Mountaineers have noted that as they climb, for example, up to the 12,633-foot Humphreys Peak in the San Francisco Peaks in Arizona, plant life changes radically. Starting among the cacti of the Sonoran Desert, one climbs into a pine forest at 7,000 feet and a treeless alpine tundra at the summit. It may seem that plants at a given altitude are associated in what can be called “communities” – groupings of interacting species. The idea is that over time, plants that require particular climate and soil conditions come to live in the same places, and hence are frequently to be found together. Scientists who study the history of plant life are known as paleobotanists, or paleobots for short. They build up a picture of how groups of plants have responded to climate changes and how ecosystems develop. But are these associations, which are real in the present, permanent?
A great natural experiment took place on this planet between 25,000 and 10,000 years ago, when small changes in the earth’s orbit and axis of rotation此文共有24页 上一页 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 下一页 |
