THEO ALLOFS/MINDEN PICTURES/FLPA
NEWS & VIEWS RESEARCH capture performance persist in the presence of water, and the adsorption–desorption cycle can operate at elevated temperatures (between 100 and 150 °C, for example). The ability to function at high temperatures is pivotal for many applications, because the temperature of the flue gas from fuel combustion is typically higher than ambient temperatures. The mechanism also raises issues that might affect industrial-scale applications of the MOFs. For example, rapid step-like CO2 adsorption might be accompanied by a sudden release of heat. This would need to be dissipated quickly so that the material does not warm spontaneously, thus desorbing the CO2 prematurely. McDonald and colleagues’ study takes full advantage of the crystalline nature of MOFs. Porous materials with extremely high surface areas can be made from robust amorphous polymer networks10, but it is hard to imagine the observed cooperative mechanism working efficiently in a disordered material. Indeed, the mechanism is a function of the precise relative placement of the chemical groups in the framework, and of the strength of the metal– ligand bonds. This mechanistic feature brings to mind biological systems, such as enzymes, whose functions are also determined by the relative placement of organic species around specific metal centres. As the authors note, their magnesium-containing MOF shows strong structural similarities to the active site of the ribulose-1,5-bisphospate carboxylase/oxygenase enzyme (Rubisco), which is responsible for biological CO2 fixation (Fig. 1). This may not be a coincidence: the superior CO2-capture capacity of the magnesium MOF at low CO2 pressures might shed light on why magnesium ions are found in Rubisco, which also functions at low CO2 concentrations. The authors’ work therefore suggests that future developments in carbon capture might be bioinspired. ■
BI O GEO C HE M I ST RY
Signs of saturation in the tropical carbon sink The carbon sink in the land biosphere has grown during the past 30 years, taking up much of the carbon dioxide produced by human activities. The first signs of this growth levelling off have been spotted in Amazon forests. See Letter p.344 LARS O. HEDIN
I
ntact tropical forests offer a world-class ecosystem service: they absorb and store large amounts of the greenhouse gas carbon dioxide that is emitted to the atmosphere by human activities. The amount of new carbon stored each year in these forests — in the form of growing tree stems, new leaves and roots, and increased soil organic matter — is equivalent to roughly half of all the carbon scrubbed from the atmosphere by the land biosphere1,2. But will these tropical forests continue to accumulate carbon unabated in the future? In this issue, Brienen and co-authors3 (page 344) report the first evidence that the carbon sink in intact tropical forests may be becoming saturated. Their result is based on a remarkable 30-year study of around 189,000 trees from 321 forest plots distributed across the Amazon basin. As early as 1895, Svante Arrhenius noted4 that the global circulation of carbon between the uptake of CO2 by vegetation and the decay
of organic matter is sufficiently fast that, over time, the two processes must equilibrate, except for periods when large quantities of carbon are “subtracted from the circulation”. Analyses1,5 of the global land carbon sink have shown such an unbalanced circulation, with vegetation removing CO2 from the atmosphere at a rate that greatly exceeds the re-release of the gas to the atmosphere. Encouragingly, these analyses revealed that the land carbon sink has grown during the past three decades, because vegetation has responded positively to the increased availability of CO2 (a key component of photosynthesis) in the atmosphere. As a result, the land sink now equals or exceeds the ocean sink, and has shown no signs of levelling off. But all has not been well with this picture. Those concerned with the constraints on vegetation growth caused by limiting resources (such as nutrients, water, heat or light) have warned against the idea that the land carbon sink will continue unchecked in the future6. Moreover, inclusions of nutrient constraints
Andrew I. Cooper is in the Department of Chemistry, University of Liverpool, Liverpool L69 3BX, UK. e-mail: [email protected] 1. International Energy Agency. CO2 Emissions From Fuel Combustion: Highights (IEA, 2013); see go.nature.com/9dpily 2. McDonald, T. M. et al. Nature 519, 303–308 (2015). 3. Rochelle, G. T. Science 325, 1652–1654 (2009). 4. Sumida, K. et al. Chem. Rev. 112, 724–781 (2012). 5. Dawson, R. et al. J. Am. Chem. Soc. 134, 10741–10744 (2012). 6. Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. Science 341, 1230444 (2013). 7. Mottillo, C. & Friščić, T. Angew. Chem. Int. Edn 53, 7471–7474 (2014). 8. Serre, C. et al. J. Am. Chem. Soc. 124, 13519–13526 (2002). 9. Sato, H. et al. Science 343, 167–170 (2014). 10. Ben, T. et al. Angew. Chem. Int. Edn 48, 9457–9460 (2009). This article was published online on 11 March 2015.
