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1 .<-NARRATOR:->Listen to a part of lecture in an environmental science class.
1 .<-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.
1 .Tundra is a flat land with very little vegetation.
2 .Just a few species of plants grow there because the temperature is very cold, and there's not much precipitation.
3 .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.
4 .Beneath this active layer is the second layer called "permafrost", which is frozen all year around, and is impermeable to water.
1 .<-FEMALE STUDENT:->So because of the permafrost, none of the plants that grow there can have deep roots, can they?
1 .<-FEMALE PROFESSOR:->No, and that's one of the reasons that shrubs survive in the arctic.
2 .Shrubs are little bushes; they’re not tall, and being low to the ground protects them from the cold and wind.
3 .And their roots don't grow very deep, so the permafrost doesn't interfere with their growth. OK?
1 .Now since the temperatures have been increasing in Arctic Alaska, the growth of shrubs has increased.
2 .And this has presented climate scientists with a puzzle...
1 .<-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?
1 .<-FEMALE PROFESSOR:->Good question! And the answer is “both.”
2 .The size of the shrubs has increased and shrub cover has spread to what was previously shrub-free tundra.
3 .Ok, so what's the puzzle—warmer temperatures should lead to increased vegetation growth, right?
4 .Well, the connections are not so simple.
5 .The temperature increase has occurred during the winter and spring—not during the summer.
6 .But the increase in shrubs has occurred in the summer.
7 .So, how can increased temperatures in the winter and spring result in increased shrub growth in the summer?
1 .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.
2 .These microbes enable the soil to have more nitrogen, which plants need to live and they remain quite active during the winter.
3 .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.
4 .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.
5 .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.
6 .Areas with more new nutrients are the areas with the largest increase in shrubs.
1 .<-FEMALE STUDENT:->[Interrupting]But, what about run-off in the spring, when the snow finally melts?
2 .Won't the nutrients get washed away?
3 .Spring thaw always washes away soil, doesn't it?
1 .<-FEMALE PROFESSOR:->Well, much of the soil is usually still frozen during peak run-off.
2 .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.
1 .But, as I was about to say, there's more to the story.
2 .The tundra is windy, and as snow is blown across the tundra it’s caught by shrubs …and deep snowdrifts often form around shrubs.
3 .And we've already mentioned the insulating effect of snow.
4 .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.
5 .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.
1 .<-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.
1 .<-FEMALE PROFESSOR:->Well, shrub expansion has occurred in other environments, like semiarid grassland and tall grass prairies.
2 .And shrub expansion in these environments does seem to persist, almost to the point of causing a shift.
3 .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.