填空题 A．accumulates B．veiled C．fascinating D．pasture
E．procedure F．barren G．terminating H．endure
I．moisture J．secondary K．stimulates L．lodged
M．texture N．brief O．machine
The forest from which man takes his timber is the tallest and most impressive plant community on earth. In terms of man"s 11 life it appears permanent and unchanging, save for the seasonal growth and fall of the leaves, but to foresters it represents the climax of a long succession of events.
No wooded landscape we see today has been forest for all time. Plants have minimum requirements for temperature and 12 and, in ages past, virtually every part of earth"s surface has at some time been either too dry or too cold for plants to survive. However, as soon as climatic conditions change in favor of plant life, a(n) 13 sequence of changes occurs, called a primary succession.
First to colonize the 14 lands are the lowly lichens, surviving on bare rock. Slowly, the acids produced by these organisms crack the rock surface, plant debris 15 , and mosses establish a shallow roothold. Ferns may follow and, with short grasses and shrubs, gradually form a covering of plant life. Roots probe even deeper into the developing soil and eventually large shrubs give way to the first trees. These grow rapidly, cutting off sunlight from the smaller plants, and soon establish complete domination closing their ranks and forming a climax community which may 16 for thousands of years.
Yet even this community is not everlasting. Fire may destroy it outright and settlers may cut it down to gain land for 17 or cultivation. If the land is then abandoned, a(n) 18 succession will take over, developing much faster on the more hospitable soil. Shrubs and trees are among the early invaders, their seeds carried by the wind, by birds and 19 in the coats of mammals.
For as long as it stands and thrives, the forest is a vast 20 , storing energy and many elements essential for life.