【托福阅读题型：Savanna Formation句子插入题】托福阅读中句子插入题是重要题型之一。根据托福阅读题型不同来进行专项练习，是短期提高托福阅读能力的有效方法。同学们需要多加练习哦!

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　　托福阅读句子插入题：Savanna Formation

　　【Paragraph 5】Most savannas probably experience mild fires frequently and major burns every two years　or so. Many savanna and dry-forest plant species are called pyrophytes, meaning they are adapted in various ways　to withstand occasional burning. Frequent fire is a factor to which rain forest species seem unable to adapt,　although ancient charcoal remains from Amazon forest soils dating prior to the arrival of humans suggest that　moist forests also occasionally burn. Experiments suggest that if fire did not occur in savannas in the Americas,　　species composition would change significantly. When burning occurs, it prevents competition among plant　species from progressing to the point where some species exclude others, reducing the overall diversity of the　ecosystem. But in experimental areas protected from fire, a few perennial grass species eventually come to　dominate, outcompeting all others. ■Evidence from other studies suggests that exclusion of fire results inmarkedly decreased plant-species richness, often with an increase in tree density. ■There is generally little doubt　that fire is a significant factor in maintaining savanna, certainly in most regions.

　　【Paragraph 6】■On certain sites, particularly in South America, savanna formation seems related to　frequent cutting and burning of moist forests for pastureland. ■Increase in pastureland and subsequent overgrazing　have resulted in an expansion of savanna. The thin upper layer of humans (decayed organic matter) is destroyed　by cutting and burning. Humus is necessary for rapid decomposition of leaves by bacteria and fungi and for　recycling by surface roots. Once the humus layer disappears, nutrients cannot be recycled and leach from the soil,　converting soil from fertile to infertile and making it suitable only for savanna vegetation. Forests on white, sandy　soil are most susceptible to permanent alteration.(Savanna Formation)

　　Look at the four squares [■] that indicate where the following sentence could be added to the passage.

　　In addition, humans have contributed to the conditions favoring the formation of savannas.

　　Where would the sentence best fit?

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