Figure 1 | Giants of the rainforest. Trees in intact rainforests, such as the Tongkoko Reserve, Indonesia (pictured), can grow to be enormous. 1 9 M A RC H 2 0 1 5 | VO L 5 1 9 | NAT U R E | 2 9 5
© 2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS in global land models have predicted 7 dramatically reduced carbon uptake over time compared with unconstrained models. But despite these concerns, there has been no field-based evidence of a slowdown in the growth rate of the carbon sink across tropical, temperate or boreal forests — until now. Brienen et al. measured the size and growth rate of individual trees in each of the forest plots and for each census interval in their study, and recorded whether trees died or appeared as new recruits within the plots. In this way, they documented not only a 30% deceleration of carbon uptake by vegetation in intact Amazon tropical forests over the past two decades, but also that this trend depended on changes in both forest productivity (rate of carbon capture per unit of area and time) and the tree mortality (rate of biomass carbon loss per unit of area and time). The individual tree demographic information is crucial, because it shows that the drop in forest biomass growth occurred even though individual tree productivity increased or stayed the same. This points to increased mortality and turnover of forest biomass as the proximate mechanisms underlying the observed reduction in carbon-sink strength. Any field census based on forest plots is sensitive to a particular class of artefact, in which trends in carbon exchange, productivity and mortality that occur naturally as forests age8 can be misinterpreted as indicators of a slowing carbon sink. The growth of young forests naturally decelerates as they age, whereas mortality tends to increase over time. Brienen and colleagues excluded both artefacts by analysing the observed basin-wide patterns in detail and by showing that they do not depend on the age of the plots at each census interval. The study does not, however, resolve the cause of the basin-wide drop in biomass growth. The observed trends might result from increased forest turnover in response to elevated levels of atmospheric CO2, but such a response is inconsistent with what would be expected on the basis of plant physiology, and still begs the question of the underlying mechanism. A more likely explanation is that the influence of one or more constraining factors — such as water availability, temperature stress or nutrient limitation — has increased as forest biomass has expanded. Carbon uptake is sensitive to droughts9 and temperature10, but the timing of the observed basin-wide trends in biomass and tree mortality do not fit the pattern expected from historical drought events9. Surprisingly, we know much less about how soil nutrients influence the growth and mortality of the very large trees that inhabit intact tropical forests (Fig. 1), including whether the availability of nitrogen, phosphorus or possibly even molybdenum impose fundamental constraints. But the observed slowdown in forest carbon uptake indicates that the ability of individual trees to trade photosynthetically
fixed carbon for access to rare soil nutrients (by ‘priming’ processes caused by microbes that decompose soil organic matter11, or by symbiotic associations of plants and myco rrhizal fungi or nitrogen-fixing bacteria12) may not be sufficient to overcome the constraint on forest biomass growth. Brienen and co-workers’ findings raise new questions about the land carbon sink and its future dynamics in Earth’s coupled carbon– climate system. If the tropical carbon sink is becoming saturated, then over what timescales will this happen, in what regions and under what biophysical conditions? The CO2 component of climate change may become substantially more difficult to manage and abate in the future if the findings from the Amazon basin apply more generally to the land carbon sink. We need a better understanding of whether carbon uptake in forests worldwide is slowing, including those in temperate or boreal climatic regions. Perhaps most fundamentally, there is an urgent need to resolve the mechanisms and dynamics that govern the land carbon sink, and the constraints that may cause it to
