60.60°N, 7.53°E) in 2019 and Siikaneva (Finland, 61.83°N, 24.19°E)20,21,52 in 2021. The campaign times and major vegetation types at the three sites are listed in Supplementary Table 1, and more details about the flux measurements can be found in Seco, et al.24. and Vettikkat, et al.26.

Community Land Model 5 and numerical experiments
CLM543 is the land component of the Community Earth System Model (CESM) and can simulate the land surface and terrestrial ecosystem response and feedback to the weather system and climate change. MEGAN v2.1 is coupled to CLM5 as the BVOC emission module. We used CLM5 to test the influence of updating the isoprene temperature response curves on the global isoprene scale. We compared two CLM5 simulations, one with the default MEGAN and the other with the updated MEGAN using the strong Arctic isoprene temperature responses reported in this study. The emission factors are based on the MEGAN v2.1. The model components set is I2000Clm50BgcC5 (http://www.cesm.ucar.edu/models/cesm2/clm/), and CLM5 is driven by the Global Soil Wetness Project Phase 3 (GSWP3) reanalysis climate forcing dataset with a 3-hour intervals. The spatial resolution of the CLM5 runs is 0.9° × 1.25°, and the numerical experiments cover the period from 1950 to 2009 with the monthly outputs. We adopted the default initial condition of CLM5 that comes from the steady state of the model and treated the first 10 years as the spin-up time. Our analysis is based on the model outputs for the years from 1960 to 2009.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The article presents the temperature sensitivity and emission potential of sedges in the\nArctic region. The authors highlight that the Arctic sedges have a larger temperature\nsensitivity, in comparison to other Arctic species e.g. Arctic willow, using environmental gas\nchamber experiments. The authors used MEGAN to simulate and compare the isoprene\nfluxes eddy-covariance measurements at 3 high-latitude sites. The results presented are\nquite interesting and should be useful to the community. The article is well written, and we\nrecommend its publication to Nature Communication, after the authors have addressed the\nfollowing comments.\n1. Why is the 10-day average temperature used for Carex spp, since neither the AE nor EF\nshows a statistically significant value for 5-15 days (Supplementary Figure 4)? I think this is\nimportant to address since the emission potential of Carex spp is the highest. Therefore, it is\nimportant to address the choice of 10-day average temperature for Carex spp. What would\nthe impact of, say, a 2-day temperature range be on the Carex spp emission potential?\nWould it still have a higher emission potential compared to Eriophorum Spp?\n2. Supplementary Figure 4: The right axis is the emission factor, and the left axis is the\nactivation energy.\n3. It is difficult to see the differences in the temperature response for Guenther 2012 and\nSedges for T<30 C. Please consider replotting it in a way that clearly shows the distinct\ntemperature response curves, including, perhaps, an inset plot. Currently, the differences in\nthe temperature range < 30oC is not visible.\n4. Supplement Table 2: There doesn’t seem to be a huge difference in the R2 or RMSE for\nthe 3 sites, especially Abisko and Finse. A hypothesis or t-test would be useful to assess the\nsignificance of the change in RMSE.\n5. Figure 2: Why does the default setup predict higher flux in springtime in Finse, even\nthough the only change from the default model is the updated emission factors and\ntemperature response for sedges and Arctic grass? Is it due to updated vegetation fractions?\nIf so, it would be better to state it in the text. The table T1 can also be updated to indicate\nthe old vegetation fractions in the default model case. I suspect the higher isoprene flux in\nspring is not only due to updated vegetation type; they should also affect the summertime\nflux emissions. Factoring in Figure 1a, if one considers the temperature response curves for\ndefault Megan (Guenther 2012) and the updated emission from the sedges, they appear to\nbe similar for T<30 C, which makes this increase in springtime isoprene flux puzzling.\n6. Figure 3: Why is there minimal relative change in isoprene emissions in the northern\nregions (Norway, Finland and Sweden) where comparisons with the flux measurements\nwere performed? There seems to be really high relative changes in the Russian Siberian\nregion, and it is hard to assess that the changes in the sedges emission potential can have\nsuch a huge impact here? How can you justify the changes in the Russian region without\nadequate comparison to any flux measurement data from the site?\n7. How is the inhibition of isoprene by CO2 treated in the model?\n8. Why is the deactivation energy set to 230 KJ/mol? Figure 1b shows that activation\nenergies can exceed 250 KJ/mol at lower 10-day average temperatures.\n9. Although the authors claim that isoprene emissions changed by 20% between 2000 and\n2009 and speculate on the potential impacts on SOA, O3, and CH4, it would be valuable to\nsee the perceived changes in all species concentrations using global model simulations. At\nleast for the SOA formation, which is pointed to increase, a more sensitive study is required.\nAs higher isoprene emission decreases the OH concentration, there will be less SOA\nformation through the oxidation of monoterpenes. It is well-known that monoterpenes\nhave a much higher SOA formation potential than isoprene.", "reviewer_2": "The comments will come from my post-doctoral student, as this was a common review.", "reviewer_3": "High-temperature sensitivity of isoprene emissions has been reported in different Arctic\necosystems for the last decade or so. The exact reason is not well known. Such high-\ntemperature sensitivity has many implications for our understanding of plant physiology,\nhigh-latitude atmospheric chemistry, and climate systems. This manuscript has contributed\nto this knowledge gap in two ways. They have identified that sedges, and not all Arctic\nvegetation, exhibit a very strong temperature response, thus key contributors to some of\nobserved the ecosystem level sensitivity. Using the chamber experiments, this work also\nexplored the mechanism for such high-temperature sensitivity in Arctic plants and used the\ninformation to inform an updated MEGAN algorithm to account for it. It shows that the\nupdated MEGAN can simulate the landscape/ecosystem level isoprene fluxes better than\nthe default MEGAN, particularly in those low-emission periods. The results of how\ndifferences in MEGAN perditions between the updated and default MEGAN for current and\nhistorical emissions are also of high interest.