There have not been many studies of the longevity of most polycarpic plant species. The logistic problems involved and the consideration of mortality are more closely related to the size of the plant than to its age. Knowledge of the longevity in many species, particularly polycarpic herbs, is very poor.
Many herbs, such as buttercups and clovers, live for five to twenty-five years. Other herbs, such as blazing star, milkweed, and some goldenrods, may live for twenty-five to fifty years. Some shrubs, including blueberries and sumacs, can live for thirty to seventy years. Trees such as gray birch, pin cherry, and trembling aspen live for fifty to one hundred fifty years. Conifers such as hemlock, white pine, and red spruce have longevity in the range of two hundred to three hundred years, with some trees living five hundred to six hundred years. Hardwood trees such as sugar maple, white oak, sycamore, and beech can live for a similar duration. The oldest trees are the redwoods, at thirteen hundred years, and the bristlecone pines, at three thousand to five thousand years.
Most plants do not live to their maximal potential, succumbing to environmental factors. The average age that plants attain is well below the maximum life span. Beginning in the 1960's, ecologists began to examine the length of time that individual plants in a population remain alive. Although the actual patterns differ greatly from one species to another, plants typically suffer heavy mortality shortly after germination. For most species, fewer than 10 percent of newly emergent seedlings survive for two months. After that point, there are additional losses, although the death rate slows.
Some plants produce seeds that can remain dormant for many years. For example, seeds of many weed species, including monocarpic and polycarpic herbs, can remain dormant for seventy years in abandoned farm soil. Under experimental conditions, seeds of some of these species were found to be capable of germinating after one hundred years. Extreme examples of seed longevity can be seen in species of goosefoot and lotus, both of which have survived for more than fifteen hundred years. The variety of plant life cycles appears to be related to the earth's widely divergent habitats.
|
