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NEWS
10 September 2020

Fires are releasing record levels of carbon dioxide, partly because they are burning ancient peatlands that have been a carbon sink.

Alexandra Witze

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Northern fires (like the one shown here in the Novosibirsk Region of south Siberia) released record-setting amounts of carbon dioxide this year.Credit: Kirill Kukhmar/TASS/Getty

Wildfires blazed along the Arctic Circle this summer, incinerating tundra, blanketing Siberian cities in smoke and capping the second extraordinary fire season in a row. By the time the fire season waned at the end of last month, the blazes had emitted a record 244 megatonnes of carbon dioxide — that’s 35% more than last year, which also set records. One culprit, scientists say, could be peatlands that are burning as the top of the world melts.
Peatlands are carbon-rich soils that accumulate as waterlogged plants slowly decay, sometimes over thousands of years. They are the most carbon-dense ecosystems on Earth; a typical northern peatland packs in roughly ten times as much carbon as a boreal forest. When peat burns, it releases its ancient carbon to the atmosphere, adding to the heat-trapping gases that cause climate change.

Nearly half the world’s peatland-stored carbon lies between 60 and 70 degrees north, along the Arctic Circle. The problem with this is that historically frozen carbon-rich soils are expected to thaw as the planet warms, making them even more vulnerable to wildfires and more likely to release large amounts of carbon. It’s a feedback loop: as peatlands release more carbon, global warming increases, which thaws more peat and causes more wildfires. A study published last month1 shows that northern peatlands could eventually shift from being a net sink for carbon to a net source of carbon, further accelerating climate change.
The unprecedented Arctic wildfires of 2019 and 2020 show that transformational shifts are already under way, says Thomas Smith, an environmental geographer at the London School of Economics and Political Science. “Alarming is the right term.”
Zombie fires
The fire season in the Arctic kicked off unusually early this year: as early as May, there were fires blazing north of the tree line in Siberia, which normally wouldn’t happen until around July. One reason is that temperatures in winter and spring were warmer than usual, priming the landscape to burn. It’s also possible that peat fires had been smouldering beneath the ice and snow all winter and then emerged, zombie-like, in the spring as the snow melted. Scientists have shown that this kind of low-temperature, flameless combustion can burn in peat and other organic matter, such as coal, for months or even years.
Because of the early start, individual Arctic wildfires have been burning for longer than usual, and “they’re starting much farther north than they used to — in landscapes that we thought were fire-resistant rather than fire-prone”, says Jessica McCarty, a geographer at Miami University in Oxford, Ohio.

Sources: Copernicus Atmosphere Monitoring Service/European Centre for Medium-Range Weather Forecasts; Hugelius, G. et al. Proc. Natl. Acad. Sci. USA 117, 20438–20446 (2020)

Researchers are now assessing just how bad this Arctic fire season was. The Russian Wildfires Remote Monitoring System catalogued 18,591 separate fires in Russia’s two easternmost districts, with a total of nearly 14 million hectares burnt, says Evgeny Shvetsov, a fire specialist at the Sukachev Institute of Forest, which is part of the Russian Academy of Sciences in Krasnoyarsk. Most of the burning happened in permafrost zones, where the ground is normally frozen year-round.
To estimate the record carbon dioxide emissions, scientists with the European Commission’s Copernicus Atmosphere Monitoring Service used satellites to study the wildfires’ locations and intensity, and then calculated how much fuel each had probably burnt. Yet even that is likely to be an underestimate, says Mark Parrington, an atmospheric scientist at the European Centre for Medium-Range Weather Forecasts in Reading, UK, who was involved in the analysis. Fires that burn in peatland can be too low-intensity for satellite sensors to capture.
The problem with peat
How much this year’s Arctic fires will affect global climate over the long term depends on what they burnt. That’s because peatlands, unlike boreal forest, do not regrow quickly after a fire, so the carbon released is permanently lost to the atmosphere.
Smith has calculated that about half of the Arctic wildfires in May and June were on peatlands — and that in many cases, the fires went on for days, suggesting that they were fuelled by thick layers of peat or other soil rich in organic matter.

