These tree belts effectively protect the farmland against shifting sand.是什么意思？

These tree belts effectively protect the farmland against shifting sand.

这些防护林带有效地保护了农田,使之免受流沙侵袭.

these

adj. 这些的；pron. 这些；

例句
There's lots of money about these days for schemes like this.
近来这类计划资金充足。

tree

n. 树，木料，树状图，宗谱；vt. 把…赶上树，使处于困境，把鞋型插入（鞋内）；

例句
If the soil is allowed to dry out the tree could die.
如果听任土壤变干，树木可能会枯死。

belts

n. 腰带( belt的名词复数 )，区域，传送带，带状物；

例句
The film belts along at a cracking pace.
影片节奏紧凑。

effectively

adv. 有效地，实际上，事实上；

例句
The border, effectively closed since 1981, will be opened as of January the 1st...
实质上从1981年就已关闭的边境将于1月1日开放。

protect

vt. 保护，保卫，贸易保护，备款以支付；

例句
It has been claimed that wine helps protect against heart disease.
有人声称葡萄酒有助于预防心脏病。

the

art. 指已提到的人（物），指说话人与听者已知的人（物），用于独一无二的事物前，与形容词最高级和序数词连用；

例句
He did have a real knowledge of the country...
他对该国的确知之甚多。

farmland

n. 农田，耕地；

例句
The company owns a chunk of farmland near Gatwick Airport.
该公司拥有盖特威克机场周边的大片农田。

against

prep. 反对，对…不利，紧靠，以防；

例句
His hand brushed against hers; it could have been either accidental or deliberate.
就在有意无意间，他的手轻轻拂过她的手。

shifting

n. 移位，转移，偏移，狡猾；v. 改变(shift的现在分词)；adj. 移动的，多变的，狡诈的；

例句
...the shifting geopolitics of the post-cold-war era have changed the thinking behind aid.
冷战之后地缘政治的变幻改变了人们对援助背后的思考。

sand

n. 沙，沙滩，沙色；vt. 撒沙于，用砂纸磨光；

例句
... the mile of white sand beach bordered by palm trees and tropical flowers
四周种有棕榈树和热带花卉的长达1英里的白色沙滩