关闭图标
log图标

toefl.viplgw.cn

  • 使用手机注册
  • 使用邮箱注册
  • 手机号不能为空!

    验证码不能为空!

    用户名不能为空!

    密码不能为空!

  • 邮箱不能为空!

    验证码不能为空!

    用户名不能为空!

    密码不能为空!

已有账号? 登录到雷哥托福
关闭图标
log图标
  • 使用手机找回密码
  • 使用邮箱找回密码
  • 手机号不能为空!

    验证码不能为空!

    密码不能为空!

  • 邮箱不能为空!

    验证码不能为空!

    密码不能为空!

又想起来了
加入生词本

listen

英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

已添加
×

我要举报草莓小菇凉评论

用户头像
草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

请选择举报类型:

举报电话:400-6021-727    举报QQ:2095453331
×
logo图标
分享到雷哥托福

分享成功图标分享成功

邀请名师点评成功,管理员正在安排老师进行点评。

继续做题 返回首页
支付雷豆失败图标 雷豆余额不足 购买雷豆 返回
报告题目错误
请选择错误类型:
请描述一下这个错误:

取消

TPO-31-Savanna Formation

Located in tropical areas at low altitudes, savannas are stable ecosystems, some wet and some dry consisting of vast grasslands with scattered trees and shrubs. They occur on a wide range of soil types and in extremes of climate. There is no simple or single factor that determines if a given site will be a savanna, but some factors seem to play important roles in their formation.

Savannas typically experience a rather prolonged dry season. One theory behind savanna formation is that wet forest species are unable to withstand the dry season, and thus savanna, rather than rain forest, is favored on the site. Savannas experience an annual rainfall of between 1,000 and 2,000 millimeters, most of it falling in a five- to eight-month wet season. Though plenty of rain may fall on a savanna during the year, for at least part of the year little does, creating the drought stress ultimately favoring grasses. Such conditions prevail throughout much of northern South America and Cuba, but many Central American savannas as well as coastal areas of Brazil and the island of Trinidad do not fit this pattern. In these areas, rainfall per month exceeds that in the above definition, so other factors must contribute to savanna formation.

In many characteristics, savanna soils are similar to those of some rain forests, though more extreme. For example, savanna soils, like many rain forest soils, are typically oxisols (dominated by certain oxide minerals) and ultisols (soils containing no calcium carbonate), with a high acidity and notably low concentrations of such minerals as phosphorus, calcium, magnesium, and potassium, while aluminum levels are high. Some savannas occur on wet, waterlogged soils; others on dry, sandy, well-drained soils. This may seem contradictory, but it only means that extreme soil conditions, either too wet or too dry for forests, are satisfactory for savannas. More moderate conditions support moist forests.

Waterlogged soils occur in areas that are flat or have poor drainage. These soils usually contain large amounts of clay and easily become water saturated. Air cannot penetrate between the soil particles, making the soil oxygen-poor. By contrast, dry soils are sandy and porous, their coarse textures permitting water to drain rapidly. Sandy soils are prone to the leaching of nutrients and minerals and so tend to be nutritionally poor. Though most savannas are found on sites with poor soils (because of either moisture conditions or nutrient levels of both), poor soils can and do support lush rain forest.

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 in markedly 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.

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 thin upper layer of humus (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.

位于热带低海拔地区,稀树草原是一个稳定的生态系统,有些湿润而干燥,由广阔的草原和疏散的树木和灌木组成。它们发生在广泛的土壤类型和极端的气候条件下。没有简单或单一的因素来决定一个特定的地点是否是一个稀树草原,但有些因素似乎在其形成过程中起着重要的作用。

 

