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1 Chapter 1 Origin of the Evergreen Forests Conifers and the Ice Age Of all the people who enter a northern forest, only a handful ever ask themselves these two questions: Where has the forest come from? And why are the great majority of its trees conifers rather than deciduous, broadleafed trees—“broadleafs” for short. The answers are by no means obvious. The questions have engaged the interests of ecologists and motivated years of research. What has been investigated and measured is not, for the most part, observable on a hike in the woods, but the hike would certainly be more interesting for somebody knowing about the questions, and the answers that have been discovered so far. Consider the first question, where have the trees come from? The only certain answer is that the trees in the area we are concerned with, the area that was ice-covered at the end of the last ice age, must have descended from ancestors that lived elsewhere. About 18,000 years ago, when the ice sheets of the most recent ice age had reached their maximum extent (figure 1.1), they covered nearly all of northern North America. The glaciated areas must have been like Greenland and Antarctica were until recently—barren expanses of ice, devoid of plant life. Then, as now, the ice was on the verge of disappearing. Conditions in the unglaciated regions near the margin of the ice sheets must have been bleak. The contrast with conditions now is worth contemplating. 2 the world of northern evergreens Once the climate began to warm up and the ice sheets started shrinking, newly exposed land became available for small plants, and hardy evergreen forests gradually invaded the area. Seeds were blown in from the south. At the time of maximum glaciation, evergreen forests stretched across the continent south of the ice margin; even the Great Plains were forested. East of the continental divide the most abundant trees were spruces and jack pines (the evidence that allows us to visualize the forests of the distant past is described in chapter 3). The forests south of the ice on the west coast had a richer mix of tree species and may have been much like they are now. The northward march of the forests into the newly ice-free land was inevitably slow. There was no soil to start with—only lifeless mixtures of boulders, gravel, sand, and clay, laid bare by the melting ice. The development of soil adequate for trees must have taken a considerable time. Different Figure 1.1. The area in North America covered by ice sheets at the end of the last ice age. origin of the evergreen forests 3 species of trees arrived to occupy their present geographic ranges at different times. Those that survived the ice age farther south than the spruces and jack pines had farther to come. And those that require shade had to wait until forests of sun-loving trees were casting enough shade for the shade-growers to invade. For example, hemlock is believed to have reached what is now northern Michigan about 3000 years later than white pine.1 The climate continued to change as it has done throughout time, independently of the current global warming (the topic of chapter 13). It was thought to have warmed fairly steadily between 18,000 and 10,000 years ago, except for a temporary cold interval around 12,000 years ago. In any case, warming resumed and temperatures reached a maximum about 8000 years ago, even before the ice sheets had completely melted. Because of the very long time required to melt huge masses of ice at the thenprevailing temperatures, probably around 2° Celsius higher than now (2010), two remnants of the original ice sheets persisted, one on each side of Hudson Bay, until 6500 years ago. They finally disappeared after the time of maximum warmth. Because of the slight downward trend of temperatures until very recently, some species of evergreen have lost ground and do not, nowadays, grow as far north as they did 8000 years ago. The Advantages of Being Evergreen The second of the two questions posed at the beginning of this chapter was, why are the great majority of the trees in the northern forests evergreen conifers? Why conifers rather than broadleafs? To say that conifers are better adapted to the environment is no answer. The question then simply becomes, in what way are they better adapted? The answer cannot be...


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