\nThe update in MEGAN to account for such high-temperature sensitivity in high-latitude\nvegetation has been expected from the community for a while. I am pleased to see the\nauthors report the update here. I expect it will be incorporated into many other\ncommunity models and thus will help facilitate more research into this topic. The manuscript is well-\nwritten, and the experiments are well-designed. I don’t have any major concerns about the\npublication of this work. As I said, the updated MEGAN is a much-needed development.\nA few minor questions/suggestions:\n- Emission factor response to high-temperature sensitivity vs temperature response\ncurve/gamma T: How to attribute some of the sensitivity to the changed emission factor\n(Arctic willow), while in other species (sedges), it is due to the temperature response curve?\n- Has the updated MEGAN changed the emission factors for Arctic vegetation? I was not\nsure if it had been updated or not.\n- There appears to be a bit of repetition between the last paragraph of the ‘Abstract’ and\nthe first two paragraphs in ‘Results and Discussion’.", "reviewer_4": "This manuscript conducted species-specific chamber experiments, which demonstrate a\npronounced temperature response of isoprene emissions from Arctic sedges. This finding\nsheds light on previous underestimations of the temperature effect on isoprene emissions\nin high-latitude tundra ecosystems. By updating this temperature response curve for sedges\nand the dependence of emission capacity on the previous day temperature for willows, the\nmanuscript shows a substantial underestimation in both the magnitude and long-term trend\nof isoprene emissions in the high-latitude regions. These findings are novel and of\nsignificance to improve model simulations of BVOC emissions. I’d recommend accept this\nmanuscript after addressing the following comments.\n1. To make the manuscript suitable for Nature Communications, I’d recommend the authors\npresent a more comprehensive introduction of BVOC emissions by emphasizing the\ninterdisciplinary nature of their study. In particular, I’d recommend the authors explain why\nplants emit isoprene, and biologically why isoprene emissions are sensitive to temperature,\nand why there is large uncertainty of the temperature sensitivity of isoprene emissions.\nI found the discussions about the enzyme activity very interesting, but I don’t see any\ndiscussions about this in introduction.\n2. The manuscript is overall well-structured, but the methodology section could benefit\nfrom further clarification to enhance its comprehensibility. For example, in the MEGAN\nmodel and temperature response curve section, the authors wrote “The updated short-term\ntemperature response curve for sedges”, “long- term temperature response for boreal\nbroadleaf deciduous shrub”. The manuscript does not explain the short-term vs. long-term,\nand the 10-day average temperature is considered as short-term?\n3. The temperature sensitivity of isoprene emissions from sedges is mainly derived from\nshort-term experiments, but it’s unclear to me whether the short-term temperature\nsensitivity will hold true at longer time scales. With the global warming, plants may adapt to\nwarmer environment, and their sensitivity to temperature may also change.\n4. The manuscript concludes by discussing the likely increased high-latitude isoprene\nemissions in response to increasing heatwaves and general warming and the impacts on\nregional chemistry and climate system, but such discussions are most qualitative. To\nfortify the manuscript's impact, it would be beneficial to delve into the broader\nimplications by quantitatively assessing the impacts of emission changes on global\nradiative forcing and atmospheric chemistry.\nMinor comments:\n1. Regarding the leaf chamber experiments: previous studies have suggested a dependence\nof BVOC emissions on soil moisture, but in the chamber experiments plant samples were\ndetached from soil and submerged into water, how might this affect the results?\n2. “Isoprene flux measurements”, add one sentence to state what the flux measurements\nare used for to improve readability.\n3. What are the uncertainties on the isoprene emissions by assuming 10-day average\ntemperature in the framework?\n4. For Fig. 2 model prediction vs observation, is R2 similar for cold and warm periods? Why\ndoes R2 vary a lot across different sites?\n5. Supplementary Fig. 4: AE on left axis, EF on y axis.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review and\nto provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Reviewers\nWe have revised the paper according to the reviewers’ suggestions and comments.\nIn addition, we added one more corresponding author, Riikka Rinnan. The point-by-point\ncomments are given below.\nReviewer #1:\nThe article presents the temperature sensitivity and emission potential of sedges in the\nArctic region. The authors highlight that the Arctic sedges have a larger temperature\nsensitivity, in comparison to other Arctic species e.g. Arctic willow, using environmental\ngas chamber experiments. The authors used MEGAN to simulate and compare the\nisoprene fluxes eddy-covariance measurements at 3 high-latitude sites. The results\npresented are quite interesting and should be useful to the community. The article is\nwell written, and we recommend its publication to Nature Communication, after the\nauthors have addressed the following comments.\nResponse: Thank you so much for your precious time and comments. We are very\nwilling to revise the paper according to your comments and suggestions. The point-by-\npoint comments are given below.\n1. Why is the 10-day average temperature used for Carex spp, since neither the AE nor\nEF shows a statistically significant value for 5-15 days (Supplementary Figure 4)? I think\nthis is important to address since the emission potential of Carex spp. is the highest.