Acorn drawing science11/21/2023 Also, white oak acorns almost all ripen the same year they are pollinated, sometimes germinating before they even fall. Species in the other major lineage, the white oak group, have no bristles on their leaves, and the leaves generally contain more soil-enriching nutrients when they fall than those of red oaks do. In most red oak group species, pollen takes a full year from the time it lands on the female flower to fertilize the seed, so that acorns-the fruits of these trees-pollinated in one year only ripen in the next. One of these, the red oak group, is composed of species with bristle-tipped leaves. In the Americas, oaks are dominated by two evolutionary lineages that you may already know. The differences between major groups of oaks are readily apparent to even a casual observer. It is a remarkable evolutionary success story, one that will have important implications for predicting how these essential trees will fare in the face of climate change-and for developing management plans to ensure their survival. But recent advances in genome sequencing and analysis have allowed us and our colleagues to reconstruct a detailed picture of the origin, diversification and dispersal of oaks. For decades scientists could only speculate about much of the evolutionary history of oaks because of gaps in their fossil record and limitations of the biomolecular techniques used to infer evolutionary events from the DNA of living organisms. To understand forests, then-their biodiversity, food webs and contributions to human well-being-one must understand how oaks came to rule them. This astounding variety, along with the fact that the oaks in this region account for more forest tree biomass than any other woody plant genus in North America and Mexico, makes them the single most important group of trees in the continent's forests. Approximately 60 percent of all Quercus species live here. Oaks are especially prominent in the Americas. Indeed, oaks have proved so valuable to people that we have immortalized them in legends and myths for centuries. And they have shaped human culture, feeding us with their acorns and providing wood to build our homes, furniture and ships. They help clean the air, sequestering carbon dioxide and absorbing atmospheric pollutants. They foster diversity of organisms across the tree of life, from fungi to wasps, birds and mammals. Oaks are keystone species, foundational to the functioning of the forests they form across the Northern Hemisphere. Over the course of some 56 million years, oaks, which all belong to the genus Quercus, evolved from a single undifferentiated population into the roughly 435 species found today on five continents, ranging from Canada to Colombia and from Norway to Borneo. But that was about to change, with the spread and extraordinary diversification of what would eventually become some of the most ecologically and economically significant woody plants in the world: the acorn-bearing, wind-pollinated trees we call oaks. The deciduous broad-leaved forests that now cover 11 percent of North America north of Mexico were in their infancy. The northeastern U.S., for its part, ranged from broad-leaved (as opposed to needle-leaved) evergreen forest to deciduous forests of ginkgo, viburnum, birch and elm, among other species. was dominated by tropical rain forest, complete with primates. In the Canadian High Arctic, which today harbors relatively few tundra plant species, year-round temperatures above freezing nurtured a rich and diverse flora Ellesmere Island in far northern Canada, across from the northwestern coast of Greenland, was home to alligators and giant tortoises. The continent's plant and animal communities were dramatically different. A sea had just closed up in the middle of the Great Plains, and the Rocky Mountains had not yet attained their full height. Back then, at the dawn of the Eocene epoch, the earth was warmer and wetter than it is today. If you were dropped into virtually any region of North America 56 million years ago, you probably would not recognize where you had landed.
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