become saturated as we continue to titrate the biosphere with increasing amounts of atmospheric CO2. ■ Lars O. Hedin is in the Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08540, USA. e-mail: [email protected] 1. Ballantyne, A. P., Alden, C. B., Miller, J. B., Tans. P. P. & White, J. W. C. Nature 488, 70–72 (2012). 2. Pan, Y. et al. Science 333, 988–993 (2011). 3. Brienen, R. J. W. et al. Nature 519, 344–348 (2015). 4. Arrhenius, S. Phil. Mag. J. Sci. 41, 237–276 (1896). 5. Le Quéré, C. et al. Earth Syst. Sci. Data 5, 165–185 (2013). 6. Hungate, B. A., Dukes, J. S., Shaw, M. R., Luo, Y. & Field, C. B. Science 302, 1512–1513 (2003). 7. Gerber, S., Hedin, L. O., Keel, S. G., Pacala, S. W. & Shevliakova, E. Geophys. Res. Lett. 40, 5218–5222 (2013). 8. Fisher, J. I., Hurtt, G. C., Thomas, R. Q. & Chambers, J. Q. Ecol. Lett. 11, 554–563 (2008). 9. Gatti, L. V. et al. Nature 506, 76–80 (2014). 10. Clark, D. A., Clark, D. B. & Oberbauer, S. F. J. Geophys. Res. Biogeosci. 118, 783–794 (2013). 11. Lambers, H., Mougel, C., Jaillard, B. & Hinsinger, P. Plant Soil 321, 83–115 (2009). 12. Batterman, S. A. et al. Nature 502, 224–227 (2013).
N EUR O SC I ENCE
Hot on the trail of temperature processing Two studies investigate how information about temperature is processed in the brains of fruit flies, and reveal that different neuronal pathways transmit heating and cooling signals to higher brain regions. See Letters p.353 & p.358 TJ F L O R E N C E & M I C H A E L B . R E I S E R
P
eople often seek a cool breeze on a hot summer’s day, and delight in a roaring fire on a cold winter’s evening — the desire to stay in a thermal ‘comfort zone’ is a powerful driver of our behaviour. Because temperature is both a sensory stimulus and a factor that directly affects metabolic processes, it is not surprising that nearly all animals avoid unfavourable temperatures1. But what is the biological basis of temperature preference? Two studies2,3 in this issue track the flow of temperature information into the brains of fruit flies (Drosophila melanogaster). They reveal that the brain extracts different qualities of temperature at the very first connection, and does so using distinct mechanisms for hot and cold. Fruit flies have long been the workhorse of genetic studies, but the animals’ behavioural repertoire is also noteworthy. Like most arthropods, flies cannot regulate their core temperature internally. Instead, they move in search of favourable temperatures, which
2 9 6 | NAT U R E | VO L 5 1 9 | 1 9 M A RC H 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
makes them ideal for studies of thermosensing. D. melanogaster exhibits exquisite sensitivity to small temperature changes4, and can even associate thermal conditions with other senses, such as sight and smell, to remember locations and avoid ominous odours5. Studies have defined the organs6, neurons7,8 and receptor proteins9,10 that fruit flies use to avoid unfavourable temperatures. Their primary sensory neurons are in the antenna, which is a veritable Swiss army knife of sensory functions. Individual thermoreceptor neurons are mainly excited by small temperature changes — hot thermoreceptors are strongly excited by heating and weakly inhibited by cooling, and cold thermoreceptors have the opposite response3,8. Genetically silencing these peripheral thermoreceptor neurons, or removing their hot or cold receptor proteins, prevents the fly from avoiding harmful heat or cold7,8. Thermoreceptor neurons project to an area of the fly brain called the proximal antennal protocerebrum (PAP), in which their output terminals (axons) segregate into hot and cold zones8.
