新托福考试必备：新托福TPO（1-24）听力原文文本TPO9

　　Lecturer: So since we're around the topic of global climate change and its
　　effects, in Alaska, in the northern Arctic part of Alaska, over the last thirty years
　　or so, temperature has increased about half a degree Celsius per decade, and
　　scientists have noticed that there've been changes 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 the second layer called "permafrost", which is
　　frozen all year around, and is impermeable to water.

　　Female Student: 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 in the ground protect
　　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 is presented to climate scientists with a puzzle...

　　Male Student: I'm sorry, when you say the growth of shrubs has increased, do
　　you mean 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 shrub cover 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 connections are 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 increase in 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 student: 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 student: 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.