\nTherefore, it is important to address the choice of 10-day average temperature for\nCarex spp. What would the impact of, say, a 2-day temperature range be on the Carex\nspp emission potential? Would it still have a higher emission potential compared to\nEriophorum Spp?\nResponse: Thank you so much for your question. Species of the grass-like Carex spp.\nand Eriophorum spp. both belong to the Cyperaceae family, and thus we have assumed\nthey would exhibit similar behaviors under changing temperatures until there is more\nevidence that there is a difference in their behavior. Since we have more measurements\nfrom Eriophorum spp. (n=9) than from Carex spp. (n=5), we had greater confidence in\nusing Eriophorum spp. data to construct the model used in this study for grass tundra\necosystems. Additionally, the relatively small number of measurements for Carex spp.\n(n=5) is a contributing factor to why Carex spp. did not show statistical significance for\nthe 5-15 day period, even though the correlation coefficient was very close between the\n2-day and 5-10 day data. In the main manuscript, the decision to use a 10-day average\ntemperature as the indicator for long-term warming was primarily based on the\nEriophorum spp. measurements, but this approach may carry some uncertainty. More\nrigorous follow-up experiments, with a larger number of replicates in a walk-in\nenvironmental chamber, are needed to investigate if there are differences between\nthese genera and to determine the precise timescale for this long-term impact.\nIn addition, Carex spp. did not exhibit a higher emission capacity than Eriophorum spp.,\nbut Salix spp. did (Fig. R1). Salix spp., which are isoprene-emitting shrubs in the Arctic,\nwere thoroughly investigated for their temperature response in the Arctic environment in\nour previous study (Wang et al., 2024). As shown in Fig. R2, the minute-to-hour\ntemperature response of Salix spp. closely matches the Model of Emissions of Gases\nand Aerosols from Nature (MEGAN). However, the emission capacity of Salix spp. is\nhighly correlated with the average temperature of the preceding day (Fig. R3). For the\nmodeling component of this study, we utilized the temperature response curve derived\nfrom our previous study (Wang et al., 2024) to update the model for boreal shrubs as\noutlined in the Methods Section.\n[... (omitted figures R1-R6 for brevity as per instructions to keep verbatim text)]\n2. Supplementary Figure 4: The right axis is the emission factor, and the left axis is the\nactivation energy.\nResponse: Thank you for pointing that out. We have corrected that.\n3. It is difficult to see the differences in the temperature response for Guenther 2012 and\nSedges for T<30 C. Please consider replotting it in a way that clearly shows the distinct\ntemperature response curves, including, perhaps, an inset plot. Currently, the\ndifferences in the temperature range < 30°C is not visible.\nResponse: Thank you so much for your suggestion, and it is very helpful! The updated\nFig. 1 with the caption at Line 186 is as:\n“Fig. 1. (a) presents the temperature responses of isoprene emissions from this and previous\nstudies in the northern high-latitude regions. (b) is the same plot as (a), but only for temperatures\nunder 30°C; (c) shows the relationship between the isoprene temperature sensitivities and\nemission capacities of sedges. The short-term temperature response curves of sedges up to 35 °C\nfrom this study is showed by the orange solid line, and the orange shading represents the 95%\nconfidence intervals. The short-term temperature response curves of tundra ecosystem from previous\nstudies are also presented by lines with different colors and patterns. GC, BC and EC represent ground\nchamber experiments, branch chamber experiments and eddy-covariance measurements. The\ntemperature response curves are normalized to the emission level when the leaf temperature equals\n30 °C. The temperature curves in Tang, et al. 39 and Li, et al. 22 came from the ground chamber\nobservations of mixed local vegetation at the Abisko site. Li, et al. 22 also did the branch chamber\nexperiments for Salix myrsinites L. (purple solid line). The site in Seco, et al. 18(2020) is located in a\nsedge-dominated fen near the Abisko-Stordalen site. The Abisko measurements in Seco, et al. 21 (2022)\nhappened at a different location within the same Abisko-Stordalen area on an ombrotrophic permafrost\nplateau. The Finse site in Seco, et al. 21 (2022) is a tundra with mixture of fen and heath vegetation with\nshrubs and lichens. The Siikaneva site is in a fen dominated by moss, sedges and dwarf shrubs, and\nsurrounded by Scots pine forest.23 (c) presents an inverse relationship between the activation energies of\nthe isoprene temperature response and the isoprene emission factors for Eriophorum spp. (circle) and\nCarex spp. (triangle). The green dashed line in (b) shows the activation energy in the Model of Emissions\nof Gases and Aerosols from Nature (MEGAN)13. The colors of markers denote the average temperatures\nover the previous 10 days, and emission factor is defined as the level of isoprene emission at a leaf\ntemperature of 30 °C and a photosynthetic photon flux density (PPFD) of 1,000 μmol m−2 s−1.”\n4. Supplement Table 2: There doesn’t seem to be a huge difference in the R2 or RMSE\nfor the 3 sites, especially Abisko and Finse. A hypothesis or t-test would be useful to\nassess the significance of the change in RMSE.\nResponse: Thank you so much for your suggestion. RMSE is a single-number metric,\nand we cannot apply the t-test to RMSE. Instead, we assess the significance of model\nerror change (mean absolute error, MAE). The results for MAE and the corresponding t-\ntest results have been added to Supplementary Table 2 as follows:\n“Supplementary Tab. 2. The performances of models. The statistics of the different temperature\nresponse curve models at the Abisko-Stordalen, Finse, and Siikaneva sites with the least square fitting.\nRMSE and MAE are short for the root mean square error and mean absolute error in the unit of nmol m-2\ns-1, respectively. T-tests were applied to test the significance between the differences of MAE.