- 2018年01月31日11:04 来源：小站整理
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Narrator: Listen to part of a lecture in an environmental science class.
Lecturer: So since we're on the topic of global climate change and its effects, in Alaska, in the northern Arctic part of Alaska, over the last ... oh... thirty years or so, temperatures have increased about half a degree Celsius per decade, and scientists have noticed that there’s been a change in surface vegetation during this time. Shrubs are increasing in the "tundra". Tundra is flat land with very little vegetation.
Just a few species of plants grow there because the temperature is very cold, and there's not much precipitation. And because of the cold temperatures, the tundra has two layers: top layer, which is called the active layer, is frozen in the winter and spring, but thaws in the summer. Beneath this active layer is a second layer called "permafrost", which is frozen all year around, and is impermeable to water.
Female Stu: So because of the permafrost, none of the plants that grow there can have deep roots, can they?
Lecturer: No, and that's one of the reasons that shrubs survive in the Arctic. Shrubs are little bushes. They're not tall and being low to the ground protects them from the cold and wind. And their roots don't grow very deep, so the permafrost doesn't interfere with their growth. OK? Now since the temperatures have been increasing in Arctic Alaska, the growth of shrubs has increased. And this has presented climate scientists with a puzzle...
Male Stu: I'm sorry, when you say the growth of shrubs has increased, do you mean that the shrubs are bigger, or that there are more shrubs?
Lecturer: Good question! And the answer is both. The size of the shrubs has increased and shrubcover has spread to what was previously shrub-free tundra. Ok, so what's the puzzle? Warmer temperatures should lead to increased vegetation growth, right? Well, the connection is not so simple.
The temperature increase has occurred during the winter and spring, not during the summer. But the increase in shrubs has occurred in the summer. So how can increase temperatures in the winter and spring result in increased shrub growth in the summer? Well, it may be biological processes that occur in the soil in the winter, that cause increased shrub growth in the summer, and here's how: there are "microbes", microscopic organisms that live in the soil.
These microbes enable the soil to have more nitrogen, which plants need to live and they remain quite active during the winter. There're two reasons for this: first, they live in the active layer, which, remember, contains water that doesn't penetrate the permafrost.
Second, most of the precipitation in the Arctic is in the form of snow. And the snow, which blankets the ground in the winter, actually has an insulating effect on the soil beneath it. And it allows the temperature of the soil to remain warm enough for microbes to remain active. So there's been increased nutrient production in the winter. And that's what's responsible for the growth of shrubs in the summer and their spread to new areas of the tundra. Areas with more new nutrients are the areas with the largest increase in shrubs.
Female Stu: But, what about run-off in the spring, when the snow finally melts? Won't the nutrients get washed away? Spring thaw always washes away soil, doesn't it?
Lecturer: Well, much of the soil is usually still frozen during peak run-off. And the nutrients are deep down in the active layer anyway, not high up near the surface, which is the part of the active layer most affected by run-off. But as I was about to say, there's more to the story. The tundra is windy, and the snow is blown across the tundra, it's caught by shrubs. And deep snow drifts often form around shrubs.
And we've already mentioned the insulating effect of snow. So that extra warmth means even more microbial activity, which means even more food for the shrubs, which means even more shrubs and more snow around etc.. It's a circle, a loop. And because of this loop, which is promoted by warmer temperatures in winter and spring, well, it looks like the tundra may be turning into shrub land.
Female Stu: But will it be long-term? I mean maybe the shrubs will be abundant for a few years, and then it'll change back to tundra.
Lecturer: Well, shrub expansion has occurred in other environments, like semiarid grassland, and tall grass prairies. And shrub expansion in these environments does seem to persist, almost to the point of causing a shift. Once is established, shrub land thrives, particularly in the Arctic, because Arctic shrubs are good at taking advantage of increased nutrients in the soil, better than other Arctic plants.