Savannas通常会经历相当长时间的旱季。在稀树草原形成背后的一个理论是,潮湿的森林物种不能经受旱季,因此热带稀树草原而不是热带雨林更受青睐。 Savannas每年降雨量在1000至2000毫米之间,大部分降雨量在5至8个月的雨季。尽管一年中可能会有大量的降雨落在热带稀树草原上,但至少在今年的一部分时间里,这种干旱压力最终导致了草的生长。南美北部和古巴大部分地区都存在这种情况,但许多中美洲的稀树草原以及巴西沿海地区和特立尼达岛都不适合这种模式。在这些地区,每月的降雨量超过了上述的定义,所以其他因素必须有助于稀树草原的形成。

 

在许多特征中,稀树草原土壤类似于一些热带雨林的土壤,尽管更为极端。例如,稀树草原土壤(如许多雨林土壤)通常是氧化物(由某些氧化物矿物主宰)和超溶剂(不含碳酸钙的土壤),具有高酸度和显着低浓度的诸如磷,钙,镁,和钾,而铝水平高。一些稀树草原发生在潮湿,涝渍的土壤上;其他人在干燥,多沙,排水良好的土壤。这似乎是矛盾的,但这只意味着极度的土壤条件,对于森林来说太湿或太干,对于稀树草原来说是令人满意的。更温和的条件支持潮湿的森林。

 

积水的土壤出现在地势平坦或排水不畅的地区。这些土壤通常含有大量的粘土,容易变得饱和。空气不能穿透土壤颗粒之间,使土壤贫氧。相比之下,干燥的土壤是沙质和多孔的,它们的粗糙结构允许水迅速流失。桑迪土壤倾向于营养物质和矿物质的浸出,因此往往是营养差。尽管大多数稀树草原是在土壤贫瘠的地方(由于湿度条件或两者的营养水平)发现,贫瘠的土壤可以并且支持茂密的雨林。

 

大多数的稀树草原很可能经常发生轻度火灾,每两年大面积烧伤。许多稀树草原和干燥的森林植物物种被称为pyrophytes,这意味着他们适应各种方式来承受偶尔燃烧。经常发生的火灾是雨林物种似乎无法适应的一个因素,尽管亚马逊森林土壤中的古老的木炭在人类到来之前的年代表明,潮湿的森林也偶尔会燃烧。实验表明,如果在美洲的稀树草原没有发生火灾,物种构成将发生显着变化。当发生燃烧时,它阻止了植物物种之间的竞争进展到一些物种排斥其他物种的位置,从而减少了生态系统的整体多样性。但在防火的实验区,一些多年生草本植物最终占据主导地位,胜过所有其他种类。来自其他研究的证据表明,排除火灾导致植物物种丰富度显着降低,通常树密度增加。通常毫无疑问,在大多数地区,火灾是维持热带草原的重要因素。

 

在某些地区,特别是在南美洲,稀树草原的形成似乎与湿地森林频繁割裂和燃烧有关。草地的增加和随后的过度放牧已经导致了热带草原的扩张。腐殖质(腐烂的有机质)薄薄的上层被切割和燃烧破坏。腐殖质对于由细菌和真菌迅速分解叶子和通过表面根部回收是必需的。一旦腐殖质层消失,营养物质不能被回收和从土壤中浸出,将土壤从肥沃土壤转化为贫瘠土地,使其仅适用于稀树草原植被。白色沙土上的森林最容易受到永久性的影响。

中文翻译

The word “prolonged” in the passage is closest in meaning to:

  • Savannas typically experience a rather prolonged dry season. A Predictable B Destructive C Lengthy D Unproductive prolonged
正确答案: C

网友解析

  • 用户头像

    十三个  发表于 2018-11-22 19:57:27

    可以根据词根分析,原文中prolonged,词根为long,长度。Prolonged为延长的。选项中C,length本来是长度的意思,lengthy变为形容词格式,符合原词词义。

题目讨论

  • 用户头像

    kisenana  发表于 12-07 16:31:29

    词缀也是判断词义的一种方式

    • (0)
    • |
    • 回复(0)
    • |
    • 举报
  • 1

雷哥托福微信

  • 微信公众号:toeflgo

    二维码
  • 小托君:Toefl12345

    二维码
查看详情图片

您已提交评论成功

确定