\nSite Abisko-Stordalen Finse Siikaneva\nMAE MAE MAE\n- R2 Slope RMSE R2 Slope RMSE R2 Slope RMSE\n(p<0.05) (p=0.22) (p<0.01)\nUpdated\nMEGAN 0.81 0.84 0.45 0.24 0.68 0.74 0.58 0.28 0.90 0.89 1.02 0.87\nv2.1\nDefault\nMEGAN 0.78 0.73 0.48 0.21 0.64 0.62 0.60 0.27 0.83 0.80 1.37 0.64\nv2.1\n”\n5. Figure 2: Why does the default setup predict higher flux in springtime in Finse, even\nthough the only change from the default model is the updated emission factors and\ntemperature response for sedges and Arctic grass? Is it due to updated vegetation\nfractions? If so, it would be better to state it in the text. The table T1 can also be\nupdated to indicate the old vegetation fractions in the default model case. I suspect the\nhigher isoprene flux in spring is not only due to updated vegetation type; they should\nalso affect the summertime flux emissions. Factoring in Figure 1a, if one considers the\nt considers the\ntemperature response curves for default Megan (Guenther 2012) and the updated\nemission from the sedges, they appear to be similar for T<30 C, which makes this\nincrease in springtime isoprene flux puzzling.\nResponse: Thank you for your question. In the default model simulation, the\nuncertainties of vegetation cover fraction have been included in the total emission\nfactors (E) as presented in Equation (10) in the Method section of the manuscript.\nThese factors represent the emission capacity of the entire ecosystem (or the footprint\nof the flux measurements) by considering leaf-level emission factors, biomass, and\nvegetation cover fraction. Because all environmental factors are consistent across all\nplant functional types (PFTs), it is not necessary to differentiate the vegetation types\nfrom those in the default MEGAN model. Therefore, we can use E to represent the total\nemission capacity for the area covered by the flux tower footprint.\nWe fitted the \"best\" estimation of the emission factor for the Finse site using the least\nsquares fitting method for the observed fluxes over the entire season. After that, we still\nobserved an overestimation of isoprene during the spring, a phenomenon also reported\nby Seco et al., 2022. We believe this is caused by the inability of the temperature\nresponse curves to adequately represent the response to long-term (including seasonal)\nwarming. The emission of isoprene is proportional to the emission factors in the default\nMEGAN model. As you mentioned in your question, if we aim to capture the high values\nof isoprene in summer, adjusting the canopy emission factor will lead to an\noverestimation in colder periods like spring.\nIn addition, the temperature response curve we proposed is a dynamic temperature\nresponse curve relating to the 10-day averaged temperature, and the temperature curve\nfor sedges shown in Fig. 1 was fitted by all the measurements we have. As shown in\nFig. R4, the sedges could respond to a cold environment and have a higher activation\nenergy for the temperature response curve, which would lead to a much lower value of\nγ . The 10-day averaged temperature during spring to early summer at the Finse site is\nΤ\nfrom -3.7 to 7.0 °C, so the temperature response curve for the sedges could be\nsignificantly lower than that of the default MEGAN model as shown in Fig. R4.\nAs you suggested, we revised the corresponding statement about the emission factors\nin the revised paper as in the Method section at Line 467:\n“We fitted the model to get the optimal Ei for the default model and CFi for the updated\nmodel using the least-squares method to get rid of the uncertainties associated with\nvegetation fraction input.”\n6. Figure 3: Why is there minimal relative change in isoprene emissions in the northern\nregions (Norway, Finland and Sweden) where comparisons with the flux measurements\nwere performed? There seems to be really high relative changes in the Russian\nSiberian region, and it is hard to assess that the changes in the sedges emission\npotential can have such a huge impact here? How can you justify the changes in the\nRussian region without adequate comparison to any flux measurement data from the\nsite?\nResponse: Thank you so much for this very good question. This change is likely related\nto the vegetation distribution and the differences in temperature responses between the\nArctic sedges and shrubs as shown in Fig. R5. As mentioned in the main manuscript,\nwe updated the temperature response curves for both Arctic grass and boreal\ndeciduous shrubs plant functional types (PFTs) based on this study and our previous\nstudy on Salix spp. (Wang et al., 2024). For the sedges (represented by Arctic grass),\nthe new model simulates a decrease in isoprene during the cold period compared to the\ndefault model, which was found when we compared the model results with the flux\nmeasurements. In that case, the model predicts a general decrease in isoprene\nemissions for the Arctic grass-dominated area, including Fennoscandia. However, the\nboreal deciduous shrub-dominated regions, including the Russian Siberian region, show\nan increase due to the updated shrub temperature response curves. Direct isoprene\nemission measurements do not exist for the Russian Siberian region. However, indirect\npotential evidence is provided by the Ozone Monitoring Instrument (OMI) formaldehyde\n(HCHO) measurements, and Stavrakou et al. (2018) suggested that the interannual\nvariability of HCHO in Russian Siberia could be explained by the biogenic isoprene\nemissions. Nevertheless, we believe that more direct measurements are crucial to\nvalidate the model and understand the biogenic isoprene emissions in Siberia. We\nadded the discussion we had here in the manuscript at Line 304 as:\n“The simulations predict a notable increase in isoprene emissions in the Russian\nSiberian regions dominated by boreal deciduous shrubs. The model results presented\nby Stavrakou et al.46 suggested that the interannual variability of Ozone Monitoring\nInstrument (OMI) formaldehyde (HCHO) measurements in Siberia could be explained\nby biogenic isoprene emissions. However, more in-situ measurements are crucial to\nvalidate the model and understand the biogenic isoprene emissions in Siberia.”\n7. How is the inhibition of isoprene by CO2 treated in the model?