| During the last ice age, ice covered a large part of North America, and it was impossible for trees or any type of plant life to grow on these vast frozen expanses. Several small areas, especially in what is now Alaska, did escape the ice cover, but even these areas around the edges of the glaciers were cold, bleak, and barren. |
| Tundra and desert existed in the northern U.S. just south of the ice sheets. Farther south, however, in what is now the southern U.S. there were large evergreen forests, and even parts of the Great Plains were covered in trees. Because of the colder climate, these forests resembled forests which now exist much farther north. Southeast on the Florida Peninsula, conditions were drier, and at certain times this area may have been a complete desert. |
| As the ice began to melt, more land became exposed, but it was a long time before trees could move into these new areas. There was no soil to support any trees; the melting ice sheets uncovered mainly sand, gravel and boulders. When soil eventually began to develop, trees moved northwards at various speeds. Some species had waited out the ice age farther south, and therefore had farther to move. Other species simply required richer soil and couldn't yet survive in the newly ice-free areas. | There is much disagreement over what types of forest habitats existed in which regions. Even though the ice was now beginning to melt, conditions in a lot of areas were still fairly cool and dry. Pollen data and evidence of insects or animals helps to determine the various conditions over time. By the time boreal forests reached as far north as Washington state, evidence of insect life shows that the climate had warmed considerably. |
| There was now a complete ice-free corridor running from Alaska to the southern edge of the melting ice sheets. Shrubs began to grow on the Alaskan and northern Canadian tundra. At about this same time, deciduous forests began to increase in the Eastern U.S. |
| By now, the ice sheets were greatly reduced in size. Lakes were forming from meltwater, and open coniferous forests were developing around them. But forest couldn't move into new areas instantly, so there was always a margin of barren land around the edges of the ice sheets. A long lag time existed between the melting of ice on a certain site and the subsequent development of forests. |
related topics
[boreal forests] [soil] [plants] [pollen] [insects]
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