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科学美国人60秒:新树种的碳储存速度是燃烧前的四倍

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Emily Schwing: This is 60 Second Science, I’m Emily Schwing.

艾米丽·施温:这里是 60 秒科学,我是艾米丽·施温。

The boreal forest is the largest terrestrial biome in the world. It covers 14 percent of the land on earth.

北方森林是世界上最大的陆地生物群落。它覆盖了地球上 14% 的土地。

And, today, I’m with my dogs on the north side of Denali National Park and Preserve.

今天,我和我的狗来到德纳利国家公园和保护区的北侧。

Schwing: I’m walking my dogs in the boreal forest. It’s also known as taiga and this is where 11 percent of the world’s carbon is stored. It covers 1.3 billion acres of the earth's surface.

施温:我带着狗穿过北方森林。它也被称为针叶林,这里储存了世界 11% 的碳。覆盖了13亿英亩的地球表面。

[Sound of dogs playing]

This forest is actually really beautiful. There’s these black spruce trees.... They’re stumpy, sometimes really short and goring in different directions. There’s lichen that hangs off of the dead branches, bright green lichen. Today, there’s a skim of snow on the ground overtop of this feather moss that just covers the forest floor.

这片森林其实真的很美。有这些黑云杉树……它们很矮,有时很短,朝不同的方向生长。枯枝上挂着青苔,鲜绿色的青苔。今天,在这片刚刚覆盖森林地面的羽毛苔藓之上,有一层薄薄的雪。

[Bird calls, Dog whistles]

There's a magpie on a tree branch right in front of me....

我面前的树枝上有一只喜鹊……

[Sound of crunching in snow]

Wildfire is a normal part of the lifecycle for a boreal forest, but as the climate warms, this ecosystem is becoming increasingly vulnerable. That vulnerability risks turning forest from carbon sink to carbon source. When the boreal forest burns, it has the potential to release gigatons of carbon into the atmosphere.

野火是北方森林生命周期的正常组成部分,但随着气候变暖,这个生态系统变得越来越脆弱。这种脆弱性有可能将森林从碳汇变成碳源。在北方森林燃烧时,它有可能将数十亿吨的碳释放到大气中。

Jill Johnstone: Much of that carbon is actually in the forest floor.

吉尔·约翰斯通:大部分碳实际上存在于森林地表中。

Jill Johnstone has been studying boreal forest ecology for decades. She does research through Canada’s Yukon University, and with the University of Alaska, Fairbanks, Alaska.

吉尔·约翰斯通几十年来一直在研究北方森林生态学。她通过加拿大育空大学和阿拉斯加费尔班克斯的阿拉斯加大学进行研究。

Johnstone: It's this undecomposed plant remains that accumulate through time. And when a fire comes through the forest, it can just singe the surface of those accumulated plant remains, or it can burn deeply into them like a severe burn combusts most of that forest floor. That results in a pulse of released carbon into the atmosphere in the form of smoke and other aerosols and small carbon dioxide molecules. And so that carbon is essentially lost from storage that burns away.

约翰斯通:这些未分解的植物残渣会随着时间积累。当一场大火穿过森林时,它只会烧焦那些堆积的植物残渣的表面,或者它可以像严重的起火一样深深地烧进森林的大部分地面。其结果是碳以烟雾、其他气溶胶和二氧化碳小分子的形式释放到大气中。所以碳本质上是在储存过程中消耗掉的。

Schwing: This past spring, a group of scientists released their findings from a 15-year study that looked at how boreal forests regenerate after wildfire. Johnstone was a co-author. [Michelle C. Mack et al., Carbon loss from boreal forest wildfires offset by increased dominance of deciduous trees]

施温:今年春天,一组科学家公布了他们在一项为期 15 年的研究中的发现,该研究着眼于野火后北方森林如何再生。约翰斯通是合著者。

Johnstone: Well, one of the things that we've been noticing in the past couple of decades is that fires are burning differently in the boreal forest than we understand them to have burned in the past. And as a result of that, how the forest regenerates is changing.

约翰斯通:嗯,在过去的几十年里,我们注意到的一件事是,北方森林中的大火燃烧方式与我们理解的过去燃烧方式不同。因此,森林的再生方式正在发生变化。

Schwing: Black spruce and fir are the dominant tree species in the boreal forest. They’re coniferous. They’re good at withstanding cold and they grow really slowly.