\nResponse: Thanks you so much for your question. We adopted the constant CO of\n2\n284.7 ppmv in the CLM5 for simulating the longterm trend in BVOCs. According to the\nmethod proposed by Heald et al. (2009), our estimations for isoprene emission will have\na 9.78% overestimation without considering the trend of CO from pre-industrial times to\n2\n2009 (Fig. R6). The CO inhibition of isoprene emission is an important but highly\n2\nuncertain process that was not the focus of this study. For example, Potosnak et al.\n(2014) demonstrated that the inhibition effect of CO on isoprene could be eliminated at\n2\nincreased temperature for the tundra dwarf willows and this behavior has been\nconfirmed by recent studies (Sahu et al., 2023). Because of the uncertainties and\npotential interactions between CO and temperature responses, we treated the CO\n2 2\nexactly the same in our experiments, and it will not affect our evaluation of the impact of\ntemperature response curves on isoprene emissions in the high latitude regions.\n8. Why is the deactivation energy set to 230 KJ/mol? Figure 1b shows that activation\nenergies can exceed 250 KJ/mol at lower 10-day average temperatures.\nResponse: The activation and deactivation energies in the Arrhenius-type temperature\nresponse function are used to describe the increase and decrease stages of isoprene\nunder different temperatures (Rasulov et al., 2015). The deactivation energy of 230\nkJ/mol for the default MEGAN model (Guenther et al., 2006;Guenther et al., 1993) was\nfitted from chamber experiments for eucalyptus, sweet gum, aspen, and velvet bean.\nHowever, in our experiments, we did not observe a decrease in isoprene even though\nthe leaf temperature reached 40 °C, which is beyond the normal leaf temperature\nobserved in high latitudes. In that case, the new algorithm did not contain a deactivation\nenergy term and only had an activation energy term to describe the increase stage of\nisoprene emission response to temperature.\n9. Although the authors claim that isoprene emissions changed by 20% between 2000\nand 2009 and speculate on the potential impacts on SOA, O3, and CH4, it would be\nvaluable to see the perceived changes in all species concentrations using global model\nsimulations. At least for the SOA formation, which is pointed to increase, a more\nsensitive study is required. As higher isoprene emission decreases the OH\nconcentration, there will be less SOA formation through the oxidation of monoterpenes.\nIt is well-known that monoterpenes have a much higher SOA formation potential than\nisoprene.\nResponse: We appreciate taking up this topic. As you identified, we didn't quantitatively\nevaluate the potential atmospheric impacts of the change in isoprene emissions but only\nbriefly discussed this based on literature. We agree that we could expect a decrease in\nsecondary organic aerosols (SOA) caused by the decrease of OH. However, to fully\nunderstand the chemistry involved in this change, we need thorough simulation studies\nto quantitatively evaluate the change. Therefore, in this study, we focus entirely on\nexplaining this high-temperature response curve and its impact on isoprene emissions\nin high-latitude regions. The evaluation of chemical impact would be our next step as a\nnew independent project. For the revised manuscript, we changed the upper arrow for\nSOA change in Fig. 5 to a question mark as:\n“\nFig. 5. Schematic representation of isoprene emission increase and atmospheric chemical changes\ninduced by the response of sedges and willows to Arctic warming. Warming will increase isoprene\nemission from Arctic ecosystems. This change in isoprene emissions due to warming can alter tropospheric\nchemistry, resulting in extended methane lifetime, altered tropospheric ozone concentration, and changed\naerosol formation. These changes can influence the local radiation energy balance and exacerbate climate\nfluctuations.”\nReviewer #1 (Remarks on code availability):\nWe have not reviewed the code.\nResponse: Data and code are open access through the Zenodo platform:\nhttps://zenodo.org/records/11090365. We updated the code package with a README\nfile.\nReviewer #2:\nThe comments will come from my post-doctoral student, as this was a common review.\nResponse: Thank you so much for your precious time and efforts.\nReviewer #2 (Remarks on code availability):\nAll data were avaiable.\nResponse: Data and code are open access through the Zenodo platform:\nhttps://zenodo.org/records/11090365. We updated the code package with a README\nfile.\nReviewer #3:\nHigh-temperature sensitivity of isoprene emissions has been reported in different Arctic\necosystems for the last decade or so. The exact reason is not well known. Such high-\ntemperature sensitivity has many implications for our understanding of plant physiology,\nhigh-latitude atmospheric chemistry, and climate systems. This manuscript has\ncontributed to this knowledge gap in two ways. They have identified that sedges, and\nnot all Arctic vegetation, exhibit a very strong temperature response, thus key\ncontributors to some of observed the ecosystem level sensitivity. Using the chamber\nexperiments, this work also explored the mechanism for such high-temperature\nsensitivity in Arctic plants and used the information to inform an updated MEGAN\nalgorithm to account for it. It shows that the updated MEGAN can simulate the\nlandscape/ecosystem level isoprene fluxes better than the default MEGAN, particularly\nin those low-emission periods. The results of how differences in MEGAN perditions\nbetween the updated and default MEGAN for current and historical emissions are also\nof high interest.\nThe update in MEGAN to account for such high-temperature sensitivity in high-latitude\nvegetation has been expected from the community for a while. I am pleased to see the\nauthors report the update here. I expect it will be incorporated into many other\ncommunity models and thus will help facilitate more research into this topic. The\nmanuscript is well-written, and the experiments are well-designed. I don’t have any\nmajor concerns about the publication of this work. As I said, the updated MEGAN is a\nmuch-needed development.\nResponse: Thank you for taking the time to review our manuscript and for your insightful\ncomments. We are pleased to hear that you find our work on the high-temperature\nsensitivity of isoprene emissions in Arctic ecosystems and the updates to the MEGAN\nalgorithm to be a valuable contribution to the field. The point-by-point responses are\ngiven below.