The evergreen is not actually immortal; its leaf growth patterns just differ from its deciduous cousins. Scale-like conifers, long needles and broad-leaf evergreen leaves, unlike deciduous leaves, last for more than a year. The tree only appears to remain green because the current year’s growth is hiding the leaves that are dying.
Evergreens evolved in the prehistoric rainforests; dinosaurs feasted on them. As the earth’s climate shifted, conifers adapted to almost every climate type except the arid deserts. Early peoples considered it sacred because of its continually green color and decorated their homes with its branches at the time of the winter solstice in hope that spring would return to their lands. Early Christians adopted the tree and its promise of the return of the light for their mid-winter celebration of Christmas.
The most widely distributed group of trees and other plants known as evergreens are conifers, a group within the family of plants called gymnosperms. Gymnosperms' rugged features relate to their place on the evolutionary ladder between ferns and angiosperms or flowering plants. New leaves protect the old and their seeds do not grow within a fruit as do angiosperms, giving their family its Latin name, meaning “naked seeds.”
The secret to the evergreen’s seeming immortality is that it engages in photosynthesis at a slow but steady rate and spends its energy efficiently. This efficiency and low level of activity serves to create more persistent foliage than deciduous plants with their rapid metabolism. Some evergreen leaves can remain green for as long as 40 years according to a paper by Son and Gower in the scientific journal Biogeochemistry.
Evergreen leaves, no matter how persistent, eventually die and are replaced with thickening woody growth on branches. The oldest needles on the tree’s interior change color, fading from green to yellow and some to rust color before turning brown and falling in winter. In the spring, evergreens grow from the tips of the previous year’s growth, providing an ever-green camouflage for older leaves as they die.
The evergreen’s long-lived foliage makes it unique in the tree family. It holds moisture in its tough needles that protect it from heat and cold. Hardy evergreens grow in temperate areas around the world. They give color to winter landscapes; gardeners in almost any climate can grow some variety of evergreen tree, shrub or herbaceous perennial.
Not all evergreens are conifers and not all conifers keep their leaves year-round. Mahogany, mountain Laurel and rhododendron, all angiosperms, are broadleaf evergreens; most are tropical or semi-tropical plants. The tamarack, native to tall mountains and cold climates, is a conifer whose leaves turn yellow and drop in autumn. (See Resource 1)