施温:黑云杉和冷杉是北方森林中的主要树种。它们是针叶树,擅长抵御寒冷,而且他们的成长速度非常缓慢。

Johnstone: Particularly fires that burn deep into the forest floor and they change the seed beds and that affects which tree species regenerate well. And as a result we're seeing to what we call alternative trajectories.

约翰斯通:特别是燃烧到森林深处的大火,它们改变了苗床,影响了树种再生。结果,我们看到恢复过程转变为我们所说的替代轨迹。

Schwing: A changing climate means fires are becoming more severe. It’s burning off the organic layer of soils that support species like black spruce. So after a severe fire, the environment is better suited for deciduous tree species like birch and aspen to get a foothold. These trees grow much faster.

施温:气候变化意味着火灾变得更加严重。它正在烧掉支持黑云杉等物种的土壤有机层。所以在大火之后,环境更适合桦树、白杨等落叶树种获得立足点。这些树长得更快。

And here’s where Johnstone and her colleagues discovered an unexpected twist. Even though extreme fires are burning up those carbon rich layers of organic soil and releasing that carbon into the atmosphere in the short term—what grows back is even more carbon hungry than what came before it. Johnstone says this new kind of boreal forest is storing carbon four times faster than it would have before it burned.

在这里,约翰斯通和她的同事们发现了一个意想不到的转折。尽管极端大火正在烧毁富含碳的有机土壤层,并在短期内将这些碳释放到大气中,但生长回来的植物比之前的植物更需要碳。约翰斯通说,这种新型北方森林的碳储存速度是燃烧前的四倍。

Johnstone: I think that I would have anticipated something like twice as fast, not necessarily four times as fast. And the reason that this number is really interesting, this increase in rate or are so remarkable to us is that prior to this point, we always assumed that the stands that burned severely were a net source of carbon to the atmosphere, meaning they lost more carbon in the fire than they could hope to reaccumulate before the next fire came through.

约翰斯通:我想我会预料到速度会快两倍,不一定是四倍。这个数字真的很有趣,这个速度的增加或者对我们来说如此显着的原因是,在此之前,我们一直认为严重燃烧的树丛是大气中碳的净来源,这意味着它们在大火中失去的碳比它们希望在下一次火灾发生前重新积累的碳要多。

Schwing: Ecologists like Johnstone have worried for years about the amount of carbon that could be lost to the atmosphere from wildfire in the boreal forest, because that could lead to what she calls a “runaway feedback loop with climate change.”

施温:多年来,像约翰斯通这样的生态学家一直担心北方森林野火可能会导致大量碳流失到大气中,因为这可能会导致她所说的“气候变化的失控反馈循环”。。

Johnstone: And so this example of more rapid accumulation of carbon in deciduous broadleaf stands is a way of thinking of putting the brakes on the runaway train. And it's really valuable to recognize that by changing that type of vegetation that recovers after fire or we actually change the carbon accumulation rate.

约翰斯通:所以这个落叶阔叶林中碳快速积累的例子是一种让失控的火车刹车的思维方式。通过改变火灾后恢复的植被类型,或者我们实际上改变碳积累率,认识到这一点非常有价值。

Johnstone says the change in vegetation regime might help bring the system back into “control”—at least for a bit. There are limits to this seeming self-regulation.

约翰斯通说,植被状况的变化可能有助于使系统重新进入“控制”状态——至少在一段时间内。这种表面上的自我调节是有局限性的。

Johnstone: So let's change the metaphor to driving a bus down the highway. And what we're concerned about is that as far as climate warming leads to more fire weather, we get more severe and active fires burning in the boreal forest that releases more carbon. And that's just pushing the accelerator pedal down on this bus.

约翰斯通:所以让我们把比喻改成在高速公路上开公共汽车。我们担心的是,就气候变暖导致更多火灾天气而言,我们会在北方森林中燃烧更严重和活跃的火灾,从而释放更多的碳。这只是在这辆公共汽车上踩下油门踏板。

Schwing: The added carbon accumulation, as the forest regime regenerates and shifts from coniferous to deciduous trees—that’s like putting the parking brake on while driving the bus, says Johnstone.

