Life Cycle of a Conifer

Cedar, redwood, hemlock, spruce, and pine are all conifers. Conifers are gymnosperms. That is, they are trees, or occasionally shrubs, that bear naked seeds in cones. They never bear flowers, as most people understand flowers. In the commonest plants, the angiosperms, flowers are an essential part of reproduction, but conifers do without them.

Conifer leaves are needles, scales, or at any rate linear. Most conifers don’t drop their leaves in fall, or any other season. More precisely, they do drop their leaves, but not all at once. (The larch and the dawn redwood, though, are conifers that do lose their leaves in fall.) Conifers continuously drop aged or damaged leaves and replace them with new ones. By not dropping them all at once, they save the energy it would take to make new ones, and take advantage of the sun’s energy throughout the year.

Gymnosperms are very different from the more evolved angiosperms, the flowering plants. Their life cycle is different too. Conifers, for example, generally have male and female structures on the same tree, but in different locations. The structures are called strobili.

The female structure contains many scales arranged spiral-fashion into a cone. Each scale has a bract and a pair of ovules, egg-forming structures. The ovule has two parts, the nucellus and the integument surrounding it. A passageway, the micropyle, passes through the integument to the nucellus.

Male strobili are smaller. They are made of modified leaves called microsporophylls, gathered around an axis. Each microsporophyll has two microsporangia. The microsporangia contain elements that develop into grains with four cells each. Each grain contains half the genetic material needed to make a seed, and thus a new tree. The grains are pollen. When the golden pollen is ripe, the male strobilus releases it into the wind, and then withers away.

During pollination, fresh pollen lands on the female strobilus, and sticks to fluid it has secreted. As the fluid dries and shrinks, it draws the pollen through the micropyle to the nucellus. There the pollen germinates, and forms a tube. The female then produces four megaspores. These each contain half the genetic information for a new conifer, just as each pollen grain does.

One of the female megaspores develops into a megagametophyte, and the others die. Then, very slowly, the megagametophyte forms archegonia, reproductive structures holding egg cells.

Finally, the pollen tube arrives at the egg cell. It releases two sperm into it. One degenerates and one unites with the egg. This forms the zygote, at last. The seed then develops, with an embryo tree within.

The entire process can take two years or more. Angiosperms, flowering plants, can reproduce much more quickly. Yet many gymnosperms, once they sprout, can live for hundreds, even thousands, of years.

When the seeds are ripe, most conifers open their cones and release the seeds into the wind. Once shed, the seeds may travel far, often sailing on wing-like spurs. Generally, they fall to the forest duff and wait until conditions are right for them to germinate and grow.

Lodgepole pines are conifers that do not germinate without fire. The cones stay tight shut until a forest fire roasts them. Then they open, to shed seed upon the unshaded soil. That is why pure stands of lodgepole pine are often first to appear after a forest fire. The lodgepoles are followed by other trees, which sprout in their cool shade.

In general, forests renew themselves by disturbance. When a small fire, a landslide, an avalanche or other interruption causes a break in the forest cover, new trees or other plants hurry to take advantage of the sun and space. Other plants may establish themselves first, but the long-lived conifers generally return to dominance in areas that suit their growing requirements. At least, they have so far.

Lodgepoles live for hundreds of years, and bristlecone pines live for thousands. Their life cycles make humans seem like mayflies that flicker curiously about their weathered trunks, and then are gone.