Official 09 Set 3

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Shrubs in Tundra

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What is the lecture mainly about?
  • A. Factors involved in the increased growth of shrubs in Arctic Alaska

  • B. How temperature increases might be affecting the permafrost in Arctic Alaska

  • C. Why nutrient production of microbes in the soil in Arctic Alaska is declining

  • D. Reasons that grasslands are turning into tundra in Arctic Alaska

显示答案 正确答案: A

我的笔记 编辑笔记

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    NARRATOR:Listen to a part of lecture in an environmental science class.

    FEMALE PROFESSOR: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 a 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 temperature, 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?

    FEMALE PROFESSOR: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 STUDENT:I'm sorry, when you say the growth of shrubs has increased … um, do you mean that the shrubs are bigger, or that there are more shrubs?

    FEMALE PROFESSOR: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 increased 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 STUDENT:[Interrupting]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?

    FEMALE PROFESSOR: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 as snow is blown across the tundra it’s caught by shrubs …and deep snowdrifts 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 them, 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.

    FEMALE PROFESSOR: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 it’s 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.

  • 旁白:请听一段环境科学课上的演讲。

    教授:那么我们一直在讨论的话题是关于全球气候变化及其影响,在阿拉斯加,其北部的北极地区,在过去大概三十年间,气温以大概每十年0.5摄氏度的水平上涨。而科学家们已经注意到气温上涨时植被表面也发生了变化,苔原上的灌木丛数量增加。

    苔原即植被甚少的平坦地面。苔原上的植物种类不多,因为其气温非常低,降雨量少。而由于气候非常寒冷,苔原有两层,上面一层叫做活土层,春冬两季是结冰状态,夏天解冻。下面第二层叫做“永久冻土层”,全年冰冻,水也渗透不进去。

    学生:那么由于永久冻土层的存在,没有植物可以将根长到里面去是吗?

    教授:是的,而这也是为什么灌木丛能够在北极存活的原因之一。灌木丛是小型的矮树丛,距离地面的高度不高,这使得其不受风寒所害。而它们的根也不会长得很深,这样永久冻土层便不会阻碍了其生长。是吧?

    那么由于北极阿拉斯加气温逐渐上涨,灌木丛也随之生长。而这就给气候科学家带来了难题。

    学生:不好意思,当你说到灌木丛生长加剧的时候,你是说灌木丛变得越来越大还是越来越多?

    教授:问得好!答案是两者兼具。灌木丛的尺寸增大,蔓延到原先没有灌木丛的苔原地区。那么这个难题是什么呢?气温上升应该导致植被生长的加剧,对吧?嗯,其关系不是这么简单的。冬春两季气温上升,夏天却不变。而灌木丛生长加剧则发生在夏天。那么冬春两季气温上涨是怎么导致夏天灌木丛的生长加剧的呢?

    嗯,这也许是冬季土壤中发生的生物过程,导致夏天灌木丛生长加剧。现在说说是怎样的过程:土壤中有“微生物”即微小的有机体。这些微生物可以让土壤中植物生长所需的氮含量增加,而冬天的时候它们仍然会保持活跃状态。有两个原因:首先,微生物在活土层中生长,记住,活土层中所含水分是不能渗透到永久冻土层的。其次,北极的降水大多是以下雪的形式出现的。而冬天的雪会覆盖整个地面,实际上是对雪下面的土壤产生隔热效果,这就让雪下土壤的温度保持足够温暖,微生物便可以活跃生长。因此冬天的营养物生长加剧,而这就说明了为什么夏天灌木丛会加剧生长并蔓延到苔原的新区域中去。营养物多的地方即灌木丛生长最多的地区。

    学生:那么,春天雪融化的时候呢?营养物不会被冲洗掉吗?春天雪融化一般都会造成水土流失,不是吗?

    教授:在雪融化高峰期的时候大多数土壤还处在结冻期。而营养物处于活土层底部,不是在最容易被冲刷掉的土层表面。

    但正如我刚要说的,这背后还有很多原因。苔原上风大,雪被吹过苔原地区,落在灌木丛上,这样灌木丛上经常有厚厚的积雪。而我们已经说过关于雪的隔热效果。所以土壤中没有散发的温暖意味着更多的微生物活动,也就是给灌木丛提供更多食物,更多的灌木丛,更多的积雪等等...这是一个循环。而正由于这样的循环,其在冬春两季气温变暖加剧了该循环,这样,苔原看起来就像是被转换为灌木林地。

    学生:这是长期的么?就是也许几年间灌木丛会长得很茂密,之后又回到苔原的状态。

    教授:嗯,在其他环境中也发生了灌木丛扩张现象,像半干旱草原和草丛草原。在这些环境中,灌木丛扩张似乎还在持续,几乎到了转变的程度。一旦扎根土壤,灌木林地就会生长茂盛,特别是在北极地区,因为北极地区的灌木丛比其他植物更善于利用土壤中的营养成分。

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    题型分类:主旨题
    原文定位:

    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, temperature has 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".
    选项分析:
    文章开头提到了环境变化的影响和shrub,后续文章内容主要讨论影响shrub 增长因素,全文总结,A选项正确。

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