施温:约翰斯通说,随着森林制度的再生和从针叶树向落叶树的转变,增加的碳积累——就像在驾驶公共汽车时踩下刹车。

Johnstone: So, you're slowing down that acceleration, but it's not enough to stop it completely. And as long as we are continuing to pump carbon into the atmosphere. Particularly through our own fossil fuel combustion, then we're going to keep accelerating. But this feedback of vegetation change in the boreal forest is at least pushing back against that acceleration to slow us down. And we need to use all the tools available to us to help slow that bus down because it's headed to go over a cliff.

约翰斯通:所以,这是减慢了加速,还不足以完全阻止它。只要我们继续将碳排放到大气中。特别是通过我们自己的化石燃料燃烧,我们就会继续加速。但是,北方森林中植被变化的这种反馈至少正在抵制这种加速,以减慢我们的速度。所以需要使用我们可用的所有工具来帮助减慢公共汽车的速度,因为它正驶向悬崖。

Schwing: Discovering this natural shift took some sleuthing.

施温:发现这种自然转变需要一些调查。

Johnstone: Well, this research really played out like a detective novel. At first we noticed, Oh, the fires seem to be doing different things and asking questions about, well, if the fire, the way the fire burns changes, how does that change the forest regeneration? And that required a bunch of experiments and following patterns of forest regeneration for five, 10,15 years.

约翰斯通:嗯,这项研究真的像一部侦探小说。一开始,我们注意到,哦,火灾似乎是重要的事情,并提出了一些问题:如果火灾,大火燃烧的方式发生了变化,这将如何改变森林再生?这需要进行一系列实验并遵循森林再生模式,5年、10年、15年。

Schwing: Deciduous trees like birch and aspen live for about 100 years, so the team couldn’t simply wait for one or two centuries to find out what happens in the forest.

施温:桦树和白杨等落叶树的寿命约为 100 年,因此团队不能简单地等待一两个世纪来了解森林中发生了什么。

Johnstone: So we went out and found all these different age stands across the boreal forest in Alaska and sampled them and tried to make sure that we had a good comparative set of ages that could be linked together as being probably starting from the same initial conditions and then just have different ages. And that piecing together is like doing a jigsaw puzzle.

约翰斯通:所以我们走出去,在阿拉斯加的北方森林中发现了所有这些不同年龄的林木,并对它们进行了取样,试图确保我们有一组很好的可比较的年龄组,这些年龄组可能从相同的初始条件开始联系在一起。然后是不同的年龄组。拼凑起来就像拼图一样。

Schwing: The pieces of the puzzle that are still missing are the ones that help explain how we pump the brakes before that bus reaches the cliff.

施温:仍然缺少的拼图部分,有助于解释我们如何在公共汽车到达悬崖之前刹住车。

For Scientific American, I’m Emily Schwing.

以上是《科学美国人》,艾米丽·施温报道。

Emily Schwing: This is 60 Second Science, I’m Emily Schwing.

The boreal forest is the largest terrestrial biome in the world. It covers 14 percent of the land on earth.

And, today, I’m with my dogs on the north side of Denali National Park and Preserve.

Schwing: I’m walking my dogs in the boreal forest. It’s also known as taiga and this is where 11 percent of the world’s carbon is stored. It covers 1.3 billion acres of the earth's surface.

[Sound of dogs playing]

This forest is actually really beautiful. There’s these black spruce trees.... They’re stumpy, sometimes really short and goring in different directions. There’s lichen that hangs off of the dead branches, bright green lichen. Today, there’s a skim of snow on the ground overtop of this feather moss that just covers the forest floor.

[Bird calls, Dog whistles]

There's a magpie on a tree branch right in front of me....

[Sound of crunching in snow]

Wildfire is a normal part of the lifecycle for a boreal forest, but as the climate warms, this ecosystem is becoming increasingly vulnerable. That vulnerability risks turning forest from carbon sink to carbon source. When the boreal forest burns, it has the potential to release gigatons of carbon into the atmosphere.

Jill Johnstone: Much of that carbon is actually in the forest floor.

Jill Johnstone has been studying boreal forest ecology for decades. She does research through Canada’s Yukon University, and with the University of Alaska, Fairbanks, Alaska.

