Changes Since the Last Ice Age |
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When most of us hear the word “migration” we think mainly of birds or other animals. Few of us would think of trees. And yet to survive climate change during and since the last ice age, trees and plants have climbed and descended mountains and traipsed across continents in every direction. When the last ice age began to release the Earth from its wintry grip, warmer temperatures nibbled away at the southern margin of the Laurentide, and tundra plants began to re-colonize the newly exposed soil. Many of the boreal species that had sought refuge in the southern latitudes began to “relocate” to the north. Spruce and northern pines, both of which had become established in the South began to retreat northward on the heels of the ice sheet 18,000 years ago. Around 15,000 years ago, the ice age’s dominant spruce species, P. critchfieldii, had gone extinct. By 12,000 years ago, the southern limit of remaining spruce and northern pines extended little farther than mid-continent, while their northern limit reached almost to Newfoundland, Canada. Fir and birch require more precipitation than spruce, and lagged the northward trek by several thousand years. |
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The end of the full glacial episode began the Holocene period, our modern era. The gradual warming experienced during the Holocene was punctuated by several flickers in climate, during which conditions would briefly become cooler, but overall the Earth was becoming warmer and most probably wetter. Between 12,000 and 9,000 years ago, spruce, fir, northern pines, and birch were all coexisting south of the edge of the glacier, which still covered much of Canada. Rapid increases in warmth during this period—probably in the form of summer temperature increases—caused spruce to decline, and northern pines dominated the early boreal forest. This change in species abundance is what Davis is talking about when she says that in many cases, forests that have existed since the last ice age are unlike any we have today. According to Davis, we have to be cautious about thinking that a whole forest ecosystem, has ever migrated, en masse, in response to climate change, or that it could do so again. “The important aspect of boreal forest migration is that the forest didn't migrate as a community. Individual species shifted ranges and fluctuated in abundance. Spruce was very much less abundant about 10,000 years ago than it is today. As spruce is such a signature species for boreal forests, can we really say we had a boreal forest at that time? Certainly it was very different from the boreal forest of today.” |
Since the height of the last ice age, the geographic range and abundance of tree and plant species in North America have changed, with many modern boreal species migrating northward. The images above show changes from 21,000 to 12,000 years ago in pine, spruce, birch, and non-grass prairie vegetation. Increasing color intensity represents increasing concentration of pollen, which is proportional to the amount of that species in a given area. The Laurentide Ice Sheet is pale blue, and areas where no data were collected are white. Spruce and pine were found in abundance in the central United States for several thousand years. About 12,000 years ago, the Great Plains began to appear more prairie-like, with spruce and pine retreating northward. (Images courtesy Department of Geological Sciences at Brown University, the National Center for Ecological Analysis and Synthesis, and the Department of Geography at the University of Oregon) | ||
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Even as recently as 9,000 years ago, both spruce and birch, by that time well established in Canada and the northern United States, were still not settled into the present range, and actually began to spread southward once again. Around 6,000 years ago, the last of the continental ice sheets had melted, and the boreal forest was beginning to resemble its current self. |
Around 9,000 years ago, the ice age was ending, and the Earth was entering the Holocene interglacial period, when moisture as well as temperature is generally thought to have been increasing. The images above show changes in North American vegetation from 9,000 years ago to the present. In each case, increasing color intensity represents increasing concentration of pollen, as in the images above. Southern pine species began to take hold again in the southeast, while spruce and birch were migrating farther north. (Images courtesy Department of Geological Sciences at Brown University, the National Center for Ecological Analysis and Synthesis, and the Department of Geography at the University of Oregon) | ||