And the August study1 found that there are nearly four million square kilometres of peatlands in northern latitudes. More of that than previously thought is frozen and shallow — and therefore vulnerable to thawing and drying out, says Gustaf Hugelius, a permafrost scientist at Stockholm University who led the investigation. He and his colleagues also found that although peatlands have been helping to cool the climate for thousands of years, by storing carbon as they accumulate, they will probably become a net source of carbon being released into the atmosphere — which could happen by the end of the century.
Fire risk in Siberia is predicted to increase as the climate warms2, but by many measures, the shift has already arrived, says Amber Soja, an environmental scientist who studies Arctic fires at the US National Institute of Aerospace in Hampton, Virginia. “What you would expect is already happening,” she says. “And in some cases faster than we would have expected.”

doi: 10.1038/d41586-020-02568-y

References

1.
Hugelius, G. et al. Proc. Natl Acad. Sci. USA 117, 20438–20446 (2020).

2.
Sherstyukov, B. G. & Sherstyukov, A. B. Russian Meteorol. Hydrol. 39, 292–301 (2014).

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Affiliations

Department of Biology, University of Miami, Coral Gables, FL, USA
K. J. Feeley, C. Bravo-Avila, B. Fadrique & T. M. Perez

Fairchild Tropical Botanic Garden, Coral Gables, FL, USA
K. J. Feeley, C. Bravo-Avila & T. M. Perez

Universidad Nacional de Colombia Sede Medellín, Medellín, Colombia
D. Zuleta

Forest Global Earth Observatory, Smithsonian Tropical Research Institute, Washington DC, DC, USA
D. Zuleta