NEWS & VIEWS RESEARCH capture performance persist in the presence of water, and the adsorption–desorption cycle can operate at elevated temperatures (between 100 and 150 °C, for example). The ability to function at high temperatures is pivotal for many applications, because the temperature of the flue gas from fuel combustion is typically higher than ambient temperatures. The mechanism also raises issues that might affect industrial-scale applications of the MOFs. For example, rapid step-like CO2 adsorption might be accompanied by a sudden release of heat. This would need to be dissipated quickly so that the material does not warm spontaneously, thus desorbing the CO2 prematurely. McDonald and colleagues’ study takes full advantage of the crystalline nature of MOFs. Porous materials with extremely high surface areas can be made from robust amorphous polymer networks10, but it is hard to imagine the observed cooperative mechanism working efficiently in a disordered material. Indeed, the mechanism is a function of the precise relative placement of the chemical groups in the framework, and of the strength of the metal– ligand bonds. This mechanistic feature brings to mind biological systems, such as enzymes, whose functions are also determined by the relative placement of organic species around specific metal centres. As the authors note, their magnesium-containing MOF shows strong structural similarities to the active site of the ribulose-1,5-bisphospate carboxylase/oxygenase enzyme (Rubisco), which is responsible for biological CO2 fixation (Fig. 1). This may not be a coincidence: the superior CO2-capture capacity of the magnesium MOF at low CO2 pressures might shed light on why magnesium ions are found in Rubisco, which also functions at low CO2 concentrations. The authors’ work therefore suggests that future developments in carbon capture might be bioinspired. ■
BI O GEO C HE M I ST RY
Signs of saturation in the tropical carbon sink The carbon sink in the land biosphere has grown during the past 30 years, taking up much of the carbon dioxide produced by human activities. The first signs of this growth levelling off have been spotted in Amazon forests. See Letter p.344 LARS O. HEDIN
I
ntact tropical forests offer a world-class ecosystem service: they absorb and store large amounts of the greenhouse gas carbon dioxide that is emitted to the atmosphere by human activities. The amount of new carbon stored each year in these forests — in the form of growing tree stems, new leaves and roots, and increased soil organic matter — is equivalent to roughly half of all the carbon scrubbed from the atmosphere by the land biosphere1,2. But will these tropical forests continue to accumulate carbon unabated in the future? In this issue, Brienen and co-authors3 (page 344) report the first evidence that the carbon sink in intact tropical forests may be becoming saturated. Their result is based on a remarkable 30-year study of around 189,000 trees from 321 forest plots distributed across the Amazon basin. As early as 1895, Svante Arrhenius noted4 that the global circulation of carbon between the uptake of CO2 by vegetation and the decay
of organic matter is sufficiently fast that, over time, the two processes must equilibrate, except for periods when large quantities of carbon are “subtracted from the circulation”. Analyses1,5 of the global land carbon sink have shown such an unbalanced circulation, with vegetation removing CO2 from the atmosphere at a rate that greatly exceeds the re-release of the gas to the atmosphere. Encouragingly, these analyses revealed that the land carbon sink has grown during the past three decades, because vegetation has responded positively to the increased availability of CO2 (a key component of photosynthesis) in the atmosphere. As a result, the land sink now equals or exceeds the ocean sink, and has shown no signs of levelling off. But all has not been well with this picture. Those concerned with the constraints on vegetation growth caused by limiting resources (such as nutrients, water, heat or light) have warned against the idea that the land carbon sink will continue unchecked in the future6. Moreover, inclusions of nutrient constraints
Andrew I. Cooper is in the Department of Chemistry, University of Liverpool, Liverpool L69 3BX, UK. e-mail: [email protected] 1. International Energy Agency. CO2 Emissions From Fuel Combustion: Highights (IEA, 2013); see go.nature.com/9dpily 2. McDonald, T. M. et al. Nature 519, 303–308 (2015). 3. Rochelle, G. T. Science 325, 1652–1654 (2009). 4. Sumida, K. et al. Chem. Rev. 112, 724–781 (2012). 5. Dawson, R. et al. J. Am. Chem. Soc. 134, 10741–10744 (2012). 6. Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. Science 341, 1230444 (2013). 7. Mottillo, C. & Friščić, T. Angew. Chem. Int. Edn 53, 7471–7474 (2014). 8. Serre, C. et al. J. Am. Chem. Soc. 124, 13519–13526 (2002). 9. Sato, H. et al. Science 343, 167–170 (2014). 10. Ben, T. et al. Angew. Chem. Int. Edn 48, 9457–9460 (2009). This article was published online on 11 March 2015.
