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Listen to a part of lecture in an environmental science class.
獨白:請聽下面一段環境科學課上的演講。
Lecturer:So since we're around the topic of global climate change and its effects,
講師:那么我們一直在討論關于全球氣候變化及其影響的話題，
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,
在阿拉斯加，其北部靠近北極的地方，在過去大概三十年間，氣溫以大概每十年一攝氏度的水平上漲。
and scientists have noticed that there've been changes in surface vegetation during this time.
而科學家們已經注意到氣溫上漲時植被表面也發生了變化。
Shrubs are increasing in the "tundra".
“苔原”上的灌木叢數量增加。
Tundra is flat land with very little vegetation.
苔原就是植被甚少的平坦地面。
Just a few species of plants grow there because the temperature is very cold, and there's not much precipitation.
苔原上的植物種類不多，因為其氣溫非常低，降雨量少。
And because of the cold temperatures, the tundra has two layers:
而由于氣候非常寒冷，苔原有兩層:
top layer, which is called the active layer, is frozen in the winter and spring, but thaws in the summer.
最上面的一層被稱為活土層，春冬兩季是結冰狀態，夏天解凍。
Beneath this active layer is the second layer called "permafrost", which is frozen all year around, and is impermeable to water.
下面第二層叫做“永久凍土層”，全年冰凍，水也滲透不進去。
Female Student:So because of the permafrost, none of the plants that grow there can have deep roots, can they?
女學生:那么由于永久凍土層的存在，沒有植物可以將根長到里面去是嗎？
Lecturer:No, and that's one of the reasons that shrubs survive in the Arctic.
講師:是的，而這也是為什么灌木叢能夠在北極存活的原因。
Shrubs are little bushes.
灌木叢是小型的矮樹叢。
They're not tall and being low in the ground protect them from the cold and wind.
距離地面的高度不高，這樣就保護其不受風寒所害。
And their roots don't grow very deep, so the permafrost doesn't interfere with their growth.
而它們的根也不會長得很深，這樣永久凍土層便不會阻礙其生長。
OK? Now since the temperatures have been increasing in Arctic Alaska, the growth of shrubs has increased.
沒問題吧？那么由于北極阿拉斯加氣溫逐漸上漲，灌木叢也隨之生長。
And this is presented to climate scientists with a puzzle.
而這就給氣候科學家帶來了難題。
Male Student:I'm sorry, when you say the growth of shrubs has increased,
男學生:不好意思打斷，當你說到灌木叢生長加劇的時候，
do you mean the shrubs are bigger, or that there are more shrubs?
你是說灌木叢變得越來越大還是越來越多？
Lecturer:Good question!
講師:問得好！
And the answer is both.
答案是兩者兼具。
The size of the shrubs has increased and shrub cover has spread to what was previously shrub-free tundra.
灌木叢的尺寸增大，蔓延到原先沒有灌木叢的苔原地區，
Ok, so what's the puzzle?
那么這個難題是什么呢？
Warmer temperatures should lead to increased vegetation growth, right?
氣溫上升應該導致植被生長的加劇，對吧？
Well, the connections are not so simple.
嗯，其關系并非如此簡單。
The temperature increase has occurred during the winter and spring, not during the summer.
冬春兩季氣溫上升，夏天卻不變。
But the increase in shrubs has occurred in the summer.
而灌木叢生長加劇則發生在夏天。
So how can increase temperatures in the winter and spring result in increased shrub growth in the summer?
那么冬春兩季氣溫上漲是如何導致夏天灌木叢的生長加劇的呢？
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:
嗯，這也許是一個發生在冬天土壤里的生物過程，導致夏天灌木叢生長加劇。現在說說是怎樣的過程:
there are "microbes", microscopic organisms that live in the soil.
土壤中有“微生物”即微小的有機體存在。
These microbes enable the soil to have more nitrogen, which plants need to live and they remain quite active during the winter.
這些微生物可以讓土壤中的氮含量增加。氮是植物生長所需，而冬天的時候微生物會保持活躍狀態。
There're two reasons for this:
有兩個原因:
first, they live in the active layer, which, remember, contains water that doesn't penetrate the permafrost.
首先，微生物在活土層中生長，請記住活土層中所含水分是不能滲透到永久凍土層的。
Second, most of the precipitation in the Arctic is in the form of snow.
其次，北極的降水大多是以下雪的形式出現的。
And the snow, which blankets the ground in the winter, actually has an insulating effect on the soil beneath it.
而冬天的雪會覆蓋整個地面，實際上還會對雪下面的土壤產生隔絕效果。
And it allows the temperature of the soil to remain warm enough for microbes to remain active.
這就讓雪下土壤的溫度得到保持，微生物便可以活躍生長。
So there's been increase in nutrient production in the winter.
因此冬天的營養物生長加劇。
And that's what's responsible for the growth of shrubs in the summer and their spread to new areas of the tundra.
而這就說明了為什么夏天灌木叢會加劇生長并蔓延到苔原的新區域中去。
Areas with more new nutrients are the areas with the largest increase in shrubs.
營養物多的地方即灌木叢生長最多的地區。
Female student:But, what about run-off in the spring, when the snow finally melts?
女學生:那么，春天的時候雪一融化營養物不會被沖洗掉嗎？
Won't the nutrients get washed away? Spring thaw always washes away soil, doesn't it?
春天雪融化一般都會造成水土流失，不是嗎？
Lecturer:Well, much of the soil is usually still frozen during peak run-off.
講師:在雪融化高峰期的時候大多數土壤還處在結凍期。
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.
而營養物處于活土層底部，不是在最容易被沖刷掉的土層表面。
But as I was about to say, there's more to the story.
但正如我剛要說的，這背后還有很多原因。
The tundra is windy, and the snow is blown across the tundra, it's caught by shrubs.
苔原上風大，雪被吹過苔原地區，落在灌木叢上，
And deep snow drifts often form around shrubs.
這樣灌木叢上經常有厚厚的積雪。
And we've already mentioned the insulating effect of snow.
而我們已經說過關于雪的隔絕效果。
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..
所以土壤中沒有散發的熱氣意味著更多的微生物活動，也就是給灌木叢提供更多食物，更多的灌木叢，更多的積雪等等。
It's a circle, a loop.
這樣一環扣一環。
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.
而正是由于這樣的循環，其在冬春兩季氣溫變暖加劇了該循環，這樣，苔原看起來就像是被轉換為灌木林地。
Female student:But will it be long term?
女同學:那這樣循環持續的時間長嗎？
I mean maybe the shrubs will be abundant for a few years, and then it'll change back to tundra.
就是也許幾年間灌木叢會長得很茂密，之后又回到苔原的狀態。
Lecturer:Well, shrub expansion has occurred in other environments, like semiarid grassland, and tall grass prairies.
講師:嗯，在其他環境中也發生了灌木叢擴張現象，像半酸性草原和草叢草原。
And shrub expansion in these environments does seem to persist, almost to the point of causing a shift.
而灌木叢擴張似乎不會延續，就算達到了轉變的地步，
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.
一旦扎根土壤，灌木林地就會生長茂盛，特別是在北極地區，因為北極地區的灌木叢比其他植物更善于利用土壤中的營養成分。