Johnstone: It's this undecomposed plant remains that accumulate through time. And when a fire comes through the forest, it can just singe the surface of those accumulated plant remains, or it can burn deeply into them like a severe burn combusts most of that forest floor. That results in a pulse of released carbon into the atmosphere in the form of smoke and other aerosols and small carbon dioxide molecules. And so that carbon is essentially lost from storage that burns away.

Schwing: This past spring, a group of scientists released their findings from a 15-year study that looked at how boreal forests regenerate after wildfire. Johnstone was a co-author. [Michelle C. Mack et al., Carbon loss from boreal forest wildfires offset by increased dominance of deciduous trees]

Johnstone: Well, one of the things that we've been noticing in the past couple of decades is that fires are burning differently in the boreal forest than we understand them to have burned in the past. And as a result of that, how the forest regenerates is changing.

Schwing: Black spruce and fir are the dominant tree species in the boreal forest. They’re coniferous. They’re good at withstanding cold and they grow really slowly.

Johnstone: Particularly fires that burn deep into the forest floor and they change the seed beds and that affects which tree species regenerate well. And as a result we're seeing to what we call alternative trajectories.

Schwing: A changing climate means fires are becoming more severe. It’s burning off the organic layer of soils that support species like black spruce. So after a severe fire, the environment is better suited for deciduous tree species like birch and aspen to get a foothold. These trees grow much faster.

And here’s where Johnstone and her colleagues discovered an unexpected twist. Even though extreme fires are burning up those carbon rich layers of organic soil and releasing that carbon into the atmosphere in the short term—what grows back is even more carbon hungry than what came before it. Johnstone says this new kind of boreal forest is storing carbon four times faster than it would have before it burned.

Johnstone: I think that I would have anticipated something like twice as fast, not necessarily four times as fast. And the reason that this number is really interesting, this increase in rate or are so remarkable to us is that prior to this point, we always assumed that the stands that burned severely were a net source of carbon to the atmosphere, meaning they lost more carbon in the fire than they could hope to reaccumulate before the next fire came through.

Schwing: Ecologists like Johnstone have worried for years about the amount of carbon that could be lost to the atmosphere from wildfire in the boreal forest, because that could lead to what she calls a “runaway feedback loop with climate change.”

Johnstone: And so this example of more rapid accumulation of carbon in deciduous broadleaf stands is a way of thinking of putting the brakes on the runaway train. And it's really valuable to recognize that by changing that type of vegetation that recovers after fire or we actually change the carbon accumulation rate.

Johnstone says the change in vegetation regime might help bring the system back into “control”—at least for a bit. There are limits to this seeming self-regulation.

Johnstone: So let's change the metaphor to driving a bus down the highway. And what we're concerned about is that as far as climate warming leads to more fire weather, we get more severe and active fires burning in the boreal forest that releases more carbon. And that's just pushing the accelerator pedal down on this bus.

Schwing: The added carbon accumulation, as the forest regime regenerates and shifts from coniferous to deciduous trees—that’s like putting the parking brake on while driving the bus, says Johnstone.

Johnstone: So, you're slowing down that acceleration, but it's not enough to stop it completely. And as long as we are continuing to pump carbon into the atmosphere. Particularly through our own fossil fuel combustion, then we're going to keep accelerating. But this feedback of vegetation change in the boreal forest is at least pushing back against that acceleration to slow us down. And we need to use all the tools available to us to help slow that bus down because it's headed to go over a cliff.

Schwing: Discovering this natural shift took some sleuthing.

Johnstone: Well, this research really played out like a detective novel. At first we noticed, Oh, the fires seem to be doing different things and asking questions about, well, if the fire, the way the fire burns changes, how does that change the forest regeneration? And that required a bunch of experiments and following patterns of forest regeneration for five, 10,15 years.

Schwing: Deciduous trees like birch and aspen live for about 100 years, so the team couldn’t simply wait for one or two centuries to find out what happens in the forest.

Johnstone: So we went out and found all these different age stands across the boreal forest in Alaska and sampled them and tried to make sure that we had a good comparative set of ages that could be linked together as being probably starting from the same initial conditions and then just have different ages. And that piecing together is like doing a jigsaw puzzle.

Schwing: The pieces of the puzzle that are still missing are the ones that help explain how we pump the brakes before that bus reaches the cliff.

For Scientific American, I’m Emily Schwing.


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