Authors
K. J. Feeley

C. Bravo-Avila

B. Fadrique

T. M. Perez

D. Zuleta

Corresponding author
Correspondence to K. J. Feeley. More

NPFWs are generally considered to have negative effects on the fig tree–fig pollinator mutualism according to theoretical studies2,4,6. Many empirical and experimental studies have justified this conclusion. Some studies show that NPFWs have a negative effect and found a negative correlation between the offspring number of NPFWs and the offspring number of pollinator and seeds9,13,14. For instance, Patel19 found no correlation between the offspring number of pollinator and NPFWs in their survey , and Peng et al.20 recorded a positive correlation between the offspring number of NPFWs and pollinator in figs of F. hispida . Our capacity to detect the real cost of development of different types of NPFWs and their exact effect on the mutualism is limited by many interaction factors13 and non-experimental empirical studies can underestimate or fail to detect the impact of NPFWs on this obligate mutualism when overall consideration was taken with different studies of pollinator-NPFW interactions in consideration18.
For early-ovipositing gallers, a remarkable feature of those species is that their galls usually fill the space of B-phase syconium cavities and can greatly hinder pollinator movement and oviposition, thus result in their specific consequences on the mutualism24. For example, Conchou et al.13 studied Ficicola spp. which is a genus of early-ovipositing large-sized gallers hosted by F. guianensis and found a substantial negative impact of this genus on the production of both pollinators and seeds. Here, we present another case that studies the effect of an early gall inducer.
Our study has explored the effect of early-ovipositing gallers on the growth of figs (fig size of matured figs) for the first time. In our study, it turns out that early-ovipositing gallers can greatly depress the growth of the figs. The mechanical injury caused by the insertion of the ovipositor of S. testacea to the tiny figs may be responsible for the depression of the growth of the Fig25. Another possible reason is the gall-inducing process26. Jansen-González26 et al. studied the gall-inducing process and larval feeding strategy of pollinating and non-pollinating fig wasp species associated with F. citrifolia and found that the way non-pollinating galler induces gall is quite different from pollinators and they exploit plant resources more aggressively26. With the involvement of the pollinators, the growth of figs improved greatly. The existence of both seeds and larvae of the pollinator, both of which is essential to the fitness of fig trees, may contribute to the improvement of the growth of the figs.
The variation trend of development ratio is consistent with fig volume, which can be explained by the saction mechanism applying to uncooperative cheaters27,28,29,30,31. Jander et al.27,28,29 found that the Ficus tree can sanction the pollen-free pollinators through decreasing their offspring development ratio and increasing the abortion rate of the unpollinated figs. Almost all the non-pollinating figs wasp species lay their eggs in the female flowers but not spread pollen to the fig, which is analogous to the cheaters of pollinators. It makes sense that the same sanction mechanism works on the non-pollinating fig wasps. When only S. testacea oviposit in the fig, most of the figs aborted, and the fig tree invests little nutrition for the remaining figs that survived. With the increase of oviposition and pollination by pollinators (2Cf + St treatments versus St treatment), and the decrease of mechanical injury by NPFWs (2Cf treatments versus St treatment), fig trees invest more nutrition to those figs and the development ratio of the galls increases. Jander et al.28 argued that sanctions can be modular or individual. In a modular sanction, all the offspring produced in the fruit is punished due to involvement of non-cooperative individuals. Our experiment can be a good test of this hypothesis as we can investigate accurately the development of the pollinator and the cheater (S. testacea) respectively. By comparing the theoretical value and the actual number of galls for 2Cf + St treatment, we can explore the source that leads to the decrease of the overall development ratio in the 2Cf + St treatment compared with the 2Cf treatments, thus determine if the sanction works at fig level (modular) or not. We found there is no significant difference between theoretical value and the actual number for 2Cf + St treatment, which suggests that the development ratio of the two species is independent of each other, so the lower development ratio of 2Cf + St treatment compared with 2Cf treatment was caused mainly by the low development ratio of S. testacea. For 2Cf + St treatment, the development ratio of pollinator didn’t decrease, and being pollinated didn’t change the development ratio of S. testacea, which means pollination didn’t increase the nutrition provided to galls of. S. testacea. Our results show that the sanction is not at a fig level, which is inconsistent with the sanction to the cheaters in pollinators (pollen-free pollinators)28,30. Perhaps the sanction mechanism is related to the identification of the cheaters. When the host can identify the cheaters, the sanction can work at individual level where they may only sanction the offspring of cheaters, such as S. testacea in this study. If the host can’t identify the cheaters, the sanction may work at fig level (modularly) through sanctioning all the wasps reproduced in the fig, such as the cheaters in the pollinators.
The production of seeds is influenced greatly by the wasp S. testacea. A former study shows that galls produced by early-ovipositing gallers often fill B-phase syconium cavities and hinder pollinator movement and oviposition24, which can explain the decline of the seed production. In our study, the oviposition behavior of pollinators was much less affected than the pollen spreading behavior due to the block of galls produced by S. testacea.
For the treatments with the pollinators, figs with S. testacea (2Cf + St treatment) have more galls than figs without S. testacea (2Cf treatment). This indicts that the existence of galls produced by S. testacea has little effect on the egg-laying behavior of the pollinator. Since pollinators lay their eggs in the female flowers one by one and the number of eggs to be laid is much less than the number of pollen to be spread, it’s easy to understand that oviposition is less affected than pollen spreading.
Our study shows that S. testacea has an obvious negative effect on the fig tree-fig pollinator mutualism. Oviposition by S. testacea leads to a drastic decrease in seed production, thus may harm the maintenance of stability of fig tree-fig pollinator mutualism if the fig is excessively parasitized by S. testacea. We think the mechanical injury from oviposition, the galling process, and the blocking of cavity by the galls of S. testacea may be responsible for the negative consequences. For figs in which only S. testacea oviposit, the total galls are much less than figs in which pollinators were introduced. There are two possible reasons. One possible reason is that the low density of the population of S. testacea results in the low oviposition rate. Another possible reason is that the fig tree may sanction the unpollinated figs, ie, the figs in which only S. testacea oviposit. Wang et al.30,31 found that the sanction mechanism also works on F. racemosa. Further more, they found that the sanction strength became stronger with an increase in foundresses30 (the wasps that enter the figs to oviposit). If too many S. testacea oviposit in figs, the figs are aborted more easily due to sanction by the tree. Only figs that contain a small number of galls survive.
Although many studies9,11,13,14,15,16,17,18 have shown that NPFWs can have a negative effect on the fig tree-fig pollinator mutulism due to the reduction in pollinator offspring or the seed production, some study has found that the NPFWs also can play a positive role in maintaining the stability of this obligate mutualism32. For example, parasites may stabilize and maintain the fig and fig wasp system through their effects on within- and between-tree reproductive phenology32. More specifically, oviposition by NPFWs can result in the asynchrony of the development of the figs, and increase the probabilities of pollinators finding oviposition sites, which is good for the maintenance of this mutualism32. More

