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Not for nothing do we call Raleigh the City of Oaks.
When land is abandoned in central North Carolina, it does not lie bare for long. There is a distinctive sequence of plants that takes it over: first weeds like crabgrass and horseweed, then broomsedge and colorful wildflowers such as the bidens and goldenrod so brilliant this month. Within a few years these plants disappear under vigorously growing trees: loblolly and shortleaf pines, mostly, with sweetgum and perhaps redcedar or sugar maple. After a few decades pass, we have a forest. Botanists call this process succession and the characteristic sequence of plants causing the succession is called the sere.
There are thousands of acres of successional pine forests in the Triangle area. But the pines don't last. They get old, pine bark beetles attack them, and broadleaf trees begin to poke up between them. It may take more than 100 years, but almost everywhere in our area the sere ends with nut-bearing trees: oaks of a dozen kinds, several species of hickory, and, on moist sites, American beech.
When European settlers first arrived on the upper Neuse and Cape Fear, they found a magnificent forest of oak and hickory (and chestnut, gone now due to chestnut blight). This forest, extending from Georgia to Michigan and from Massachusetts to Arkansas, was and still is the world's largest nut-bearing forest. When you walk outside this month, you can hear the acorns falling.
Most people can name the two animals most dependent on the bounty of nuts falling this month: the gray squirrel and the blue jay. The squirrel's scientific name is Sciurus carolinensis and it is perhaps more common in the Carolinas than anywhere. Of course, we don't always see squirrels and jays in oak or hickory trees, but we rarely see them if there are no oaks or hickories nearby.
There are so many kinds of oak in North Carolina, it's difficult to learn them all. But everyone can recognize the two main groups: the white oaks and the red or black oaks. Red and black oaks have dark bark, often ridged or striped, and their leaves have bristly points on the ends of the lobes. White oaks have lighter, often shaggy bark, and their leaves, though often lobed, have rounded ends and points, with no bristly tips.
Both blue jays and squirrels bury acorns and hickory nuts for their winter food supply, and this month we see them hard at work at that task. Jays play a major role in spreading oaks to new sites, because they often carry the nuts hundreds of yards before burying them. Scientists have calculated that at the end of the ice age, oaks spread northward at exactly the rate expected as a result of this dispersion of nuts by blue jays.
Red and black oaks are spread by both jays and squirrels, white oaks mostly by jays. Squirrels don't bury white oak acorns as much; they prefer to eat them on the spot instead. Interested scientists have tried to figure out why squirrels prefer white oak acorns to those of red oaks. It turns out that red and black oaks acorns have higher levels of tannin, a bitter chemical used in tanning and dyeing. This makes them somewhat distasteful to squirrels, probably. But there's more to the story. Red and black oak acorns are also much higher in fat than white oak acorns, and squirrels need the fat to survive through the winter. So its' a trade-off for the squirrels: high fat is good, tannin is bad.
What tips the equation, apparently, is that white oak acorns sprout faster than those of red and black oaks. Not only are white oaks acorns tastier to squirrels; if not eaten quickly it may not be possible to eat them at all. So squirrels probably have evolved an aversion to tannin so that they will bury the longer-lasting red and black oak acorns and eat the white oak ones right away, while the eating is good.
Studying this web of relationships among species is what ecologists do. The eastern oak-hickory forest is a major ecosystem, one we see around us every day. It is an ancient and complex system, full of surprises, and the processes that keep it going are right under our noses every day. There is still much to learn about them, despite all the hours of study by our scientists.
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First posted October 1, 1997. Revised and links checked October 12, 1998. Features remain online as long as they remain current; they may be updated if new information becomes available.
Copyright © 1998, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
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