OFFICIAL9 What is the lecture mainly about?

[00:00.00]Narrator: Listen to part of a lecture in an environmental science class.
[00:07.99]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 … 30 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—[00:27.22]shrubs are increasing in the tundra. [00:30.35]Tundra is flat land, with very little vegetation.
[00:33.38]Just a few species of plants grow there because the temperature’s very cold and there’s not much precipitation. [00:39.03]And because of the cold temperatures, tundra has two layers. The top layer, which is called the active layer, is frozen in the winter and spring, but thaws in the summer. [00:49.11]Beneath this active layer is a second layer called permafrost, which is frozen all year round and is impermeable to water.
[00:58.10]FEMALE STUDENT: So, because of the permafrost, none of the plants that grow there can have deep roots, can they?
[01:03.79]FEMALE PROFESSOR: No, and that's one of the reasons that shrubs survive in the arctic. [01:08.07]Shrubs are little bushes; [01:09.80]they’re not tall, and being low to the ground protects them from the cold and wind. [01:13.93]And their roots don’t grow very deep, so the permafrost doesn’t interfere with their growth. OK? [01:19.93]Now, since the temperatures have been increasing in arctic Alaska, the growth of shrubs has increased. [01:26.47]And this has presented climate scientists with a puzzle.
[01:29.76]MALE STUDENT: Um, 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?
[01:37.49]FEMALE PROFESSOR: Good question. And the answer is “both.” [01:40.15]The size of the shrubs has increased and shrub cover has spread to what was previously shrub-free tundra. [01:48.77]OK. So, what's the puzzle—[01:50.97]warmer temperatures should lead to increased vegetation growth, right? [01:55.62]Well, the connection’s not so simple.
[01:58.23]The temperature increase has occurred during the winter and spring—not during the summer. [02:03.46]But, the increase in shrubs has occurred in the summer. [02:07.42]So, how can increased temperatures in the winter and spring result in increased shrub growth in the summer?  [02:14.01]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.
[02:29.63]These microbes enable the soil to have more nitrogen, which plants need to live, and they remain quite active during the winter. [02:37.05]There are two reasons for this. First, they live in the active layer which, remember, contains water that doesn’t penetrate the permafrost.
[02:45.46]Second, most of the precipitation in the arctic is in the form of snow. [02:50.68]And the snow which blankets the ground in the winter actually has an insulating effect on the soil beneath it … [02:57.06]and it allows the temperature of the soil to remain warm enough for microbes to remain active.[03:04.79]So there’s been increased nutrient production in the winter, [03:08.33]and that’s what’s responsible for the growth of shrubs in the summer and their spread to new areas of the tundra. [03:14.35]Areas with more nutrients are the areas with the largest increase in shrubs.
[03:18.58]FEMALE STUDENT: But what about runoff in the spring, when the snow finally melts? [03:22.84]Won't the nutrients get washed away? [03:24.93]Spring thaw always washes away soil, doesn’t it?
[03:28.22]FEMALE PROFESSOR: Well, much of the soil is usually still frozen during peak runoff. [03:32.19]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 runoff. [03:41.23]But, as I was about to say, there’s more to the story.  [03:45.15]The tundra is windy, and as snow is blown across the tundra it’s caught by shrubs …[03:50.16]and deep snowdrifts often form around shrubs.
[03:53.20]And we’ve already mentioned the insulating effects of snow … [03:57.06]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. [04:08.66]It’s a circle, a loop. [4:10.23]And because of this loop, which is promoted by warmer temperatures in the winter and spring … well, it looks like the tundra may be turning into shrubland.
[04:19.58]FEMALE STUDENT: But will it be long-term? [04:21.70]I mean, maybe the shrubs will be abundant for a few years, and then it’ll change back to tundra.
[04:26.61]FEMALE PROFESSOR: Well, shrub expansion has occurred in other environments, like semiarid grassland and tall grass prairies. [04:33.50]And shrub expansion in these environments does seem to persist … almost to the point of causing a shift. [04:39.67]Once it’s established, shrubland thrives. Particularly in the arctic, because arctic shrubs are good at taking advantage of increased nutrients in the soil—better than other arctic plants.