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Evergreen Trees

Trees and shrubs are divided into two groups, based on whether or not they possess complete flowers and fruit (angiosperms,) or have naked seeds that are encased in a woody cone (gymnosperms.) Angiosperms are commonly called “hardwood,” or “deciduous” trees, because their wood contains chemicals such as lignin which makes it hard and heavy, and they shed their leaves in the fall in temperate regions such as Maryland.   Gymnosperms are more commonly referred to as “evergreens,” or “conifers,” since most have needle-like leaves that persist year round, or they bear their seeds in cones.

Giant Sequoia at Sequoia National Park, California
David Barrow, FCFCDB Member at the base of the tree
Photo by Jan Barrow

Conifers are a more primitive tree, and were the dominant group during the age of dinosaurs.  The dominant conifer canopy was eventually surpassed by hardwoods about the time mammals first appeared on the scene because evergreens were dependent on wind or gravity to fertilize and distribute their seed, whereas angiosperms had flowers and fruit to attract pollinators, along with wind and gravity. This provided more chances for success, and enabled more opportunities for cross-pollination in hardwoods. Deciduous trees could also drop their leaves and enter a state of dormancy when conditions were not especially appealing for growth and survival, such as wintertime or during a prolonged drought, whereas evergreens did not have that advantage. In addition, it normally takes longer for cones to mature, compared to fruits,  and some cones will remain closed for years unless unusual circumstances occur, such as when they are exposed to very hot temperatures ( e.g. forest fires.)  As such, hardwoods had a distinct advantage in reproduction and adjusting to changes in climatic conditions, compared to the evergreens.

Despite their more primitive nature, conifers are still a major part of our forests, especially in arid regions, high altitude mountainous areas, or northern latitudes where cool climates prevail.  The presence of smaller, waxy needles prevent excessive water loss, so evergreens can inhabit areas where water may be scarce due to dry climate or shorter growing seasons.  The internal structure of the woody “tracheid” cells and needle-like leaves also provide a very efficient means to transport water and nutrients up and down the tree, enabling  evergreens to conserve water and efficiently transport it to all parts of the plant. This same adaptation also allows evergreens to grow to a very large size— especially trees like giant sequoia, redwood trees, western red cedar, and sugar pine.  Because evergreens don’t shed all their needles at once like deciduous trees, they do not expend as much energy as hardwoods do when leafing out in the spring. This adaptation allows evergreens to survive is some soils that might be too barren for hardwoods.

Conifers are found in every region throughout the world, but they are especially plentiful in cooler climates and drier areas such as the boreal and artic regions and in mountainous areas. Throughout the globe, conifers are grouped into seven families, and within these families are 68 genera and about 600 species of cone-bearing trees. Some of the more common genera found throughout the North America are: pine, spruce, fir, hemlock, yews, true cedars, junipers, hemlocks, meta sequoia, larch, and gingko. Future articles will overview some of the more popular species in these genera. 

Article by Mike Kay, DNR Forester for Frederick County

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