\nA few minor questions/suggestions:\n- Emission factor response to high-temperature sensitivity vs temperature response\ncurve/gamma T: How to attribute some of the sensitivity to the changed emission factor\n(Arctic willow), while in other species (sedges), it is due to the temperature response\ncurve?\nResponse: Thank you for your question. We conducted individual experiments to\ndistinguish the temperature response of willows and sedges. In our previous study\n(Wang et al., 2024) on Arctic willows (Salix spp.), we found that the minute-to-hour\ntemperature response curve is very close to the MEGAN model. In contrast, the\nisoprene emission factors of willows exhibited a more substantial than expected\nresponse to the mean ambient temperature of the previous day, as shown in Fig. R3\nabove. In that study, we tried to explain the high temperature sensitivity with the change\nof emission factor of willows; however, the change in willow emission factors cannot\nfully explain the high temperature response curves observed in other whole ecosystem\nmeasurements, as shown in Fig. R7.\nThe emission factors and the temperature response curves of sedges both respond to\nthe 10-day average temperature as shown in Fig. R8. During a cold period, the sedges\nexhibit a strong temperature response curve, and emissions could increase significantly.\nWhile going through a warm period, sedges would be less sensitive to minute-to-hour\ntemperature changes as indicated by a lower activation energy, while the emission\nfactors would increase correspondingly. Integrating these two behaviors, the whole\necosystem is likely to exhibit very low isoprene emissions during the cold period, and\nemissions could increase rapidly with the rise in temperature, reaching a high level after\na warming period.\nThe evidence of different patterns can also be found in ecosystem-level measurements.\nAs shown in Fig. R9 (Seco et al., 2022), the temperature curves derived from the Abisko\nand Finse sites differ from each other, even though they are both high-latitude tundra\nsites. The potential explanation is that the isoprene emission at Finse is dominated by\nsedges and willows, while at Abisko, it is dominated only by sedges. In that case, the\ntwo sites exhibit two different high-temperature response curves.\n- Has the updated MEGAN changed the emission factors for Arctic vegetation? I was\nnot sure if it had been updated or not.\nResponse: Thank you for your question. We updated the leaf-level emission factors for\nour site-scale simulations and fit the vegetation fraction to the measured landscape-\naverage emission factors. The leaf-level emission factors were 12.0 nmol m-2 s-1 for\nsedges and 6.5 nmol m-2 s-1 for the willows, respectively, which were based on our glass\nchamber measurements for this study and Wang et al. (2024). As shown in Tab. 1 in the\nmanuscript, the estimated vegetation cover fractions were within the expected range for\nthese sites indicating that our estimations of emission factors for sedges and willows are\nin a resonable range.\nFor the CLM5 experiments, we used the default PFT-level emission factors from\nMEGAN to assess the impact of temperature response curves. Seco et al. (2022) report\nthat the emission factor for the Arctic grass PFT in MEGANv2.1 closely matches the\nlandscape-scale flux measurements, suggesting that about 10% of the vegetation,\nspecifically sedges, on the tussock tundra are isoprene emitters. However, for boreal\ndeciduous shrubs, emission factors vary depending on the species, as not all deciduous\nshrubs emit isoprene, according to our measurements. In this case, a detailed species\ncomposition distribution map would be required to determine the accuracy of the\nemission factors for boreal deciduous shrubs. Substantial efforts are required to\nprecisely map the emission factors for deciduous shrubs in high-latitude ecosystems.\n- There appears to be a bit of repetition between the last paragraph of the ‘Abstract’ and\nthe first two paragraphs in ‘Results and Discussion’.\nResponse: Thank you for pointing this out. We removed the repetitive part in the first\nparagraph in “Results and Discussion”, and the current first two paragraphs in “Results\nand Discussion” at Line 88 are now:\n“Our species-level gas exchange chamber experiments showed that the main isoprene\nemitters among the species measured at Toolik Field Station (TFS), Alaska, USA, are\nsedges, a major component of Arctic graminoid plants, and willows, a major component\nof Arctic woody plants (Supplementary Fig. 1). Other studies have also indicated that\nthe Salix spp., Carex spp. and Eriophorum spp. exhibit significantly higher isoprene\nemission levels compared to other tundra species, e.g., Betula spp. and Cassiope\nspp.25,29,31-33 and Sphagnum spp. 34-36. The temperature response of willows in the Arctic\ncannot explain the high temperature sensitivity of isoprene emissions from high-latitude\necosystems30.\nArctic sedges studied here show a more pronounced temperature response than other\nplant species, including Arctic woody willow shrubs and any of the plant responses used\nto develop the MEGAN model. Our data confirm that the sedges are responsible for the\nheightened temperature responses of isoprene emissions from high-latitude\necosystems (Fig. 1a). We calculated the Q10 coefficient for isoprene emissions from\nsedges (Carex spp. and Eriophorum spp.) and willows between 25 and 35 °C. The Q10\ncoefficient represents the isoprene emission rate change with a 10 °C rise of the leaf\ntemperature. The Q10 values of Carex spp. (15.6 ± 8.8) and Eriophorum spp. (9.1 ± 7.0)\nare much higher than the Q10 of the Arctic willows (3.2 ± 1.8), which is close to the Q10\nof the MEGAN model (2.91) (Supplementary Fig. 2). We applied the Arrhenius equation\nto model the exponential temperature response curves of Carex spp. and Eriophorum\nspp. (refer to the Methods section), where the activation energy (Eq. 5) denotes the\ntemperature sensitivity of isoprene emission. Our findings indicate that the temperature\nresponse curves of both Carex spp. and Eriophorum spp. exhibit high temperature\nsensitivity (or high activation energy) up to 35 °C (Supplementary Fig. 3). However, the\nactivation energy and R2 decrease beyond 40 °C, suggesting a slower increase rate of\nisoprene emissions from both species (Supplementary Fig. 3).”