Figure 1 | Giants of the rainforest. Trees in intact rainforests, such as the Tongkoko Reserve, Indonesia (pictured), can grow to be enormous. 1 9 M A RC H 2 0 1 5 | VO L 5 1 9 | NAT U R E | 2 9 5
© 2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS in global land models have predicted 7 dramatically reduced carbon uptake over time compared with unconstrained models. But despite these concerns, there has been no field-based evidence of a slowdown in the growth rate of the carbon sink across tropical, temperate or boreal forests — until now. Brienen et al. measured the size and growth rate of individual trees in each of the forest plots and for each census interval in their study, and recorded whether trees died or appeared as new recruits within the plots. In this way, they documented not only a 30% deceleration of carbon uptake by vegetation in intact Amazon tropical forests over the past two decades, but also that this trend depended on changes in both forest productivity (rate of carbon capture per unit of area and time) and the tree mortality (rate of biomass carbon loss per unit of area and time). The individual tree demographic information is crucial, because it shows that the drop in forest biomass growth occurred even though individual tree productivity increased or stayed the same. This points to increased mortality and turnover of forest biomass as the proximate mechanisms underlying the observed reduction in carbon-sink strength. Any field census based on forest plots is sensitive to a particular class of artefact, in which trends in carbon exchange, productivity and mortality that occur naturally as forests age8 can be misinterpreted as indicators of a slowing carbon sink. The growth of young forests naturally decelerates as they age, whereas mortality tends to increase over time. Brienen and colleagues excluded both artefacts by analysing the observed basin-wide patterns in detail and by showing that they do not depend on the age of the plots at each census interval. The study does not, however, resolve the cause of the basin-wide drop in biomass growth. The observed trends might result from increased forest turnover in response to elevated levels of atmospheric CO2, but such a response is inconsistent with what would be expected on the basis of plant physiology, and still begs the question of the underlying mechanism. A more likely explanation is that the influence of one or more constraining factors — such as water availability, temperature stress or nutrient limitation — has increased as forest biomass has expanded. Carbon uptake is sensitive to droughts9 and temperature10, but the timing of the observed basin-wide trends in biomass and tree mortality do not fit the pattern expected from historical drought events9. Surprisingly, we know much less about how soil nutrients influence the growth and mortality of the very large trees that inhabit intact tropical forests (Fig. 1), including whether the availability of nitrogen, phosphorus or possibly even molybdenum impose fundamental constraints. But the observed slowdown in forest carbon uptake indicates that the ability of individual trees to trade photosynthetically
fixed carbon for access to rare soil nutrients (by ‘priming’ processes caused by microbes that decompose soil organic matter11, or by symbiotic associations of plants and myco rrhizal fungi or nitrogen-fixing bacteria12) may not be sufficient to overcome the constraint on forest biomass growth. Brienen and co-workers’ findings raise new questions about the land carbon sink and its future dynamics in Earth’s coupled carbon– climate system. If the tropical carbon sink is becoming saturated, then over what timescales will this happen, in what regions and under what biophysical conditions? The CO2 component of climate change may become substantially more difficult to manage and abate in the future if the findings from the Amazon basin apply more generally to the land carbon sink. We need a better understanding of whether carbon uptake in forests worldwide is slowing, including those in temperate or boreal climatic regions. Perhaps most fundamentally, there is an urgent need to resolve the mechanisms and dynamics that govern the land carbon sink, and the constraints that may cause it to
