|Plant Life Spans|
The longest-lived organisms are plants. For example, one bristlecone pine tree in eastern California is forty-nine hundred years old, and some creosote bushes, also in California, are estimated to be about twelve thousand years old. People have long recognized this variation in plant longevity, but the understanding of plant life spans improved greatly after research during the 1960’s.
Types of Life Spans
The life span of an individual plant depends upon two factors. The first is the innate, genetically determined potential for longevity. The second is the effects of the environment, including soil and weather conditions, competing plants, disease-causing microbes, and herbivores.
Historically, people have classified the life spans of plants into three categories: annuals, biennials, and perennials. Annual plants live for up to one year. Biennials live for approximately two years. Perennials live for more than two years, often for several decades, even centuries.
While this categorization is useful in many ways, botanists have come to recognize that it is inaccurate, especially for plants that growunder natural conditions.
Plant life histories are now classified mainly according to the number of times that each individual normally reproduces before it dies. Two main categories are recognized using this system: monocarpic plants and polycarpic plants. Monocarpic plants reproduce once before they die (mono means “one”; carpic means “fruits”).
Polycarpic plants reproduce several or many times before they die (poly means “many”). Some botanists have defined a third group, the paucicarpic plants, that are intermediate between the two. Paucicarpic plants reproduce up to five times (pauci means “few”).
Monocarpic plants have a general life history that involves four separate stages: germination, vegetative growth, reproduction, and death.
The period of vegetative growth is very important to the monocarpic plant because during this time the plant manufactures and stores starch, which is rich in energy. When reproduction occurs, all that stored energy is devoted to producing flowers, fruits, and seeds; none is saved for the following year.
The plant literally reproduces itself to death. Monocarpic plants vary greatly in their longevity, and it is possible to recognize several subcategories: ephemerals, annuals, obligate biennials, facultative biennials, and long-lived monocarpic perennials.
Ephemerals are plants that germinate, grow, reproduce, and die within a few weeks or months. They are typically found in environments in which conditions favor active plant growth for only a short period of time during the year, such as a desert.
Desert ephemerals spend most of the year as seeds. When a heavy rainstorm occurs, the seeds germinate, and the new plants grow quickly and reproduce before the soil dries out. One species in the Sahara Desert can complete its life cycle in as little as ten days.
Annuals are plants that progress from germination to true seed within a six- to twelve-month period. Botanists recognize two major subcategories of annual plants. One is the summer annual, which germinates in the spring: The plant grows vegetatively during the summer and reproduces during the autumn. Examples of summer annuals include touch-me-not, common ragweed, and goosefoot.
The second subcategory is the winter annual, in which germination occurs in the fall, vegetative growth occurs in the winter, and reproduction occurs in the spring. Daisy fleabane and winter wheat are examples of winter annuals.
Obligate biennials are monocarpic plants that germinate and grow vegetatively over the course of one year and throughout much of a second year. At the end of the second year, the plant always reproduces, sets seed, and dies (hence the designation “obligate”).
During the 1960’s and 1970’s, some botanists doubted that obligate biennials existed in nature. Studies conducted during the 1970’s and 1980’s demonstrated that some plants, such as the white and yellow sweet clovers, are indeed obligate biennials.
Facultative biennials are monocarpic plants that have the ability to germinate, grow, and reproduce within two years. They can behave as biennials only when they grow under favorable conditions, with adequate moisture, light, and soil nutrients.
More commonly, these plants grow under stressful conditions—either infertile soils or high competition. On such sites, they grow vegetatively for three, four, or even five years before they reproduce. Examples of facultative biennials include wild carrot, foxglove, burdock, teasel, and thistle.
Long-lived monocarpic perennials are able to live formany years or a fewdecades before they reproduce—once—and then die. Well-known examples include bamboo and plants from the arid southwestern United States, such as species of Yucca and the century plant Agave. These may reproduce only after they attain an age of sixty, eighty, or even one hundred years.
Polycarpic and Paucicarpic Plants
Paucicarpic and polycarpic plants normally reproduce more than once before they die. They are able to survive for at least one year following reproduction and hence are true perennials. Paucicarpic plants are short-lived herbs that may die after reproduction but more commonly live to reproduce two, three, or four times before dying.
Paucicarpic plants are therefore intermediate between the true monocarps and the true polycarps. Examples include the common and English plantains, which are weeds found in lawns and fields throughout temperate North America and Europe.
True polycarpic plants survive to reproduce many times during their lifetimes and usually remain alive for at least ten years. Unlike the monocarps, polycarps do not expend all of their energy in reproduction.
They save some of their energy and maintain part of the plant for the post-reproductive period. In seasonal climates, some of that energy must be directed to forming structures that allow the plant to survive the unfavorable season—a cold winter or a rainless period.
These structures are called perennating buds, and they differ from plant to plant in their location relative to the ground surface. In some plants, called cryptophytes (the prefix crypto means “hidden”), the perennating buds are buried several centimeters under the ground.
Examples of cryptophytes includemilkweed, iris, and onion. Conversely, hemicryptophytes (hemi means “partial”) have their perennating buds at the soil surface; a good example is the dandelion. Both cryptophytes and hemicryptophytes are herbaceous plants, never producing an aboveground woody structure.
Phanerophytes are polycarpic plants that do produce an aboveground woody structure—the perennating buds are borne above the ground surface. Some phanerophytes are shrubs that have several shoots.
Examples of shrubs include lilac, blueberry, hawthorn, hydrangea, rhododendron, and many dogwoods and willows. A second category of phanerophytes is the trees, which typically have a single woody stem emerging from the rootstock.
In theory, most species of polycarpic plants can live for decades, if not centuries, under ideal conditions. Many do not appear to have a maximum life span because they rejuvenate their tissues with each reproductive period, as in some polycarpic herbs or because the tissues that they accumulate do not putmuch of an added strain on the plant, as in many phanerophytes.
In nature, such polycarpic plants are not killed by old age. External factors such as herbivory (consumption by animals), fire, severe weather, disease, and competition from other plants contribute heavily to die-off among individuals. Other polycarpic plants form senescent tissue that has tens their death.
Potential and Real Life Spans
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 golden rods, 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.