\nReviewer #3 (Remarks on code availability):\nThere is no README file but the file names seem to be intuitive. I didn't check if the\ncode could be executed.\nResponse: Data and code are open access through the Zenodo platform:\nhttps://zenodo.org/records/8339419. We updated the code package with a README\nfile.\nReviewer #4:\nThis manuscript conducted species-specific chamber experiments, which demonstrate a\npronounced temperature response of isoprene emissions from Arctic sedges. This\nfinding sheds light on previous underestimations of the temperature effect on isoprene\nemissions in high-latitude tundra ecosystems. By updating this temperature response\ncurve for sedges and the dependence of emission capacity on the previous day\ntemperature for willows, the manuscript shows a substantial underestimation in both the\nmagnitude and long-term trend of isoprene emissions in the high-latitude regions. These\nfindings are novel and of significance to improve model simulations of BVOC emissions.\nI’d recommend accept this manuscript after addressing the following comments.\nResponse: Thank you for your encouraging comments on our manuscript and for\nrecognizing the value of our research. We have addressed the points you've mentioned\npoint-by-point.\n1. To make the manuscript suitable for Nature Communications, I’d recommend the\nauthors present a more comprehensive introduction of BVOC emissions by\nemphasizing the interdisciplinary nature of their study. In particular, I’d recommend the\nauthors explain why plants emit isoprene, and biologically why isoprene emissions are\nsensitive to temperature, and why there is large uncertainty of the temperature\nsensitivity of isoprene emissions. I found the discussions about the enzyme activity very\ninteresting, but I don’t see any discussions about this in introduction.\nResponse: Thank you for your suggestion. We added one paragraph to better introduce\nplant isoprene emissions in the Introduction section at Line 46:\n“Isoprene is the most abundant reactive BVOC emitted globally and in the Arctic3,13,14.\nIsoprene can help vegetation to tolerate abiotic stresses15, and isoprene can act as a\nsignaling compound to stimulate plant defense mechanisms during stress periods16.\nIsoprene is synthesized from dimethylallyl diphosphate (DMADP) derived from the\nmethyl erythritol 4-phosphate (MEP) pathway through the enzyme isoprene synthase\n(IspS)17. Isoprene emission is controlled by environmental conditions, especially\ntemperature and solar radiation18. Thus, a rapidly warming climate in the Arctic is\nfavorable for increasing the emission of isoprene19-22. The temperature response curves\nof isoprene emission, used in the current earth system models (ESMs) and the\nchemistry transport models (CTMs), are based on measurements of a few temperate\nplants13,23, and a typical isoprene temperature response curve has a Q10 of about 3\nwhich is thought to be driven by the influence of temperature on substrate supply and\nthe activity of IspS17. However, recent whole-ecosystem measurements suggest that the\ntemperature response of isoprene emissions in high-latitude tundra ecosystems has a\nQ10 over 8, which is also much higher than that predicted by the widely used BVOC\nemission model, the Model of Emissions of Gases and Aerosols from Nature\n(MEGAN)21,24-28.”\n2. The manuscript is overall well-structured, but the methodology section could benefit\nfrom further clarification to enhance its comprehensibility. For example, in the MEGAN\nmodel and temperature response curve section, the authors wrote “The updated short-\nterm temperature response curve for sedges”, “long- term temperature response for\nboreal broadleaf deciduous shrub”. The manuscript does not explain the short-term vs.\nlong-term, and the 10-day average temperature is considered as short-term?\nResponse: Thank you for your suggestion. The short-term refers to the minute-to-hour\ntemperature response in this manuscript, and long-term impact refers to the influence of\n1-10 day average temperatures. We agree that more explanation is needed, so we\nrevised the methods section at Line 425 as:\n“MEGAN considers the BVOC responses to the long-term temperature, defined as the\ntemperature of the past one day or longer, and the current temperature, reflecting\nchanges on a minute-to-hour scale. The default short-term temperature response curve,\ng , for isoprene in MEGAN is…”\nT\n3. The temperature sensitivity of isoprene emissions from sedges is mainly derived from\nshort-term experiments, but it’s unclear to me whether the short-term temperature\nsensitivity will hold true at longer time scales. With the global warming, plants may\nadapt to warmer environment, and their sensitivity to temperature may also change.\nResponse: Thank you for your comment. Our results address the question of short-term\ntemperature responses of sedges. When we conducted the short-term temperature\ncurve experiments, our sedge samples went through periods with varying temperatures.\nAs shown in Fig. 1(b), the activation energy of the temperature response curve for\nsedges would decrease with a higher 10-day average temperature. This phenomenon\nsuggests that the isoprene emissions from sedges will be less sensitive to short-term\ntemperature changes and would be similar to those of temperate plants after a warming\nperiod, which is represented in the MEGAN model. This aligns with your suggestion that\nthe sensitivity of sedge isoprene emissions to temperature could change in response to\nthe general warming and cooling of the climate on approximately a 10-day timescale.\nHowever, we also note that the emission capacity of these sedges could increase when\ntheir sensitivity decreases. In that case, we would still expect increased isoprene\nemissions in a warming climate.\n4. The manuscript concludes by discussing the likely increased high-latitude isoprene\nemissions in response to increasing heatwaves and general warming and the impacts\non regional chemistry and climate system, but such discussions are most qualitative. To\nfortify the manuscript's impact, it would be beneficial to delve into the broader\nimplications by quantitatively assessing the impacts of emission changes on global\nradiative forcing and atmospheric chemistry.