become saturated as we continue to titrate the biosphere with increasing amounts of atmospheric CO2. ■ Lars O. Hedin is in the Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08540, USA. e-mail: [email protected] 1. Ballantyne, A. P., Alden, C. B., Miller, J. B., Tans. P. P. & White, J. W. C. Nature 488, 70–72 (2012). 2. Pan, Y. et al. Science 333, 988–993 (2011). 3. Brienen, R. J. W. et al. Nature 519, 344–348 (2015). 4. Arrhenius, S. Phil. Mag. J. Sci. 41, 237–276 (1896). 5. Le Quéré, C. et al. Earth Syst. Sci. Data 5, 165–185 (2013). 6. Hungate, B. A., Dukes, J. S., Shaw, M. R., Luo, Y. & Field, C. B. Science 302, 1512–1513 (2003). 7. Gerber, S., Hedin, L. O., Keel, S. G., Pacala, S. W. & Shevliakova, E. Geophys. Res. Lett. 40, 5218–5222 (2013). 8. Fisher, J. I., Hurtt, G. C., Thomas, R. Q. & Chambers, J. Q. Ecol. Lett. 11, 554–563 (2008). 9. Gatti, L. V. et al. Nature 506, 76–80 (2014). 10. Clark, D. A., Clark, D. B. & Oberbauer, S. F. J. Geophys. Res. Biogeosci. 118, 783–794 (2013). 11. Lambers, H., Mougel, C., Jaillard, B. & Hinsinger, P. Plant Soil 321, 83–115 (2009). 12. Batterman, S. A. et al. Nature 502, 224–227 (2013).
N EUR O SC I ENCE
Hot on the trail of temperature processing Two studies investigate how information about temperature is processed in the brains of fruit flies, and reveal that different neuronal pathways transmit heating and cooling signals to higher brain regions. See Letters p.353 & p.358 TJ F L O R E N C E & M I C H A E L B . R E I S E R
P
eople often seek a cool breeze on a hot summer’s day, and delight in a roaring fire on a cold winter’s evening — the desire to stay in a thermal ‘comfort zone’ is a powerful driver of our behaviour. Because temperature is both a sensory stimulus and a factor that directly affects metabolic processes, it is not surprising that nearly all animals avoid unfavourable temperatures1. But what is the biological basis of temperature preference? Two studies2,3 in this issue track the flow of temperature information into the brains of fruit flies (Drosophila melanogaster). They reveal that the brain extracts different qualities of temperature at the very first connection, and does so using distinct mechanisms for hot and cold. Fruit flies have long been the workhorse of genetic studies, but the animals’ behavioural repertoire is also noteworthy. Like most arthropods, flies cannot regulate their core temperature internally. Instead, they move in search of favourable temperatures, which
2 9 6 | NAT U R E | VO L 5 1 9 | 1 9 M A RC H 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
makes them ideal for studies of thermosensing. D. melanogaster exhibits exquisite sensitivity to small temperature changes4, and can even associate thermal conditions with other senses, such as sight and smell, to remember locations and avoid ominous odours5. Studies have defined the organs6, neurons7,8 and receptor proteins9,10 that fruit flies use to avoid unfavourable temperatures. Their primary sensory neurons are in the antenna, which is a veritable Swiss army knife of sensory functions. Individual thermoreceptor neurons are mainly excited by small temperature changes — hot thermoreceptors are strongly excited by heating and weakly inhibited by cooling, and cold thermoreceptors have the opposite response3,8. Genetically silencing these peripheral thermoreceptor neurons, or removing their hot or cold receptor proteins, prevents the fly from avoiding harmful heat or cold7,8. Thermoreceptor neurons project to an area of the fly brain called the proximal antennal protocerebrum (PAP), in which their output terminals (axons) segregate into hot and cold zones8.