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Trabaque tufa record
Trabaque Canyon (40.36° N; 2.26° W; 840 m above sea level) is located in the central Iberian Peninsula (Fig. 1). At this site, tufa deposits precipitate as freshwater carbonates downstream of overflow karst springs. During the last interglacial period, tufa precipitated continuously at the studied site while water level of the aquifer was high enough for upstream springs to discharge13. Outcrops of the studied tufa deposit are preserved in the margins of Trabaque River over a distance of 500 m downstream of overflow karstic springs. The studied tufa deposit is 12 m thick, with a gentle ramp morphology, and a simple stratigraphy of sub-horizontal tufa beds that covered the full section of the narrow canyon. The accumulation of tufa created a small lake upstream the ramp, which prevented erosive events while the deposit was active, because most of the river bedload was accumulated in the basin of the lake. This configuration favoured the lack of erosive episodes in the tufa and the deposition of a continuous record. The tufa deposit was partially eroded by subsequent fluvial incision once the tufa accretion ceased and detrital sediments filled the lake basin and started to flow over the ramp during floods. The tufa deposit is mostly composed of well-cemented intra-clastic and peloidal carbonate particles13. The deposit comprises tufa beds 0.02–1 m thick that typically extend tens of metres downstream. At the base of the section, the tufa lies over loose fluvial sediments of sandy silt, whereas at the top of the section there is an erosive scar, and recent gravitational deposits overlay the tufa preserved in the slopes of the canyon.
Figure 1

Pictures of Trabaque Canyon and the studied deposit. (a) Trabaque Canyon. The river flows according to yellow arrows. The red ellipse shows the location of the main section where the deposit was sampled. The inlet map shows the location of Trabaque Canyon within the Iberian Peninsula. (b) View of most of the studied Trabaque tufa section. The base and top of the section are missing from this panorama. The centre of the valley bottom is to the left of the image and the slope of the canyon to the right. The river flowed from the position of the observer towards the tufa deposit. The picture shows gravitational pulses GP-2 and GP-3 that interdigitate with the tufa deposit, and their disappearance from the bottom of the valley after GP-3. (c) Detail of GP-3 gravitational deposit. (d) Detail of the alternation between well-cemented and loose tufa beds at the top of the section.

Full size image

The base of the deposit section is characterized by nearly 4 m of tufa sediments in the centre of the valley, laterally interdigitating with gravitational deposits towards the slopes (Fig. 1). These gravitational deposits partially invaded the bottom of the valley during three distinct pulses. These gravitational deposits occurred during periods of enhanced slope processes due to the decrease in vegetation cover on the canyon slopes during prolonged dry periods. The evidence of local erosion recorded by the gravitational deposits is consistent with other proxies that record local and regional erosion and that are displayed in Fig. 2. Thus, independent evidence of erosion is also recognized from the increase of insoluble residue (IR) particles in the tufa, recorded by the percentage of silt IR. IR particles were transported to the tufa by the river or by the action of wind. The increase of these particles in the tufa is interpreted as enhanced erosion, not only from the catchment but also from outside the basin. Higher concentrations of Si and Al are also interpreted as proxies of soil erosion from areas with silicate substrates inside or outside the catchment. The increase of micro-charcoal particles in the tufa is also interpreted as a sign of enhanced soil erosion. Charcoals were incorporated to the tufa during floods or transported by the wind after the occurrence of fires, as well as from the erosion of soils that accumulated charcoals from previous fire events. In any case, the increase of micro-charcoals in the tufa record suggests soil erosion due to the lost of vegetation cover. Major events of local and regional erosion occurred synchronously (Fig. 2), supporting that the common decreases in vegetation cover that resulted in erosion events were related to periods of reduced precipitation.
Figure 2

Record of the Trabaque tufa deposit. (a) Simplified lithological log of the Trabaque record. Patterns represent gravitational deposits (black) with distinct three pulses, well-cemented tufa sediments (light grey), and alternation of well-cemented and loose tufa sediments (dark grey). (b,c) Tufa δ18O and δ13C records. Isotope values at each date (dots) are the average of 3 sub-samples and blue/red line is a 3-point running mean. The grey shade shows the 1σ variability of the three sub-samples along the record. (d) Concentration of Si and Al. (e) Silt-sized insoluble residue in tufa as percentage of the total sample. (f) Counts of micro-charcoal particles  More