\nResponse: Thank you for your suggestion. We agree that a quantitative assessment of\nthe atmospheric chemistry impacts of isoprene changes would be valuable next step.\nHowever, it is beyond the scope of this study to design and conduct the quantitative\nsimulations needed to investigate the impact of warming on atmospheric chemistry in\nhigh-latitude regions. For this study, we focus on reporting our chamber experiments\nand isoprene emission modeling.\nMinor comments:\n1. Regarding the leaf chamber experiments: previous studies have suggested a\ndependence of BVOC emissions on soil moisture, but in the chamber experiments plant\nsamples were detached from soil and submerged into water, how might this affect the\nresults?\nResponse: Thank you for your question. The water stress that is reflected in soil\nmoisture could affect isoprene emission. If the plant is in a stressed situation, isoprene\nemission could be affected by changes in leaf temperature and substrate supply, which\nhave been investigated in our previous studies (Seco et al., 2015;Wang et al., 2022).\nFor this study, we didn't observe the behavior of drought when we collected the\nvegetation samples. We also didn't see any inhibition, or other changes, of\nphotosynthesis behavior when conducting our experiments, which indicates that the\nplants were not stressed.\n2. “Isoprene flux measurements”, add one sentence to state what the flux\nmeasurements are used for to improve readability.\nResponse: Thank you for your suggestion. We added one sentence as:\n“Isoprene flux measurements from three high-latitude sites were used to validate the\nmodels.”\n3. What are the uncertainties on the isoprene emissions by assuming 10-day average\ntemperature in the framework?\nResponse: Thank you for your question. We can qualitatively see that the behavior of\nsedges is related to long-term temperature changes as shown in Fig. R10. However, we\ndo not think our experiments can accurately determine the exact timescale for this\nbehavior, and our adoption of the 10-day average temperature is based on the current\nmodel framework and our experience with Eriophorum spp. We expect to conduct more\nexperiments in the future to quantitatively investigate this acclimation behavior.\n4. For Fig. 2 model prediction vs observation, is R2 similar for cold and warm periods?\nWhy does R2 vary a lot across different sites?\nResponse: We defined the cold and warm periods based on a leaf temperature\nthreshold of 20°C. As shown in Fig. R11, the updated model shows better performance\nthan the default MEGAN model. We observed a difference in model performance\nbetween different sites. The model also performs better at the Abisko and Siikaneva\nsites in general, and the updated model shows much greater improvement at the Finse\nand Siikaneva sites than at the Abisko site. We speculate that the difference in R2 is\nrelated to the heterogeneous landscapes at these sites. Not all ecosystems have\nhomogeneous distributions of isoprene emitters (Seco et al., 2020), but our model\nassumes a homogeneous source of isoprene emission. The primary isoprene source is\nsedges at the Abisko and Siikaneva sites, but at the Finse site, it includes both sedges\nand willows. In that case, a heterogeneous source of isoprene at the Finse site will\naffect the flux measurements and the model simulations.\n5. Supplementary Fig. 4: AE on left axis, EF on y axis.\nResponse: Thank you for your comment. We have made the correction.\nReviewer #5:\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: Thank you for accepting the review invitation for our manuscript. We have\naddressed the comments from the co-reviewer."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The response to the review provided by the authors was satisfactory. I recommend the\npublication of this work in Nature communications.", "reviewer_2": "Based on our comments, the authors have improved the manuscript, and we recommend\npublishing it in its current state.", "reviewer_4": "The authors have addressed most of the reviewers' comments. The only comment I have is\nabout the time scales of the temperature response. Fig. 5 talks about long-term warming,\nbut the reply to Q3 of Reviewer 4 indicates this study addresses short-term changes of\ntemperature response. I'd suggest the authors better clarify this in the manuscript.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers."}, "author_response": "Response to Reviewers\nReviewer #1:\nThe response to the review provided by the authors was satisfactory. I recommend the\npublication of this work in Nature Communications.\nResponse: Thank you for your positive feedback and recommendation. We appreciate\nyour support.\nReviewer #2:\nBased on our comments, the authors have improved the manuscript, and we\nrecommend publishing it in its current state.\nResponse: Thank you for your feedback and recommendation. We appreciate your\nsupport and are glad our revisions meet your approval.\nReviewer #4:\nThe authors have addressed most of the reviewers' comments. The only comment I\nhave is about the time scales of the temperature response. Fig. 5 talks about long-term\nwarming, but the reply to Q3 of Reviewer 4 indicates this study addresses short-term\nchanges of temperature response. I'd suggest the authors better clarify this in the\nmanuscript.\nResponse: Thank you so much for your comments. We think it is a good point, and the\nlong-term warming here refers to a 1-2 week scale change in temperature for us, which\nis addressed in this study. We agree that we didn’t address the change in temperature\non a scale of months to years in this study, which can also be called long-term warming.\nIn that case, we revised our discussion in the manuscript on page 4 as follows:\n“The isoprene emitters, including sedges and willows30, would respond to both short-\nterm, intense heatwaves and long-term warming from minutes to weeks scales by\nincreasing their isoprene emissions (Fig. 5).”\nReviewer #5:\nI co-reviewed this manuscript with one of the reviewers.\nResponse: Thank you for your collaborative review. We appreciate the thorough\nevaluation and feedback from both reviewers."}]

Decision letter