Listen to part of a lecture in an ecology class, Professor: We've been talking about the factors that influence the development of certain ecosystems. we've looked at the large role that climate plays，but today I wanted to touch on an ecosystem where fire actually plays a bigger role than climate, savannas. Now savannah are usually flat grasslands with scattered trees. They are more common in Africa in both high rainfall areas and low rainfall areas. But regardless of where they are, almost all savannas experience bushfires to one degree or another. And those fires really play a central role in how savanna vegetation is maintained. Yes, Mike, Male students: So savannas in high rainfall areas also have a lot of fires? That seems strange to me. I mean you would think that the weather，the area，the fewer number of fires you'd have, Professor: It's counter intuitive，but the key here is seasonality，which is a distinctive feature of savannas. So high rainfall areas can have dry seasons and that's when the high rainfall savannas burn. After a long dry season has caused the grass to become especially dry. Very dry grass is almost like t it will burn very easily and very quickly. Also high rainfall areas tend to produce very high grass, which means a lot of fuel for the fire. In fact even the driest savannas in low rainfall areas only burn when above average rainfall has allowed plants to grow enough to provide fuel for the fire. all right, Since we're talking about ecosystems，let's go back to the vegetation of savannahs and how fires affected, As you might imagine，over time，savanna vegetation has adapted very well to fire. Trees and shrubs have a very thick bark that helps protect them from the fire. Savannah trees are separated pretty far apart from one another，and their crowns are high above the ground, So crown fires are almost impossible, A crown fire burns in the leafy Top of a tree and quickly spreads from tree Top to tree Top when trees are close together. But still，some plant species recover more quickly than others. In savannah，some plants need seeds to reproduce and others do not. Plants without seeds undergo what we call vegetative regeneration. Vegetative regeneration means the plants use their own root systems to regenerate. Those plants are the ones that recover from fires more quickly because they can sprout from underground buds，after a fire, Female student: Are there a lot? I mean how frequent are these fires？ Professor: It varies with climate condition, Some fires are deliberately started by people. Those occur pretty much year round to clear land for cultivation and keep excessive growth down that sort of thing. They are extremely common，whereas natural fires are a result of specific rare events like when lightning strikes during a thunderstorm, Male student: Don't they worry about completely destroying native species or something？ Professor: Well, Remember plants re let's say you have an area of land you want to clear for cultivation, so you can grow vegetables on it or let animals graze on it. You're probably going to have to set a controlled fire in order to clear that land, But once that land is clear，the lifespan of local plants and animals is actually probably going to increase. Among other things，the ashes left by the fire help create a soil that's much more rich with nutrients，which is good for both plants and animals. It really benefits the ecosystem, I'll give you an example from right here in this country. In the Midwest of the United States,without the existence of carefully controlled fires, the savannah oak tree would be in serious danger of extinction. The oaks themselves are fire resistant and the fire clears out the weeds and smaller plants that cling to the base of the trees，which makes the trees healthier and allows them to live longer. Now let me be clear，fire is a real danger in forests, where trees grow closer together and their bark is thinner, important not to overlook that. But in savannas they're almost necessary,